CHAPTER 5

SURFACE AND SLOPE PROTECTIVE MEASURES

5.1 Introduction

Properly designed slope protection and stabilization has to include two components: a vegetational-biological and a mechanical-structural component. For maximum effect, both components must be integrally planned prior to road construction.

Properly designed and planted vegetative covers play a significant role in preventing surface erosion and shallow mass failures. The function of root systems of live plants on shallow soils on steep slopes is that of a binder for individual soil particles or aggregates. They act in several ways to increase slope stability: (1) they bond unstable soil mantles to stable subsoils or substrata, (2) they provide a cover of a laterally strong fine root systems close to the surface, and (3) they provide localized centers of reinforcement in the vicinity of individual trees where embedded stems act like a buttress pile or arch-abutment on a slope.

The structural-mechanical component can consist of conventional retaining walls, either the gravity or cantilever type, or a reinforced earth structure. Structural-mechanical stabilization techniques are called for in cases where the potential for deep-seated slope movement or high, lateral earth stresses exists.

A simplified flow chart is shown in Figure 94 which indicates the appropriate combination of methods to either maintain or achieve a stable and erosion-free slope. Implicit in any slope stability discussion is the effect of water and the importance of proper drainage. Mechanical drainage structures, such as culverts, ditches, water bars, is discussed in Chapter 4.3, 4.4, and 4.5. In addition to mechanical controls, however, vegetation can provide a form of "biological" drainage through plant transpiration. Root systems can effectively dewater soil mantles during their active growing season, but often the periods of most danger from sloe failure and erosion do not coincide with peak transpiration periods.

More detailed information concerning biotechnical slope stabilization, the combination of vegetative and structural components can be found in Gray and Leiser (1982), Volgman (1979) and Schiechtl (1978, 1980).

5.2 Surface Protection Measures

The simplest and most cost effective means of stabilizing bare soil surfaces is through the use of vegetation or mulches. The objective of all surface stabilization techniques is to establish, as rapidly as possible, a dense vegetative cover to minimize available sources for sediment. Native plants generally require less expense and maintenance as well as being visually harmonious with the natural landscape. Many exotic species have been cultivated specifically for erosion protection and may also be suitable.

The body of research that points to road construction as the major cause of stream sedimentation in mountainous environments also indicates that surface erosion on severely disturbed soils such as road fills is highest immediately following disturbance and decreases rapidly over time. This suggests that stabilization measures must be employed during and immediately following construction. The methods chosen must provide rapid benefits, hence merely seeding disturbed areas may not provide much relief. Transplanting living plants, fertilizing, or mulching exposed soil surfaces may be required to achieve the desired level of protection.

Figure 94. Selection criteria for slope stabilization methods

5.2.1 Site Analysis

In order to ensure success of any revegetation effort, it is necessary to prepare an overall plan which considers the climate, vegetation, and microsite (soils, microclimate, slope, and aspect).

Climatic information should center on rainfall frequency distribution and amount. Likewise, average temperature, minimum/maximum temperatures, heating degree days and number of frost free days are important points to consider. The vegetation analysis includes the suitability of native or exotic (introduced) plant species for the specific area in question. Here, the focus should be set on inventorying the entire spectrum of plant species that occupy a given site. The survey should note the particular microsites, soils and aspects in which different species grow. Typical points to consider include:

- which plants do well in a wide range of conditions
- which plants make good seed crops
- which plants root readily when partially buried or resprout from roots cut during construction
- which plants have the best attributes for erosion control (rapid, dense growth; growing season; rooting characteristics).

Microsite evaluation should consider factors such as microclimate, aspect, topography and soils. The microclimate is primarily affected by variations in the radiation balance and the immediate surroundings. Changes in the radiation balance will affect microclimate regime and surface temperatures -- two extremely important factors for plant survival. Examples are changing the surface color or establishing a vapor barrier. Installing a vapor barrier to reduce evaporation losses will result in increases in the surface temperature. By changing the surface to a light color, the radiation absorbed by the surface can be reduced to compensate for the temperature rise. For further information on the interaction of microclimate and vegetation, the reader is referred to Geiger (1961, 1966).

Aspect and topography may reveal the need for specific site treatment either for plant survival and/or local site stabilization and slope preparation to allow for plant establishment. Wet and dry areas should be mapped for determining the need for special dewatering treatments or adoption of a particular seed mix. Likewise slope angles greater than 40 degrees are often difficult to revegetate, except in cases where slopes consist of decomposing bedrock or have a uniform, rocky subsoil. Assessment of the degree of local surface erosion (e.g. shallow versus deep seated) will determine the need for shallow rooted plant cover or a more deeply rooted plant species.

Soil analysis should consider the local soil profile and determine the predominant soil horizon present on the finished surface. Road cuts or fills may expose layers, strata, or horizons which may be significantly different from the surface soil which supports a given plant species community. Factors to consider are pH, salinity, nutrient levels, and texture (water holding capacity).

5.2.2 Site Preparation

In order to ensure success of any revegetation effort, it is necessary to prepare a proper seedbed. This may include reshaping the slope if gravity will cause "ravelling" of loose soil. A 1:1 slope ratio or better is recommended to provide a good seeding surface. Slopes of 6 meters (20 feet) or more should be broken up with small ditches or flat benches on the contour. Roughening the slope along the contours will reduce the chance of rilling and will provide small depressions which retain the seed. Oftentimes, construction work or tree and brush removal generally leave sites sufficiently scarified to permit seed to reach mineral soil.

Site preparation efforts on shallow soils may permanently damage the seeding site. The small volume of soil interlacing rocks may fall in the ditch line and be lost. Likewise, loose soil sidecast on fill slopes is extremely prone to erosion. Compaction of this sidecast material with one pass of a sheepsfoot roller will secure the soil to the slope and provide roughened surface for planting. Fill slopes often are best treated with brush-layering or wattling in order to provide added mechanical stability. Typically, fill slopes are more prone to deep seated erosion (rilling or gullying) than are cut slopes.

5.2.3 Seeding and Planting

Type of seed, plant or cutting will determine the most appropriate planting technique. Herbaceous species typically germinate rapidly when compared to woody species. Woody species often must be planted to greater depths than herbaceous plants and may need mulching to keep them from drying out before germination takes place. Woody plants often require protection from herbivores and rodents because of their slow growth.

In addition to providing a dense, fibrous mat of protective material, seeded grasses and legumes[1] improve the organic and nutrient balance of the soil. They also act as "nurse plants" to young native species by providing shade and thereby reducing moisture loss from the soil. Grass seeding is often considered detrimental to tree regeneration, although this need not be the case. For instance, in southeast Alaska, grass seeding of exposed mineral soil helps establish spruce and hemlock seedlings by reducing the disruptive influence of frost heave and by retarding alder invasion. Grass species can also be selected such that competition with tree species for vital soil moisture during critical growth stages is minimized.

Mixtures of at least three plant species is recommended to assure continuous, even protection across a slope. In addition to factors mentioned at the beginning of this section, other factors to consider in selecting an appropriate mixture include:

- slope stability, angle, aspect, and exposure
- general climatic conditions, including conditions at the time of planting
- competitive ability of species to be planted in relation to native weed species or desired ultimate vegetation establishment
- susceptibility to foraging by livestock, rodents, and game
- visual and esthetic considerations
- physical and chemical characteristics of the soil.

It is impossible to recommend specific grass seed mixtures in this document. Likewise, seeding rates depend upon the number of live germinant seeds per unit weight and not simply on seed weight. For example, one kilogram of subterranean clover contains 34,000 seeds whereas one kilogram of timothy grass has 590,000 seeds--a 17.3 fold difference. In general, 1,100 to 1,600 live pure seed per square meter (100 to 150 per square foot) are sufficient seed densities for roadside erosion control in temperate climates (Berglund, 1978). It may be desirable to increase this rate in critical areas--culvert and bridge installations and road fill slopes--and decrease it in less critical or arid areas. Because of wide variations between sites and adaptability of individual grass and forb species around the globe, appropriate specialists should be consulted in each case in order to tailor the seed mixture to site conditions. These specialists include soil scientists, agronomists, ecologists, range conservationists, wildlife biologists, and landscape architects.

Generally, a vigorous, fast-spreading legume is included in the seed mixture because of its beneficial effects in replenishing soil nitrogen. Care must be taken, however, in ensuring that the chosen legume has been treated with an innoculant of the associated root bacteria. A problem associated with most legumes is their high palatability to livestock, deer, elk, and other game. Grazing animals will trample the soil and mechanical structures and create a more erosive condition than existed prior to the treatment. It is therefore recommended that legumes not be included in seed mixtures for sites readily accessible to game animals, cattle, sheep, or goats unless the legume is known to be unpalatable to animals (Adams, et al., 1983).

Road construction oftentimes results in the loss of the very thin mantle of fertile topsoil leaving a relatively infertile residual subsoil. Fertilizers are often required to provide young plants with sufficient nutrients. Again, variability from site to site requires the expertise of a specialist in order to determine proper fertilizer selection and application rates. In general, fertilizer prescriptions are developed on the basis of the amount of total nitrogen in the soil. If a soil test shows total nitrogen to be greater than 0.2 percent, no fertilizer is needed.

Fertilization normally occurs together with seeding either prior to or near the end of the rainy season. Two applications--one prior to and one after the rainy season--are extremely effective. Refertilization may also be needed in following years due to reductions in vigor of the crop. If fertilizer costs are prohibitive or supplies limited, it may be desirable to concentrate efforts on such key areas as large fills and culvert and bridge emplacements.

[1] Any one of a large group of plants of the pea family (Leguminosae). Because of their ability to store and fix nitrogen, legumes, such as alfalfa, are often used in rotation with other cash crops to restore soil productivity.

5.2.4 Application Methods

Techniques used in establishing grasses include hand-operated cyclone seeders, truck-mounted broadcast seeders, seed drills, and hydroseeders. Drilling is best as it places the seed directly in the soil at a controlled depth and seeding rate, but may be impossible on steep cut banks and fills. Hydroseeding is the application of seed, fertilizer, and mulch in a slurry of some sort of viscous water soluble binder, such as wood fiber, from a truck-mounted tank. This method is most suitable for large areas and steep slopes where plastering of materials is necessary to achieve uniform coverage. It is also expensive and sometimes impractical due to climatic, terrain, or road access conditions. Hand planting is generally effective for small areas and is often the least expensive. Covering the seed with at least 0.5 to 1 centimeter of soil is critical. Rainfall may help cover it, but raking or dragging seeded areas with tire chains, sections of cyclone fence, or similar objects is the most effective.

Soils which are heavily disturbed or which have little surface organic material to retard water runoff need protection afforded by any readily available mulching materials. Such materials include excelsior, straw, shredded logging residue or slash, and slurried wood or ground paper fibers. Excelsior provides the best protection but is very expensive. Straw mulch is very effective when applied at a rate of 5.5 metric tons per hectare (2 tons per acre) and secured to the surface either mechanically by punching it into the surface with the end of a shovel or chemically with a liquid "tackifier" such as emulsified asphalt. Table 36 shows the effectiveness of different mulches subjected to a rainfall rate of 64 mm (2.5 in) per hour on a 20 percent slope with 15 cm (6 in) of sift loam over compacted calcareous till. Figure 91 outlines a decision matrix to use in order to choose the most effective erosion control combination for a given set of site and climatic conditions.

Table 36. Erosion control and vegetation establishment effectiveness of various mulches on highways in eastern and western Washington. Soils: silty, sandy and gravelly loams, glacial till consisting of sand, gravel and compacted silts and clays (all are subsoil materials without topsoil addition). Slope lengths: approximate maximum of 50 m (165 ft). Application rates: Cereal straw - 5,500 kg/ha (2 t/ac); Straw plus asphalt - 5,500 kg/ha (2 t/ac) and 0.757 I/Kg (200 gal/t), respectively; Wood cellulose fiber- 1,345 kg/ha (1,200 Ibs/ac); Sod - bentgrass strips 46 cm (18 in) by 1.8 m (6 ft) pegged down every third row.

Figure 95. Selection criteria for surface cover establishment methods in relation to erosion risk.

When using straw as a mulch, it is recommended that only "clean" straw be used to prevent the introduction of noxious plants. Wood fiber should be applied at a rate of 1.4 to 1.6 metric tons per hectare (0.5 to 0.6 tons per acre). At higher rates, wood fiber improves erosion control but inhibits plant establishment. When mulching follows seed and fertilizer application, rather than in combination with seeding and fertilizing (as is the case sometimes with hydroseeding), there is a much greater chance that seed will be in direct contact with mineral soil and will germinate more readily. Hydroseeding a fiber-seed-water slurry can entrap 60-70 percent of the seed in the mulch layer.

5.2.5 Wattling and filter strips

Oftentimes grass cover alone is insufficient to prevent erosion on long, steep slopes. Wattling or filter strips work to break the slope into short segments so that the kinetic energy of water flowing over the surface is dissipated. Many different methods and materials can be employed to achieve this objective. Heede (1975) successfully used submerged burlap strips 30 cm (1 ft) wide, placed vertically into the ground on the contour 0.5 to 1 meter (1.5 to 3 ft) apart from each other to control rilling in semi-desert regions. Filter windrows can be fabricated from slash accumulated during road construction and can easily be constructed simultaneously along with the road (Cook and King, 1983). A rough estimate of production rates for windrow construction during one demonstration is 52 m/hour (170 ft/hour) using a track-mounted Caterpillar 235 hydraulic pull shovel (a large backhoe). Sediment trapping efficiency was estimated at between 75 and 85 percent. Windrows in this demonstration consisted of logs not less than 0.45 m (18 in) diameter secured against undisturbed stumps, rocks, or trees on fill slopes immediately above and parallel to the toe of the slope. Slash (tops, limbs, and brush not exceeding 15 cm (6 in) in diameter and 3.7 m (12 ft) in length were then placed above the logs in neat piles (also see Chapter 6.3.3, Figure 119 ).

Table 36 is included to provide guidance in determining appropriate windrow widths based on the length of the slope and type of material used to construct the windrow.

Wattling (Figure 96) consists of combined mechanical and natural stabilization techniques in which stakes are placed on the contour 0.5 m (1.6 ft) apart and at 1.2 m (4 ft) intervals between rows. A trench is then dug 20 cm (8 in) wide and 25 cm (10 in) deep against or immediately above the contour stakes. Bundles of live vegetative materials (such as Salix spp., Bambusa spp., Cassia sepium or other locally available material) 13 cm (5 in) in diameter and 3 m (10 ft) long are placed in the trench overlapping end and tail. The wattling bundles are then covered with soil so that part of the branches and leaves above ground are left visible. It is very important that the soil is worked thoroughly into the interstices of the wattles. During the installation workers should walk on the wattles as much as possible to insure maximum compaction and working the soil into the bundles.

Incorrect installation of wattles may actually aid in soil slumping because of collection of water in the wattle trenches. It is therefore important that soil is thoroughly worked into the wattles and no trench remains to be filled with water. Likewise firm staking is important particularly in areas where frost heaving is a problem. An average 10 person crew can treat 200 m² to 250 m² (2,000 to 2,500 ft2) in a day (Sheng, 1977b). More detailed information on wattling procedure and installation can also be found in Kraebel (1936), Grey and Leiser (1982), and Schiechtl (1978, 1980).

Table 37. Windrow protective strip widths required below the shoulders[1] of 5 year old[2] forest roads built on soils derived from basalt[3], having 9 m cross-drain spacing[4], zero initial obstruction distance[5], and 100 percent fill slope cover density[6].(U. S. Environmental Protection Agency, 1975).

Figure 96. Preparation and installation procedure for contour wattling, using live willow stakes Shown are (a) stems of cut brush wattles; (b) live willow stakes that have rooted sprouts; (c) inert construction stakes driven through wattles; (d), (e), and (f) vegetation (grasses, shrubs, and trees) (after Kraebel, 1936).

5.2.6 Brush Layering

Contour brush layering (Figure 97) involves embedding green branches of shrubs or trees on successive horizontal layers into the slope. Brush layering is different from wattling in that (1) branches are placed into the slope perpendicular to the strike instead of parallel creating better resistance to shallow shear failure, (2) staking is not required, (3) brush layers and surfaces can be reinforced with wire mesh or other material, (4) brush layers can be incorporated into the construction process of a fill. That is, brush layers are laid down, the next lift of soil is placed and compacted, and the process is repeated.

Contour brush layering is comparable to the "reinforced earth" concept where the cuttings or branches act in the same fashion as the reinforcing strips.

Figure 97. Brush layer installation for slope stabilization using rooted plants for cut slope and green branches for fill slope stabilization.

According to Schiechtl (1978, 1980) three brush layering techniques may be used. The first technique uses brush layers consisting of rooted plants or rooted cuttings only. Approximately 5 to 20 rooted seedlings per meter are required (See Figure 97).

The second technique utilizes green cuttings or branches from alder, cottonwood or willow. On cut slopes, cuttings from 0.5 to 2.0 meters in length are used. On fill slopes, cutting length can vary from 2.0 to 5.0 meters. This method is particularly suited for use in critical and sensitive areas.

The third technique is a combination of the first two methods where rooted seedlings or cuttings are installed together with branches or cuttings. From 1 to 5 rooted cuttings per meter are required.

In all three methods, the material should be placed with the butt ends slightly dipping into the fill (20 percent) and the tips protruding a few centimeters. Vertical spacing of brush layers can vary from 0.5 to 1.5 meters depending on soil type, erosion hazard, slope angle and length of slope. A good practice is to vary the vertical spacing on long slopes with short spacings at the bottom and increasing the spacing towards the upper end of the slope.

A variation to the contour brush layering approach where the layers are positioned along the contours or horizontally is to arrange the layers at a 10 - 40 percent incline. This variation is called for on wet, heavy soils or slopes with numerous small springs. Water collecting in the berms or brush layers is drained off and does not stagnate and infiltrate into the slope.

Installation procedures typically proceed from bottom to top. Fill slope installation is simple. However, care should be taken that the brush layer dips into the slope at least 20 percent. The next soil layer is placed on top and compacted. Cut slope installation requires the opening of a ditch or berm. As with fills, work progresses from bottom to top. The excavation of the upper berm is used for filling-in and covering the lower brush layer.

5.2.7. Mechanical Treatment

Mechanical surface stabilization measures consist of diversion ditches and terraces, serrations, or scarification and can be used in conjunction with vegetative methods discussed above. These methods generally require detailed engineering design and location. T. C. Sheng (1977a) discusses several different methods for the construction of bench terraces together with tables providing design information and costs.

Serrations consist of steps of 60 to 120 cm (2 to 4 ft) cut vertically and horizontally along the normal, intended slope gradient. After treatment, the slope is seeded, fertilized, and mulched as discussed in Chapter 5.2.3. The steps provide improved seedbeds free of sliding forces normally experienced on steep slopes. Serrations are only effective on cut slopes of soft rock or similar material that will stand vertically or near vertically for a few years in cut heights of approximately one meter. Likewise, this method is not applicable to soil types where the rate of slough is so high that vegetative cover is buried and destroyed. If acceptable slope material is soft, the slope should be allowed to slough before seeding until about one-third of the steps are filled. Otherwise, grass may be destroyed by the excessive rate of initial slough.

Roughened or scarified slopes may not be as esthetically pleasing to the eye as smoothly graded cut and fill slopes, but they are far more effective in increasing infiltration and impeding runoff. Scarified slopes also provide small depressions for the retention of seed and also help mulch to better adhere to the slope. Roughening may be accomplished by several means including deep cleated bulldozers traveling up and down the slope, sheepsfoot rollers, rock rippers, and brush rakes mounted on bulldozers. The path of the roughened slope should trend perpendicular to the direction of flow.

5.3 Mass Movement Protection

Deep seated mass failures can be dealt with in three ways. The methods are categorized by the way they affect soil stability.

1. Avoidance Methods: Relocate road on a more stable area (for large, unstable fills probably by far the most appropriate approach).

2. Reducing Shear Stress: This is achieved through excavation of unacceptable materials. It creates a reduction in soil weight and can be accomplished by: a) removal of soil mass at the top of the potential slide, b) flattening of cut slopes above the road, c) benching of cut slopes.

3. Increasing Shear Strength: This is achieved through retaining structures. They can be grouped into a) rock buttresses at the toe of fill slope, b) cribs or gravity retaining walls at toe of fill or cut, and c) piling walls, likewise at the toe of fills or cuts.

Engineering and structural methods for stabilizing slopes can be grouped into four categories:

1. Excavation and filling techniques. This would include excavating the toe of an earth flow until successive failures result in a stable slope, removing and replacing failed material with lighter, more stable material, or recompacted debris, excavating to unload upper portions of a mass failure, and filling to load the lower portions of a mass failure (most likely in conjunction with other loading or restraining structures).

2. Drainage techniques. This would include efforts to remove or disperse surface water (as discussed in Chapter 4), drainage of tension cracks, using rock fill underlain by filter cloth to prevent upward migration of water into the road prism, insertion of trench drains, perforated, horizontal drains, or drainage galleries, insertion of vertical drains or wells discharged by syphons, or pumps, and electro-osmosis (the use of direct current passing between wellpoints and steel rods placed midway between the rods to increase the drainage rate) for drainage of low permeability soils.

3. Restraining structures. These include retaining walls, piles, buttresses, counterweight fills, cribs, bin walls, reinforced earth, and pre-stressed or post-tensioned soil or rock anchors (Figure 98). Organizations such as highway departments and railroads have developed charts and tables giving earth pressures for the design of retaining walls that require a minimum of computation. Nearly all of these charts and tables are based on the Rankine formula which describes earth pressures as a function of unit weight and internal angle of friction of the backfill material.

4. Miscellaneous techniques. Grouting can be used to reduce soil permeability, thereby preventing the ingress of groundwater into a failure zone. Chemical stabilization, generally in the form of ion exchange methods, is accomplished by high pressure injection of specificion exchange solutions into failure zones or into closely spaced pre-drillled holes throughout the movement zone. Heating or baking of clay soils can sometimes improve their strength, and, rarely, freezing of soils will help gain temporary stability. Localized electro-osmosis can be used to form in situ anchors or tie-backs. Suppression of natural electro-osmosis can be used to reduce unfavorable groundwater pressures. Blasting is sometimes used to disrupt failure surfaces and to improve drainage.

Figure 98. Types of retaining walls.

For correcting cut or fill failures, a detailed investigation into the reason for the failure, particularly the position and geometry of the failure surface and other potential failure surfaces, is required prior to prescribing ameliorative measures. The neutral line concept, discussed by Hutchinson (1977) and Sidle, et al. (1985), is of particular interest in assessing the impact of cuts and fills on factors of safety. The neutral line describes where the load attributed to fill material will have no effect on the original factor of safety. If the load falls up-slope of the neutral line, the factor of safety will decrease; if it is downslope of the neutral line, the factor of safety will decrease.

The use of any of these stabilization techniques requires extensive site specific investigations into the mechanics of soils, groundwater, and bedrock occurring on the site. It is advisable to utilize the most experienced geotechnical or highway engineer available in order to provide the most effective design possible. As can be inferred from the above discussion, any of these techniques will be quite costly to design and install. Furthermore, the success of such measures in functioning adequately through time is highly dependent on the skill of the design engineer and the degree of maintenance employed after construction. Hence, avoidance of areas where structural stabilization measures are required will result in considerable short term and long term cost savings, and the major opportunity for reducing landslide risk is at the route planning stage.

The purpose of a retaining structure is to provide stability against sliding or failure and protection against scour and erosion of a slope, or the toe or cutface. The typical retaining structure on forest roads is a gravity retaining wall which resists earth pressure by the force of its own weight. Excavation and/or fill volume can be significantly reduced particularly on steep side slopes (see also discussion in Chapter 3.2).

The volume of cribs or retaining walls should be 1/6 to 1/10 of that of the total moving mass to be retained. As a rule, the foundation or base should at least extend 1.2 to 2.0 meters below the slip plane in order to be effective.

The forces acting on a retaining wall are similar to those acting on a natural slope. These forces are be grouped into resisting forces (forces resisting failure) and driving forces (forces causing failure) as illustrated in Figure 99.

Failure of a retaining wall can be brought about through

- sliding along its base
- bearing capacity failure
- overturning

Figure 99. Forces acting on a retaining wall.

For low toe walls or toe-bench structures it is usually possible to use standard designs. Standard designs have been developed on the basis of soil mechanics and past performance. Standard designs have been developed by a number of sources and are typically available on request. These sources include manufacturers of retaining wall systems (e.g. gabions, crib walls, welded-wire walls, geotextiles), trade associations (e.g. American Wood Preservers Institute), and state and federal agencies (e.g. Forest Service, Highway Administration, local transportation departments). Standard designs can be used safely provided they conform with the local conditions The factors or conditions to consider include maximum wall height, surcharge conditions, strength and finish of structural members, inclination requirements, construction requirements, backfill material, soil conditions at base, and groundwater conditions.

An example of a standard log crib wall is shown in Figure 100. A thorough discussion of timber walls is provided by Schuster et al. (1973). Timber crib walls and gabion structures can withstand some limited, differential base settlement without a significantly affecting the retaining action. Drainage characteristics of the backfill and crib material is important because of potential water pressure build-up. Most standard designs assume free draining sand or gravel fills.

Gabions are rectangular containers made of heavy steel wire and filled with cobble-sized rocks (10 to 30 cm in diameter). A typical gabion retaining wall is shown in Figure 101. Advantages of gabion structures are ease of construction, tolerance of uneven settlement, and good drainage characteristics. Gabion walls are particularly suited in areas where only small, fragmented rocks are available. Typically, they can be built without heavy equipment. Both crib and gabion walls lend themselves to incorporation of vegetative systems to provide additional strength over time as well as providing a more esthetically pleasing appearance.

Gravity retaining structures utilizing standard designs are typically limited to a height of less than 6.0 meters. Structures requiring a larger height have to be designed based on site-specific soil mechanical conditions.

Figure 100. Example of a standard crib wall design. (Wash. State Dept. of Highways).

Figure 101. Low gabion breast walls showing sequence of excavation, assembly, and filling. (From White and Franks, 1978).

LITERATURE CITED

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