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Forest roads in the tropics - I


JEAN LE RAY, Conservator of Waters and Forests, is on the staff of the Centre technique forestier tropical, Nogent-sur-Marne, France.

I. General characteristics, roads on compacted oil: and planning for alignment


IN MOST cases the problem of transporting logs and timber in tropical forests is very different from that in temperate forests, for tropical forests have seldom been subjected to rational management. Up to now most tropical countries have not had the necessary means or time to provide their forests with a system of roads or railways where they are required. However, world forest product requirements can only be satisfied by increasing the exploitation of the great tropical forest areas in different countries.

Two factors in recent decades have made possible the rapid development of the utilization of tropical woods. In the first place, studies carried out on the properties of the timber have made it possible to use some tropical woods for their particular characteristics. Thus okoumé, Aucoumea klaineana from west Africa (Gabon, Spanish Guinea, Congo), and the Shoreas (seraya in Borneo, meranti in Sarawak, lauan in the Philippines) from southeast Asia are at the moment in demand for the manufacture of plywood.

Secondly, modern methods of forest exploitation permit the economic extraction of these woods in spite of unfavorable natural conditions. Of the different stages in exploitation which make it possible to bring the wood from the actual place where the tree has grown to the log yard or to the point of export, transport presents the greatest and most costly difficulty.


According to local conditions, exploiters have used and are using today transport by water, by narrow gauge private railway, by forest road and by public railway. Each of these methods has its own draw-backs and each is examined briefly here in order the better to indicate their respective roles.

In every country it was by waterways that the forest was originally exploited. Rivers and lagoons provide a means of transport which is indeed slow but sure, and very cheap over long distances, whether the logs are floated or transported on barges or lighters. The public railway provides for the forest through which it passes a means of transport over long distances which is very fast, safe and relatively cheap. Unfortunately, when the areas immediately surrounding rivers and railway lines have been exhausted, the exploiters have to fell trees growing further and further from these means of transport and carry the logs to the nearest river or railway station. There are two solutions to the problem which can be used over long distances: the narrow gauge railway and the forest road; both are built at the sole expense of the exploiter and are used only for the transport of wood.

Around the years 1920-25, the narrow gauge railway became increasingly general in many logging operations, first with steam locomotives and then with diesel engines. It had long been thought that difficulties due to a very wet climate and to the weight of the logs would not allow the construction and, above all, the use of the roads by heavy trucks. The first results obtained in making roads of compacted earth, following the first work of Proctor in California, U.S.A., have shown all the advantages that can be derived from this new technique in making roads where there is no hard material.

These roads of compacted earth have made it possible to lay out a widespread road system in the heart of the tropical forest. Thus the opening up for exploitation of forests far away from rivers and railways has become a reality and the potential wealth of the vast tropical forests can be mobilized. With modern techniques, when the efficiency of a narrow gauge railway - with, say, a gauge of 60 centimeters - and road haulage is compared, the latter is always preferred.

The cost of making the formation for a single-track 60 centimeter gauge railway in a level area is less than that of building a road for logging trucks. When the forest is on uneven ground, a road which can have gradients up to 6 or 8 percent requires less elaborate work than a narrow gauge railway which cannot function on a slope of more than 3 to 3.5 percent. The equipping of a track with rails and sleepers, etc. for a rail weight of 20 to 30 kilograms per meter costs about the same as that of the construction itself, while the completion of a road only requires work on drainage and on compaction which is relatively simple. From the point of view of maintenance, a narrow gauge railway needs two or three times more manual labor per kilometer than a road. Nowadays, specialized equipment needing very little manual labor is used to provide mechanized road maintenance.

The proportion of transport costs proper are definitely less for a narrow gauge railway than for a forest road. For long distances, the slowness of trains traveling at about 10 to 15 kilometers per hour is a disadvantage compared with the average speed allowed for trucks, which is 40 to 50 kilometers per hour. On the other hand, a permanent forest railway can ensure a regular rhythm of transport independent of rainfall, whereas road transport can be stopped by rain.

To sum up, extraction by means of a forest railway can provide a solution adapted to the special case of a very rich forest on fairly level ground, where the rainfall is high and there is a long period of exploitation on the same concession. On the other hand, in every case it can be confidently advised that a system of roads would be more economical.

The construction of forest roads should always be undertaken with an eye to economy, in relation to the difficulties of the terrain and the limited and temporary nature of the traffic which these roads have to carry. In particular, it is out of the question to give these roads a protective covering of bitumen or tar, which would be prohibitive in cost. Furthermore, the building of these roads cannot be carried out according to the normal methods of planning and construction usually adopted for permanent public roads: such methods would result in too high a cost. But it must not also be concluded that the construction of forest roads should avoid following all rules and thus be left to the initiative of incompetent constructors. Forest roads demand special skill and precise local knowledge from the forester. The success of exploitation depends largely on the cost of construction and the state of the road system for the transport of the wood.

The present study, which is based on experience / gained in the forests of west Africa, is an attempt to give an account of the results which can be obtained most economically. It comprises five sections, as follows:

1. Principles of forest roads, their use, and their profiles and cross sections
2. Principles of road construction from compacted soil, the properties of the soil and the composition of the road base
3. Study of the alignment
4. Process of construction: clearing, earthworks, compaction, leveling, drainage maintenance
5. Problems of choice of equipment and its maintenance

1. General characteristics of forest roads


Unlike roads used by the general public, forest roads serve a limited purpose. Their specialized function stems from five unusual characteristics: light traffic, traffic mostly in one direction, haulage, traffic of long and heavy trucks. The following specific points are, therefore, of importance.

1. The traffic on forest roads is usually limited in numbers, being restricted to the extraction of forest produce and the activities connected with it. With unmanaged forests which are sporadically exploited, as is the case with most tropical forests, transport will be of short duration, but it may be seasonal in the case of forests under systematic management. The forester, whether he be the manager or the exploiter, has to balance the cost of construction against the volume of the yield from the felling coupes. This is, in its turn, limited by the possibility of the forest and by the current commercial demand. The road builder's concern is to construct roads as economically as possible, and he is compelled to find a solution which is a judicious compromise between the natural conditions of the area and the actual needs of the traffic which he envisages. On the one hand, to construct a road for too low a yield prevents the exploiter from extracting the total volume estimated; on the other hand, the cost of a road constructed to too high a standard cannot be met by the value of the produce from the forest which this road serves. The work of studying and constructing forest roads must always be considered and carried out in the light of existing economic conditions.

2. Almost all the transport is in one direction, going from the forest to the points of consumption or redistribution. The tendency is for the extraction to be toward landings or depots where the loads are deposited and sorted or to sawmills and factories. The landings are situated in the neighborhood possibly of a wharf, a waterway (tributary, river or lagoon), a junction on a main road carrying heavier traffic, or a station on a main railway. Wood-users (mostly sawmills) are the concentration points for forest produce. Thus the profile of a forest road, as of a forest railway, can and indeed must have somewhat different characteristics in the two directions. Vehicles returning empty to the forest can manage steeper descents than those they climb when traveling laden.

3. Forest traffic is concerned essentially with light vehicles for communication and with vehicles for transporting long and heavy logs. Communication vehicles, used to carry personnel, staff or workmen, are of the light ordinary touring, farming or military type. Jeeps, pickups, station wagons, vans or ordinary touring cars are the most popular

These vehicles can go everywhere that trucks can go. It is the log-carrying truck which in fact determines the design of the road. These trucks are slow, heavy vehicles, usually composed of a truck with two or three back axles pulling a special pole trailer or semitrailer usually having one and sometimes two axles. An up-to-date model is a 150 h.p. motor truck with a semitrailer which makes a unit with a wheel base of 15 meters, an over-all length of about 20 meters and a laden weight of 30 to 35 tons. These articulated vehicles should be able to take curves, climb slopes at a reasonable speed and descend hills in perfect safety. The ruling gradients should remain slight even at the expense of a certain increase in the length of the route.

4. The number of journeys past a given point on the main road serving a logging area will never be excessive, even during periods of intense activity. A total of 20 vehicles a day in the two directions could be considered a good average. In some exceptional circumstances, 30 journeys might be made. Circulation remains such that the movement of vehicles is independent of each other there is no problem of too much traffic. At the most, safety rules may impose some control at special points, for example, blind corners, approaches to bridges, crests of hills. Generally speaking, roads for only a single line of traffic are adequate.

5. A forest road will be used for the haulage of wood products harvested over all the forest area. It is always a connecting road with the object of approaching as closely as possible to the felling coupe so as to reduce the short haul. The shortest possible route is not always chosen irrespective of cost. If a shorter route can only be made at a higher cost of construction, it must be abandoned in favor of a longer but less expensive one.

The above points justify the following rules which are peculiar to forest roads:

1. All expenditure for permanent construction of too high a standard should be discarded in favor of a more economic temporary solution adapted to the present needs.

2. Maximum ruling gradients uphill in the direction of the forest (returning unladen) can be considerably more than the downhill ruling gradients coming out (traveling laden). It will be seen later that 8 and 4 percent, or even 12 and 6 percent can be accepted according to the terrain.

3. The selected gradients of the route up and down should be as easy as possible, as the majority of the traffic consists of heavy logging vehicles.

4. Generally a forest road consists only of a single track with some widening at special points: on bends and tops of slopes.

5. Taking the exploitation area as a whole, a longer winding route can be more economical than one shorter and more direct.


In the extraction of forest produce each road or section of road does not have exactly the same part to play, nor is it subjected to the same amount of traffic. The characteristics of each section of road depend on its function in the road system, whether it is in the management of the forest or, which is much the same thing, serving a forest concession in the course of exploitation.

The object of exploitation is to carry away from the forest all the timber from the stump to a specified place or a specified outlet, often serving the whole forest. Nearby logs are dragged to a landing or depot which is accessible to trucks. Each landing is linked by a branch road, which will carry the produce toward larger and busier roads, till the outlet is reached on the public highway, waterway or permanent railway. Thus each successive section carries traffic which grows in proportion to the distance of the felling area from the public highway, waterway or railway.

An example will show the role of each section of road according to the volume of produce to be carried and the length of time it will be in use. To simplify matters, let it be supposed that the forest to be exploited is uniform, the terrain level and that the state of the market justifies exploitation at an average of 10 cubic meters per hectare. If the maximum distance of the short haul is 1 kilometer, each section on this road will receive logs from areas 1,000 meters apart; thus each section of 1 kilometer will serve a compartment of 2 × 1 kilometers, or 200 hectares, from which 10 × 200, or 2,000 cubic meters, will be extracted. Each time the road is extended by 1 kilometer, the volume it carries on each section will increase by 2,000 cubic meters. In other words, on a given section of 1 kilometer the amount of traffic will be as many times 2,000 cubic meters as there are or will be kilometers of road in existence. The length of time during which transport will take place on each section can also vary between wide limits, from a week or two for the landings furthest away to several months for the nearer sections. At each road junction, the traffic grows by the addition of that brought in by the converging roads and varies in accordance with the distance from the original site. This statement is obvious, but it is necessary to explain that, if extraction roads can be roughly constructed at their extremities, it is absolutely essential to give considerable care to the layout and to the construction when the road has to serve for several months.

Roads for exploiting the forest can be classified into three categories: main roads, secondary roads and feeder roads (Figure 1).

The main road

This takes the timber to the point where it meets the public highway, serving the whole concession. Every activity connected with the exploitation is organized along this road, which is the backbone of the concession. The volume of produce to be extracted can vary from a few thousand to several hundred thousand cubic meters. This roadway must remain in a good state for several, sometimes even 20, years and must be passable the whole year round, including during the rainy season. Its length depends essentially on the geographical situation and the importance of the landings; it can be extended progressively to 50 kilometers, and more. For this backbone the question of the alignment and the construction of the road are obviously the greatest and most complex problems. It often happens that finally this road is incorporated in the public highway system. This main route should in practice be in existence before felling starts.

FIGURE 1. - Layout for a system of forest roads leading to a river suitable for floating and for barges. The number of knee indicates the importance of the road.

Secondary roads

These give access to the area in the process of exploitation, merging into the main road. Their function consists in carrying the timber felled during one or two seasons only. Their length of service, as far as the transport of logs is concerned, may be but a matter of months - two years at the most. In exceptional circumstances these secondary roads can carry traffic required by the general needs of exploitation (transport of equipment, camping requirements, etc.). Thus, as will be seen later on, at the beginning of each season, at least one secondary road should be prepared in advance, so that the surface has the chance to settle anyway during one rainy season.

Feeder roads

These are short sections which allow logging trucks from the secondary roads to reach the landings. Their length, which is always relatively short, varies from a few hundred meters to less than one kilometer. It sometimes happens that some landings are placed along the secondary road. If the area of forest served by the landing can be cleared in one operation, the feeder road is only used for a few weeks at the most, and then it is abandoned.

To sum up, the composition of a road system for exploitation should conform to the following pattern:

1. The main road must be laid out, built and maintained with all the resources available.
2. Secondary roads are made to last only for a short time and do not need maintenance.
3. Feeder roads need only the minimum necessary to reach the felling coupes.


For every road the alignment, profile and cross sections are planned. Their characteristics are specified in the following standard documents: a map fixing the line of the road; a longitudinal profile; a typical cross section defining the normal layout and width of the formation; and cross sections to illustrate the layout at individual curves, embankments, cuttings, etc.

The planning of forest roads does not usually entail the preparation and drawing up of such standard documents. The conditions for carrying out the work and the amount of traffic are such that a plan showing the line of the road and some cross sections will suffice; for example, the side of a hill, an embankment, a cutting, a blind corner.

It is now possible to specify the factors to be considered in the study of a forest road, that is to say of a road planned particularly for exploitation:

1. The line of the road is determined mainly from the general configuration of the terrain of the forest to be served.

2. The cross section must be chosen according to the requirements of long heavy vehicles.

3. The normal cross section must provide for a single line of traffic with passing places for vehicles at very slow speeds.

4. The cross section will need to be modified in places to meet local variations in terrain and soil.


To facilitate further study, the standard cross section is now to be examined, thus defining the different parts of a road (Figure 2).

FIGURE 2. - Normal cross section of a road.

In every cross section can be distinguished:

(a) the carriageway made for the passage of vehicles;
(b) the formation, between the ditches or tops of embankments, including the carriageway and the shoulders;
(c) the roadway between the extreme limits of the earthworks;
(d) the road reserve, which corresponds to the limits of the ground affected by the road, that is, between the fences, if any.

The conception of the road reserve arises only in areas where rights of property or occupation are demarcated on the ground. The roadway corresponds to the width of stumping in places where the road passes through wooded areas. The formation is wide enough to allow passing or overtaking of vehicles on single track roads. The carriageway of a forest road is nearly always a single track because of the relatively small number of vehicles using it each day.

Each cross section must comply with three essential conditions: it must ensure drainage of the road; maintain stability of vehicles; and allow passing and overtaking.

Cross section of carriageway

A convex shape is nearly always used; this makes for stability of the carriageway and allows rain falling on the road to run off to the sides.

In practice, the costs of waterproofing or sealing material for the surface, for example, bitumen, tar and cement, are too expensive for their use to be economic on forest roads. Such waterproof surfacings are used only on roads with properly constructed bases. It will be seen in Section 2 that it is essential to avoid standing water, seepage and, of course, gullying. The choice of slope in the cross section must be a compromise; it must be steep enough to ensure the rapid runoff of water and gentle enough to prevent any gullying. It is desirable to ensure that the run-off of rain water should be in the form of a sheet of water of reasonably uniform depth, thus reducing infiltration and gullying to the minimum. Any puddles increase seepage and lead to localized run-off in rivulets and not in uniform sheets of water. These rivulets tend to converge and result in a network of small gullies.

The surface drainage of the road is easier the narrower the road. The most effective slope is from 3 to 5 percent. To avoid puddles or an accumulation of water it is better to make a slope which increases from the center to the edge. The shape of the road is usually defined in terms of the camber, that is, the slope from the center to the sides.

Table 1. gives the elevation of the center of the carriageway over the edges for roads of different widths and degrees of camber.


Width of road

Side elope or gradient

3 percent

5 percent










The construction of the formation by mechanical means, which is becoming increasingly general, facilitates making cross sections having a uniform slope rather than a surface with a variable curve. Thus, when grading a road, it is convenient to make two uniform slopes which are easily checked with a template.

It should be noted that the amount of the side slope must be limited so as to provide a suitable base for the dual wheels of vehicles and trailers.

Width of carriageway

The width of a carriageway depends on the amount of traffic envisaged for the road. In most cases there is only one exploiter who is interested in the amount of traffic working on one concession. Thus it is only the requirements for exploitation which have to be considered. Bearing in mind the effect of construction costs on the net cost of exploitation, the width of the road must be adapted to the amount the road will be used by heavy trucks; these will constitute the essential element in the traffic.

It is sometimes thought that the need for logging trucks to pass each other makes it necessary to plan for a double track. This solution involves both heavy and unnecessary expenses which it is quite easy to avoid. Indeed, passing and overtaking can be achieved without difficulty if the lighter vehicle reduces speed and makes use of the shoulder. It must be agreed that priority is always given to the laden vehicle. The empty logging truck, or the light vehicle, can get out of the way on to the shoulder and allow the laden vehicle which is traveling on the crest of the road, to pass. When a light vehicle is overtaking a logging truck it will overtake as on a normal road, by using the right hand shoulder where the rule of the road is to drive on the left. No accident need be feared if sufficient visibility is ensured at all points by a good alignment in both plan and profile, and by subsequent maintenance in controlling the regrowth of vegetation at strategic points.

On a single-track road, all vehicles should keep to a moderate pace, priority being given to those which are laden. In this case a roadway of 3.50 meters together with the shoulders should be sufficient to allow for 20 to 30 vehicles a day.

The few limitations which this width imposes on the traffic are very light in view of the considerable economies it allows. A wide road always tempts drivers to go very fast, thus causing accidents. By way of comparison, experience has shown that on a modern busy road the width of each track should be at least 3.50 meters to allow for minimum traffic at the rate of 200 vehicles an hour.

FIGURE 3. - Above, a ditch dug by hand; below, one made by a grader.

In cases where additional traffic above the usual needs of exploitation can be foreseen, a roadway of 4 meters between the shoulders becomes necessary.

Shoulders (or verges)

These can be of two types: flush or raised.

Along metaled roads made of macadam, which are still the most usual in temperate climates, the shoulders or pavements stand about 10 centimeters above the road.

Roads on stabilized soil, on the other hand, are always made so that the shoulders continue the camber of the roadway - these are flush shoulders. Their transverse slope should be at least equal to that of the road to facilitate the complete run-off of water - say, 4 to 5 percent toward the ditch. They can be made use of by light vehicles when they are passing or being overtaken on a single track road. The width of the shoulders often varies and is only limited by the need for loaded trucks to pass each other. In practice, a minimum width of 1.5 to 2 meters is sufficient for all needs.


Ditches can have a traditional trapezoidal section, or a triangular section if they are dug out and maintained by the grader. The width at the top is 1 to 1.50 meters as required (Figure 3).

To ensure drainage of the subsoil under the carriageway the top of the ditch should be 50 centimeters lower than the road. Ditches must be planned sufficiently wide to fulfill their function. There are two stumbling blocks to be avoided: deposits of debris or silt which may block the drains, and erosion which may threaten their shape. The longitudinal slope should be greater than the minimum to avoid silting up, but also less than the maximum of about 5 percent to avoid gullying which might destroy the shoulders.

It should be remembered that ditches exist into which the side drains empty themselves and also the catch-water drains which prevent the erosion of the sides of cuttings.

In conclusion, the roadway comprising the carriageway, berm and ditches between the extreme limits of the earthworks, extends over a width of at least 8 to 11 meters on a straight alignment. To allow for earthworks and subsequent maintenance of the road in a wooded area, clearing and stumping must be done on a strip in accordance with the needs of these operations; the width of the strip must be chosen to meet each particular case.


It can be taken as a basic approximation that the alignment of the center of a road is a succession of straight lines connected by arcs of a circle. On the ground, the alignment is a compromise between the characteristics of the road and the peculiarities of the terrain. The more the land is cut by valleys, the more the alignment must to avoid heavy earthworks.

A curve can be considered as a special place which slows down traffic; thus each curve is the subject of a special study to satisfy the rules governing the safety and the capacity of the vehicles. Three factors in turn will be examined:

(a) stability of vehicles under the action of centrifugal forces; banking;
(b) visibility, particularly in cuttings or on bends;
(c) ability of vehicles to negotiate curves.

Stability of the vehicle - minimum radius - banking

In the action of turning, a vehicle is submitted to centrifugal forces. Its stability is only ensured if it can stay on the road without slipping. Every driver has experienced the tendency to skid toward the outside when his vehicle is traveling fast round a sharp bend. This tendency is greater the shorter the radius of the curve and the more slippery the road. To overcome this difficulty and to give vehicles more stability on a bend it is possible to raise up or bank the outer part of the curve. At a low speed, therefore, the minimum radius of a curve is given by the outside radius of the turning circle of the vehicle. For long vehicles like trucks and tractors with semitrailers, this radius is from 15 to 20 meters but, to avoid sudden braking by heavy vehicles, it is preferable to allow a minimum radius far greater than that of the turning circle. The length of this radius is a compromise between the necessity for slowing down which affects the flow of traffic and the cost of construction which affects the usefulness of the road. One may discriminate between a normal limiting radius which should be adopted and a minimum radius to suit exceptional cases.


Normal radius

Minimum radius


On fairly even ground



On very uneven ground



Below a radius of about 300 meters, plans should be made to bank the curves.

For a curve of a given radius, the banking should be higher when the surface of the road has a poorer vehicle-holding capacity. However, a bend cannot be raised too much on an earth carriageway; the banking must remain moderate in order not to hinder slow vehicles and not to cause cross gullying The slope should not exceed 5 percent on an earth road. On a banked curve the camber is not kept and the roadway is given a constant side elope or cross fall.

The transition of the carriageway from a banked curve to the normal carriageway of a straight alignment must be gradual in order to maintain the stability of the vehicles. It is generally agreed that the transition should be made progressively on a gradient of 1 percent, that is, over a length equal to 100 times the greatest height of the bank. On a bend with the normal limiting radius (100 meters) the banking can be up to a maximum of 250 centimeters, which will mean a progressive transition over a length of 25 meters. This maximum distance corresponds to an elevation on the outside edge made entirely above the profile.

Sight distance - stopping distance

Every driver knows that there is a certain delay between the moment he notices an obstacle and the moment he begins to slow up his vehicle and put on the brake. This delay in reaction varies from ½ second to 2 seconds. Similarly, there is a delay between the moment when the brakes begin to act and when the vehicle has stopped in front of the obstacle. It is obvious that the faster the vehicle is traveling the longer is this delay. It is therefore necessary to plan and build a road in such a way that at any given moment a driver can see enough of the road in front of him to allow him to stop before he reaches an obstacle. This distance is called the stopping distance. As the obstacle can be a vehicle coming in the other direction, the minimum sight distance must be twice the stopping distance.

Several factors must be taken into account in determining this distance, in particular: the height above the ground of the driver's eye; the height of the dangerous obstacle to be avoided; the speed of the vehicles; the grip of the tires on the road's surface. The calculations which determine the distances needed in each case will emerge in the course of this study.

Table 2 shows the sight distances needed according to maximum speeds allowed for the vehicles on the road which is being studied.


Maximum speed allowed

Stopping distance

Sight distance















NOTE. The figures are obtained from the formula:

where D = stopping distance in meters.
V = velocity of vehicle in kilometers per hour

The first part of the formula is the distance traveled during the delay in the driver's reaction, and the second part the distance traveled between applying the brakes and stopping.

The minimum sight distance must be verified for every bend: this is particularly important in cuttings where the inside bank juts out and hides the road. This sight distance imposes fairly stringent conditions even for curves of small radius taken at a reduced speed. Bearing these points in mind, it is possible to improve the visibility by several methods.

Figure - Visibility berm (indicated by arrow) on sharp curves in a cutting.

In the most usual case, the inside of the bend is clearer, not to the level of the carriageway but to that of the eye of the driver, that is to say, in practice, to about 1.0 to 1.25 meters above the center of the carriageway. This is called a "berm of visibility" (Figure 4).

On a bend the two streams of traffic can be separated without an excessive widening of the carriageway. It is sufficient to place a line of stout stakes (8 to 15 centimeters in diameter and 1 to 1.5 meters in height) along the center of the road. This arrangement is very effective on sharp bends and on the top of hills (Figure 5).

When the terrain requires a curve of a very small radius (less than 50 meters) on a steep side slope of more than 80 percent for a section of road including both cutting and embankment, it is sometimes easier to build two parallel tracks than to make a single track of the same length. Making two tracks, one for each direction and stretching for 2 to 300 meters results in less extensive earthworks, quicker construction and easier drainage on curves (Figure 6).

Widening curves - negotiation of curves by vehicles

Articulated vehicles, such as logging trucks with semitrailers, have difficulty in negotiating small radius curves. When the direction of the front wheels is changed suddenly, the back wheels of the tractor describe a trajectory of increasing curvature. They tend to cut the corner. The vehicle is thus taking up more space than its width along a straight line. This extra width in the curve must be planned by a symmetrical widening on both sides of the curve or on the inside. This extra width in the bend is gradually decreased till it reaches the straight alignment before the beginning and end of the curve (Figure 7).


Radius of curve (R)

Extra width (S)

Length of tangent (T)














Siting curves

There are special places where curves cannot be allowed even when the configuration of the terrain would normally demand it. Curves are to be avoided on embankments, in cuttings or at a junction with a main road.

Siting bridges

Forest bridges are nearly always temporary and narrow. Even if their condition does not require vehicles to stop before crossing, it is essential to have the bridge approaches in a straight line for at least 30 meters to reduce the danger of accidents (Figure 8).

Two curves in opposite directions should be separated as much as possible by a straight alignment - 40 meters on easy terrain and 10 meters in broken or uneven country. It is a mistake to make an isolated sharp curve on uniform ground and run the risk of the driver not being prepared for it.


The longitudinal profile is best made in a series of continuous curves. In fact it is composed of straight lines connected by vertical arcs. Sudden variations in slope should be avoided at all Costs, for instance, dips crossing the road and humpbacks. These are dangerous points for vehicles and unpleasant for passengers.

FIGURE 5. - Cross-section of road at the top of a hill. Arrow on left indicates the natural ground level.

FIGURE 6. - Double track for traffic on the sharp curve in a cutting.

The profile should fulfill several conditions:

(a) ensure the run-off of water;
(b) avoid a marked slowing down on the part of heavy trucks on slopes;
(c) avoid too sudden braking on steep slopes;
(d) ensure good visibility at all points.

Flow of water - gullying

Standing water on the carriageway should be avoided at all costs on all roads, especially on earth roads, as are most forest roads. A slight slope is therefore necessary and always preferable to a level section. A slope of 1 percent should be considered the minimum. In this case the camber should be the maximum possible. It has already been seen how the need for good drainage imposes maximum and minimum limits on the camber of the cross section.

On the other hand, the scouring due to gullying caused by rainwater grows very quickly as the slope becomes steeper on earth or stabilized roads. Above a slope of 5 percent scouring gets worse and therefore necessitates particularly extensive maintenance. As the gradient becomes steeper and the slope longer the scouring due to gullying increases.

FIGURE 7. - Gradual widening on a curve.

FIGURE 8 - Curves on the approach to a bridge.

Vehicles on ascents and descents

It is necessary to know the relative importance of the different resistances that a truck has to overcome in order to understand properly the influence of rising gradients on the passage of logging trucks, which are often fully laden. It is known that, in a general way, resistance to movement is composed of four elements: resistance to rolling in the driving axles and free axles; resistance of the air; the weight (when ascending or descending); and acceleration when the speed is not uniform.

Whereas the resistance to rolling and air only absorbs a small part of the power developed by the engine, the weight on the contrary takes far the greatest part of this power. It is usual to see trucks slow down and labor on hills. Any slowing down does not allow the truck to pick up and the driver is forced to reduce his speed sometimes under very difficult. Various theoretical studies have shown that heavy trucks when laden cannot climb slopes of more than 6 percent except at a very slow speed and at the cost of excessive fuel consumption and wear and tear on the engine and transmission.

In the same way, descents mean incessant braking which is partly achieved by special systems or appliances called speed retarders.

To limit the pulling effort imposed on vehicles, slopes on small radius curves should be easier than on the straight. A rule can be laid down that a rising gradient on a curve should never be more than 5 percent.

Bearing in mind the effect on gullying and on transport expenses, the following limits on gradients should not be exceeded:

Type of terrain




Fairly even ground



Very uneven ground




This problem only arises in the case of passing, as traffic on forest roads is so small. Hill tops are often dangerous points.

At these points it is easy and economical to construct two one-way tracks separated by a line of stout stakes 8 to 15 centimeters in diameter and 1.0 to 1.5 meters high. The two tracks cover the whole of the formation, thus reducing or eliminating the shoulders (Figure 5).

2. Roads on compacted soil

The construction of roadways capable of carrying heavy vehicles is not dependent on the presence of hard material which has been transported at great expense. Experience has shown that, when the moisture of the natural soil is within certain limits, the soil can perfectly well carry road traffic; this is the case with a dry clayey or a damp sandy soil. It is also known that these qualities are modified and can disappear when the moisture content changes. Systematic studies, originally undertaken in the United States and later followed in many countries, have led to the development of a technique for stabilizing soils. This technique has the immense advantage of making the quick construction of a road possible by the addition of less expensive material than a macadamized road. The transport of the material and its addition is restricted to one application for improving the soil. The technique of making roads of compacted soil is based on the study of the properties of soil: the particle size distribution, and the effect of water and soil compaction.

The roadway is made up by a combination of different layers made to withstand the passage of the vehicles and to distribute their weight to the sub-base. Thus it is necessary that we should be familiar with the action of the vehicles on the carriageway and with the properties of the soil.


Vehicles destroy the surface in various ways. The weight of the vehicles is transmitted by pressure through the tires. In considering the amount of inflation of the tires, the pressure on the road is slightly more than the tire pressure (about 10 percent). It is the tire pressure which determines the destructive action on the upper layers, whereas it is the weight of the axles which contributes to the wear of the sub-base.

The wheels create a tangential stress on the road when accelerating, braking or skidding. These various functions cause wear to the tires and to the road. Impact caused by irregularities on the road add a dynamic effect to the static action of the weight of the vehicles. This explains why holes become deeper so quickly and justifies very regular maintenance. Thus it can be said in general terms that on a road with a soft surface the resistance of the soil at a given point depends essentially on the thickness of the carriageway above this point. The planning of this thickness therefore depends on the quality of the soil of the sub-base.



When soil is examined, particles or grains can be distinguished which differ greatly in size. These different particles have been named according to their dimensions:

Coarse grained soils:


larger than 20 millimeters


between 2 and 20 millimeters

Coarse sand

between 0.2 and 2 millimeters

Fine grained soils:

Fine sand

between 0.02 and 0.2 millimeter (i.e., 20 and 200 microns)


between 2 and 20 microns


less than 2 microns.

It is possible by physical analysis to measure the weight of both small and large particles to a given size which exist in the soil under consideration. This analysis is carried out by sifting the grains above 0.1 millimeter and by sedimentation of the particles below 0.1 millimeter. Soils can be found in which all the size particles are distributed and which are capable of being compacted (as will be seen later). liver terraces are often composed of alternating layers of varying thickness of sands and gravels which can be used after a simple separation process. There are also certain layers composed of a mixture of gravel (as understood for road material), coarse sand, fine sand, silt and clay which can be used in their original state without any screening. They are like the unsorted material from a quarry. Special mention should be made of unsorted lateritic material which comes from certain layers of granular laterites. In these different soils it is the fine particles, particularly clay in the presence of water, which give cohesion to the soil. This cohesion is due to capillary forces which bind the fine particles together in the soil.

Function of water

The presence of clay and water plays a very important part in the behavior of soil. It is known that clay has the advantage of drying up slowly and of keeping its cohesion in dry weather, and the disadvantage of being unstable when wet. Very permeable soils, such as sand, are less affected by water. Nonporous soils with a large amount of clay do not soak up water well. On the other hand, intermediate soils like silt are the most sensitive to the action of water.

The group of particles smaller than 0.42 millimeter (passing through a No. 36 B.S. sieve) make up the matrix and have an essential function in the behavior of the soil in relation to water. The amount of water in the matrix determines the changes in the consistency of the matrix which passes from a paste-like state, called fluid, to a plastic state where the soil keeps its shape and can be kneaded by hand but loses its shape under a light load. The presence of organic matter or humus increases the sensitiveness of the soil to water. For this reason the surface of the soil which contains decomposing vegetable matter must always be removed from the surface of the road before earthworks start.

Stabilization and compaction

All natural soil in situ as well as loose soil, contains many empty spaces which are filled with air; these spaces lead to shrinkage under the influence of pressure. To be able to use any soil as the base of a carriageway, it must be stabilized, that is to say, it must be improved so that it can carry traffic even under unfavorable conditions of wetting or drying. This improvement can be carried out by several methods: by compaction, by altering the particle size distribution, or by changing the properties of the matrix. In practice it is compaction which is most general on forest roads.

Compaction consists of reducing the apparent volume of the soil, that is, by reducing the empty spaces and increasing the density of the soil. It aims at arranging the particles in such a way as to give the greatest density so as to reduce the possibility of absorption. Compaction is carried out by means of special machines, the description and use of which are given in Section IV. The machines are sheep's foot rollers, pneumatic tired rollers and vibratory rollers.

The results depend mostly on the water content of the soil to be compacted. A small quantity of water acts as a lubricant and helps in arranging the particles in relation to each other and the expulsion of the air from the voids; when there is an excess of water the air bubbles remain in the soil and absorb the efforts of the roller. With every soil there is an optimum moisture content at which compaction is easiest. In the field the water content of soil is likely to vary considerably and it is necessary to aim at a water content near to the optimum. The following empirical test can be applied: the optimum moisture content is reached if, when a handful of soil is squeezed firmly, the imprint of the fingers can be seen but water does not ooze out through the fingers, the ball of earth should become smooth when it is rolled once or twice in the hand. According to its conditions, the soil has either to be scarified to aerate it and facilitate evaporation or to be watered to increase the water content. It is clear, therefore, that the seepage of water later on into the carriageway should be avoided as it would have the effect of decreasing the load-bearing capacity. Evaporation plays an essential part on unsurfaced or earth roads in reducing the water content and increasing the cohesion of the particles.

It will be seen later how these theories can be translated into practice. In the ease of public works, compaction operations require numerous laboratory tests to check the water content and to measure the dry density. These tests, which are quite easy to carry out, have to be numerous in order to follow the variations of soil along the road, but they have not yet become the usual practice in forest works, where reliance is usually placed upon the knowledge of the men in the field who are helped by a certain number of practical guiding principles plus their own experience.


Function of the different layers

On a public road built to carry heavy traffic several different layers can be distinguished, each of which is intended to resist definite forces. From the top to the bottom, these layers consist of the surface layer, the road base and the sub-base (Figure 9).

The surface takes the vertical forces caused by the load and the horizontal forces caused by braking. It must resist shearing and have great cohesion. As a rule it is made with a bituminous binder to which the wheels adhere well.

The road base, 10 to 20 centimeters in thickness, must above all resist vertical forces; it must be compact and well bound.

FIGURE 9. - Above the layers of a carriageway for a main public highway; below, the two layers of an earth forest road.

The sub-base, which is thicker than the road base and which has to resist moderate vertical forces, is generally made from a cohesionless material.

The formation is the surface of the subgrade or natural ground after the earthworks have been completed. It must be remembered that it is possible to place a lower layer between the sub-base and the natural earth to stop water rising by capillarity from a water table and to drain away water infiltrating from above.

In practice, this succession of three beds of material chosen to resist different forces is not found in roads for forest exploitation which have been made of compacted soils. Indeed, the quality of each of these layers has to be decided a priori, according to the importance of the layer and the relative price of the materials. Two different layers can barely be distinguished: the natural earth and an improved layer (Figure 9).

Natural earth

Once the organic material has been taken off and the surface raised by pushing in material from the sides, the natural soil is adequate for making the sub-base and is thus a continuation of the natural earth. Most soils which can be readily compacted and which are not very sensitive to water are suitable.

The soils are easier to compact when composed of particles of all sizes. In this case the distribution of particles of different sizes makes it possible by compaction to have greater density and fewer voids. The fewer the small particles, especially clay, that the soil contains, the less sensitive it is to water. Thus the following soils are particularly suitable:

(a) coarse-grained soils composed of a mixture of coarse and fine gravels with little or no fines;
(b) gravel with fines which are more or less silty or clayey;
(c) sandy soils with little or no fines.

Fine sands, slightly silty, would be satisfactory. The principal quality essential for this standard of road is slight sensitivity to water. Good drainage by ditches and evaporation must be ensured.

The improved layer

Above the compacted natural soil there is an improved layer, which corresponds to the road base on main roads. The material to be used should be carefully chosen so that it can resist the local forces which can be developed under traffic. Pebbles of more than 30 to 40 millimeters must be avoided; this facilitates spreading and leveling and prevents the surface from being torn up by the traffic. As in the case of the sub-base, a good particle size distribution makes it possible to stabilize this surface by compaction. The material should be sufficiently hard not to be crushed by the traffic; this fault which modifies the sizes of the particles and increases plasticity is found in certain granitic sands and in certain schists. The thickness of the improved layer varies from about 15 to 20 centimeters before consolidation. It is composed of a mixture of natural gravel comprising pebbles, gravel, sand and a few fines. This material is sometimes taken from open quarries in fluvial deposits situated in the main bed of rivers.

In the tropics, this layer is often composed of unsorted laterite consisting of coarse particles and some fine material. Coarse particles more than 1 millimeter in diameter are composed of ferruginous concretions of varying hardness and they make the compacted layers more resistant to shearing. The fine material contains oxides of iron and aluminum which harden in the air and are not readily affected by water. The laterite is easily compacted by rubber tires or simply by the flow of traffic. Moreover, it is often the only hard material available in tropical regions.

The materials which make up the improved layer are taken from natural deposits that are always heterogeneous. The amount of concretions varies from one level to another; the large compact kidneys sometimes present must be avoided. Very often beds which appear suitable are not at all thick and the clay content increases in the lower levels until these become no longer suitable. The foreman must exercise constant vigilance when these deposits are being examined and used.


The mechanical properties of fine-grained soil varies with the moisture content. We have seen that Lion is bound up with the presence of clay and water, and also that clay is very sensitive to water. In addition, we have seen that strength increases with compaction and that maximum compaction can only be obtained by having a suitable moisture content. Once this maximum compaction is obtained, it is essential to prevent any further introduction of water which always causes trouble. Roads are nearly always spoiled by the unforeseen decrease in strength either of the soil or of one of the layers which compose the roadway.

The measures to be taken should tend to:

(a) prevent penetration of rainwater into the carriageway;
(b) ensure rainwater running off the road;
(c) ensure drainage of the various layers;
(d) prevent water rising by capillarity;
(e) facilitate evaporation from the surface.

Penetration of rainwater is restricted by the consolidated layers being less permeable. It is obvious on economic grounds that roads used exclusively for forest work are never given a waterproof surface based on cement or bitumen. Therefore it is only the compactness of the top layer and the camber which limit the superficial penetration of rainwater. When earthworks are carried out in the rainy season it is advisable not to stop work until the road is cambered and the rainwater can be certain of running off into temporary ditches. This shape which need only be like a roof with two slopes is made by the grader working after the bulldozer. Maintenance work, especially in very rainy areas, regularly renews the camber to avoid water standing in puddles.

The removal of water by ditches should also take place as quickly as possible (Figure 10). Lateral ditches are not intended just to collect the water which has fallen on the road, but to take it away toward the natural outlets like streams or to main drains. A lateral ditch full of water, which cannot empty itself because of inadequate or insufficient outlets, leads to seepage under the neighboring carriageway and thus reduces its strength. After heavy rain, the surface of the road can be dried out to a depth of several centimeters by the action of wind and sun. All the same, ruts will occur if water from a neighboring ditch, which often #says full of water or is very slow in emptying itself, can seep through to the lower layers.

It has already been seen that the gradient of the ditches should be more than about 1 or 2 percent in order to avoid deposits of sediment, mud or sand, but less than 5 percent to avoid gullying which would destroy the berms. It is essential, therefore, to make sufficient outlets to ensure the removal of water after heavy rain. These outlets should be very numerous in both the following cases:

(a) where the side ditch has a gentle slope of 1 to 2 percent, where the water will only flow away slowly;
(b) where the side ditch has a steep slope (5 percent), where water flows away quickly and can lead to rapid erosion.

FIGURE 10. - The function of ditches and outlets: left, a ditch with no drainage; right, a ditch of adequate depth.

Where parts of the road are on hillsides the water in the inner ditch should be taken under the road by an adequate number of culverts. The best way of ascertaining whether ditches and outlets are sufficient to carry all the rainwater is to visit the site immediately after very heavy rain. It will then be seen that the outlets are nearly always inadequate.

Whenever the road is really saturated with water,- whether by rainwater, water standing in side ditches or water rising from below by capillarity,- permanent drainage must be ensured. There are several means of accomplishing this. The side ditches can be dug so that their top is at least 50 centimeters below the level of the road; when this ditch is slow in emptying the eventual risks from a prolonged soaking will then be limited only to the deeper layers. When the road is built on swampy ground and where the level of the water table is near the surface, it is recommended that a fairly thin porous layer should be spread between the natural soil and the layers in the embankment; this layer will prevent water rising by capillarity and will drain off this water as well as water infiltrating from above. A depth of about 10 centimeters of sand and gravel would be enough. It might be convenient to place this layer on a bed of fascines or small poles which would prevent it mixing with the soil underneath.

Evaporation from the surface of the road depends directly on the amount of exposure to the air. In the forest the sun's rays are screened by large trees bordering the formation. Their long shadows prevent the road from drying, particularly in the early morning. When they are felled, the period of exposure to the sun is increased and the aeration of the surface is improved. The larger the clearing in the forest, the better the air circulation. Therefore, there should never be a tree with its crown standing over a road; this causes shade on the road, and drops of water continue to drip onto the surface long after each fall of rain.

It is difficult to specify the minimum width for clearing to open up the road and to give adequate light. It depends on numerous factors: the average height of the stand, the orientation of the road, the direction of prevailing winds, the nature of the soil. The higher the stand, the wider should be the opening. The side exposed to the morning sun dries faster than the other side. A road orientated according to the prevailing winds will dry faster than one orientated in a different direction. Roads on clay soil should have more light than those on sandy soil. A main road should always be cleared to a greater width than a secondary road (Figure 11).

FIGURE 11 - Openings for roadways in the forest. Left, a narrow opening suitable for sandy soils, near small trees and with early exposure to the sun; right, a wide opening for clayey soils, near tall trees and with late exposure to the sun.

Some road builders are of the opinion that the clearing on each side should be at least equal to the roadway; thus, at the time of construction, they plan an opening in the stand three times the width of the road including ditches. The best rule is that after 8.30 or 9 a.m. there should be no shade on the formation between the side ditches. Any tree giving shade at that time should be felled.

Various rules have now been formulated which govern the protection of roads against deterioration and decrease in strength caused by water. The builder of forest roads should always keep these rules in his mind; they should give him a "water obsession." He should tell himself that, if the layers of the road are soaked with water or if there is traffic on a road which has lost its strength, deep ruts and holes will develop which cannot be repaired during the whole of the rainy season. But if he is careful to take all the precautions just specified, the depth of the softened surface of the carriageway should be sufficiently slight for it to dry out in a few hours, and the road can then bear the transport of logs. Except in the height of the rainy season, the exploiter has many dry days at his disposal in which to operate his transport.

3. Planning the alignment

It is often said that road making is too expensive anyway for it to be possible to spend precious time planning the alignment. It should be enough, when considering the cost of this planning compared with the expense of construction, to realize that, if a saving can be made by shortening the total length and reducing the amount of earth to be moved, such planning is an excellent investment.

To clarify this statement, let it be supposed that the limit of preliminary planning for a section of road is to walk over it at the most four to six times. The distance is 3 to 6 kilometers, covered in a day by a party composed of a party chief, a foreman with a compass and a gang of about 10 men. A preliminary study of the alignment, according to the principles which follow, would cost about 2 to 3 percent of the total expenditure. It should be noted that, in the United States, 5 percent of the total cost is often allowed for the preliminary reconnaissance. What is this expense, which corresponds per kilometer to only 2 or 3 hour's work by a D7 tractor, compared with the thousands of cubic meters of earthworks which can be saved? Apart from other indirect, but tangible, advantages gained by a good alignment, the gradients can be adapted to the vehicles, the bends need not slow down the vehicles unduly, and the logging trucks can be fully loaded and can travel at a normal speed without undue wear.

A fundamental principle guides all planning: the main lines of the alignment are decided in advance by a method of successive approximations on maps, plans or rough sketches on an increasing scale, which are later checked by reconnaissance carried out on foot over the ground.

Two essential aspects of planning should be underlined: methodical study and walking over the route. Nothing replaces the methodical and progressively detailed study of the route finally selected. This study necessitates walking over the chosen route several times in each direction to complete and verify on the spot information obtained in previous reconnaissances. This information will be of value when planning in the office, during the periods of study and thought which alternate with reconnaissance in the forest.

The fixing of the alignment and the various reconnaissances can only be undertaken by a person who is duly qualified (party chief or engineer) and who has experience of forest exploitation. The function of a topographer, who is only a topographer, is to make the maps, lay out the trace, and follow an alignment or a given gradient; delicate operations, indeed, but more or less mechanical. A topographer should not select the alignment, for this requires all the experience of the party chief or an engineer and results in a compromise between the configuration of the ground to be traveled over and the road which he envisages building for subsequent exploitation. Judgment and experience have the strongest influence on the choice of the best alignment, an alignment resulting in the least amount of earthworks in constructing a road, the characteristics of which have been previously decided. It is an illusion to think that "looking for a route" can be done only on the ground. Study and reconnaissance on foot are complementary. Indeed, the over-all picture and decisions can only be made in camp, in conditions of calm reflection where it is possible to appraise the available information of all kinds and where, by weighing them all together, the fundamental decision can be reached. On the other hand, basic information which helps to clarify the subsequent choice can be obtained only in the field. To persist in wishing to limit the investigation to the site can only result in an unmethodical survey of a large area at the cost of prolonged effort, and will not produce anything more than a few bits of information more or less relevant to each other.


There are five phases in the systematic and increasingly detailed study of the terrain.

1. examination of general information from maps, aerial reconnaissance and aerial photographs;
2. drawing up of provisional alignments in the light of the information collected;
3. detailed reconnaissance on the ground of the possibilities of the provisional alignment;
4. fixing the final alignment by correcting the provisional one in the light of information gamed by the reconnaissance on the ground;
5. marking out and staking the selected alignment with regard to the detailed irregularities found on the ground.

In practice, some of these phases will be found to overlap or even to be simultaneous according to the experience and capabilities of the person in charge of the planning. But each one of these phases is a real stage in the process of planning. Actually a systematic effort of this kind will not immobilize the party chief longer than any of his other usual duties.

Phase 1. Examination of all the documents

First, how to study the general direction of the alignment will be discussed, and then how to resolve the general problem of fixing it between two points.

The first phase consists in making a rough sketch of the general direction of the alignment. For this all the available cartographic documents on the region must be used.

General maps. The first step is to find out what maps of the district exist. In spite of their imperfections, these maps can, in the first stage, be a considerable

help. Some information will be found in them which will be quite easy to check, for instance, the lines of ridges, the principal rivers, waterfalls or rapids on important rivers. In fact, these various maps are only useful for explorations on a small scale; for an average concession (10,000 hectares), maps or sketches on a scale of 1/200,000 will not furnish much information apart from the location of large rivers or of watersheds, the crossing of which may present difficult problems.

Aerial reconnaissance. The great advantage of aerial reconnaissance is that the whole forest zone can be seen. A detailed plan of the different flights should be made beforehand. These flights should be plotted on a small scale map or rough sketch, even if they are not very accurate. The plan can be for flights of two kinds: either for a grid with a spacing of from 5 to 10 kilometers, or for flights between two points consisting of prominent landmarks which are easy to identify, such as the corner of a forest, a waterfall or rapid, a river junction, crossroads, an outlier or a village. During each flight the distances are noted down in minutes together with the speed of the aeroplane. (At a speed of 180 kilometers an hour, one minute corresponds to 3 kilometers of flight.) When flying over each prominent point, the time should be noted to pinpoint the feature later on the map.

The altitude of the flight should be from 400 to 1,000 meters, according to the cloud layer and the capabilities of the aeroplane. Light aeroplanes with high wings are used to facilitate observation by the crew; the plane should be capable of flying at a ground speed of less than 200 kilometers per hour. Drift due to wind is to be avoided. Each journey in a straight line should last only about half an hour. The crew will consist of a pilot, and one or two observers mutually checking their plotting.

An aerial reconnaissance can also be made at other stages in the planning. Thus an aeroplane can equally well be used for a general systematic survey of the zone to be walked over (first phase) as for a special reconnaissance (third phase) over a limited area or even over a particular point like a hill or a cliff which has to be negotiated.

If aerial photographs, even small-scale ones of about 1/50,000 are available, they should be used. They are, in fact, expressive, true and complete pictures of the terrain on which all the important topographical details can be marked. Each photograph can be examined qualitatively and angles for direction can be measured on it. In fact, only angles from the principal point on the center of each photograph are used; distances are not measured from this point.

With vertical photographs of fairly even ground, it is possible to obtain a good assembly of several strips. Care should be taken to even out as far as possible irregularities from strip to strip, and to keep a good superimposition of common features in the photographs of each strip. The result obtained is a mosaic which can serve as a very useful provisional map. This mosaic can be photographed, but it must not be forgotten that the errors in putting individual photographs together can be considerable. The mistake of measuring distances must be avoided.

Whenever possible, stereoscopic examination of a pair of photographs is preferable to every other test. The pair, of course, is the common part of two consecutive photographs of one strip, that is, two taken successively by the aeroplane on the same flight. Stereoscopic examination allows the simultaneous study of the planimetric details and the nature of the topography. It gives the impression of examining a small rough model of the ground. This examination, though fairly easy, needs some preliminary training. It consists of learning to use a stereoscope correctly and to interpret the stereographic picture obtained.1

1In this connection the Institut géographique national (IGN) of France has issued a paper on the stereoscopic examination of photographs and a collection of stereo-graphic pairs with an interpretation of the different details which can be identified. They can be obtained by writing to the Service commercial de l'IGN, 107 rue la Boétie - Paris 8e - Ely 48-17 or to La Photothèque, 2, avenue Pasteur, Saint-Mandé‚ (Seine).

Special inventory maps. The only really useful document is the sketch map of the whole area to be exploited. It is made from information gathered at the time of the inventory when lines were cut across the forest to be exploited. It is worthwhile describing briefly this method of exploration which can make up for the lack of large-scale maps and provide an inventory of the exploitable trees in the concession.

The whole section of forest to be inventoried is divided into rectangular compartments with the principal lines running north and south and the secondary ones from east to west. The compartments have a rectangular shape and an area which varies according to the distances between the lines: it may be 1,000 × 250 meters (= 25 hectares) or 500 × 200 meters (= 10 hectares). This system of lines makes up a topographical grid on which one can immediately pick out the topographical features and the position of exploitable trees. In the fair copy of this rough sketch of the whole area (hereafter called the inventory map) the usual scale is 1/20,000 or 1/10,000. The basic truth must be accepted that it is impossible to have a plan for a road or for exploitation at moderate cost without a good preliminary inventory.

A preliminary examination of the inventory map makes it possible to pick out the best zones in the forest in which a road of predetermined specifications can later be constructed. Marked on this sketch may be:

(a) the areas to be exploited and therefore to be provided with roads;
(b) obligatory points, such as a narrow part of a stream for a crossing or a saddle which could be a point to cross a ridge of hills;
(c) places to avoid: marshy land, or land under water in the rainy season which would need an expensive embankment and which might often be unstable;
(d) areas where food crops have been grown in the past and without exploitable trees, but where the absence of stumps would make crossing easier.

Unfortunately the inventory map is only concerned with the area covered by the license or concession. There is always very little information about the areas outside the boundaries of the concession which the main extraction road has to cross until it meets either the public highway or a water way. It is for this area that aerial observation or aerial photographs can be of the very greatest help.

Phase 2. Provisional alignment

The cartographic documents are thus completed little by little by information gleaned in all possible ways: general maps, aerial reconnaissance, aerial photographs and inventory maps. These basic documents are subjected to a systematic examination and from them one or several provisional alignments can be chosen. This examination has two stages.

The first stage is to become thoroughly familiar with the configuration of the terrain. The surface relief of the ground must be assimilated. This relief is the result of erosion, that is, by the disintegration of the higher parts of the land and by the carrying away of the debris to the lower parts by water, usually from rain.

It is convenient to mark the position of characteristic lines: that is, the lines of ridges and of valleys. The watersheds or ridge lines are the upper intersection of two adjacent slopes. Valley bottoms or lines where water running off the surface join, and which are often followed by streams or watercourses, are the lower intersection of two adjacent slopes. Lines of the same nature divide and change direction rather like a roof. The convergence of water toward the lowest points cause the valleys to flow into each other and thus form a network. Between two valleys there is a ridge line; these lines form a system enclosing the network of valleys.

If the details of the valley lines and the ridge lines are marked in systematically, a picture of the ground emerges (Figure 12); from this the essential features of the land can be picked out; this greatly facilitates its study.

It is not generally known at the time of the inventory how much delay there will be in planning the extraction roads, but it often happens that, as work cannot be carried out under the best conditions, the inventory operations immediately precede the planning of the alignment. In this case these two operations are given to the same person. When marking in his topographical data, the head of the survey party will give special attention to any information which could be useful for planning the road. He will note rocky places, swamps, and very steep places unsuitable for an inexpensive alignment. He will be especially careful to mark the easy places, such as the lower saddles on ridges and river banks suitable for bridge sites. The sum total of this information will facilitate the whole planning, will indicate the parts of a provisional alignment needing further study and will thus reduce the time taken subsequently in detailed reconnaissances.

FIGURE 12. - Lines showing the features of the terrain.

FIGURE 13. - A curve of a large radius M R N. preferable to a double curve M p q r N.

In the second place, a provisional alignment is made, step by step. In practice, it is a matter of fixing obligatory points and in planning a provisional section between two successive points. In spite of the variety in the form of the terrain, the different cases observed in studying maps and sketches can be summed up in a few standard situations which are examined below.

On flat ground the only difficulty that can present itself comes from any kind of obstacle. Long straight lines should be avoided at all costs. It is better to shift the alignment immediately outside the area of obstacles to avoid a succession of deviations. Indeed an arc M P N (Figure 13) is never much longer than the direct route represented by the cord M T N. Thus it is better to avoid at one stroke a group of obstacles, for example, rocks or a number of large stumps, than to go round each one: the line M R N is preferable to the line M p q r N.

On uneven ground, where there are marked ridges or valleys, the choice of line must be tentative. If the obligatory points are all obvious, then the solution follows naturally, but this is rare. In practice, the determination of the alignment is always a compromise between the length to be constructed, the maximum gradient and a limited amount of earthworks. Nearly always each particular case can be put into one of four categories, which we will now examine:

(a) Two points in the bottom of the same valley

When the two points, A and B. are on the same bank of the valley, it is enough for the line to be placed above the areas which will be inundated when the water is high. For example a line like A p1 p2 p3 B (Figure 14) is preferable, even though a little longer, if it reduces embankments or replaces a fairly large bridge (pl) with two culverts which would be easier to build.

When the two points A and C (Figure 14) are on different banks of the same water course, a point must be sought along the stream where, without going far out of the way, it is possible to make a crossing which will not be too expensive. It is often better to cross the valley at as high a point as possible so that only minor works are involved. A line along A p1 p2 P4 C is often better than A p3 p4 p5 C.

FIGURE 14. - The two points to be joined are in the bottom of the same valley, but A and B are on the same bank while A and C are on opposite banks. The alignments shown as + - + - + - + are preferable to those shown as - - - -

FIGURE 15 - The two points to be joined are situated in the same basin, D on the slope and E in the valley. The alignment + - + - + - + is preferable to - - - -

(b) Two points in the same basin, one on the elope and the other at the bottom of the valley

There is often a tendency to come down from the highest point D (Figure 15) to the bottom of the valley by the steepest permissible gradient and then follow the valley to the lowest point. A better solution, wherever possible, is to follow a direct alignment keeping an average gradient.

In practice, however, it would often be better to avoid the direct line D P5 E with a large bridge p5 and adopt instead D p6 p5 p4 E. The latter is certainly longer and requires three small bridges, P6, p5 and p4, but these can be built economically on a combined profile on the side of a hill, partly cut and partly on an embankment. This layout has the following advantages: it keeps a gentle gradient, is not expensive to maintain, allows a light amount of traffic at all seasons and has the advantage of serving a more extensive forest area at a reasonable cost.

FIGURE 16. - The two points to be joined are on opposite slopes of the same valley (L and M) or of the same ridge (M and N). The alignments + - + - + - + are preferable to - - -

(c) Two points on opposite sides of the flame valley or of the same ridge

In the case of a valley. The most direct line such as L P1 M (Figure 16), which would be the most obvious one at first, involves two steep slopes, x and y, and a relatively important crossing P1 situated between approaches on two high embankments. This line would give a difficult profile to travel over, high maintenance costs because of the steep gradients and be expensive to improve at a later date. On the other hand, the line L P2 p1 M should always be preferable. It comes down into the valley on a gentle gradient, crosses the valley at a point where a less expensive crossing would be sufficient and climbs on a gentle gradient up the other side. In spite of being longer, it can be finished more quickly, often keeping a combined profile on the side of the hill, and the whole cost would be less than would have been spent on L P1 M.

In the case of a ridge. In this situation, the direct line as in M T N (Figure 16) is only possible if the gradient encountered remains below the limit. If it has the advantage of being the shortest route, it has the drawback of requiring a deep cutting in crossing the ridge-top T. In a special case where large embankments are necessary to cross adjacent valleys beyond M or N. this direct line can again be adopted, provided that the length of the eventual cutting remains resonable in terms of the earth to be moved, say in fact 50 to 80 meters with one bulldozer.

Beside these two special cases, there is still the choice between two alternatives: either to go round the ridge on as gentle a gradient as possible, making the road on the side of a hill on a combined profile M F N. or to follow an intermediate alignment M D N along the ruling gradient necessitating a small cutting D and an acceptable increase in the length of the route.

The choice of the best route will often be settled by other conditions. the nature of the soil or the location of the trees to be exploited. The examination of the preceding cases leads to the pronouncement of the following practical rule: In relation to a straight line joining two points, the alignment across a valley should be toward the head of the valley, and across a ridge it should be toward the lower part of the ridge.

(d) Two points at the bottom of two valleys separated by several spurs

The first thing is to look for saddles, that is, places where the ridges can be crossed. One of the preceding cases will apply where, by using a normal alignment, successive slopes can be cut through obliquely by a line on a uniform gradient and a combined profile.

Thus, by reference to the preceding rules, the general direction of a main road can be looked for to avoid an excessive lengthening of the route which would influence the turn-round of the trucks. On the other hand, it can be an advantage to lengthen a secondary road by keeping to very easy gradients, and so give direct access to a larger area and make it possible to collect easily timber situated in the immediate proximity.

The study of an inventory map can result in one or two provisional theoretical alignments or outlines of alignments To settle the question as to which will be the final plan, it is necessary to study the configuration of the ground in more detail and to compare the plans with the actual ground during special reconnaissances.

Phase 3. Special reconnaissances on the ground

One or two provisional alignments kept in the office should be compared with the configuration of the ground in the course of the special reconnaissances. It has been said that these alignments comprise successive sections planned in suitable areas and separated by obligatory points. Each suitable area and each obligatory point should be the subject of a detailed reconnaissance. The best time to do this is during the rainy season; it is then that characteristics of the soil, the limits of marshy places and the width and level of water courses can best be appreciated. A reconnaissance carried out in the dry season is certainly much more pleasant, but it can cause errors which can later be very annoying. The following equipment is necessary for this undertaking: a map of the whole district with the provisional alignments, a compass, a clinometer, a 20-meter measuring tape and, if possible, an aneroid barometer. The few stakes which will be needed should be easily found in a forest.

The aneroid barometer. The use of the barometer for measuring the relative heights is based on the variation of atmospheric pressure which goes down when the barometer is raised in the atmosphere. The difference in atmospheric pressure between two points depends on the general atmospheric pressure in the metereological sense and on the average temperature at the time of the observations. It is enough to know the difference in pressure between the two points to get the difference in altitude or level. To counterbalance errors, it is a good idea to check frequently, that is, to describe closed circuits, passing the same place twice during the same reconnaissance, taking readings for example every hour or every two kilometers. It is essentially the differences in altitude that can be measured fairly accurately. An aneroid barometer is used because the variations in the expansion of the mechanism are compensated when due to changes in temperature. This instrument is composed of a cylindrical box in which there is a vacuum and where an internal spring keeps the faces in equilibrium against the atmospheric pressure. The movements of the box are amplified by a mechanism which controls the movement of a hand. There is a choice of two models - a pocket barometer, about the size of a large watch, or a precision instrument (weight with case, 1,300 grams), which can give a probable error of 2 meters. The dial of a leveling barometer is either barometric or graduated directly in meters.

Essential points to remember. When considering a crossing over a watercourse, a check must be made that there is not another place nearby which had been overlooked when the map was studied and which would be more suitable. To determine the highest flood level of the water the masses of debris brought down by the floods should be looked for, and also traces of slime left on the stems of plants in the neighborhood. If it likely that the route will cross the line of a ridge or a watershed between two valleys, the line of the main ridge and the lines of the secondary ridges or spurs are walked over systematically. It is often essential to work up the valleys with the reconnaissance equipment to discover the highest part suitable for a crossing. The relative heights of the characteristic high and low points are read on the barometer.

The nature of the terrain met with should be looked at from the angle of the ease or otherwise of carrying out earthworks - cuttings or embankments. The limits of seasonal swamps on which embankments may sink down are noted with care. Loose ground can cause a fall of rock or local landslides when cuttings are opened up. Rocky areas which would need explosives are noted and their limits are explored. In the same way a line of springs could indicate a clayey outcrop and the road should be kept well away from this.

FIGURE 17. - Lyre clinometer: left open; right, shut.

Areas where sand, clay or a mass of lateritic gravel are predominant are specially noted for later use in the construction of the road.

All this detailed information gleaned from the reconnaissance should be noted and marked on the map, if possible in the field.

Alignment on a uniform gradient. In a general way, it would not be likely that a final alignment would be drawn up at this stage based on the provisional one envisaged from the first examination of the map. However, it is possible to note, especially from high points, where to lay out on the ground an alignment on a uniform gradient. Described below is a simple process which can be followed by the assistant topographers and is the best method to use when, as is usually the case, a detailed map is not available. It is in two stages.

At the first stage, look for a possible alignment on a given gradient with no earthworks on the road axis and without taking into consideration the length or the bends. Several successive attempts should be made, each time decreasing the gradient. Thus at first an alignment could be planned with a gradient of, say, 6 percent, then a second with 4 percent. These theoretical alignments, obtained by a method to be described later, are shown in the form of dotted lines.

At the second stage, choose from these lines the one which, for a given gradient, is the shortest and the least tortuous, that is, where the angles between the straight sections are as open as possible. Usually this choice can be made on the ground, but when there is still some doubt as on uneven ground, it is enough to plot the lines already obtained with the help of a measuring tape, compass and clinometer and then arrive at a considered decision.

This operation depends on the use of the clinometer of Col. Goulier (Figure 17) which is the ideal instrument for rapid reconnaissance and which has not yet been superseded. This instrument is held a few centimeters from the eye by a finger through the ring. A collimator system composed of a large magnifying glass makes it possible to see the figures on a graduated scale marked on an unpolished surface in the instrument. The picture of the scale obtained through the instrument appears to be in the same plane as the object in view and can be compared with it. The instrument is balanced by a counterweight in the form of a lyre (hence its name lyre-clinometer) so that the magnifying glass occupies the same position in relation to the vertical when it is suspended from its ring; in this position the line of sight passing through the zero of the graduated scale is horizontal. The makers usually have two models: one having a gradient scale graduated in 0.5 percents from - 40 to + 40 degrees, the other with two scales of from - 100 percent to 0 and from 0 to + 100 percent. The first model is adequate for road planning. The degree of accuracy possible is 0.25 meters in 100 meters. It is impossible to repeat too often that a clinometer is indispensable to all road planning.

Alignment on a given gradient. How is it possible to demarcate an alignment on the ground following a given gradient, particularly along a slope? This problem is too often inadequately solved, as follows: some sort of alignment is made more or less at random while earthworks are in progress; it then becomes apparent that the gradient is Still too steep and cannot be accepted. With the help of successive cuttings and embankments, requiring many hours of bulldozing, the party chief strives to obtain a gradient which can be accepted. The result of these efforts tends to be unsatisfactory in spite of large and costly earthworks: the profile remains undulating with unduly steep gradients.

It is an illusion to think that the planning of a profile with a uniform gradient can be accomplished without the help of any topographical instrument. Anyone who has tried to walk over a slope following a horizontal line or a line with a low gradient of from 3 to 5 percent has soon realized that there is always a tendency to climb toward the top of the slope describing a line where the gradient easily rises to 20 percent and more: this gradient nearly always exceeds the maximum acceptable on an earth extraction road. This is why there should be no hesitation in replacing these ineffectual methods by a very simple topographical operation. Moreover, the demarcation of a line on a uniform gradient is a special operation in laying out a road trace. The topographical equipment to be used comes down to this: a survey chain 20 meters in length, a compass, and a clinometer.

The party consists of an assistant forester, a foreman and four laborers, two carrying the chain. A few other workmen - say from two to five - have the task of clearing the undergrowth to improve vision: a total of an assistant forester, foreman and four to seven men.

FIGURE 18. - The image of the gradient scale with the sighted object as see, through the clinometer. Readings: at the top of the hat, + 65 percent (or 6.5 percent); at the top of the staff, + 50 percent (or 5 percent).

FIGURE 19. - The terrain has the same slope as the line of eight.

It is often more convenient to start work from the highest point on the road to be made, whether it is on a saddle or some point on a ridge. Indeed, the lowest point is often not absolutely fixed and in a forest it is easier to see more of the terrain looking toward the bottom of a slope. All that is required are a straight staff or stick and a clinometer which are used as follows:

The head of the party, carrying the clinometer, begins by measuring the height of his eye above the ground when he is using the instrument to measure a gradient; say, for example, it is 1.60 meters. He puts a square of white paper (or newspaper) from 5 to 10 centimeters wide at the same height on a staff or a very straight stick. To read on the clinometer the gradient of the line joining the bearer and the bearer of the staff it is enough to be able to see the square of paper. The line of sight between the eye of the operator and the square of paper is parallel to the line which joins the two stations (Figure 19). An experienced operator would be content to note on the man carrying the staff the point of his hat or of his face which corresponds to the same height of the clinometer (Figure 18); this avoids using a staff which it is not always easy to hold upright.

This process makes it possible to mark out gradually, bit by bit, a line on a given gradient. The only precaution necessary is to see that the staff is held upright on the ground.

An operator with some common sense very soon acquires the experience which will enable him to mark out quickly an alignment on a given gradient. This process has the advantage of being very simple.

As soon as the limits of this line on a given gradient are laid out, it is a good idea to mark each station by a stake or a small pole driven at least 30 centimeters into the ground so that the line will not be lost. To be able to see this stake easily the tip of the stake is often split with a cutlass or machete, and a piece of paper is slipped in which remains wedged into the top of the stake (Figure 20). It is a good idea to put the number on a blaze at the top of the stake either in red pencil or with a ball pen. When the whole route is staked out in this way a trace can be opened at once for subsequent inspections. The process just described is much the easiest way to lay out an alignment on the side of a hill.

FIGURE 20. - Staff with a square of paper for sighting with a clinometer: the marks L3 and N demarcate the road trace.

Phase 4. Selection of the final alignment

The comprehensive map, duly completed from sketches and information of all kinds gathered from the terrain, is now going to serve as a basis for deciding on the final alignment. The special reconnaissances undertaken with the map in hand have led to an intimate knowledge of the terrain, the form of which has now become familiar.

It is after this, in a quiet time for thought and study, that the different obstacles and their importance can be appreciated. It is thus, for example, that instead of looking for a long detour difficult to construct round a ravine with steep slopes, the idea of a bridge across it might present itself as a less expensive solution. In the same way, a cutting not much longer than another could, without any other obstacles, constitute a short cut preferable to a very much longer alignment. In these two cases, conditions in the field are not conducive to careful thought, even with an available sketch. All the important factors to be considered are not available - time is always limited and surroundings, with the heat and mosquitoes, are not favorable for protracted thought, especially if two very different local alignments have to be compared quickly with each other. Pressed by circumstances, it is difficult not to arrive at a solution which has not been properly thought out. Why not there and then take advantage of the best and calmest conditions to make the choice in a moment of relaxation, after having weighed in turn all the different points to be considered?

In a general way it must be understood that a given alignment is the result of a compromise between contradictory requirements: for example, minimum gradient and minimum earthworks; minimum earthworks and length of route. But the given factors - earth - works, admissible slopes, length of route - can be appreciated in terms of money. A rapid calculation, making it possible to compare the cost of construction and the cost of extraction, will always throw a new light on the matter. For example, take the following special case: There are two possible routes from A to B; the shorter requires considerable earthworks and will cost about 600,000 francs and the longer (by about 2 kilometers) will cost 350,000 francs;2 both have the same gradients. Which will be the better route? The decision will depend a great deal on the subsequent use of the road. Indeed, each cubic meter transported over these two extra kilometers calls for an additional expenditure of 20 francs (if 10 francs is the average price of the transport of a cubic meter per kilometer). The extra Cost of the short route in relation to the long route, that is 250,000 francs, would correspond do the transport of 250,000/20 or 12,500 cubic meters. It must be concluded from this that for anything less than a turn round of 12,500 cubic meters the shorter route would be the one to choose. In other words, the longer route would be better for a secondary road, while the main road should follow the shorter route.

2US $ 1.00 = 4.90 French francs.

FIGURE 21. - On a hillside the easiest slopes are looked for.

When he makes his choice the party chief should bear in mind the following general observations which are dictated by experience.

1. On fairly even ground it is always better to build the road on a ridge or near it; by keeping to the shoulders that he meets there the builder will save in earthworks and will find drainage easy, for he can dispense with a good number of culverts and embankments.

2. On uneven ground, the main road will pass from one valley to another; each secondary road will serve a compartment corresponding to the whole of a secondary valley, thus avoiding crossing the ridge, which is always expensive.

3. On a hillside, especially in regions of marked topography, the least steep slopes must be looked for (Figure 21). The top of the steepest part of the slopes is usually a line where the gradient changes (the false crest); this line borders the plateau and an endeavor should be made to build the road along it. Near the base, especially in valleys with a flat bottom, the road can be made immediately below or close to the foot of the slope.

4. In a valley alignment, on the other hand, it is better to keep as low as possible, but above the flood plain; in a wide flat valley, instead of crossing numerous water courses near their confluences and where they are widest, it is better to economize with culverts or small bridges. If, however, the road has to go through a narrow valley with steep sides, fewer deep depressions should be crossed and the cuttings will be less deep.

5. On the side of a hill, when a constant gradient has to be kept, a combined profile is chosen (Figure 21) as the earthwork is nil along the center of the alignment itself even if the transverse gradient is low; this makes the cutting settle better. On the other hand, if the transverse gradient is steep, a profile consisting largely of a cutting is better than a combined profile (Figure 22), as it brings the road into the hillside; if the amount of the cutting is increased gradually at least the road has a better foundation and the drains are more effective.

6. A deep cutting has two disadvantages: in the first place, the surface at the bottom is shut in, less sun reaches it and it is slower to dry; secondly, when the excavation is in progress strata of earth rich in clay may be found and this is exceedingly difficult to stabilize. The remedy lies in extending the alignment somewhat thus reducing the depth of the cutting.

7. To facilitate the making of an embankment, especially for the approach to a bridge, it is obviously very economical to choose a place where extra earth is easily available (Figure 23).

8. Bearing in mind the nature of the terrain, it is better to plan a low embankment on marshy ground and as shallow a cutting as possible on rocky ground.

9 In valleys, straight lines are the best in the first place, always endeavoring to plan as open angles as possible between them, so that the joining curves should be easy to construct; but in uneven ground, it is often better to lay out the curves as nearly as possible along the contours of the ridges or in the heads of the valleys.

FIGURE. 22 - It is better to make a profile mostly in a cutting than a balanced combined profile. 1. Profile booed on an equal amount of cutting and embankment. 2. Profile resulting from an increase in the cutting and giving a more stable roadway.

FIGURE 23. Embankment for a bridge approach.

FIGURE 24. - The parts of a curve joining two lines.

Phase 5. Staking out the alignment on the ground

Demarcation consists of showing on the ground the special points along the axis of the alignment. These are the ends of the straight section, the points of entry and exit of curves, the apex of the curves and the places where the gradient changes. Staking out consists in marking on the ground the exact position of the road to be built:

First, stakes are placed on the center line every 10 meters in curves with a radius of more than 100 meters and every 5 meters in curves of smaller radii.

Secondly, stakes are placed on the top of the batter of the cuttings, to limit the width of stumping. The stakes consist of pieces of hard wood about 10 centimeters in diameter and 50 centimeters long.

Straight alignments are laid out by eye with the help of posts put in three at a time. There is little point in using a transit or any other instrument. Care must be taken to see that the general alignment keeps as closely as possible to the gradient allowed. Experience has shown that the actual layout always has a tendency to be slightly shorter than that staked out; care must therefore be taken never to exceed a gradient 1 percent less than the ruling gradient. It is for this reason that an alignment on a constant gradient made according to the instructions of an earlier paragraph will be laid out with an actual gradient for example of 5 percent when the limit planned was 6 percent.

Laying out curves. Any curve which is intended to join two alignments A' AS and B' BS meeting at S is defined by three principal points (Figure 24): the two transition points of the curve and the alignments, known as the point of entry into the curve A and the point of exit from the curve B. and, the apex of the curve C which is situated on the arc of a circle at the intersection of the bisector of the angle made by the two lines A' A and B' B. Generally curves, which are the arcs of a circle, are laid out by eye. This method, which at least has the advantage of simplicity, is not always very quick - it merely results in laying out a curve with a varying radius. These badly laid out curves are often the cause of accidents through sudden sharp braking.

Daily experience shows that carriageways deteriorate very quickly on curves, which indicates that they should be laid out with the largest possible radius.

To lay out a curve exactly several methods can be followed, but they have the double disadvantage of requiring the use of special tables and of having to stand at the apex S of the two alignments or of walking over the alignments A-AS and B-BS or the cord AB joining the two transition points A and B. These different points are not always easily accessible before earthworks have begun.

Given below is a method proposed by E.W. Fobes as well adapted to the needs of foresters, which has the advantage of being simple and easy to follow.3 All that is required is a survey tape (or chain) of 10 or 20 meters and a graduated staff of 2 meters.

3E. W. Fobes of the Madison Laboratory, Wisconsin (U.S.A.) Improved alignment of logging roads reduces hauling costs. R 1637-40 June 1957.

FIGURE 25. - Laying out an arc of a circle by Fobes' method.


Radius of the curve

Length of the ½ cord read on the tape. AC=AE

Length of the offset read on the staff. CD=DE

































































First set a post or stake at the chosen point, for example, the entry point of a curve (point A, Figure 25).

Then choose the distance which separates two successive posts. It is obvious that the posts must be nearer together on curves of small radius than on curves of large radius. In practice, a distance of between 10 and 20 meters is chosen between posts for a main road. Suppose the distance is 10 meters. Select on the straight alignment BA a point C between A and the point of intersection S (which is inaccessible) so that AC = 10 meters. Put a post at D along the graduated staff placed perpendicular to AC. The point D will be on the curve; its position is determined by the lengths of AC and CD which are chosen in advance in relation to the radius (Table 4). To obtain a new point on the curve, it is sufficient to lay down a line DE where DE= CD, to put a provisional stake at E, then to extend AE to F so that EF = AE = AC. The point F is the second point of the curve. A further point H is found on the curve by putting provisionally a post at G so that FG = DE = CD and then putting a stake at H making GH = DG = AC.

By repeating this operation until the other straight alignment C'A'B' is reached, a curve is described which is an arc of a circle of the required radius.4

4 The radius R of the arc of the circle is obtained from the formula R2 = AC2 + (R - CD)2.

At the first attempt it is unlikely that the point of contact A' will be exactly on the alignment C'B'. All that is needed is to begin again using a slightly different length on the graduated staff. With a little experience the curve required can be found at the second attempt. Table 4 shows what lengths to choose on the tape (or chain) and the graduated staff in relation to the radius of the curve which is required.

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