Cost-Effective and Sustainable Road Slope Stabilization and Erosion Control (2012)

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8 CHAPTER TWO THE BASICS Good slope stabilization has three essential elements: proper planning and site investigation, understanding the soil, and knowing the surface and subsurface water con- ditions. This chapter summarizes literature and interview results on all three topics and provides additional resources for follow-up information. PLANNING AND SITE INVESTIGATION When conducting slope stabilization work, it is more cost- effective to proactively apply appropriate techniques to con- trol erosion, stabilize slopes, and maintain the site than to repair them after they have failed. A full assessment of the site should be completed before a slope stabilization tech- nique is selected. The project plan developed by Howell (1999) for Roadside Bio-engineering: Site Handbook is a good example of how to develop a successful slope stabiliza- tion implementation plan (Table 1). Howell (1999) suggests that planning occur over at least 1 year, if possible, and that time be allowed in the second year following construction for site monitoring and maintenance. It is widely recognized that the more front-end work one can do to understand the site, the more likely it is that the best possible treatment will be selected. During the initial planning stages, one should think broadly at the watershed level and consider topography, geography, geology, and so on. It may be helpful to consult historical rainfall and snowfall records for the site, geologic maps, nearby slope stability, records on previous work completed at the site, and previous work above and below the slope. Changes to construction practices may be necessary to allow for longer maintenance periods. Consider using a multidisciplinary team of soil scientists, botanists, geologists, hydrologists, ecologists, landscape architects, and geotechnical engineers to gain further information about the site characteristics. The aim of the site investigation is to (1) recognize actual or potential slope movements and (2) iden- tify the type and cause of the movement (Turner and Schuster 1996). This information will help in selecting the most appro- priate prevention and correction strategy. When starting a site investigation, the following five items need to be defined: • Purpose of the site or road; • Scope of the site, including topography, geology, ground- water, weather, and slope history; • Extent of the project or area of the work site; • Depth of the instability and/or stable support layer; and • Duration of the project (Turner and Schuster 1996). Signs of slope instability may include slumped soil (Fig- ure 3); tension cracks; eroded material at the base of the slope (Figure 4); hummocky and broken or uneven terrain; leaning trees (Figure 5); water seeps, ponds, or channels; or other signs of surface erosion. Useful Points • In the planning phase, consider the timing of each proj- ect component (S. Jennings, personal communication, April 12, 2011). • At the planning level, consider all options and keep a broad focus (S. Romero, personal communication, May 11, 2011). • Consider using experienced engineers and contractors (B. Johnson, personal communication, April 18, 2011). • Know the areas of expertise of potential contractors (S. Romero, personal communication, May 11, 2011). TABLE 1 STEPS TO IMPLEMENT BIOENGINEERING Planning Design Implementation Maintenance • Initial work plan • Prioritize work • Divide site into segments and assess • Determine engineering and bioengineering techniques to be used • Design engineering and bioengineering techniques • Select species • Calculate quantities, rates, and finalize budget • Plan for plant needs • Arrange for implementation and required documents/ permits • Prepare for plant propagation • Make site arrangements • Implement engineering and bioengineering techniques • Monitor work • Maintain site Source: Howell (1999).

9 FIGURE 3 Unstable slope caused by freeze–thaw cycles in Alaska (Courtesy: J. Currey). FIGURE 4 Shallow cut failure just below grass root depth. Use deep-rooted vegetation for slope stabilization (Courtesy: G. Keller). FIGURE 5 Slumped slope with leaning trees (Courtesy: G. Keller). • Consider using an experienced project manager on site who can coordinate efforts and operations for all aspects of the project (K. Mohamed, personal commu- nication, April 26, 2011). • Consider conducting a life cycle analysis for all treat- ments before they are used (A. Faiz, personal commu- nication, May 6, 2011). • Every project is unique, and each treatment needs to be tailored to the site (A. Faiz, personal communication, May 6, 2011). • Talk with knowledgeable local personnel to understand the types and nature of problems in that area (G. Keller, personal communication, April 26, 2011). Additional Resources for Planning and Site Investigation Adams, P.W. and C.W. Andrus, “Planning Secondary Roads to Reduce Erosion and Sedimentation in Humid Tropic Steeplands,” In Proceedings of Research Needs and Applica- tions to Reduce Erosion and Sedimentation in Tropical Steep- lands, Fiji Symposium, IAHS-AISH Publ. No. 192, June 1990. Clayton, C.R.I., N.E. Simons, and M.C. Matthews, Site Investigation: A Handbook for Engineers, Halsted Press, New York, N.Y., 1982. Ethiopia Roads Authority, Design Manual for Low Volume Roads Part A, B and C, Final Draft, Apr. 2011 [Online]. Avail- able: http://www.dfid.gov.uk/r4d/PDF/Outputs/AfCap/ Design-Manual-for-Low-Volume-Roads-Part-A.pdf. Howell, J., Roadside Bio-engineering: Site Handbook, His Majesty’s Government of Nepal, Ganabahal, Kath- mandu, 1999 [Online]. Available: http://onlinepubs.trb.org/ Onlinepubs/sp/Airport/RoadsideBioengineering.pdf. Hunt, R.E., Geotechnical Engineering Investigation Handbook, CRC Press, LLC, Boca Raton, Fla., 2005. Hearn, G.J. and R.W. Weeks, Principles of Low Cost Road Engineering in Mountainous Regions, with special ref- erence to Nepal, Himalaya, C.J. Lawrence, Ed., Transporta- tion Research Library Overseas Road Note 16, Berkshire, United Kingdom, 1997. Sara, M.N., Site Assessment and Remediation Handbook, CRC Press, LLC, Boca Raton, Fla., 2003. Turner, K.A. and R.L. Schuster, Eds., Special Report 247: Landslides Investigation and Mitigation, National Academy Press, Washington, D.C., 1996. SOIL TYPES AND SOIL MECHANICS Soil mechanics is the study of the engineering behavior of soil under different stress conditions. The basic components of soil are soil particles (grains), water, and air. The rela-

10 tionship among these components provides several impor- tant index properties, such as density, moisture content, and degree of saturation. Other characteristics that are impor- tant for classifying the soil and engineering soil structures (including slopes) are index properties, such as grain size distribution, Atterberg limits (particularly the plastic and liquid limits), and soil shear strength. Soil classification systems provide a means of grouping and identifying the expected behavior of soils. Laboratory tests for the grain size distribution, plastic limit, and liquid limit of a soil are used for classification within the Unified Soil Classification System (USCS), which is the most widely used system. The AASHTO soil classification system was originally developed to classify soils for assessing their suit- ability as a subgrade or pavement layer. There are significant differences between the USCS and AASHTO systems, but the AASHTO system is still commonly used by departments of transportation (DOTs) and pavement engineers (Holtz et al. 2011). Grain size distribution is determined by a sieve analysis for the coarse-grained fraction of soils (above No. 200 sieve with 0.075 mm opening size). If the size distribu- tion of the fine-grained portion of soil is desired, a hydrom- eter is used. However, the more relevant property of the finer fraction of soils is plasticity index (PI). The plasticity index is the range in water content at which the soil behaves as a plastic solid. The PI is calculated from results of tests for the liquid limit and plastic limit of the finer portion (smaller than No. 40 sieve with 0.425 mm opening size) of the soil. Once the grain size distribution, plastic limit, and liquid limit are known, the soil can be classified by USCS using ASTM 2487, Holtz and Kovacs (1981), Das (2007), Holtz et al. (2011), or any introductory geotechnical engineering or soil mechanics book. Figure 6 shows a few typical grain size distributions, and Table 2 provides typical compaction and drainage characteristics for USCS soil groups. Soils fail in shear. If the shear stress is greater than the shear strength of the soil, it will fail. Thus, it is important to know the shear strength of soil. The strength properties of soil are described in terms of friction (φ) and cohesion (c). These properties are determined from laboratory tests, such as the direct shear test and triaxial test. The triaxial laboratory test can be conducted under a variety of drain- age conditions to provide parameters appropriate for drained and undrained analyses. Whether loading on soil could be thought of in terms of “drained” or “undrained” conditions depends on the permeability of the soil, the rate at which the load is applied, and the time period of interest (short or long term) after the load is applied (Holtz and Kovacs 1981; Duncan and Wright 2005). • “Undrained signifies a condition where changes in loads occur more rapidly than water can flow in or out of the soil. The pore pressures increase or decrease in response to the changes in loads. • Drained signifies a condition where changes in load are slow enough, or remain in place long enough, so that water is able to flow in or out of the soil, permitting the soil to reach a state of equilibrium with regard to water flow. The FIGURE 6 Three typical grain size distributions (Holtz et al. 2011).

11 – Weathering – Cyclic loading. • Increases in Shear Stress – Loads at the top of the slope – Water pressure in cracks at the top of the slope – Increase in soil weight resulting from increased water content – Excavation at the bottom of the slope – Drop in water level at the base of a slope – Earthquake shaking. Geotechnical failures based on loss of shear strength are complex. For this reason, strongly encourage use of supple- mental material referenced in the Additional Resources for Soil Types and Mechanics. Water and the presence of clay minerals play a significant role in many of these processes, particularly those associated with decreases in shear strength. The percentage of clay in a soil and the activity of clay minerals are reflected qualitatively by the value of the pore pressures in the drained condition are controlled by the hydraulic boundary conditions, and are unaffected by the changes in load” (Duncan and Wright 2005). The fundamental requirement for a stable slope is that “the shear strength of the soil must be greater than the shear stress required for equilibrium” (Duncan and Wright 2005). Thus, factors that contribute to slope instabilities could be linked to decreases in shear strength and/or increases in shear stress. Duncan and Wright (2005) list the following processes responsible for these changes: • Decreases in Shear Strength – Increased pore pressure (reduced effective stress) – Cracking – Swelling (increase in void ratio) – Development of slickensides – Decomposition of clayey rock fills – Creep under sustained loads – Leaching – Strain softening TABLE 2 USCS SOIL CLASSIFICATION AND TYPICAL PROPERTIES USCS Soil Classification Compaction Characteristics and Recommended Equipment Drainage and Permeability Value as an Embankment MaterialGroup Symbol Group Name GW Well-graded gravel Good: tractor, rubber-tired, steel wheel, or vibratory roller Good drainage, pervious Very stable GP Poorly graded gravel Good: tractor, rubber-tired, steel wheel, or vibratory roller Good drainage, pervious Reasonably stable GM Silty gravel Good: rubber-tired or light sheep’s foot roller Poor drainage, semipervious Reasonably stable GC Clayey gravel Good to fair: rubber-tired or sheep’s foot roller Poor drainage, impervious Reasonably stable SW Well-graded sand Good: tractor, rubber-tired or vibratory roller Good drainage, pervious Very stable SP Poorly graded sand Good: tractor, rubber-tired or vibra- tory roller Good drainage, pervious Reasonably stable when dense SM Silty sand Good: rubber-tired or sheep’s foot roller Poor drainage, impervious Reasonably stable when dense SC Clayey sand Good to fair: rubber-tired or sheep’s foot roller Poor drainage, impervious Reasonably stable ML Silt Good to poor: rubber-tired or sheep’s foot roller Poor drainage, impervious Fair stability, good compaction required CL Lean clay Good to fair: sheep’s foot or rubber- tired roller No drainage, impervious Good stability OL Organic silt, Organic clay Fair to poor: sheep’s foot or rubber- tired roller Poor drainage, impervious Unstable, should not be used MH Elastic silt Fair to poor: sheep’s foot or rubber- tired roller Poor drainage, impervious Fair to poor stability, good com- paction required CH Fat clay Fair to poor: sheep’s foot roller No drainage, impervious Fair stability, expands, weakens, shrinks, cracks OH Organic silt, Organic clay Fair to poor: sheep’s foot roller No drainage, impervious Unstable, should not be used Pt Peat Not suitable Should not be used Should not be used Source: NAVFAC (1986).

12 rated and weakened. Construction of roads may also modify the surface and subsurface flow pattern of water, causing no flow or reduced flow in some natural channels but concen- trated flow in others (Shrestha and Manandhar 2010). In general, water management measures in slopes consist of surface and subsurface drainages that transport water to natural drainages safely and as quickly as possible (GSPW 2003). Roadway drainage is the control of water within the road, including moving water off the road surface and remov- ing excess subsurface water that would infiltrate the road base and subgrade (Orr 1998). To understand how to manage water at each site rainfall, topography, catchment area, ground sur- face conditions, soil parameters, groundwater conditions, and existing natural and artificial drainage systems should be studied and assessed to determine the required drainage solu- tion (Turner and Schuster 1996). If necessary, a combination of both surface and subsurface drains can be used to manage surface and groundwater conditions (GSPW 2003). For each site, a water management plan should answer the following questions: • Where is the water source? • Where does the water come to the surface? • How is the water interacting with the different soil and rock types? • Is an artificial drainage system needed for the slope? • Can vegetation alter the hydrology and improve slope stability? A good water management plan will include conserva- tion of natural systems that interact with the road, which can be considered in the design and construction phases (Shres- tha and Manandhar 2010). Examples include adding rolling dips and low-water fords that follow natural topography, and using bridges and surface stabilization as needed (Adams and Andrus 1990). DRAINAGE MEASURES Drainage of water from the road surface has significant implications for slope stability and can affect water qual- ity, erosion, and road costs (Keller and Sherar 2003). Poorly drained pavements and slopes adjacent to roads can cause premature deterioration and lead to costly repairs and replacements (Cedergren 1989). The following drainage issues should be addressed in road design and construction: • Roadway surface drainage; • Control of water in ditches and at pipe inlets and outlets; • Crossings of natural stream channels; • Wet area crossings; • Subsurface drainage; and plasticity index. For that reason PI affords a useful first indication of the potential for problems that a clayey soil poses: The higher the PI, the greater the potential for problems (Duncan and Wright 2005). In addition to water, soil erosion may also be caused by wind. He et al. (2007) found that there is a linear relationship between the logarithm of the wind velocity and the intensity of resulting erosion. They also reported on the effectiveness of three practices in preventing wind erosion of highway slopes. In descending order of effectiveness, they were hex- agonal bricks, arched frame beams, and mechanical com- paction, with the relative soil loss ratio of such treated slopes at 0.35, 0.55, and 0.91, respectively. In practical terms, the following conditions lead to instability: • Slopes that are excessively steep or that have been undercut, • Slopes that are wet or saturated, • Poorly compacted fill slopes, and • Steep slopes with shallow-rooted grasses that can be surcharged when saturated. Additional Resources for Soil Types and Mechanics Das, B.M., Principles of Foundation Engineering, 6th ed., Cengage Learning, Stamford, Conn., 2007. Duncan, J.M. and S.G. Wright, Soil Strength and Slope Stability, John Wiley & Sons, Hoboken, N.J., 2005. Holtz, R.D. and W.D. Kovacs, An Introduction to Geo- technical Engineering, Prentice-Hall, Inc., Upper Saddle River, N.J., 1981. Holtz, R.D., W.D. Kovacs, and T.C. Sheahan, An Intro- duction to Geotechnical Engineering, 2nd ed., Pearson Edu- cation, Inc., Upper Saddle River, N.J., 2011. WATER MANAGEMENT “If you only look superficially, and don’t address the water problem by considering the overall site hydrology, you can miss finding an appropriate solution” (A. Faiz, personal communication, May 6, 2011). Water management, in both cut and fill slopes, is important to protect the slopes from erosion and shallow depth instabili- ties resulting from increases in pore water pressure (GSPW 2003). Water may enter the roadway through cracks or surface defects on the road surface, or water can infiltrate through cuts and fills (Orr 1998). Capillary action may also draw moisture up from the water table, causing the road base to become satu-

13 • Selection and design of culverts, low-water crossings, and bridges. Before surface and subsurface drainage measures are installed, drainage conditions and patterns should be studied. Specific observation could be made during rainy periods to monitor flow patterns, identify areas where ponding occurs, assess potential damage, and determine preventive measures that can be used to minimize damage and to keep the drainage system functioning properly (McCuen et al. 2002). Good water drainage begins in the design and construc- tion phases of road building. Road surfaces should be shaped appropriately to keep water from accumulating on the road surface. Standing water should be avoided, as it often cre- ates or worsens potholes, ruts, and sags (Keller and Sherar 2003). Drainage ditches should be constructed only when necessary. For example, a road graded away from a cut slope (an outsloped road) without ditches disturbs less ground and is less expensive to construct than an insloped or crowned road with drainage ditches, although the fill slope may require explicit erosion control measures (Moll et al. 1997). Keep water drainage from roads and streams disconnected by using water retention basins. When installing drainage structures, make sure that there is some rational or statistical assessment of the expected flow. Howell (1999) offers the following advice for drainage design: • Always design drainage systems to run along natural drainage lines. • Choose locations for the drain so that the maximum effect can be achieved using the least amount of construction. • Always ensure that the drain outlets are protected against erosion. • Ensure that the foundation is sound, as with all civil structures. • A flexible design is usually an advantage (e.g., concrete masonry, a rigid design, can be easily cracked by the slight- est movement in the slope, resulting in leakage problems). • Compact the backfill material thoroughly during construction. • Apply appropriate bioengineering measures to enhance the effectiveness of the drain. Useful Points • On steep road grades, for example, greater than 12% to 15% (about 8:1), water becomes very difficult to con- trol (Keller and Sherar 2003). Surface Drainage Surface drainage is most commonly accomplished by proper grading of the road surface or the use of structures to chan- nel water from the road surface in a manner that minimizes effects to adjacent areas (Copstead et al. 1989). Surface drain- age systems include drains, berm drains, toe drains, drainage channels, and cascades. U-shaped gutters (Figure 7), rein- forced concrete (Figure 8), and corrugated half-pipe drains can also be used to construct drainage ditches (GSPW 2003). Surface water drains often use a combination of bioengineer- ing and civil engineering structures (Howell 1999). Armor roadway ditches and leadoff ditches with rock riprap (Figure 9), masonry, concrete lining, geotextiles, and/or grasses to protect highly erosive soils (Keller and Sherar 2003). Ditch dike structures can also be used to dissipate energy and con- trol ditch erosion. If ditch erosion is occurring, the best solu- tion may be to place additional cross-drains to disperse and reduce the amount of water that is causing the erosion. There are three main ditch shapes: V, U, and trapezoid. Each can be filled or lined (Orr 1998). V-shaped ditches are the easiest to construct; however, the bottom of the V is prone to erosion and can be difficult to maintain. U-shaped or rounded ditches are more efficient hydraulically than V-shaped ditches, are more desirable for erosion control, and are easy to main- tain. Trapezoid, or flat-bottomed, ditches are the most efficient hydraulically and can be used for ditches that carry heavier flows. The flat bottom of the trapezoid ditch helps reduce ero- sion problems and spread water flow. Ditches may be filled or lined and will act similarly to trench drains. An example of a filled ditch that behaves as a trench drain would be lining a ditch with large stone and placing a perforated pipe at the bottom of the ditch. Ditches can be lined with native earth, geotextiles, grass, stone, and/or concrete. (Orr 1998). Lining the roadside ditch with geotextiles can reduce erosion rates. Ditches require cleaning, which entails the removal of sedi- ment and vegetation from the bottom of the ditch. FIGURE 7 Down-drain (Courtesy: G. Keller). Useful Points • Place erosion protection or seeding before rainfalls on all newly exposed surfaces.

14 ideal for low-volume roads with low to moderate traffic speeds [less than 30 mph (or 50 kph)] and low average daily traffic. Consider constructing rolling dips rather than ditches with culvert cross-drains on roads with grades less than 5:1 (H:V), or 20% to 10:1 (10%) (Copstead et al. 1989; Keller and Sherar 2003). It is important that rolling dips be deep enough to provide adequate drainage, perpendicular to the road or angled 25 degrees or less, outsloped 3% to 5%, and long enough [50 to 200 ft (15 to 60 m)] to allow vehicles and equipment to pass. In soft soils, it is important to armor the mound and dip with gravel or rock. Ideal spacing of roll- ing dip cross-drains is a function of the road grade and soil type (see Keller and Sherar 2003, p. 55, Table 7.1, for recom- mended spacing). Use roadway cross-drain structures (e.g., rolling dips, culverts, open-top culverts, or flumes) to move water across the road from the inside ditch to the fill slope below the road (Keller and Sherar 2003). Space the cross-drain structures close enough to remove all surface water (see Keller and Sherar 2003, p. 55, Table 7.2, for recommended cross-drain spacing, or Copstead et al. 1989, pp. 9–11, Tables 3 and 4). Surface cross-drains not only provide effective cross drain- age, but also reduce the risk associated with plugged drain inlets, which can divert water over the road (Copstead et al. 1989). In areas of cut slope instability, frost-heave slough, or erodible ditches, properly located and constructed surface cross-drains can result in less erosion and disturbance to the surrounding watershed. Use a 3% to 5% cross-slope if cre- ating an insloped road in areas with steep natural slopes, erodible soils, or on sharp turns. Provide cross drainage with culverts, pipes or rolling dips and provide filter strip areas for infiltration and to trap sediment between drain outlets and waterways (Keller and Sherar 2003). In Table 3, Howell (1999) provides bioengineering solu- tions to go with specific surface drainage treatments. Culverts are commonly used as cross drains for ditch relief and to pass water under a road along a natural drain- age (Orr 1998; Keller and Sherar 2003). Culverts need to be properly sized, installed, and protected from erosion and scour. Culverts are most commonly made of concrete or corrugated metal, plastic pipe, and occasionally wood or • Have erosion materials ready before starting a job, in the event of rain (Orr 1998). FIGURE 8 Concrete surface drainage (Courtesy: G. Keller). FIGURE 9 Geotextile- and rock-lined ditch example of surface drainage (Courtesy: C. Gillies). Rolling dip cross-drains, or broad-base dips, are designed for dispersing surface water on roads with slower traffic (Keller and Sherar 2003) (Figure 10). Relative to culvert pipes, rolling dips usually cost less, require less mainte- nance, and are less likely to plug and fail. Rolling dips are FIGURE 10 Rolling dip profile (Keller and Sherar 2003).

15 water before it gets to the road, (2) lower the water table, and (3) remove excess free moisture (Orr 1998). Subsurface drains also collect seepage water from surface runoff and pre- vent it from raising the groundwater table (GSPW 2003). Underdrains are usually very narrow and have some form of pipe in them. They are installed by a special machine, and they may or may not be wrapped in geotextile (Orr 1998). The geotextile filters out fine-grained soils that would oth- erwise accumulate and plug the pipe. One common way to backfill the underdrain trench is placing a layer of geotextile in the trench and then placing pipe, followed by clean stone around the pipe. The other option is to fill the trench with washed concrete sand. Trench drains, or French drains, are usually installed with a backhoe or excavator, are fairly wide compared with underdrains, and may or may not have a pipe at the bottom (Orr 1998) (Figure 11). Using a pipe will greatly increase the life of the drain and help remove excess water. The trench can be lined with geotextile, then the pipe is placed, and then backfilled with clean stone ½ to ¼ in. in size. When installing subsurface drains, always use filter pro- tection such as a geotextile or properly sized sand or gravel. The purpose of the filter is to prevent migration of fine soil particles into underdrains, thereby allowing groundwater to drain from the soil without building up pressure. Even with a filter, subsurface drain pipes require periodic cleaning, which can be done using a sewer cleaner (Orr 1998). Deeper cleaning may be required if the pipe becomes completely plugged. Inlets and outlets should be cleared of debris and masonry (Keller and Sherar 2003). It is important that cul- verts have adequate flow capacity for the site, that the culvert size and shape match the needs of the site (e.g., fish passage), and that installation be cost-effective. There is a need to dissipate the energy of surface run- off as it is concentrated in natural and man-made channels. Drain outlets can be armored to dissipate energy and pre- vent erosion using rock, brush, logging slash, non-erosive soils, rock, and/or vegetation (Keller and Sherar 2003). If heavier water discharge is anticipated, check dams, intercep- tor drains, benches, and contour terracing can be effective countermeasures. Useful Points • Use closely spaced leadoff ditches to prevent accumu- lation of excessive water in roadway ditches (Keller and Sherar 2003). • Consider using a filter layer under or behind a selected treatment, such as riprap or a gabion structure. A fil- ter can be made of small gravel or a geotextile placed between a structure and the underlying soil (Keller and Sherar 2003). Subsurface Drainage Subsurface drains are used to drain shallow groundwater, less than 15 ft (5 m) below the ground surface (GSPW 2003). This includes water within the road surface, base, and subgrade materials (Orr 1998). Subsurface drains, including under- drains and French drains, serve three functions: (1) intercept TABLE 3 SURFACE DRAIN OPTIONS Structure Bioengineering Main Sites Advantages Limitations Unlined natural drainage system (rills, gullies) Grasses in the rills, and grasses and other plants on the sides Existing landslide scars and debris masses An inexpensive form of surface drain. Allows for rapid drainage There is a risk of renewed erosion from heavy rain on weak materials Unlined earth ditches Grasses and other plants on sides and between feeder arms Slumping debris masses on slopes up to 1:1 (H:V) (45°), where the continued loss of material is not a problem An inexpensive form of surface drainage There is serious erosion hazard on steep main drains. Should be used only where further erosion is not a problem. Leakage into the ground may also occur Unbound rock- lined ditches Grasses between stones, and grasses and other plants on sides and between feeder arms Can be used at almost any site, how- ever unstable, where the ground is firm enough to hold rock and water flow is not excessive A low-cost drain. Strong and flexible A membrane of polyethylene may be required to stop leakage back into the ground. Somewhat expen- sive to clean and maintain Bound cement masonry ditches Grasses and other plants on sides and between feeder arms For use on stable slopes with suit- able material for good foundations A strong structure for heavy water discharge Relatively high cost. Very inflexi- ble, high risk of cracking and fail- ure due to subsidence and undermining Open gabion ditches Grasses and other plants on sides and between feeder arms Can be used at almost any site, where the ground is firm enough to hold structure, and with heavy water discharge A large and high cost type of drain. Very strong and flexible A member of polyethylene may be required to stop infiltration of col- lected water Source: Adapted from Howell (1999).

16 flow maintained, animal guards should be installed over them, and mowing crews should be careful to not crush or damage them. FIGURE 11 Subsurface drainage (Photo and drawing courtesy of G. Keller) . Cross-drain pipes are used to pass water under a road- way from a ditch on the cut-slope side of the road to the fill- slope side of the road. The pipe is to be placed at the bottom of the fill (Figure 12). The inlet should be protected with a drop inlet structure or catch basin, and the outlet armored against erosion (Keller and Sherar 2003). Bedding and back- fill materials should be high quality, granular, non-cohesive, less than 3 in. (7.5 cm) in diameter, well compacted, and skewed 0 to 30 degrees (preferably 30 degrees) from perpen- dicular to the road (Keller and Sherar 2003). FIGURE 12 Typical drop inlet structure with culvert cross-drain (Keller and Sherar 2003). Horizontal drains have been used historically for land- slide correction, but can also be used generally for slope sta- bilization. Horizontal drains are installed to reduce excess pore-water pressure, thereby increasing slope stability (Long 1994) (Figure 13). Horizontal drains are drill-in drains that are inclined to match the subsurface geology. Horizontal drains have been shown to be a cost-effective alternative to major slope stabilization repairs, such as buttressing, when subsurface water is involved in the mechanics of failure. FIGURE 13 Outlet of horizontal subsurface slope drainage (Courtesy: M. Long) . Consider having a geotechnical expert perform a sub- surface investigation of the soil and rock characteristics in the design phase (Long 1994). If economically feasible, the following techniques are suggested: area reconnais- sance, ground survey, subsurface exploration for rock and soil type and water concentration, permeability testing, and ground and surface water mapping. Test drains should be installed to confirm final drain locations. Following installation, the site should be visited to ensure that proper drainage is occurring (see Long 1994, pp. 788–796, for design calculations). Subsurface drains are usually civil engineering struc- tures and do not normally use bioengineering measures (Howell 1999); however, bioengineering techniques can be

17 used to strengthen the slope around the drain or outlet. In Table 4, Howell (1999) provides some examples of how this can be done. Additional Resources for Water Management and Drainage Anderson, M.G., D.M. Lloyd, and M.J. Kemp, Hydro- logical Design Manual for Slope Stability in the Tropics, Transport Research Laboratory, Overseas Road Note 14, Berkshire, United Kingdom, 1997. Cedergren, H., Seepage, Drainage, and Flow Nets, 3rd ed., John Wiley and Sons, New York, N.Y., 1989. Copstead, R., K. Johansen, and J. Moll, Water/Road Interaction: Introduction to Surface Cross Drains, Water/ Road Interaction Technology Series, Res. Rep. 9877 1806— SDTDC, Sep. 1998 [Online]. Available: http://www.stream. fs.fed.us/water-road/w-r-pdf/crossdrains.pdf. FHWA, Best Management Practices for Erosion and Sediment Control, FHWA-SLP-94-005, FHWA, Sterling, Va., 1995. Guide to Slope Protection Works (GSPW), His Majesty’s Government of Nepal. Ganabahal, Kathmandu, 2003. Hearn, G.J. and R.W. Weeks, Principles of Low Cost Road Engineering in Mountainous Regions, with Special Reference to Nepal, Himalaya, C.J. Lawrence, Ed., Trans- port Research Laboratory, Overseas Road Note 16, Berk- shire, United Kingdom, 1997. Howell, J., Roadside Bio-engineering: Site Handbook, His Majesty’s Government of Nepal, Ganabahal, Kath- mandu, 1999 [Online]. Available: http://onlinepubs.trb.org/ Onlinepubs/sp/Airport/RoadsideBioengineering.pdf. Keller, G. and J. Sherar, Low-Volume Roads Engineer- ing—Best Management Practices Field Guide, Office of International Programs and U.S. Agency for International Development, USDA Forest Service, Washington, D.C., 2003 [Online]. Available: http://www.fs.fed.us/global/topic/ welcome.htm#12. Long, M.T., “Horizontal Drains: An Update on Methods and Procedures for Exploration, Design, and Construction of Drain Systems, Fifth International Conference on Low Volume Roads, May 19–23, 1991, Raleigh, N.C., Trans- portation Research Record 1291, Transportation Research Board, National Research Council, Washington, D.C., 1991, pp. 166–172. Long, M.T., “Horizontal Drains, Application and Design,” Section 6D, In The Slope Stability Reference Guide for National Forests in the United States, Engineering Staff, Forest Service, USDA, Washington D.C., Dec. 1993 [Online]. Available: http://www.fs.fed.us/rm/pubs_other/ wo_em7170_13/wo_em7170_13_vol3.pdf. McCuen, R., P. Johnson, and R. Regan, Highway Hydrology, Hydraulic Design Series No. 2, 2nd ed., FHWA-NHI-02-001, National Highway Institute, Federal Highway Administration, Arlington, Va., 2002 [Online]. Available: http://isddc.dot.gov/ OLPFiles/FHWA/013248.pdf. Moll, J., R. Copstead, and D.K. Johansen, Traveled Way Surface Shape, San Dimas Technology and Development Center, Forest Service, U.S. Department of Agriculture, Washington, D.C., Oct. 1997 [Online]. Available: http:// www.stream.fs.fed.us/water-road/w-r-pdf/surfaceshape.pdf. Normann, J., R. Houghtalen, and W. Johnston, Hydraulic Design of Highway Culverts, Hydraulic Design Series (HDS) No. 5, FHWA-NHI-01-020, Federal Highway Administration and National Highway Institute, Washington, D.C., 2001 (rev. TABLE 4 SUBSURFACE DRAIN OPTIONS Structure Bioengineering Main Sites Advantages Limitations French drain* or underd- rain with pipe Grasses and other plants along the sides and between feeder arms Almost any site where the ground is firm enough to hold the structure and the flow of water is not too heavy for this construction technique A relatively low-cost and com- mon subsurface type of drain. Very flexible. A good option for unstable slopes A membrane of permeable geotex- tile should be used. If the water flow is heavy, piping may occur under ground. The outlet should be monitored periodically Site-specific design of drain to pick up seepage water. An open ditch or a drain with a flexible gabion lining is preferred Planted grasses and other species along the sides Any slope with obvious seep- age lines Specific drains can be designed for any site, for optimum water collection Great care is needed to ensure that all seepage water is trapped by the drain. Movement in the slope may affect this Horizontal drains Plant grasses at the pipe outlet Moderate to deep-seated slides Can lower the groundwater level in the slope May or may not intercept and drain all of the groundwater Source: Adapted from Howell (1999). *Perforated pipe of durable, high-grade black polyethylene, 6 in. (150 mm) diameter with approximately 40 holes of 0.2 in. (5 mm) per 3.28 ft (or 1 m) in drainage composed of medium aggregate. Drain can be made more resistant to disruption by building it in a wire gabion casing.

18 2005) [Online]. Available: http://www.fhwa.dot.gov/engi- neering/hydraulics/library_arc.cfm?pub_number=7&id=13. Orr, D., Roadway and Roadside Drainage, CLRP Publica- tion No. 98-5, Cornell Local Roads Program and New York LTAP Center, Ithaca, N.Y., 1998 (updated 2003) [Online]. Available: http://www.clrp.cornell.edu/workshops/pdf/drain- age_08_reprint-web.pdf. Shah, B.H., Field Manual on Slope Stabilization, United Nations Development Program, Pakistan, Sep. 2008 [Online]. Available: http://www.preventionweb.net/english/profes- sional/publications/v.php?id=13232.