1 President, Forest Engineering Inc.
Concern for the environment and for rising costs has increased the importance of engineering for all levels of forest roads. It is no longer feasible to "eye-ball" design and construct a forest road that may not meet the needs of the users or has a higher probability of failure with the resulting costs and environmental degradation.
Of the four major activities (LOCATION, DESIGN, CONSTRUCTION, MAINTENANCE) that dictate the success or failure of a road structure, the CONSTRUCTION phase is often the least controlled. In many parts of the world, roads are temporarily located, and then the equipment operator attempts to DESIGN and CONSTRUCT a road that will meet all of the criteria specified. In many cases the result is that the road is overbuilt or underbuilt, thus increasing the cost, or it is not structurally sound, thus increasing the probability of failure.
This paper will stress the importance of CONSTRUCTION CONTROL and present some of the basic ways to achieve such control.
Access is a major expenditure in most forest areas, especially in the more difficult terrain conditions of mountains and swamps. Even though the helicopter is becoming more important as a harvesting system, the requirements of roads as the main mode of access for forest management remains.
With the increase in environmental pressures that attempt to minimize ground disturbance and the rise in construction costs, it is even more critical that roads are built with a minimum of soil removal while maintaining structural stability. Although some people attempt to do this with minimal engineering application, the cost of failure is becoming prohibitive. We can no longer rely on "estimates" by the construction operator to meet the requirements of a structurally sound road at minimum disturbance and cost.
As with most forest operations, the attributes of CONCERN, KNOWLEDGE and ACCOUNTABILITY still hold true for road construction as well as for timber harvesting. We must be CONCERNED about the environment as well as the cost of building roads into our forests. This means we must have KNOWLEDGE of engineering principles and apply them to the given situation and not rely on the dozer or excavator operator to solve our design problems for us. We must also be held ACCOUNTABLE for the results. The road should be structurally sound to carry the loads that it was designed for and cause the least impact on the ground at the lowest cost.
Hence the basic factors that influence the success of a road are LOCATION, DESIGN, CONSTRUCTION, and MAINTENANCE; it does not seem logical to meet only some of these factors without considering all. Throughout the world, operations are being conducted that only consider some of these factors. Roads will be "flagged" to indicate an approximate location, but nothing will be done to monitor the design. This will be left to the equipment operator.
Some companies are actually going through a complex engineering programme to locate and design the roads on paper and then leaving the operator to CONSTRUCT the road without any control. Often the completed road has little resemblance to the design.
The ACCOUNTABILITY of the finished product is becoming more important, especially in the developed countries where sanctions or fines are imposed upon landowners if a road fails and causes damage to the environment. Equally important, but not as "politically correct", is the concern · for loss of life if a road fails. Who is responsible for a road that failed because it was constructed on a side hill too steep to maintain a fill prism? With the increase in recreational use of forest roads in all parts of the world, the structural integrity of roadways will need to be known. This is impossible to determine with roads that are constructed without monitoring the construction phase.
Purpose of design and control
The purpose for road design and control has been covered generally in the introduction. Now we should get to a more detailed analysis. The generally accepted reason for engineering is "to protect the public from themselves." This means that, if something is designed and constructed according to the design, users should feel confident that they could use the structure safely within the design limits, For the organization that is funding the project, they should be confident that the structure will fulfil the purpose at the least cost and be socially (environmentally) acceptable (Plattner 1979).
For roads constructed in mountainous terrain, two of the primary structural considerations are the back slope and the fill slope stability (Figure 1). Will the back slope be stable or will it collapse into the road, closing the road to traffic or plugging the ditch line causing the road to be washed out in the first rainfall?
The back slope can be designed to meet the conditions of the soil, which would mean that if the soils were likely to erode, the back slope might be flattened. If the material consisted of rock, the slope could be steeped, thus reducing the amount of material that would have to be removed, which would lower the cost.
In the construction phase, the back slope starting point for the equipment operator is key for the road to be of the width and grade prescribed in the design. If the back slope cut is not started correctly, when the operation is completed, the roadway will not be of the designed width, grade, or location. Numerous different results could occur.
If the designed back slope ratio and resulting road width were maintained, the road location may be shifted and the grade lowered (Figure 2). If the location and grade were maintained, the back slope would need to be changed from the design, which could increase the amount of material moved or increase the back slope that could lead to failure (Figure 3).
Figure 1. Road prism showing back slopes and fill slopes
Figure 2. Back slope started at the wrong location can shift the centreline of the road or change the grade
Figure 3. A back slope that is started at the wrong location may require changes in the slope ratio to keep the centreline and grade constant
The lower fill slope or daylight location is even more critical when we consider the structural stability of the road, especially on the steeper slopes found in mountain forests. If the back cut is started too low or too high on the hillside, the location will be changed (Figure 2).
What often happens in practice is that the starting point of the back cut is estimated, and the road is built until a desired width is obtained. Unless this activity is controlled, a very unstable condition can result on steep ground (Figure 4).
Unless the vertical distance (cut) is known and obtained from a determined starting point, it is difficult to determine the daylight or grade-out location, thus a portion of the road prism will be unstable. Such practices are common and result in many road closures every year. Not only do these failures close roads, but also the material that fails often ends up in watercourses, thus violating environmental mandates. In a worse case scenario, the failure could occur while traffic is passing that point.
Figure 4. Inaccurate back cut starting point, which can result in an unstable road prism
Basic construction control
Probably the most useful and least implemented method for controlling road construction activities is the SLOPE STAKE. This simple technique, which provides information to the equipment operator and allows the road structure to be monitored during and after completion, is not used as often as it should be. This results in roads 1) built not according to design, 2) overbuilt to guarantee structural stability, or 3) built with little knowledge of whether the structure is stable or not.
Slope stakes (Figure 5) should contain the following information:
1. The horizontal distance from the beginning of the road.
2. The cut or fill vertical distance from the stake to the road grade.
3. The back slope or fill slope ratio.
4. The horizontal distance from the stake to the centreline of the road.
Reference tags are different from slope stakes in that they are set back away from the actual construction activity. These should be installed periodically along the road so that the centreline and grade can be re-established if the slope stakes are lost (Figure 6).
Once the back cut is started and the side cut and slope ratio are known, the slope stake has done its job and is expandable. Slope stakes should indicate EXACTLY where the construction is to begin so that the operator can visualize the road prism from a distance. Some organizations like to offset slope stakes in an attempt to save them during the construction phase. This eliminates one of the main uses of slope stakes and should not be done unless it is a fixed policy that they will ALWAYS be offset a fixed distance, for example, 1 m.
Slope stakes are a means of informing the equipment operator:
1. Where he should start his cut.
2. What back slope ratio he should use in the construction.
3. How many cuts should be made to get to the desired road grade.
Also, if these stakes are correctly installed, the operator should be able to see them prior to getting to the spot, thus allowing him to determine the best method of getting there.
Slope stakes and their accompanying reference stakes also provide a means for checking to see if the road was built according to the design. This means that the volume of material moved was similar to the volume predicted in the costing.
Figure 5. Slope stake location and information
Figure 6. Reference stake location and information
As previously mentioned, slope stakes are not used as often as they should be. The reasons behind this practice are numerous, but basically are because organizations are reluctant to spend up-front funds for engineering, thinking that they are a lost cost. They do not realize the savings that can be generated by moving only the necessary material to construct a road. One company in Southeast Asia, after staking all of their mainline road construction and holding the equipment operators to the design, found that they reduced the amount of material moved by 50 percent and reduced the overall road construction costs by 40 percent.
Forest organizations are less likely to apply engineering concepts to their activities than construction organizations. They have a feeling that estimates or approximations will work just as well. Only when forest land management and environmental policies dictate that a certain condition must be maintained and the forest organizations are held accountable for the results, has there been an interest in the engineering of many of the forest roads.
Setting slope stakes can be relatively expensive on steep mountain roads; therefore, many of the decisions as to where to start the back cut are left to the equipment operator with varying results. To reduce the cost of installation, various techniques have been tried in the past, with tables designed for this purpose being the most common method. With the increase in use of the computer to design roads, some attempt has been made to directly apply the computer-generated information to the field.
A programme, ROADENG, developed by Softree in Canada is an example of available computer road design packages. These programmes are based on the premise that good field information will be collected prior to the design phase. In practice, however, this is a BIG assumption that is often unrealistic.
Although the horizontal and vertical control design can be relatively precise and the volumes generated may be acceptable for cost estimation, when it gets down to the precise location of the slope stakes, the accuracy is normally suspect. Practitioners, who have used the road prism plots generated by the computer (Figure 7) to determine the locations of the slope stakes, have found that often the point is not applicable to the field conditions encountered. The information initially collected when the preliminary line was located and the ground information predicted in the design are not sufficiently accurate for direct location from the prism plots.
The most accurate way to set slope stakes is to do it on the ground at the site. This means that the following information must be known from the design plan:
1. The cut or fill at centreline.
2. Desired road width with or without ditch width.
At the site, the back slope ratio can be determined based upon design criteria or upon the determination of the material encountered. The ground slope must be measured at the site.
Figure 7. Computer-generated road prisms showing slope stake locations
From this information, the location of the road can be calculated from standard road formulae or determined from a developed set of tables. An example of slope stake tables developed and used by Forest Engineering Inc. Engineers (Aulerich 1994) is presented in Figures 8 and 9.
Figure 8. Slope stake table for a 60% ground slope, road width of 6 m
Figure 9. Diagram of a 6 m wide road on a 60% slope, 1.80 m cut at centreline, back slope ratio is 3/4:1
Constructing roads according to an engineered design is the best way to assure that roads are environmentally acceptable, economically feasible, and physically possible to meet the demands for which they are built. The best way to assure that the construction will take place is to have a programme that provides installed control points that can be initially referenced before construction, used to monitor the progress during construction, and accurately account for the results after construction. The best tool for accomplishing these tasks at all phases of the construction is by correctly installing and using slope and reference stakes. These apply to all roads where large quantities of material are to be moved and the sites present extreme conditions for maintaining structural integrity of the road.
Aulerich, Ed & Shen, Zhenyan. 1994. Engineering tables for forest roads (metric). Forest Engineering Inc., Corvallis, Oregon, USA. 80 pages.
Plattner, Edwin. 1979. The impact of forest road construction on long-term forest policy. In: Mountain forest roads and harvesting. FAO Forestry Paper No. 14. Second FAO/Austria Training Course on Forest Roads and Harvesting in Mountainous Forests. 3 June-2 July 1978.