F.C. BUBBERMAN AND A.T. VINK
F.C. BUBBERMAN and A.T. VINK are Senior Assistant Conservators of Forests in the Surinam Forest Service.
Opening up tropical swamp forests in Surinam
DURING RECENT YEARS the Surinam Forest Service has obtained experience with ditch blasting as a method of opening up valuable, but hitherto inaccessible, swamp forests. As far as the authors are aware the subject of ditch blasting in forest exploitation has never been discussed in a technical paper, though the Surinam ditch-blasting work has been briefly mentioned in some notes (5). As ditch blasting might provide a possible solution to forest transportation problems elsewhere, it is felt that a fuller discussion of the technique is warranted. In the following pages a somewhat elaborate description is presented of soil and vegetation in the Surinam swamp forests to enable the reader to evaluate the practicability of this construction method under local conditions.
Surinam (162,900 square kilometers) is situated on the northeast coast of South America in the tropical lowland rainforest zone. Mean daily air temperature remains fairly constant throughout the year at about 26°C. Mean annual rainfall lies between 2,000 and 2,400 millimeters with main rains from April to July, main dry season to November, and less sharply defined, short wet and dry seasons fitted in between December and April.
Two broad geological divisions are recognized:
1. Basement complex (Archaic to Paleozoic Age), exposed mainly in the densely forested upland region which makes up four fifths of Surinam.2. Young deposits, divided into:
(a) the White Sand series in the lowland region;
(b) the Coastal Sedimentary series in the coastal belt.
Roughly speaking the young deposits form continuous belts more or less parallel with the Atlantic coast, though narrower in the east and widening toward southwest.
Soil types and vegetation tend to follow the main geological pattern.
The infertile sands and loams of the White Sand series bear different types of xerophytic vegetation. On the fertile clays of the coastal alluvium the swamp forests are found which are the subject of this paper. The coastal plain, about 21,000 square kilometers in all, is an almost level landscape, partly inundated because of impeded drainage and covered with various types of hygrophytic vegetation. The population of Surinam (325,000) is largely concentrated in the coastal area.
LANDSCAPE AND SOILS OF THE COASTAL PLAIN
Pedologically the coastal plain is divided (from north to south) into:
(a) the young coastal plain;
(b) the old coastal plain.
Young coastal plain. This consists of sea clay, situated at or below the high water mark, upon which locally sand and shell ridges have been deposited at former coastlines. These long, low and narrow ridges occur individually or in bundles and are mainly found in central and eastern Surinam.
Old coastal plain. This is essentially a repetition of the landscape elements of the young coastal plain, but older, more eroded and at a higher elevation, to about 10 meters above sea level.
It consists of dry, fine sandy areas and marshy patches of irregular shape with a silty soil. The sandy and silty complexes are separated by low level creek systems and swamps with a clay soil, in age and origin belonging to the young coastal plain.
Like the young coastal plain, the old plain was formed by marine sediments laid down in a shallow sea behind offshore bars. Successive regressions and transgressions of the sea have modified the original landscape to a much greater extent than in the young coastal plain. The young swamps between the old islands in the clay landscape point to a transgression; the difference in elevation between old and young coastal plains indicates a considerable subsequent regression.
VEGETATION OF THE COASTAL PLAIN
Generally speaking an accretion of land is in progress in the coastal area by sedimentation, accumulation of organic matter, or both. As the accretion proceeds, the pioneer herbaceous vegetation gradually changes to low swamp forest, high swamp forest and, eventually, marsh forest. Sea or fire may retard or reverse the process.
Apart from the ridges which bear mainly a type of high forest, natural vegetation in the coastal area consists exclusively of hygrophytic vegetation types, characterized by their simple structure and poverty in species.
Of the hygrophytic vegetation types, the high swamp forest is of interest within the context of this article.
HIGH SWAMP FOREST
As the last intermediate stage in the succession from open swamp to marsh forest, the high swamp forest is found on the older parts of the young coastal plain and on the old coastal plain, especially on the old clay landscape. The forest type makes up about 36 percent of the coastal forests or about 4 percent of Surinam's total forest area (Figure 1).
Soils consist of heavy, grayish, sometimes silty clay. The swamp forest is waterlogged for the greater part of the year and remains at least damp in the long dry season. In the main rainy season the forest is inundated to a depth of 1 meter and more in places. Incompletely decomposed organic matter accumulates in a layer of peat - locally termed pegasse - on top of the clay, wherever the highly acidic, stagnant water produces the necessary anaerobic conditions. The pegasse layer varies in thickness from 0.25 to 0.75 meter to over 2 meters in former erosion gullies.
In extremely dry years the pegasse may dry out completely and constitute a considerable fire hazard. In 1963-64, vast swamp forest areas fell victim to man-caused pegasse fires.
The high swamp forest is a two-storied forest type; the upper story ranges from 20 to 30 meters in height, the understory consists of saplings of the canopy species and some palms, one of which (Euterpe) also reaches canopy size. The swamp forest trees are flat-rooted species, adapted to their habitat by mechanical supports: buttresses, like Virola surinamensis, Pterocarpus officinalis; stiltroots like Symphonia globulifera, large rootclumps like the palm species Euterpe oleracea and Mauritia flexuosa. Other common swamp trees are Triplaris surinamensis and Tabebuia species.
The swamp forest is poor in species: the longer the period of complete inundation, the lower the number of species. Stem count in the fairly open stand ranges from 110 to 130 trees (over 25 centimeters in diameter) per hectare, with a basal area of about 13 square meters per hectare.
FIGURE 1. - The high swamp forest.
VIROLA FOREST
In some high swamp forest complexes Virola surinamensis is one of the dominant canopy species, to the extent that these stands may be termed "Virola forest."
Characteristic canopy species in the Virola stands are Symphonia globulifera, Pterocarpus officinalis, Tabebuia insignis var. monophylla and the palm Euterpe oleracea.
The Virola stands - five in all ranging in size from 6,000 hectares to over 40,000 hectares and totaling 100,000 hectares - are found mainly on the low-lying areas formed by erosion of the clay landscape of the old coastal plain. Remnants of the original plain are now present as irregular shaped, higher islands bearing marsh forest or high forest, in the surrounding swamp.
Table 1 summarizes landscapes, soils and vegetation types in the coastal area and shows the position of the Virola forests in the general scheme.
Virola surinamensis, locally known as "Baboen," is common to frequent in the high swamp forest and common in marsh and riparian forest. Its range includes the Amazon region of Brazil, the Guianas, Venezuela and parts of the West Indies.
Virola is a tree with spreading plank buttresses. It attains a height of 30 to 35 meters and a diameter of 60 to 70 centimeters, occasionally reaching to 90 centimeters. The usually straight, cylindrical bole is 15 to 20 meters long.
The light, pink-brown Virola wood has gained reputation as an excellent peeling timber, and the Virola stands have developed into a valuable source of raw material for Surinam's plywood industry (1963 value of Virola plywood exports: U.S. $2.3 million, or roughly 5 percent of total Surinam exports). These facts sufficiently explain the importance of finding methods to inventory and open up the Virola reserves.
TABLE 1. - LANDSCAPES, SOILS AND VEGETATION TYPES
|
Geological division |
Type of soil |
Pedological formation |
Landscape |
Vegetation type |
Area |
|
Young deposits
|
Young sediments
|
Demerara (young coastal plain)
|
Ridge landscape |
Ridge forest |
Coast
|
|
Clay landscape |
Mangrove forest |
||||
|
Old sediments
|
Coropina. (old coastal plain)
|
Old ridge landscape |
Ridge forest Marsh forest |
||
|
Old clay landscape |
Marsh forest |
||||
|
Zanderij |
Cover sands |
Savanna |
Interior |
||
|
Basement complex |
Residual soil |
|
Granites Schists |
High forest Marsh forest |
Interior |
INTERPRETATION OF AERIAL PHOTOGRAPHS
Because of terrain conditions extensive terrestrial inventories in the swamp forests are a difficult proposition, so that aerial photographs have to be used to the fullest possible extent.
Of the entire northern part of Surinam aerial photography is available on scales 1: 40,000 and 1: 20,000. On these scales the forest types are readily discernible, but individual tree species only rarely. In the even-textured Virola forest with its low-contrast photo image, identification of single tree species is out of the question.
However, experiments indicated that on 1: 10,000 air-photographs the canopy shows up in sufficient detail to permit distinction of separate trees and - in the floristically poor Virola forest - identification of individual tree species. In 1956-60, photographic coverage on scale 1: 10,000 was prepared for all major Virola complexes.
By means of a field reconnaissance the photo image can be compared readily with the crown structure as seen from below, without interference from intermediate stories, and the construction of a photo interpretation key poses no problems. Once an interpretation key is completed for a given area, the entire stand may be inventoried easily and accurately by marking with a dot all Virola trees visible on the photos and tallying the trees in 100-hectare squares. The dots are also transferred to 1: 10,000 clot maps which clearly show the location of the richest Virola concentrations and serve as a basis for exploitation planning.
So far, Virola inventories have been completed in respect of three stands.
ACCURACY OF THE PHOTOINVENTORY
Apart from the experience of the interpreter, the interpretation error depends mainly on the floristic composition of the particular stand.
Presence of other canopy species affects interpretation results in a different way in each locality, depending on the composition of the stand. For example, the swamp forest species Tabebuia insignis, which closely resembles Virola on the aerial photographs, becomes increasingly frequent toward west Surinam.
Photo quality and the position of the sun at the time of photographing greatly influence image tone and contrast and thereby interpretation results.
|
Area |
Hectares |
Number of Virola trees |
Estimated merchantable volume, m³ |
|
Rorac |
11 300 |
41 000 |
78 000 |
|
Perica |
13 800 |
37 400 |
75 000 |
|
Tibiti |
11 900 |
11 300 |
19 000 |
Although photointerpretation reduces field work considerably, terrestrial checking of interpretation results is always necessary. Comparison of photoinventory with subsequent field tally in two sample plots totaling 34 hectares showed:
1. About 70 percent of the trees classified as Virola by a trained observer are identified correctly; 30 percent belongs to other species.2. The total number of visible - and potentially identifiable - Virola trees on the photographs equals or exceeds the number of trees identified as Virola (whether correctly or incorrectly). In other words, photointerpretation yields a conservative estimate of the actual stand density.
3. Interpretation accuracy increases with the diameter of the trees.
4. Trees over 40 centimeters in diameter, that is, of merchantable size, are always visible on the photographs.
5. Fairly often, several Virola trees grow close together and are interpreted as a single tree on the photograph.
The conclusion of the sample plot data is that interpretation of 1: 10,000 aerial photographs gives a fairly accurate estimate of the density and distribution of Virola of merchantable size.
Virola timber floats and the stands are inundated during at least part of the year so, obviously, water transportation was and is the main extraction method.
The easily accessible Virola near the river banks was removed years ago through hand-dug trenches. Faster and more efficient methods are needed to open up the extensive virgin stands away from the rivers.
In North America, Scandinavia and elsewhere, ditch blasting, that is, the use of explosives instead of mechanical tools, is a well-known construction method of draining ditches in level, waterlogged terrain. Dynamite disposes easily of vegetation, stumps, boulders and similar obstructions that would handicap ordinary earthmoving machinery. The method is versatile, easily adapted to needs and no heavy initial expenditure for capital equipment is required, nor workshops and skilled mechanics for its maintenance.
Ditch blasting consists essentially in placing explosive charges in the ground at fixed intervals along the center line of the proposed ditch. The expanding gases formed by the explosion throw out the earth in all directions, a part falls back into the ditch, but most of it is thrown clear, so that a V- or U-shaped trench is formed, the width and depth of which are determined by soil conditions and the weight of the charge. Distance between the loaded holes depends on the size of the individual charges. In ditch blasting the individual charges are usually exploded by propagation: only one charge is primed and fired, electrically or with safety fuse, the concussion of the exploding primer charge is forcefully propagated through the wet or moist soil, setting off all the other charges.
In dry soil, propagation blasting is not feasible and all individual charges must be fired simultaneously, by means of electric blasting caps or detonating fuse.
As propagation firing is both cheaper and simpler than individual ignition of all charges, it should be used whenever soil moisture conditions permit it. When the soil is sufficiently cohesive to stick together when molded in the hands, it is moist enough to allow propagation blasting. Insufficient moisture seriously reduces propagation distance; additional moisture permits greater intervals between charges.
DITCH BLASTING IN SURINAM
Experiments in this direction seemed indicated, the more so because all conditions for successful ditch blasting are fulfilled in the Virola swamps: fairly level terrain, high soil water table and a firm subsoil fairly close to the surface.
Trial shots gave satisfactory results, and in 1958 the Surinam Forest Service started a project of systematic ditch blasting to open up the Rorac stand (39-kilometer ditch, blasted between 1958 and 1961) and the Perica area (22-kilometer ditch, 1961-1963). Meanwhile a private logging company blasted 23 kilometers of ditch in the Tibiti Virola stand.
Ditches are projected on the inventory dot maps to tap the richest Virola concentrations and bring, all parts of the stand within reach of exploitation. In order to serve their purpose as extraction channels to float out logs to the river, they have to be 3 to 4 meters wide and 0.75 to 1 meter deep in the rainy season.
The information on ditch blasting in engineering handbooks is borne out by the experience obtained by the Forest Service. Blasting through soft peat gives poor results and ditching with explosives is practically impossible in sand or fine gravel. In Surinam, satisfactory blasts have been made in very fine sandy soils, but resulting ditches silt up again within a short time.
The following notes describe the ditch-blasting technique in general with special reference to experiences in the Surinam swamp forests.
EXPLOSIVES, ACCESSORIES, TOOLS
Explosives
Ditching dynamite (DD).1 This is a high explosive containing 50 percent nitroglycerine in an absorbing medium. Manufactured especially for ditch blasting, DD is highly shock-sensitive and water-resistant, enabling it to be fired by propagation in moist or wet soil. It is expensive and somewhat hazardous to handle on account of its sensitivity. In Surinam, it was used in 50-pound (23-kilogram) cases, containing about 100 cartridges ("sticks"), each of ½ lb (230 grams) and 1 ¼ x 8 inches (3.1 x 20 centimeters) in size.
1 Brand names referring to American and Canadian explosives. See also on following page.
Red Cross extra dynamites (RC).1 In this type part of the nitroglycerine is replaced by ammonium nitrate, to render them cheaper and less sensitive, i.e., safer to handle. RC 50 percent strength has adequate water resistance for use in ditch-blasting work, but RC cannot be fired safely by propagation and must be ignited by means of a DD stick placed alongside, or with blasting caps. In Surinam it was mainly used in 5-pound (2.3-kilogram) cartridges, size 3 x 16 inches (7.6 x 40 centimeters), to blast out stumps, etc., in the right of way.
1 Brand names referring to American and Canadian explosives.
Blasting agents. Cheaper and safer still than RC, these contain no nitroglycerine at all. Highly insensitive and not water-resistant they can be used only in dry boreholes and must be ignited with high explosives or suitable primers. The Surinam Forest Service has a limited, though favorable, experience with blasting agents in blasting through a high, fine sandy ridge.
To avoid long delivery times and high shipping costs the Surinam Forest Service ordered its explosives through regular shipments to a local mining company.
Accessories
Blasting caps. These are small metal tubes, containing a highly sensitive explosive, designed to ignite the main explosive charge when fired electrically (electric blasting caps) or from the sparks of a safety fuse.
Safety fuse. This is a combustible core within protective wrapping. Burning slowly when lit, it gives the blaster time to retreat while it carries the flame to a blasting cap and primer charge affixed to the other end.
Detonating fuse. An explosive core with protective covering, when lit - electrically or with a common blasting cap and fuse - it detonates, igniting in turn all other explosive charges it is placed in contact with.
Cap crimping plier. This is specially designed to trim the fuse and crimp blasting caps to the fuse.
Loading tools
Tools are necessary to make the charge holes, to space them at the correct distance and to load the explosive into the hole.
Soil punches. Made of wood or metal, these are suitable to punch shallow holes in soft soil to accommodate a single cartridge.
Hollow core punch bars. Such bars are generally used to make holes up to 0.75 or 1 meter deep in firmer soil or in soil obstructed by roots. The tool consists of a hollow rod, 4 to 5 centimeters or so in diameter, with handles, which is driven into the ground by means of repeated blows of a weighted punch bar, inserted through the hollow rod. Punch bars are available in several types: the type used in Surinam is illustrated in Figures 2 and 3. It can be constructed from iron pipe and rod in any blacksmith's shop. Punch bars tend to break at the most inconvenient moments and an ample supply should be kept on hand when loading is in progress.
In the Forest Service ditch-blasting work, it was found advantageous to use two rods to each punch bar, one rod being rammed down while the previous one is being loaded with dynamite. The explosive is tamped with the wooden tamping stick and the hollow rod is removed over the stick. This arrangement prevents any cartridge from sticking to the inside of the rod, or the punch bar from being inadvertently inserted once more into an already loaded hole, both with disastrous consequences (Figure 4).
The hollow core punch bar used in, the Forest Service ditch-lasting. Note that the 1-inch (2.54-cm) pipe in the punch was replaced by ¾-inch (1.9 cm) solid steel rod in later models, because the narrow clearance between punch and hollow rod in the original version caused rapid jamming of the punch.
FIGURE 2a. - HOLLOW ROD
FIGURE 2b. - PUNCH
FIGURE 2c. - TOOL ASSEMBLED
Post-hole soil augers. With a diameter of 7.5 to 15 centimeters, and either hand- or engine-operated, these are useful for drilling holes to 1 to 1.2 meters deep in drier clayey soil, for blasting by the so-called "posthole method" (see below). Sometimes the hole must be started with a spade to cut through a zone of interlaced roots.
Wooden tamping sticks. Rounded sticks about 1 meter long with squared ends, these are used to push the dynamite cartridges into the charge holes. (This should never be done with any sparking metal tool.) The stick may be graduated to ensure loading to a uniform depth.
A simple measuring stick is used to space the individual loads at the correct distance.
SITE PREPARATION
The ditch system, as projected on the Virola dot maps prepared from the aerial photographs, is surveyed in the field with compass and chain, and the trace is staked out.
Leveling had to be undertaken in some cases. Separated from the tidal lower rivers, the swamp forest areas are drained by natural, winding, sluggish watercourses, hardly discernible at times, that eventually find their way to the river by way of a swamp creek or by way of a tidal channel through the river levee.
In the Rorac project, leveling showed a height difference of 3 meters between the river and the swamp only 2.5 kilometers inland. In this case, a series of weirs with floodgates had to be constructed in the access canal from the river.
In the Perica project, a branch ditch was projected to cross an existing shallow drainage channel. Leveling revealed the drainage channel to be lower in elevation than the main entrance to the ditch network, so that the ditches would have been drained empty in an entirely unwanted direction if the proposed connection had been constructed.
Originally all trees were felled in a 20-meter wide strip, and all small trunks and felling slash removed in the central 10-meter zone. Subsequent trials in 10-meter wide strips with only the axis cleared of felling debris were satisfactory, though ditch cleaning costs increased slightly. This system became standard practice from 1962 onward.
To facilitate the transportation of dynamite through the rough and often waterlogged strip, a path of palm trunks is always constructed along the trace.
FIGURE 4. - Schematic sequence of punching and loading: (a) punching, (b) tamping, (c) loading.
WORKING TECHNIQUE
As a rough calculation it may be assumed that 1 lb (454 grams or 2 sticks) of 50 percent DD will move about 1 cubic yard of material (about 0.75 cubic meter): in heavy, compact clay this is reduced to about 2/3 cubic yard (0.5 cubic meter); in light, loose soil it is increased.
On this basis, specifications may be drawn up for single-line, cross-section and post-hole loading, according to soil and size of the desired ditch.. Loading schedules will be found in any handbook on blasting. It must be emphasized that much depends on soil and vegetation conditions. Test shots are the only reliable way to determine the correct amount and distribution of the explosive.
Surface water must be taken into account in the same way as the soil itself when necessary loads are computed. Therefore ditch blasting in deeply inundated swamp is uneconomic. In Surinam, blasting proceeded mainly in the dry seasons.
Loading
Single-line method. One or more DD cartridges, depending on the soil and the size of ditch required, are placed in a column in a single row, at distances of 35 to 55 centimeters, depending on the charge. The top of the uppermost cartridge should be 15 to 30 centimeters below ground level (Figure 5a).
Cross-section method. Essentially the same as the single-line method, but with cross rows offset at right angles from every other hole on the center line, with this method the center holes are usually loaded more heavily than the cross rows. The system is suitable for obtaining wider ditches or to break a heavy, tangled root mat, such as that found in the Surinam swamp forest in patches where Symphonia globulifera abounds (Figure 5b).
Most of the Forest Service ditches have been shot by the single-line column load method, with an occasional cross row to break a heavy root mat. Loads of 2, 3, 2, 3, 2, etc., DD sticks at intervals of 45 centimeters were usually adequate to produce a ditch of required width (3 to 4 meters) and depth (0.75 meter). In heavier soil or to blast through slightly higher portions of the swamp 3, 4, 3, 4, 3, etc., loads have been used to advantage. Heavy tree stumps are usually loaded with one or more 5-pound RC cartridges.
Loading is done by a crew of three: a porter to carry the dynamite cases to the loading site, a puncher, and a loader. One porter can keep pace with two loading crews if carrying distance is short. One crew loads 100 meters a day.
The actual loading of the set task seldom takes more than three to four hours, the remainder of the working time is spent in traveling to and from the work, by corials (dugout canoes) through the ditches to the campsite near the river.
Post-hole method. An effective way to blast deep ditches, especially in heavier, compact types of soil, post-holing consists in placing charges of several pounds in auger-drilled holes, at two thirds the depth of the required ditch and spaced 0.9 to 1.2 meters apart, depending on the charge. The loads are fired by propagation or by individual ignition (detonating fuse, electric caps). Because of its fewer but heavier charges per unit length as compared with the other two methods, post-holing is a suitable ditch blasting method in dry conditions, where all individual charges must be ignited (Figure 5c).
In Surinam, the post-hole method was used to advantage when it was necessary to extend the ditch through higher, relatively dry sections of the swamp. Holes are dug with spades, punched with heavy sticks or drilled with soil augers. The loads are usually made up in part of 5-lb RC cartridges, each accompanied by at least 1 stick of DD to ensure detonation.
Ignition is by propagation or with detonating fuse, depending on soil moisture content.
Post-holing was used successfully to blast through ridges 1.8 meters above the surrounding swamp, with loads up to 15 kilograms in 1.5 to 8-meter deep bore holes spaced up to 1.5 meters apart (Figure 6).
All loading systems in ditch blasting will remove stumps, boulders, logs, etc., but such obstructions obviously require an additional charge, varying from a few extra cartridges in the nearest charge hole to a charge of several pounds in a separate hole under the stump or a row of DD sticks along the buttresses. These separate charges must be connected to the main line by a row of DD cartridges spaced at the proper interval to ensure ignition by propagation,
To develop full strength the explosive must be closely confined in the charge hole. Therefore the charge is tamped down and stemmed by refilling the hole with earth. In wet work like the Surinam swamp forest ditch blasting, the holes rapidly fill with water and no further stemming is, required. A primer cartridge must never be tamped.
Firing the blast
Propagation-fired blasts are ignited by means of a primer loaded somewhere in the row of charges. A primer consists of an electric or ordinary blasting cap plus fuse, inserted into a dynamite cartridge. The different methods of preparing a primer are described in detail in the various handbooks on blasting.
FIGURE 5a. Loading patterns: single-line
FIGURE 5b. Loading patterns: cross-row
FIGURE 5c. Loading patterns: post-hole
Individually ignited charges are fired electrically or with a detonating fuse. A detonating fuse, in turn, is ignited either with an electric blasting cap or with common cap and fuse.
For reasons of safety, workmen, tools and surplus explosives must be removed to at least 100 to 150 meters from the blast area. In the moist, swamp forest soil of Surinam, materials from the blast are seldom thrown more than about 50 meters from the ditch, but stones or gravel, if present in the soil, may be propelled considerably farther.
After the primer is loaded, the safety fuse is trimmed and lit, and the blaster hastily retreats to a safer position.
After the blast, all personnel should wait for smoke and dust to settle before going near the blast area. The fumes of dynamite are toxic, causing severe headaches (Figure 7).
Only very few misfires have occurred in the entire Forest Service blasting program. In wet work like ditch blasting, misfires are usually caused by improperly seated or badly crimped caps, or by damaged or moist fuses.
Misfires, should not be approached for at least one hour, after which time it is considered safe to return and reshoot the charge.
Although DD and RC 50 percent are water resistant, they will deteriorate when left in water for a considerable time. Accordingly it is not advisable to leave a loaded section overnight because, even though the dynamite will usually ignite after one or even two days in water, propagation may be incomplete and part of the charge may fail to detonate.
Ditch cleaning
The explosion forms a V- or U-shaped trench in the soil, which slowly or rapidly fills with water, according to-the prevailing height of the water table and the porosity of the soil. In the wet Surinam swamps, the new trench is filled almost immediately. Depending on soil and vegetation, the ditch will be fairly clean or require still more work to remove lumps of clay, stumps, branches and floating debris. In Surinam it was found that deep and clean ditches can be blasted without difficulty where firm clay is found at or near the surface. In the stiff clay, the banks of the ditch may be steep without risk of crumbling and a width of 3 meters is adequate to maintain the required depth and bottom width of about 0.75 to 1 meter.
Blasting through soft, peaty soil results in shallow ditches that need much manual cleaning and deepening. The peat is pulverized by the explosion into a sluggish suspension that flows back into the ditch or floats in dense masses on the surface. Only by allowing it to settle and by gradually cleaning and deepening the ditch by hand can an acceptable depth be attained. In the soft peat the natural gradient of the banks is gentle, for which reason the trench must be made wider to obtain an adequate depth.
Forest Service ditches must be made suitable for log transportation as well as traffic with small outboards, since all personnel and materials for the project must move through them. A fairly complete cleaning is required, effected by workmen equipped with shovels, rakes, and light, hand-operated winches. Stubborn obstacles are removed by blasting.
PERSONNEL SAFETY MEASURES
The Forest Service ditch blasting was done by a permanent gang of carefully selected and reliable workmen. The old adage "familiarity breeds contempt" is fully applicable to working with dynamite. Workmen are apt to grow careless after a while, and constant supervision at all stages of the work is necessary to counteract this tendency. Not one accident has occurred in the course of the Forest Service ditch-blasting work.
All dynamites tend to cause headache when handled, because nitroglycerine widens the blood vessels. According to Surinam experience the workmen gradually build up some immunity against this unpleasant side effect of dynamite, though some persons are more sensitive than others. The use of workmen's gloves goes some way to eliminate the headache nuisance.
Each case of explosives contains a list of "do's and don'ts" and complete information will be found in any handbook on blasting. The rules on storage (special, looked powder house), transportation (in wooden vessels or wooden-bodied trucks, caps and explosives never close together), and handling are self-evident and need not be elaborated here.
CONSTRUCTION COSTS
The following cost-breakdown of Surinam ditch blasting might be modified to suit local soil, vegetation, and working conditions. It must be kept in mind that the access ditches were made in extensive, inaccessible and uninhabited swamp forest and that all personnel and explosives had to be moved in from elsewhere, through the ditches. Construction of campsites was expensive, and much working time was lost in traveling and transport.
Some basic Surinam cost figures, to allow comparison with local prices:2
|
|
S. fl. |
|
Laborers' wages³ per day |
6-7 |
|
DD 50 percent, per case (50 lb) |
32 |
|
RC 50 percent, per case (50 lb) |
24 |
|
Fuse and caps, a few cents per foot or per piece |
|
2 1 Surinam guilder (S fl.) = U.S. $0.54.
3 Including subsistence allowance, etc.
All-in costs (excluding depreciation and interest charges on camps, etc.) of the Forest Service ditches have varied from about S. fl. 5,500 to about S. fl. 7,500 per kilometer. The figures below refer to 1962, when operations were in full swing - 16.2 kilometers of ditch were blasted - and the technique had been perfected.
Construction cost 1962, project Perica: S. fl. 5,550 per kilometer (= U.S. $4,800 per mile).
Itemized, these costs can be divided as follows:
|
|
Percent |
|
Supervision |
18 |
|
Explosives |
40 |
|
100 cases DR: 50 lb per km (averaged) |
|
|
60 pieces RC: 5 lb per kin (averaged) |
|
|
Tools, equipment, camps, outboards, fuel |
10 |
|
Transportation, miscellaneous |
4 |
|
|
|
Man-days, percent |
|
|
Wages |
tracing |
6 |
28/100 |
|
Wages |
clearing the trace |
21 |
28/100 |
|
Wages |
blasting, incl. transportation of explosives |
16 |
28/100 |
|
Wages |
ditch cleaning |
15 |
28/100 |
|
Wages |
dams, weirs |
8 |
28/100 |
|
Wages |
camp building, maintenance |
7 |
28/100 |
|
Wages |
Sundays, sick leave, holidays, miscellaneous |
27/100 |
28/100 |
As apparent from the above figures, explosives are always the major cost item in ditch blasting.
BLOCKING PROCESS
Even in the opened up swamp forests logging is confined to the rain: seasons, when the loggers are able to construct shallow trenches through the inundated swamp with little effort, to float out the felled logs to the extraction ditches. To prevent the water from draining away too rapidly and to extend the timber exploitation into the dry season, permanent weirs with removable wooden gates are built in the exit channels. During the rainy seasons the gates are partly opened to create a steady current in the ditches, that aids in the floating out of the logs and cleans the channel by its scouring action. When the rains abate and the water level starts to drop, the gates are closed.
From the middle of the dry season shallow sections of the ditches run dry and weeds start to grow. Some are killed by subsequent inundation, others maintain their foothold. In the stagnant water aquatic weeds soon appear. Litter from the swamp forest, from leaves to entire trunks, aids in the blocking process. If the ditches are to remain navigable for more than just one or two logging seasons, they must be maintained.
Soil conditions determine to a large extent the type of vegetation in the ditch and thereby the amount of maintenance work required.
The ditch sections through pegasse, already shallow to start with, silt up further by the action of the rains and the water disturbance created by passing boats. Aquatic weeds, like water lilies (Nymphoides sp.) and Sara sara grasi (Camomba sp.) grow profusely, the latter eventually forming dense (up to 20-centimeter) floating mats, which in turn are colonized by grasses and rushes.
In the steep-banked, deep ditch sections through clay, silting up is negligible and aquatic weeds are less troublesome although the same species from the pegasse areas are found in places. Grasses and sedges sprout on the banks and gradually extend into the ditch. The rapidly developing secondary growth, mainly consisting of large-leaved pioneer species like Cecropia, Heliconia, etc., forms a dense overhead shade within a few years, causing the gradual disappearance of the aquatic weeds, which demand full sunlight.
Luckily, the dreaded water hyacinth has not made its appearance in the ditches so far, although it is found in Surinam.
CLEANING
Obstructions like fallen branches and trunks are removed by hand. Weeds are removed by manual labor or by spraying with herbicides. Against aquatic weeds "Silvex" (a 2-2,4,5-Trichlorophenoxy Propionic acid formulation) was found effective.
This herbicide is simply sprayed on or in the water, with a knapsack sprayer, at a rate of 8 to 9 liters per kilometer of ditch. Where much grass is present a mixture of 400 grams "Dalapon" (Sodium 2,2 dichloropropionate) per liter of "Silvex" is used. The herbicide may be diluted with water for easier spraying. Chemical weed control is undertaken in the beginning of the dry season, when the ditches are still navigable but the herbicide will not be diluted immediately by fresh swamp water. For obvious reasons chemical weed control is out of the question if the ditches are the only supply of drinking water for loggers and blasting or cleaning crews.
FIGURES 8 and 9. - Construction drawing and photograph of permanent weir and floodgate in the main exit of the Perica ditch system. The gate consists of 3 x 10-inch (7.6 x 25 cm) tongued & grooved planks which move between 2 U-beams. Water flow is regulated by removing or placing planks as required.
"Silvex" costs about S. fl. 5 per liter, so spraying costs work out at S. fl. 40 to 45 per kilometer (exclusive of labor). Manual removal of weeds, the only solution in completely blocked ditch sections, requires 25 to 30 effective man-days per kilometer or S. fl. 250 to 300 per kilometer.
It is already apparent that the ditch sections through pegasse will not last for more than a few years. Reblasting or deepening by mechanical means will be necessary if they are to be kept open. In clayey soil the ditches will probably last for a considerable time with only a minimum of maintenance, once an overhead canopy has become established and aquatic weeds become less troublesome.
Regular log extraction and a steady water flow through the ditches will keep them free from obstructions. For that reason blasting should proceed apace with exploitation, no new section being constructed until the forest along the preceding section has been exploited completely.
In the extensive swamp forests of the coastal area of Surinam, the commercially important peeling timber Virola surinamensis forms locally what might be termed Virola stands (average density 1 to 7 trees per hectare). These total some 100,000 hectares in five stands, varying in size from 6,000 to over 40,000 hectares each. They form a part of the vegetation type High Swamp Forest and occur mainly on flat, marine sediments, geologically belonging to the young coastal plain between the higher, eroded remnants of the old coastal plain. Soils consist of heavy clay, covered with a peat layer varying from less than 0.5 meter to over 2 meters in thickness.
The 20- to 30-meter high, open Virola forest contains only two or three emergent tree species besides Virola and a subcanopy of a palm species. On special 1: 10,000 aerial photographs, the individual mature (about 40-centimeter in diameter and over) Virola trees can be identified with reasonable accuracy, once an identification key has been prepared in the field. Dot maps are prepared to show the position of the mature trees. The Forest Service has to date inventoried three Virola stands, totaling 37,000 hectares.
To open up these stands for logging the Forest Service constructs 3- to 4-meter wide and 0.75- to 1-meter deep ditches through the swamp by blasting with dynamite in a 10- to 20-meter wide trace, cut and partly cleared by hand. Explosives, tools, site preparation and working technique are described. Blasting proceeds only in the two dry seasons. Construction costs vary from S. fl. 5,500 to S. fl. 7,500 per kilometer. During the rainy seasons Virola logs are floated through the ditches out to the river, for rafting and transport to the plywood factory. The water level in the ditches is regulated by permanent wooden weirs and floodgates (Figures 8 and 9).
Between 1958 and 1963 the Forest Service blasted 61 kilometers of ditch to open up some 23,000 hectares of Virola forest (two Swamp forest stands). A private timber company constructed a 23-kilometer ditch in another Virola stand.
Surinam experience shows that ditch blasting for forest exploitation purposes is a suitable technique in the clayey soils of the Surinam swamp forests, provided the peat layer is not unduly thick.
Litter and weeds gradually block the ditches, which have to be maintained by opening the weirs (scouring action of the current), manual removal of the weeds or by spraying with herbicides. In the deep and fairly clean ditch sections through clay, weeds are much less troublesome than in the shallow sections through peaty soil. Overhead shade eventually kills the weeds.
It is advisable to let blasting proceed apace with exploitation in order to make effective use of the ditches and to reduce maintenance costs.
1. CENTRE TECHNIQUE FORESTIER TROPICAL. 1962. Creusement des fossés à la dynamite. Nogent-sur-Marne. Recueil technique de l'exploitation forestier H 1.
2. Du Pont blasters' handbook. 14th ed. 1958. Wilmington, Delaware, E. I. Du Pont de Nemours & Company (Inc.), Explosives Department.
3. EIJK, J. J. VAN DER. 1954. De landschappen van Noord Suriname. [The landscapes of north Surinam.] Paramaribo, Centraal Bureau Luchtkaartering. Publication No. 15.
4. LINDEMAN, J. C. & MOOLENAAR, S. P. 1959. Preliminary survey of the vegetation types of northern Surinam. In The vegetation of Surinam. Vol. 1, Part 2. Amsterdam, van Eedenfonds.
5. SURINAM FOREST SERVICE. 1958-63. Annual reports. Paramaribo.
6. VINK, A. T. 1964. Ditching with dynamite. Paramaribo, Surinam Forest Service. (Mimeographed)
7. VOORDE, P. K. J. VAN DER. 1957. De bodemgesteldheid van het ritsenlandschap en van de oude kustvlakte in Suriname. [Soil conditions of the ridge landscape and of the old coastal plain of Surinam.] Paramaribo, Landbouw-proefstation. Bulletin No. 74.
