Village Hope Technical Report 12d. Construction_Roads. Jonathan Bart 2011 Standard VHTR disclaimer

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1 Village Hope Technical Report 12d Construction_Roads Jonathan Bart 2011 Standard VHTR disclaimer Although we have made every effort to insure the accuracy of information in Village Hope Technical Reports most were written by students or by the President of Village Hope as way to learn about an unfamiliar topic. Use them to get an introduction to - or quick overview of - a topic, for ideas, and to locate references. But please do not treat them as authoritative accounts. If you want to you use some of the information, check the references and reach your own conclusions. We are always happy to hear from people with corrections, updates, or especially with offers to revise a VHTR or write a new one. Introduction... 2 Roadbeds... 5 Design... 5 Materials... 7 Determining composition Low water crossings Under drains Wetlands Maintenance Ditches and banks Ditch lining Turn-outs Banks Maintenance Culverts and bridges Pipe material Pipe size Culvert walls Bridges Construction Maintenance Signs Sources of aggregate Production of sand and gravel

2 Equipment Costs and employment Glossary Bibliography Introduction Left: a well-drained rural road with a stable driving surface and stable ditches and slopes. Right: a poorly-drained road that is difficult to travel on and expensive to maintain. This chapter discusses low-volume rural, dirt roads, defined as dirt roads that support fewer than 400 vehicles per day and are designed for speeds less than 50 mph (80 kmh). We do not discuss paved roads or roads covered with gravel transported from off-site. A dirt road includes the following major components Road bed Ditches and banks Culverts and bridges Signs shoulder ditch crown road bed ditch bank Major road components (except culverts, bridges, and signs) 2

3 The road bed is the surface of the road between the ditches, starting at the shoulder if one exists. Banks are the slopes above ditches that transfer water to the ditch or to diversion structures such as terraces. In this manual, culverts are structures that contain the stream and support the road whereas bridges are more complex structures that make little if any change in the stream bed. A publication, Basics of a Good road by the Cornell Local Roads Program (CLRP) lists the ten commandments of a good road: In designing roads it is critical to consider the heaviest vehicles that will be using it. According the CLRP, 10,000 autos equal one 80,000 truck in terms of wear on the road. The CLRP also says that gravel roads are suitable for roads used by vehicles per day and virtually no heavy trucks. Roads used more heavily should be paved. Furthermore: Sealing an aggregate road eliminates the need for frequent grading and addition of aggregate to replace surface loss. On top of this is the often overlooked vehicle operating cost. This cost will vary per mile driven depending upon the type of road surface. For aggregate surfaced roads, the operating cost is often substantially more than for bituminous roads in the same condition. Careful analysis has shown that the total cost for an aggregate road and a surface treated road are identical with an average daily traffic (ADT) of eight. If the ADT is greater than eight, it is more economical to have a surface treatment. Keep in mind that this represents total cost and not just the cost for the highway agency. The total cost is the road construction and maintenance cost plus the vehicle operating costs. If total cost is considered, many aggregate roads should perhaps be paved. The public is unaware that their costs would actually be less if some of these roads were surface treated. The problem, of course, is budgeting the money to upgrade the road. 3

4 This passage suggests that we should consider paving many of the roads in our project area. Nonetheless, due to the costs of doing this, initially it seems best to establish dirt roads and only switch to pavement or other surfaces later. Manuals on road construction stress that the most important part of road construction is handling drainage (e.g., it is estimated that New York State spends $1 billion/year repairing damage to roads caused by improper drainage). Improper drainage leads to erosion of the road surface causing pot holes, soft spots, and wash boarding; degradation of road quality: and sediments in surface water. In addition to proper road construction, frequent maintenance, occurring as soon as problems begin to develop, are often needed to avoid serious degradation of roads. Accordingly, this chapter emphasizes achieving and maintaining good drainage. Most information in this report comes from: Center for Dirt and Gravel Roads at Penn. State University, the Cornell Local Roads Program (CLRP), at Publications.htm an online manual from the EPA at A Landowner s guide to Building Forest Access Roads by Richard L. Wiest Low-volume roads engineering by Gordone Keller and James Shear, USDA Forest Service, Other sources are cited in-text. 4

5 Roadbeds Design Most well-constructed dirt roads are center crowned meaning the center is higher than the sides. An effective roadbed moves water falling on the road to the ditches and is strong enough that traffic does not destroy the surface. Accomplishing the drainage function usually insures that the road does not suffer rapid erosion or develop ruts, pot holes or wash boarding. Recommendations for the slope with dirt or gravel roads vary from per foot (paved roads have less slope because the pavement does not impede the flow of water less than aggregates). Those recommending 0.5 note that steeper slopes cause drivers to drive in the middle of the road because they tend to slide when they drive entirely to one side of the crown. Such behavior, however, may be unavoidable when traffic is light, and the steeper slope may aid in road drainage. Roads in deep sand (which rapidly absorbs water) may have slopes from the crown of 0.25 or less and no distinct shoulder separating the road bed from the ditch. Examples of 3/4" to 1' crown with road ditches in place. Water sheds readily off the crowned road surface and into the ditches (Source EPA 2000). 5

6 Examples of 0" to 1/4" crown with no road ditches in place. Water infiltrates the soil of the sandy and flat road surface minimizing runoff from the roadway. The water that sheds from the roadway is readily removed from the road surface into the roadway edges allowing a passable lane in the center of the roadway.(source: EPA 2000) Road beds may also be in-sloping or outsloping. In-sloping is more common because the ditch catches surface run-off and prevents it from eroding the road bed and because vehicles are less likely to slip off the road. With slight slopes and good filtration upslope from the road an out sloping road may be less expensive because a welldefined ditch may not be necessary as in the picture below. An out slope crown. Note the good filtration upslope and the lack of a well-defined ditch on either side. Improperly crowned roads tend to incise, causing numerous problems (CLRP): 1. Loss of road surface material 2. Soil collapsing into deep pipe or drainage inlets 3. Soil collapsing from steep banks undercut during maintenance operations and by water flowing in ditches 4. Road edges undercut by concentrated ditch flow 5. Difficulty plowing snow and finding a location to place plowed snow 6. Pipe installations with steep, unstable banks at inlets and long, difficult to maintain outlet tail ditches 6

7 Materials In dirt roads, the road beds are made from a combination of gravel, sand, and fines (clay and silt). The gravel increases strength, improves traction, and reduces erosion. The fines (and I suspect especially the clay) act like cement holding the aggregates together. Sand has some properties of both gravel and fines, improving drainage compared to high-clay road beds but retaining moisture compared to high gravel road beds. Particle shapes also affect cohesion; irregular particles lock together, in the presence of the right level of moisture, better than spherical particles. The road bed should contain a mixture of fines and larger material that locks together when compacted. Different levels in the road require different compositions. The entries below (from the CLRP) are the percent of materials by weight. The only difference is in the fines. The surface needs them to bind materials together reducing dust and penetration by water. In the base, fines would hold water reducing strength. The CLRP also describes optional tests for elongated particles (fewer is better) and for a fractured faces test (more is better). The CLRP bulletin concludes Naturally occurring materials have a tremendous variation in gradation. Only a small amount of natural material is suitable for use in roads and streets. This statement seems to imply that a considerable amount of work must be done to process local soil or import soil from other areas. The CLRP also has come interesting data on permeability (feet/day): gravel - 30,000, sand - 3,000, fine sand and silt - 3, clay Permeability thus varies seven orders of magnitude between clay and gravel. They note that mixtures tend to have permeability closest to the finest component. They also compare capillarity (in feet), noting that high capillarity causes problems in roads: gravel - nil, coarse sand - 0.5, fine sand 1-3, silt , clay Keller and Shear (2003) also discuss composition of the road base and surface. They emphasize the importance of a strong base, especially if heavier vehicles will use the road. Compositions for dirt and gravel roads in wide use include all native soil, crushed surface aggregate or gravel 7

8 over native soil, and (best of all) crushed surface aggregate or gravel over aggregate base. In contrast, paved roads have asphalt pavement over an aggregate base and ideally have an aggregate sub-base. Pavement takes the place of the crushed surface aggregate or gravel. Thus, dirt or gravel roads should not simply have pavement added on top of the existing layers. They recommend that the surface layer be cm and the base be up to 30 cm (below). They explain the importance of having the right composition with the following diagram: Aggregate with no fines: grain-to-grain contact, variable density, high permeability, high stability when confined low otherwise, high strength, not affected by water, difficult to compact, ravels easily. Aggregated with 6-15% fines: grain-to-grain contact, resistant to deformation, maximum density, low permeability, high stability, medium strength, not affected much by water, moderately easy to compact, good road performance. 8

9 Aggregated with >15% fines: little grain-to-grain contact (aggregates floating in soil); decreased density, low permeability, low stability, low strength, greatly affected by water, easy to compact, creates dust. They present an interesting graph defining good size distributions. The Y-axis shows the cumulative distribution of grain size (but the X-axis is reversed). Consider the lower black line which describes a good size distribution for the base aggregate in wet, tropical countries. 100% passes 2.5 (or 64 mm) screen (upper end of line); 62% passes through a 1 screen; 20% passes through a #8 mess (2.4 mm) screen, and <2% passes through a 0.05 mm mesh. The figure thus provides a detailed, quantitative description of the optimal size 9

10 distributions for both the base and surface layers. They recommend 5-10% fines in wet, tropical areas for the surface layer. Soils in Sierra Leone were described by Stobbs (1963), Odeall et al. (1974), and Birchall (1979). All authors described the soils as loams or occasionally as clays implying a large fraction of clay and silt. Only Odell et al. (1974) reported levels of gravel. They found that 30% of the soil at the surface was gravel in uplands (which cover the great majority of the country) and a much lower fraction elsewhere. Horizons are on the X-axis with the uppermost one firts. 10 Within the remaining 70% (ie, the clay-silt-sand fraction) of the surface soils, fines in uplands made up 50% (brown line in the figure below above horizon 1). Thus the average fraction of fines was about 35% based on these data, substantially higher than the recommendations above ( ~2% in the base and ~8% in the surface layer). Roads with far more fines than desirable absorb water and become soft, slippery, and muddy. Because the base lacks strength, they may develop soft spots. The result is ruts, pot holes, wash boarding and erosion, especially in high rainfall areas. Since the amount of fines is substantially above the recommended level for the surface layer and far, far above that

11 recommended for the base layer, we may have to consider bring in fill for the roads or perhaps processing the local soil onsite as the road layers are being established. This subject is discussed more in the section Obtaining aggregate. Determining composition The composition of soil can be determined with a set of sieves and sieve shaker. AASHTO procedures T27, T88, or T3111 describe the methods in detail. The following sieves are used: 75µm, 425 µm, 6.3mm, 12.5mm, 19mm, 25mm 37.5mm, 50mm, and 75mm. Cornell offers the following advice (which I don t fully understand) for the use of sieves: get the total sample weight and then make sure that the total sample is washed through the No. 200 sieve before sieving. If the material is merely sieved dry, the percent silt and clay will be too low since much of the finer material will adhere to the larger sand and gravel particles. Presumably, the AASHTO procedures describe the method in detail. Although other fairly simple methods exist (two are described below), using sieves seems the most appropriate for us. The Cornell bulletin on drainage describes a simple test to determine composition: A simple test can be performed in a lab or in the field in approximately 30 minutes. The Sand Equivalent Test measures the proportion of clay-like particles in the sand size and smaller particles of a gravel. A sample of material is placed in a cylinder with a special solution and let to soak. It is then shaken and the sand particles settle out almost immediately. After twenty minutes the clay-like particles have settled to some value in the cylinder (Clay reading). The sand particles, which can support a weight, have settled to a lower value (Sand reading). The ratio of these two lines, reported as a percentage, is the Sand Equivalent. The Test is described in more detail at %20pdfs/sand%20equiv.pdf, but no information is provided on how to obtain the special liquid (water, calcium chloride, and glycerin). Note that the text does not distinguish between silt and clay. The Cornell program also offers the following field tests to determine suitability of the soil for roads: ***************************************************************************** Do-it-yourself gradation tests 11

12 Pick up two or three double handfuls of soil. Spread the material on a flat surface. Discard stones larger than approximately ¼ inch. Add enough water so that you can pack the material into a ball, but not so much as to make it mushy. Pick up a handful of the moist material, and squeeze it. It should contain enough sand to look and feel gritty. Look at your hand. For use in gravel roads the mixture should contain enough silt and clay to stain your hand slightly, but not leave it muddy. It should hold its shape while moist. If dried, the ball should still retain its shape and resist breaking. For use in base courses the moist material should not stain your hand. If dried, a ball of this material should pulverize easily in your hand. If the soil is wet, you can determine the percentage of fines by placing the soil in a glass container (like a canning jar). Add enough soil to fill the container ¼ full. Add water until the soil is just covered. Mark this level with a rubber band. Fill the jar ¾ full with water, and stir or shake vigorously. Let it settle approximately 1½ minutes. Mark the height of the soil with another rubber band. The difference between the two rubber bands represents the approximate percentage of fines. Do-it-yourself plasticity tests Pick up a lump of moist soil and knead it, removing as many large-grained particles as possible. Add enough water so you can mold the material into a ball easily, about golf-ball size. If the material clings to your hands, it is too wet! Divide the ball in half, and then in half again. Roll the small portion into a snake-like shape approximately one-half inch in diameter. A nonporous surface such as a Formica counter or a sheet of glass works well for this purpose. If you cannot form a snake at all, the soil is nonplastic, and is probably silt or fine sand. If it does not roll easily, add a drop or two of water. Continue rolling until the smallest possible snake is achieved before it crumbles. Measure the diameter of the thinnest snake. If the material is suitable for gravel roads, the diameter should be less than ⅜ inch. About ¼ inch is desirable. This material will be suitable for base courses. Some clays will hold a snake smaller than ⅛ inch. If the diameter of the snake is less than ⅛ inch, it is probably too plastic and the road will likely soften when wet. Now take the other portion of the soil you set aside earlier and knead it into a ball. Let it air dry, then crush it. Take a small jagged segment and try to squeeze it between your thumb and forefinger. Silt will turn to powder, and clay will be hard as a rock. Soil clues summary Clays - A tough snake that dries slowly; a crusty dry residue that is difficult to remove from your hands; very hard and strong when dry. 12

13 Silts - A weak or crumbly snake; powdery residue on your hands that is easily wiped or washed off; very weak and friable when dry. Silt or clay mixtures - Conflicting reactions to the tests. Silt or gravel with few clay fines - Enough clay to stain your hand when kneading a wet sample, but not enough to form a lump. Sand or gravel with silt fines - Any mixture with dusty or fairly gritty fines; weak and friable when dry. Clean sands and gravels - Water added works in immediately without making mud; does not stain your hands; no dry strength. Remember! Results of these tests are tentative. Soil identification for designing a road or for major rehabilitation should depend on professional laboratory testing. ***************************************************************************** In Sierra Leone, and much of Africa, surface soils are rich in the clay kaolinite. For example, among 20 soil samples, the average composition of minerals in the upper layer was kaolinite - 60%, chlorite 12%, gibbsite - 9%. Kaolinite, chlorite, and gibbsite are all classified as nonexpanding minerals with low plasticity and stickiness, both desirable qualities for road building. In contrast, the 2:1 expanding clays such as smectite, illite, and the micas that are common in many other climates are less desirable. 13

14 Low water crossings Low water crossings, also called fords or drifts, are sites in which water is intentionally directed across the surface of the road which is usually hardened to resist erosion and penetration by the water. Keller and Shear (2003) advocate their use as often as possible. Low water crossings are sometimes combined with culverts, the culverts handling low water and sparing drivers the nuisance of driving through water, the reinforced road surface handling high water and allowing debris to move downstream. The advantages of low water crossings are that they are not usually susceptible to clogging with debris, they are usually less expensive then installing culverts, and they can usually pass very large volumes of water during larger-than-expected floods. Their disadvantages are that traffic may be blocked or crossing may be dangerous during high water events, that avoiding erosion of channels or banks may be difficult, and that movements of fish may be impeded. 14

15 Low water crossings are also used to transfer ditch water across roads. The road surface may be raise ( grade breaks ) or lowered ( borad-based dips ). Examples are shown below. Grade breaks are slight rises on sloping sections of road that force water moving along the road bed to the side. Grade breaks are easy to build with normal machinery and require little maintenance. Grade breaks and dips slow traffic down and are only suitable on roads with low traffic volumes. The Center for Dirt and Gravel roads recommends that dips have a 3% grade and be oriented at an angle of degrees off perpendicular. Keller and Shear (2003) provides guidelines for the distance between dips as a function of road grade and soil type. Rainfall, however, must also affect the needed spacing 15

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17 Under drains Under drains are similar to tile used by farmers to drain fields. They are installed and function like tile. The CLRP says they intercept water before it gets to the road, lower the water table, and remove excess free moisture. The Center for Dirt and Gravel Roads recommends wrapping them in a fabric that excludes clay and silt while permitting water to enter. They are buried under ditches or in other locations and collect subsurface water due to springs or seepage. They help dry out water-logged areas. They may also be made of rocks wrapped in a geotextile fabric. The Center for Dirt and Gravels Roads has technical bulletins discussing under drains in general and rock under drains. The CLRP describes another configuration which they call edge drains: They note that the fill can be wrapped in fabric to prevent the immigration of fines that would fill the pipe or clog the openings in it, but they also say that no such material is needed if the pipe is surrounded with sand or washed concrete sand having particles large enough to bridge over the slots in the under drain pipe and still be able to act as a filter. They also note that a proper outlet must be provided and the drains cannot simply end in impervious soil. Localized wet spots caused by springs or seepage are frequently encountered in road building. This water must be drained away from the road bed or serious degradation of the road will occur. Single, small wet spots can be drained simply as shown below (left); larger areas require more elaborate approaches (below, right). The outlet should be a foot above the bottom of the ditch to avoid clogging. Even with proper placement, outlets should be maintained regularly. Two ways to drain local wet areas. The road is shown from above. The outlet drains exit into the ditch on the downhill side of the road 17

18 A French mattress is another way to transport water across a road. Layers of rocks, with larger rocks towards the bottom, are enclosed in a permeable fabric which keeps the rocks - especially the small ones - from spreading. Water flows through the rocks, and the rocks provide mechanical strength. According to the Center for Dirt and Gravel Roads, the French mattress is relatively new technique. They may be used in place of cross drains and perhaps even stream crossings (for small streams). They require less maintenance than culverts. On the negative side, they require importing a substantial amount of stones of different sizes, and the fabric must be purchased. 18

19 Wetlands Low, wet areas should be avoided if at all possible. When they must be crossed, the road surface must be raised well above the water table. Wateer may then be trasferred across the the road using multiple pipes or by placing large stones at the base of the roadbed. Wetland crossing with raised road bed and multiple pipes to avoid concentrating the flow. Note the rock berms and splash basins. Two options for conveying water in wetlands: pipe and thin laryer of rocks at base of road foundation (above) and thicker layer of rocks and no pipe (below). Note the use of geotextiles. 19

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21 Maintenance Maintenance of dirt roads (especially ones with light traffic) is needed mainly due to problems caused by poor drainage. Traffic and erosion often cause loss of the clay and silt, which binds together the larger particles, from the road bed. The coarser material then gradually moves to the edge of the road as in the picture below. As a result, water may be trapped in the low areas and have more time to penetrate the road bed, especially if fines have been lost. Water absorption causes the road bed to deform more easily, the sides being pushed up. Pumping, the mixing of soil due to vibration caused by traffic, may occur. Water forced to move along the road rather than laterally may also cause erosion forming ruts in the road bed. 21

22 Problems caused by poor drainage, and a brief description of remedies, are described in the following table: Problem Description and cause Remedy Dust Indicates loss of clay and silt which in turn leads to loss of cohesion and compaction and to reduced waterretention capacity. Ravelling Slipperiness Rutting Corrugations or washboards Pot holes Soft spots Loss of coarse aggregates due to heavy traffic after fines have been lost due to wind or water erosion. Caused by excessive fines (and I suspect especially with 2:1 clays). Traffic wear can break coarser particles into fines increasing their abundance. Longitudinal depressions caused by high moisture content in the subsurface soil or base, inadequate surface thickness, and/or heavy traffic loads or vehicles. Narrow longitudinal low areas caused by settlement due to excessive moisture and improper drainage. Small depressions; caused by excessive moisture and improper drainage and/or a poor mixture. Areas that are weak due to poor drainage and that depress under vehicle weight, usually leading to one of the other depression types above. Adding water to suppress dust is NOT a useful solution. Re-surfacing with a superior mixture of particles sizes is usually needed. Grading and blading may help but addition of fines and re-mixing often needed. Coarser particles should be added and mixed into the road bed. Add suitable material and re-mix. The needed material may be either coarse aggregates or fines as described above. In extreme cases, more elaborate methods such as drains and/or "geotextile fabric foundation with a crushed stone road fill" may be needed. The depressions should be filled with a proper mix of materials and the road should then be re-graded and compacted. Improved drainage may be needed. Small potholes may be filled with a proper mix of materials; large or recurring problems areas may be required more extensive re-grading and crowning with a proper mixture. Correct by improving drainage, replacing soft spots with more drainable materials, and/or by deepening ditches to improve drainage. Road bed maintenance is usually accomplished by blading and dragging by a road grader, also called a motor grader, patrol, or maintainer. Blading and dragging is a smoothing operation which pulls loose material from the side of the road or spreads wind-rowed aggregate to fill surface irregularities and restore the road crown (EPA 2000). The blade, or moldboard, is usually tilted forward at an angle sufficient to keep dirt from spilling over the top and is set at an angle of degrees. In most cases, the front wheels are tilted degrees in the direction that the aggregate should roll (figures below). 22

23 Left: Typical road grader (2005 Caterpillar 160H VHP Plus). Right: Usual angle in blading Right. Proper moldboard forward tilt for blading. Left. Proper blade and wheel angle. Grading as involving the following four steps: 1. Scratch & 2. Lower 3. Groom, 4. Compact move to center the crown moving excess with grader mixing well towards edge or truck 23

24 Each stage is described in more detail on the following pages.. 24

25 1. Scratch the road surface and move peripheral material back to the center. Loosen the road surface by scratching both lanes to the deepest point in the road cross-section (e.g., the bottom of pot holes). Dig out deep holes with a scarifier, back hoe or other equipment. Keep the tires off the crown, but scratch the crown (failure to do this causes potholes and wash boarding down the center line as below). Since proper compaction requires moisture, do only as much surface as can be completed before the surface dries out. Set the blade at about 25 degrees from perpendicular as shown above, with the outer end of the blade at the edge of the road, and move material that accumulated at the edge back towards the center. These materials tend to be large particles which must be thoroughly mixed to avoid rapid deterioration of the surface. This is accomplished as the materials rolls up the blade constantly falling onto itself. Several passes may be needed to cut the surface and then achieve the proper distribution and mixing. It is natural for excess material to accumulate in the center of the road. 2. Lower the crown Straddle the crown, turn the blade perpendicular to the road, and push the loose material on the crown to both sides. Leave the surface 1-2 higher than desired after compaction. The crown will then be flat with notches on both sides. 3. Groom the road 25

26 Turn the blade as shown below and spread the material evenly across the lane, disturbing only as much material as needed to achieve an even surface. Large material should ride down the blade and off the road but little fine material should reach the outer edge as shown in the inset (upper left). The disturbed material at the edge of the road should be at least 1.5 times the thickness of the largest particles. 4. Compaction Roll from the edge of the road towards the center but do not place the rollers or tires directly on the centerline (see picture to left). In normal grading only a few inches of surface are disturbed so the grader or a loaded truck can be used rather than a roller. 26

27 The Center for Dirt and Gravel Roads recommends using rotating carbide-tipped grader blades:, noting that they last up to 30 times longer than traditional cutting edges. Carbide tips also cut deeper, shatter rather than dig up rocks, grind off rocks and ledges, and let the operator travel faster and make fewer passes. Final grooming is also more efficient (though I did not fully understand why). The grooves left behind the blade (right photo above) also help anchor fines and enhances compaction. Finally, carbide tips are more effective in wet conditions (and in fact work best when the soil is moister than with standard grading) because they reduce clumping or balling up. This approach also uses less fuel than traditional systems. The Center, however, does not say how much more expensive carbide tips are than normal blades. Another approach for breaking up the surface is use of scarifier, perhaps mounted on the grader as shown below. Compaction is also a key step in re-surfacing the road bed. The CLRP contains the following on compactors: 27

28 Vibratory compactors are most effective on sand and gravel soils. Steel drum rollers tend to break down the individual grains of soil creating fines and lowering the permeability of the soil. For granular soils that have a tendency to break down, the rubber tired vibratory rollers are more suitable. Sheepsfoot rollers are best used on silty and clay soils. With these rollers, the ideal lift thickness is approximately 6 inches. Smooth drum rollers have limited effective depth and should be used where only thin layers or surface zones need to be compacted. As noted above, however, smaller jobs such as we will be doing probably do not require specialized equipment. 28

29 Ditches and banks Ditches convey storm runoff from the road bed and from upslope areas to appropriate outlets. They may also be used to lower the water table and to collect sub-surface moisture from the road bed (but only if the bed does not contain many fines). They should have shallow slopes (1:3 is good but 1:2 is acceptable) both to avoid erosion and so that drivers who lose control of their vehicle and enter the ditch can drive back onto the road bed. Ditches should be lined with vegetation or structural material to reduce erosion and sedimentation. Structures to reduce flow rates, such as turn-outs, are needed to avoid damage to ditches. Ditches on hills should be on the inside of the hill, otherwise water flowing down the slope may erode the road bed. Ditch size is influenced by how much runoff from upslope areas occurs. Road bed with no ditch on the uphill side of the roadway (left). Water flows across the road causing corrugating rills (left). Ditch bottoms should be 1-2 feet below the road bed. Three basic shapes are used, U-shaped, V- shaped, and trapezoidal. Ditches are sometimes also lined with tile, covered by fabric at least on the road side, and then filled with stones though the cost of the drainage tile seems to make this option impractical Sierra Leone. 29

30 A V-shaped ditch is easy to dig with a grader but the bottom is prone to erosion and difficult to maintain. U-shaped ditches are easy to dig with a back hoe and are efficient hydraulically, but the roadside bank is usually too steep. The shoulder can be flattened with a grader or excavator bucket or the operator can curl the bucket back during the final pass to make a rounded shape. Trapezoidal ditches are the hardest to construct but are most efficient hydraulically. They are generally the best shape, especially for ditches that must carry a large volume of water. Ditches normally follow the natural contours of the land but their slope should not be less than 1%, to avoid deposition, nor more than 5%, to avoid erosion. If the slope is too steep, a series of check dams can be installed. Two common problems with check dams are failure to dissipate the energy properly as water spills over the dam and failure to bed the dam deep enough to avoid erosion during storms. The figure below shows check dams with deep bedding and downstream pads to prevent scour. Most ditches have more than enough capacity so over-flowing is not a danger unless they receive very large flows from upslope (which should be prevented by terracing) or the distances between turn-outs are very large. 30

31 Ditch lining Ditches should be lined to retard water flow (reducing sedimentation and erosive force and increasing ditch capacity) and to increase resistance to erosion. Several different materials may be used to line the ditches (table below). Ditches lined only with earth are subject to severe erosion and fill in with weeds and grass. Seeding the sides with grass reduces water retardance but has little effect on erodibility and not even much effect on retardance if the grass is mowed. Allowing weeds to establish reduces erodibility as does lining the bottom with stones. Fabric or concrete reduce both erodibility and retardance to low levels but are very expensive. Concrete is also subject of undermining. For Sierra Leone lining the ditch bottoms and sides with grass or (more likely) other low, dense vegetation, seems like the best course though sedimentation could become a problem if the water is turbid and the vegetation is too dense. 31

32 Turn-outs Turn-outs are sections of ditch that carry water away from the road bed thereby removing water and maintaining suitable water levels within the ditches. Stones are often added at the turn-outs to prevent erosion as shown below. When a narrow bank is close to the road, if may be necessary to dig trench through it or bury a pipe under it. The frequency of turn-outs depends on how much water accumulates and on how large the ditches are. If too much water accumulates erosion may occur within the ditches and - in the worst case - water may overflow the ditches causing serious erosion of the road bed. EPA (2000) recommends that turn-outs generally should not be more than 500 feet apart. Diverting water this often may require culverts, discussed in the next section. Turn-outs (or tail ditches) diverting water into filtering areas. They are often placed before bridges or other structures to avoid high volumes at these sites. 32

33 Banks Banks are the areas immediately above the ditches (excluding the side of the ditch). The upper limit of the bank is arbitrary but may usefully be defined as the limit of the area that needs attention to avoid problems. Building roads on banks requires various forms of cuts and fills as illustrated below (slopes are expressed as run:rise rather than as rise:run). 33

34 Cut and tills may be made in several ways as illustrated below. In the typical fill (a), the slope is <40%. Vegetation on the slope to be filled is removed and the slope is scarified to improve adhesion of the fill material. Fill is added but is not (apparently) compacted using machinery. Slash may be placed at the bottom of the slope as a temporary barrier will vegetation is growing. The slope is 1:2 or less. With steeper, 40-60% slopes (b), more fill is needed and the downslope will be steeper. To accommodate the steeper slope, terraces can be dug into the downslope. this provides a surface for compacters. Fill is added in lifts cm thick. On even steeper, 60+% slopes (c), reinforcement in the form of a geogrid or geotextile layers, and drainage, is needed. 34

35 Different cut and fill arrangements. 35

36 The banks described above must be stabilized with rock, vegetation, wood chips, netting, or some other material. Brush piles are often placed at the bottom of slopes to provide a temporary means of reducing erosion while vegetation is growing. In Sierra Leone, low, dense, deep-rooted vegetation may not be common and an imported plant such as Vetiver should perhaps be considered. Vetiver has been used extensively throughout the world to stabilize banks because of its strong, deep roots, adaptability, and non-invasive properties. Rock armoring used to stabilize a highly erosive slope. 36

37 Rows of Vetiver used to reduce erosion. 37

38 When the methods above are not sufficient, the next step is usually to create terraces. Terraces are flattened portions of banks that trap water and move it parallel to the slope until it can be diverted. Terraces below the road reduce erosion of the bank. Terraces above a road also reduce bank erosion and also prevent water from reach the road ditch. Velocity of the water is also reduced. The terrace should usually be 4-10 wide and the slope of its banks should be no steeper than 1:2. The terrace should slope backwards, into the bank, to trap the water. Terracing above a road. Right: a terrace just after construction. The blue arrow shows ditch and direction of water. Left: terrace after native vegetation has re-established. Terracing below a road. 38

39 When slopes are even steeper, so that the methods above are not sufficient, then geo fabric or rip rap may be added to the slope. This practice is becoming more common in developed countries. The fabrics are made of jute, straw, coconut fibers, and plastic. Most are biodegradable. They are easy to install in difficult-to-reach locations and can handle more runoff than many stone blankets. The slope is prepared and seeded and the fabric is then placed over the seed. The fabric must be fixed at the top of the slope as shown in the illustration above. The fabric eventually decomposes and the vegetation stabilizes the bank. On steeper slopes, riprap may be added on topof the fabric as shown below. 39

40 40

41 If fabric and rip rap are not sufficient to stabilize the slope, then retaining walls must be constructed. They may be made of gabions (left), masonry, concrete or even tires and may be strengthened by piles. Such construction usually requires advice from an engineer but a few examples are shown below. Examples of walls used to stabilize banks. 41

42 Retaining walls may also be made by piling up rocks however this seems unwise if masons are available. In summary, when banks fail, repairs can be expensive. To avoid this, bank slopes above the road should usually not be steeper than 1:1 unless the substrate is rock, laterite, or well-cemented soil. Fill slopes below the road, which have been disturbed and must bear more weight, should usually not be steeper than 2:1. In developed countries and with high banks, it may be difficult to achieve these standards due to the amount of earth that must be moved and perhaps to land ownership issues. As a result, several methods have been developed to stabilize steep slopes. They are widely used in developed countries despite often being complex and expensive. In Sierra Leone, most slopes are gentle and as much earth can be moved as desired. It may, 42

43 therefore, be advisable to stabilize banks by creating 1:1 slopes rather than using more complex and expensive methods. This issue warrants more investigation. One exception, common in Sierra Leone, is laterite which is formed from soil but is as hard as bedrock and resistant to erosion. Laterite banks can be essentially vertical without fear of collapse. A well-stabilized, well-vegetated, cut bank, 1:1 slope. 43

44 Maintenance Ditches, and especially turn-outs, should be checked regularly after storms and any accumulating debris should be removed. Re-grading of ditches, however, should only be done when absolutely necessary, when heavy rains are not likely, and when vegetation can re-establish itself. The need for re-grading indicates that the initial design was deficient so improvements such as lining with rocks or vegetation may be needed. Ditch maintenance usually involves cleaning or re-shaping. Disturbance of the ditch, including desirable vegetation, during maintenance should be minimized. Remove only as much debris and vegetation as necessary. Seeding the ditch at the same time as maintenance may be desirable. Always work uphill in both cleaning and re-shaping. When ditches are deepened, the roadside banks needs to be moved out to maintain the proper slope as shown below. When banks fail, retaining walls must often be constructed to prevent future failure. These measures are usually expensive which is another reason for creating gentle, well-stabilized banks during the initial construction. 44

45 Culverts and bridges A culvert is a closed conduit used to convey water from one area to another, usually from one side of a road to the other side (EPA 2000). In this book, we use culvert when the water is transported in one or more pipes and bridge when the water flows through a larger, usually rectangular, concrete structure or when the road is held above the stream on pillars. Larger culverts require sidewalls to stabilize the banks as in the picture above. Two types of culverts are distinguished based on function. Stream crossing culverts transport stream water under the road. Cross-drains, or runoff management culverts, transport water in the ditches from one side of a road or driveway to the other. 45

46 Cross-drains may be needed frequently to conform to the guideline that turn-outs should generally be spaced at intervals of 500 feet or less. Cross-drains should be located at turn-outs to reduce the volume of water that ditches must carry. Culverts should cause as little horizontal or vertical deviation as possible in the flow of the water. Advantages of this approach include: Erosion around the pipe inlet and outlet caused when the water turns is reduced Flow capacity is increased because the water does not have to turn so shaprly (which reduces flow rate) Traffic loading on the pipe is reduced since only one tire at a time is on the pipe Often the pipe joints can avoid the wheel tracks Crossing pipes should be placed at ditch level and should cross the road at an angle. When culverts have more than one pipe, the distance between pipes should be 12 for pipes up to 24 in diameter, 50% of the pipe diameter for diameter pipes, and 36 for larger pipes. Debris often gets caught between the pipes so the CLRP recommends that a method be devised so that one pipe handles all the water at low flows. They also recommend 1:2 or 1:3 slopes for the bank of the road bed which means that the length of the culvert is substantially greater than the width of the road at its surface (below), especially when more than one foot of fill is used above the pipe. 46

47 Placing vertical pipes or posts (sometimes called trash racks ) upstream from the culvert is often an effective method to prevent debris from clogging the pipe. However, they may require frequent maintenance. Culvert pipes should never exit into the road bed as this will cause erosion. If any danger exists of an overflow, then an overflow protection dip should be constructed to prevent washout (below) 47

48 Road bed at a stream-crossing with cross drain and tail ditch well before stream and an overflow protection dip. Note the rip rap which will prevent erosion during an overflow event. Avoid culvert outlets in the middle of a fill slope. Use culverts long enough to extend to the toe of the slope, or use headwall structures to retain the fill material and minimize the pipe length. A vented ford (or low water crossing) is the combination of a ford and one or more culverts. In suc designs, it is expected that water will frequently flow across the surface of the road and it is reinforced accordingly. Lower culvert with overflow protection dip. 48

49 49

50 Pipe material Most culvert pipes are made from corrugated metal (usually steel or aluminum), plastic, or concrete. Metal and plastic pipes are and much cheaper than concrete pipes and are easier to install (because they are much lighter) but they do not last as long. Galvanized steel is the most common metal pipe material in the US. The steel may be mixed or coated with an aluminum alloy which makes it more durable than galvanizing without significantly increasing price. Care must be taken with aluminum pipes to avoid damage since they are so soft. The cost of shipping metal pipes to Sierra Leone may be prohibitive. If the flat metal could be shipped and the pipes formed in-country, then the shipping cost would be very low (about $2/cubic foot). Metal pipes come in several different sizes as shown below: Plastic pipes are usually made from high density polyethylene (HDPE). In a 2006 bulletin, the CLRP wrote that the use of this material was increasing rapidly. They continued as follows: It is easy to construct due to its light weight and is easy to maneuver and cut. It can be used in a variety of applications including lining existing pipes. It can be lined with a smooth section of plastic to increase flow capacity and improve strength. Plastic pipe is flammable and can be burned. The pipes are limited in size to about six feet and are uneconomical for many applications if more than four feet in diameter. New processes may provide more pipe types and sizes in the future. Corrugated HDPE pipes came in diameters of only 2 feet maximum when the report was written. Plastic pipes last years if properly designed and installed. 50

51 Concrete pipes have very long lives but are expensive to buy and construct. They can be purchased pre-formed or cast in place and many shapes are available. Solid steel is occasionally used for pipes, especially when cast-off material is available. The life-span of the material is hard to predict and the joints are very hard to seal properly. Steel pipe is also susceptible to corrosion in low ph water. 51

52 Pipe size Culverts should have the same cross-sectional area as the stream or ditch water at peak flow. Constricting the flow at culverts can cause serious problems including blow-outs which may destroy the road bed. Choice of pipe size requires that an estimate be made of the peak run-off that must pass through the culvert or under the bridge. Many estimation methods exist. The CLRP uses the Rational Method to explain the principles. This method provides an estimate of the flow in cubic feet per second, Q. A pipe with this cross-sectional area will accommodate the flow. For example, if Q = 1 ft 3 /sec and the cross-sectional area of the pipe is 1 ft 2 then the pipe can handle 1 ft 3 /sec. The equation for Q is Q = A * I * C where A = the area of the watershed in acres, I = intensity, the amount of rain in inches per hour, and C = the coefficient of runoff, a value between 0 and 1. Watershed area is readily determined using topographic maps. Intensity is determined from weather records. C may be roughly determined as follows: The metric version of the Rational Method (Keller and Shear 2003) is Forests, pastures, and cultivated land all have C values of 0.1 to 0.5. Since ground vegetation is often sparse in Sierra Leone, this suggests a C of The CLRP provides an example comparing the Rational Method to another method (TR-55). Results were very similar for one watershed but for the other the predictions from the TR-55 method were 30-60% higher than from the Rational Method. Estimates from simple models, like the Rational Method, should thus be regarded as first approximations that could be in error by 50-75%. where Q is in meters/sec 3, A is in ha, and I is in mm/hr. Another widely used method to determine flow velocity is Manning s Formula which is discussed in numerous sources. 52

53 As a rule-of-thumb, the SLRP recommends that when the watershed is less than 20 acres, the smallest pipe size should be 8 inches plus the number of acres. With existing pipes, asking local residents how much flooding occur is very effective as a reality check. 53

54 Culvert walls Walls at the ends of a pipe in a culvert are called headwalls (upstream wall) and endwalls (downstream walls). The advantages of headwalls include: preventing large pipes from floating out of the ground when they plug; reducing the length of the pipe; increasing pipe capacity; helping to funnel debris through the pipe; retaining the backfill material; and reducing the chances of culvert failure if it is overtopped (Keller and Shear 2003). The culvert walls should be concrete or masonry whenever possible but rip rap can also be used for part of all of the headwall. Culvert walls are occasionally made entirely of stone (left) but the sections adjacent to the pipe at least are usually made of concrete or rocks and mortar (right). Culvert walls generally flare outwards. This shape directs high flows towards the pipe, reducing turbulence and maximizing the flow of water through the pipe. The Center for Dirt and Gravel Roads has a technical bulletin on constructing culvert walls from stone. Culvert walls usually flare outwards (left)but other shapes may be needed when the stream course must change at the pipe (center and right). 54

55 If the bank is not too steep then riprap may provide effective bank stabilization as shown below. Note that the pipe is cut to form the walls. A geotextile filter was placed between the rock and the underlying bank. If outflows from the culvert are high (even occasionally), then structures to dissipate the energy without causing erosion may be needed. One approach, called a plunge (or splash or stilling) basin, is a pool of water at the outflow that dissipates the energy. The CLRP recommends that the pools for culverts of 30 diameter or less should be 12 deep, two culvert diameters wide, and 4 culvert diameters long, and should be lined with 6-12 stones. Splash pools for larger culverts should be designed by an engineer. They should be lines with rip rap (below left). A splash apron may also be used to dissipate energy of the water (below right). A key should be used to hold it in place. A drop inlet (or box or manhole cover) may also be used to let the water drop vertically and emerge at a lower level. This is probably more complex that we would need. 55

56 Bridges A simple box culvert A bridge, in this document, means a stream crossing in which the stream does not flow through a pipe. Bridges should be located where the stream channel is narrow, straight, and uniform and is unlikely to change course. Most bridge failures occur because the passage way is too small, the 56

57 bridge cannot bear the weight applied to it by traffic, or because of scour that undermines the foundation. Among these, scour and failure of the foundation is perhaps the most common. Foundations should be placed onto bedrock or other material that is not susceptible to scour. If foundations cannot be placed directly on bedrock, then they should be set into the ground (away from the stream course) at least m. Use scour protection, including riprap, gabion baskets, or concrete reinforcement as needed. Avoid placing abutments (walls) in the stream whenever possible. Allow freeboard (distance between the top of the anticipated high water level and the bottom of the bridge surface) of at least m. Bridges may be made of wood, concrete, and other materials but concrete is probably the best due to low cost and very long life. Simple box culverts as shown above seem likely to be appropriate for most of our stream crossings. Bridges should be inspected every 2-4 years. Typical bridge maintenance items include cleaning the deck and seats of the girders, clearing vegetation and debris from the stream channel, replacing object markers and signs, repairing stream bank protection measures, treating dry and checking wood, replacing missing nuts and bolts, and repainting the structure (Keller and Shear 2003). 57

58 58

59 Construction Culvert construction or replacement should be accomplished between rains. If this is not possible, then temporary ditches may have to be constructed to avoid serious damage to the road from flooding. Water should be diverted during culvert construction either by impounding it and pumping it across the road or by digging a temporary channel through which it can flow during construction. A temporary passage for traffic will also have to be built. A plastic silt fence or other temporary structure may be useful to impound the water until it is pumped across the road. Begin by placing stakes outside the work area marking the center line of the pipe to be installed. The trench that will hold the culvert should be at least twice as wide as the culvert and should be constructed to as near the desired grad for the culvert as possible. If multiple sections of pipe must be joined, it is often useful to start at the downstream side. The first joint is critical since it determines the location of the entire pipe. Joints should be wrapped with geotextile filter fabric and total wrap should be 1.5 times the circumference. The bottom of the pipe, at the entrance to the culvert, should be no lower than the stream bed. Placing it lower can cause a gully just upstream from the pipe which will then migrate upstream. In wet meadows, this process can lead to lowering the water table and drying out the meadow. The grade of the culvert should be as close as possible to the grade of the stream or ditch but usually between 0.5% and 1% (Keller and Shear, 2003, recommend 2% more than the ditch grade). Determining the grade may be difficult, calling for a specialist. pipe in the trench. Placing the pipe on a proper bed is essential. It should be granular fill with minimal fines. Fill the trench to a depth equal to about 1/2 the diameter of the pipe (below). Then form a depression in the fill so that the depth of the pipe in the fill will equal 1/6 th the diameter of the pipe, as shown below, and set the pipe into it. The back hoe used to dig the trench can usually be used to set the The best material for the trench is usually the natural, local material, with large stones removed. If fill must be imported, then the new material should be as similar as possible to the existing material so that the bed and sides respond in a similar way to temperature and moisture fluctuations. After the culvert is installed, fill in the trench using 6 lifts from the bottom to midway up the pipe and 12 lifts thereafter. Compact each layer carefully, especially adjacent to the culvert to prevent erosion channels there. The fill should be slightly moist so that it compacts thoroughly. 59

60 This is critical so that the pipe has good support. Roughen (scarify) the top 2 of compacted soil if needed to avoid a slick, smooth, or glossy surface. Mulch and vegetate all disturbed areas, and use silt fences or other methods to control erosion until the vegetation is established. 60

61 Maintenance Culverts should be inspected often and on a defined schedule, especially after storms. They should be checked for erosion of banks or along the pipe (piping), joint separation, bottom sag, pipe blockage, fill settling, sinkholes, sediment build-up within the culvert, effectiveness of the inlet and outlet, and any other evidence of problems. Problems may be due to stream blockage immediately upstream or downstream from the culvert. As with other aspects of road maintenance, frequent inspections and prompt action when problems are detected will avoid far more serious problems if maintenance is neglected. The CLRP provides a sample form to complete during each inspection. Erosion underneath a pipe (piping) caused by poor pipe installation. If gullies have formed either upstream or downstream from the culvert, they must be repaired or they will continue to deepen and widen. Usually the water must be removed and the stream rerouted, but in Sierra Leone repair work can usually be done late in the dry season when most streams are dry or nearly dry. One way to repair gullies is to establish temporary dams made from aggregate, gabions, or logs at frequent intervals so that the gully is converted into a series of pools. Steep banks on the sides of the gully may also have to be re-shaped and stabilized. 61

62 A temporary dam with a notch to keep water in the center of the channel. Deposition will usually occur rapidly in the pools and by the time they disappear sedimentation and vegetation will have re-established the original stream. If dams are made of aggregatee the size distribution must be such that water does not simply flow through the rocks. Keller and Shear (2003) recommend the following distribtion: Size (cm) Percent passing 60 cm cm cm 20 6 cm 10 As with other problems caused by improper drainage and erosion, gully restoration is much easier when the problem first occurs. 62

63 Signs Signs are generally classified as regulatory, warning, and guide (or informational). Consideration should be given to the need for each type of sign. Regulatory signs inform the driver of speed limits, rights-of-way, locations where stopping is required, and work zones. In the US, regulatory signs are usually rectangular, though stop signs are octagonal and yield signs are triangular. Warning signs tell drivers about potentially hazardous conditions on or next to the road such as sharp curves, trains, pedestrians, and wildlife. They may include recommended speeds but they are advisory, not regulatory. In the US, they are usually diamond-shaped with a yellow background and black letters or pictures. Guide signs help drivers reach their destinations. In the US, they have green, blue, or brown backgrounds and white letters. (add information on making and installing signs) 63

64 Sources of aggregate Since native soil is so often not suitable for either the base or surface layers of unpaved roads,, there would be great advantage to having our own gravel-making business. Such an operation would require a source and specialized equipment, both of which are discuss in this section. The depth to bedrock varies widely across Sierra Leone from 0 to dozens of feet. Steep bedrock hills, often of granite, are scattered throughout the eastern half of Sierra Leone Odell et al. 1974). Elsewhere, bedrock may be either sedimentary or granitic and may be found within a few feet of the surface or at depths exceeding tend feet (Odell et al. 1974). The layers above the bedrock are consistently described as gravelly and suggest that quarrying efforts might not need to crush the bedrock. More input from a specialist is needed but it appears that finding suitable rock to be crushed would not be difficult. Sand and gravel is produced by more than 4000 businesses operating in nearly every State in the US (in 2001). Production varied from less than 50,000 tons to >10 million tons. In the US, the National Stone, Sand and Gravel Association (NSSGA) is a major trade organization. In the UK, the Quarry Products Association is a major trade organization. If we found either limestone or dolomite, we could produce lime commercially too. An Aggregate Operators Handbook from British Columbia is available at ReportsandPublications/Pages/AggregateOperators.aspx Production of sand and gravel An overview of aggregate production is provided by Indiana DOT (undtd) (saved in my literature file). The following description is from the EPA. It describes what I think is a major operation. Sand and gravel typically are mined in a moist or wet condition by open pit excavation or by dredging. Open pit excavation is carried out with power shovels, draglines, front end loaders, and bucket wheel excavators. In rare situations, light charge blasting is done to loosen the deposit. Mining by dredging involves mounting the equipment on boats or barges and removing the sand and gravel from the bottom of the body of water by suction or bucket-type dredges. After mining, the materials are transported to the processing plant by suction pump, earth mover, barge, truck, belt conveyors, or other means. Although significant amounts of sand and gravel are used for fill, bedding, subbase, and base course without processing, most domestic sand and gravel are processed prior to use. The processing of sand and gravel for a specific market involves the use of different combinations of washers, screens, and classifiers to segregate particle sizes; crushers to reduce oversized material; and storage and loading facilities. 64

65 After being transported to the processing plant, the wet sand and gravel raw feed is stockpiled or emptied directly into a hopper, which typically is covered with a "grizzly" of parallel bars to screen out large cobbles and boulders. From the hopper, the material is transported to fixed or vibrating scalping screens by gravity, belt conveyors, hydraulic pump, or bucket elevators. The scalping screens separate the oversize material from the smaller, marketable sizes. Oversize material may be used for erosion control, reclamation, or other uses, or it may be directed to a crusher for size reduction, to produce crushed aggregate, or to produce manufactured sands. Crushing generally is carried out in one or two stages, although three-stage crushing may also be performed. Following crushing, the material is returned to the screening operation for sizing. The material that passes through the scalping screen is fed into a battery of sizing screens, which generally consists of either horizontal or sloped, and either single or multideck, vibrating screens. Rotating trommel screens with water sprays are also used to process and wash wet sand and gravel. Screening separates the sand and gravel into different size ranges. Water is sprayed onto the material throughout the screening process. After screening, the sized gravel is transported to stockpiles, storage bins, or, in some cases, to crushers by belt conveyors, bucket elevators, or screw conveyors. The sand is freed from clay and organic impurities by log washers or rotary scrubbers. After scrubbing, the sand typically is sized by water classification. Wet and dry screening is rarely used to size the sand. After classification, the sand is dewatered using screws, separatory cones, or hydro separators. Material may also be rodmilled to produce smaller sized fractions, although this practice is not common in the industry. After processing, the sand is transported to storage bins or stockpiles by belt conveyors, bucket elevators, or screw conveyors. Sand and gravel production is thus a complex, technical process involving extraction of native aggregate (often by blasting), crushing, piling, and loading. Each stage has potential problems and optimal approaches. This could be an excellent business enterprise for us, given our own need for high-quality aggregate and how much road-building is occurring in Sierra Leone. On the other hand, it would be expensive to start such a business and would require substantial technical expertise. More advice is needed on whether, when, and how we might undertake activities in this area. 65

66 Equipment (more information needed on equipment used to make road beds from scratch) Road beds are best smoothed, during either initial construction or maintenance, with a motor grader. Smaller models would probably be best for us because we will be working on small roads. Caterpillar sells 4 models varying from 103 to 136 hp and from $291K to $333K. All have 12 ft blades. John Deere sells six models varying from 145 to 283 hp. The smallest model is the 670G. I could not find new prices but a G was advertised for $215 suggesting that the smallest John Deere is probably similar in price to the Caterpillars. Volvo sells seven models. The smallest is the G930 rated at hp. I could not find a price. It appears, however, that even a small road grader new would be $300K. Used Volvo motor graders from are available (on the web site Rock&Dirt) for approximately $100K. Used Caterpillar graders of this age appear to be quite a bit more expensive. In summary, it appears that a road grader 6-8 years old will still cost $100K. Perhaps we can find a much older one for much less money and get it repaired, but this would have to be investigated carefully. Proper operation of a road grader requires excellent training and extensive experience. A technical article on operating a road grader is at We will probably need a dump truck to bring in fill at least when building new roads and perhaps when repairing existing ones. A ten-wheeler on Dirt&Gravel appears to cost $50-60K. Ditches can be built with back hoe loaders (though an articulated back hoe is said to be the ideal equipment) and can probably be repaired with a back hoe mounted on a small tractor. A John Deer backhoe can be purchased for $40-60K. We purchased a 30 hp small tractor almost new with a front end loader and back hoe for $16K. We may have access to a backhoe loader, a dump truck, and a small tractor in Sierra Leone meaning that we would only need to buy the road grader and a rock-crushing plant. Ditches are best constructed with a rubber-tired excavator with an articulated bucket (below left) but backhoes and bull dozers are also suitable. Road graders can be used but leave much more loose and disturbed soil which may then be eroded. 66