CHAPTER 5:slope

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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 beintegrally planned prior to road construction.

Properly designed and planted vegetative covers play a significant role in preventing surface erosion andshallow mass failures. The function of root systems of live plants on shallow soils on steep slopes is thatof a binder for individual soil particles or aggregates. They act in several ways to increase slope stability:!" they bond unstable soil mantles to stable subsoils or substrata, #" they provide a cover of a laterallystrong fine root systems close to the surface, and $" they provide localized centers of reinforcement inthe vicinity of individual trees where embedded stems act li%e a buttress pile or arch-abutment on a slope.

The structural-mechanical component can consist of conventional retaining walls, either the gravity orcantilever type, or a reinforced earth structure. &tructural-mechanical stabilization techni'ues are calledfor in cases where the potential for deep-seated slope movement or high, lateral earth stresses exists.

( simplified flow chart is shown in Figure )* which indicates the appropriate combination of methods toeither maintain or achieve a stable and erosion-free slope. +mplicit in any slope stability discussion is theeffect of water and the importance of proper drainage. echanical drainage structures, such as culverts,ditches, water bars, is discussed in hapter *.$, *.*, and *.. +n addition to mechanical controls, however,vegetation can provide a form of /biological/ drainage through plant transpiration. 0oot systems caneffectively dewater soil mantles during their active growing season, but often the periods of most dangerfrom sloe failure and erosion do not coincide with pea% transpiration periods.

ore detailed information concerning biotechnical slope stabilization, the combination of vegetative andstructural components can be found in 1ray and 2eiser !)3#", 4olgman !)5)" and &chiechtl !)53,!)36".

5.2 Surfc! Prot!ction M!"ur!"

The simplest and most cost effective means of stabilizing bare soil surfaces is through the use ofvegetation or mulches. The ob7ective of all surface stabilization techni'ues is to establish, as rapidly aspossible, a dense vegetative cover to minimize available sources for sediment. 8ative plants generallyre'uire less expense and maintenance as well as being visually harmonious with the natural landscape.any 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 ma7or cause of stream sedimentation inmountainous environments also indicates that surface erosion on severely disturbed soils such as roadfills is highest immediately following disturbance and decreases rapidly over time. This suggests thatstabilization measures must be employed during and immediately following construction. The methodschosen 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 re'uired to achieve thedesired level of protection.

Fi#ur! $%.&election criteria for slope stabilization methods


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5.2.1 Site Analysis

+n order to ensure success of any revegetation effort, it is necessary to prepare an overall plan whichconsiders the climate, vegetation, and microsite soils, microclimate, slope, and aspect".

C&i'tic infor'tionshould center on rainfall fre'uency distribution and amount. 2i%ewise, averagetemperature, minimum9maximum temperatures, heating degree days and number of frost free days areimportant points to consider. The vegetation analysis includes the suitability of native or exoticintroduced" plant species for the specific area in 'uestion. ere, the focus should be set on inventoryingthe entire spectrum of plant species that occupy a given site. The survey should note the particularmicrosites, 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 ma%e 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; rootingcharacteristics".

Micro"it! !(&utionshould consider factors such as microclimate, aspect, topography and soils. Themicroclimate is primarily affected by variations in the radiation balance and the immediate surroundings.hanges in the radiation balance will affect microclimate regime and surface temperatures -- twoextremely important factors for plant survival. !, !)>>".


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(spect and topography may reveal the need for specific site treatment either for plant survival and9or localsite stabilization and slope preparation to allow for plant establishment. ?et and dry areas should bemapped for determining the need for special dewatering treatments or adoption of a particular seed mix.2i%ewise slope angles greater than *6 degrees are often difficult to revegetate, except in cases whereslopes consist of decomposing bedroc% or have a uniform, roc%y subsoil. (ssessment of the degree oflocal surface erosion e.g. shallow versus deep seated" will determine the need for shallow rooted plantcover or a more deeply rooted plant species.

Soi&analysis should consider the local soil profile and determine the predominant soil horizon present onthe finished surface. 0oad cuts or fills may expose layers, strata, or horizons which may be significantlydifferent from the surface soil which supports a given plant species community. Factors to consider arep, salinity, nutrient levels, and texture water holding capacity".

5.2.2 Site Preparation

+n order to ensure success of any revegetation effort, it is necessary to prepare a proper seedbed. Thismay include reshaping the slope if gravity will cause /ravelling/ of loose soil. ( !:! slope ratio or better isrecommended to provide a good seeding surface. &lopes of > meters #6 feet" or more should be bro%enup with small ditches or flat benches on the contour. 0oughening the slope along the contours will reduce

the chance of rilling and will provide small depressions which retain the seed. @ftentimes, constructionwor% or tree and brush removal generally leave sites sufficiently scarified to permit seed to reach mineralsoil.

&ite preparation efforts on shallow soils may permanently damage the seeding site. The small volume ofsoil interlacing roc%s may fall in the ditch line and be lost. 2i%ewise, loose soil sidecast on fill slopes isextremely prone to erosion. ompaction of this sidecast material with one pass of a sheepsfoot roller willsecure the soil to the slope and provide roughened surface for planting. Fill slopes often are best treatedwith brush-layering or wattling in order to provide added mechanical stability. Typically, fill slopes are moreprone 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 techni'ue. erbaceous speciestypically germinate rapidly when compared to woody species. ?oody species often must be planted togreater depths than herbaceous plants and may need mulching to %eep them from drying out beforegermination ta%es place. ?oody plants often re'uire protection from herbivores and rodents because oftheir slow growth.

+n addition to providing a dense, fibrous mat of protective material, seeded grasses and legumesA!Bimprove the organic and nutrient balance of the soil. They also act as /nurse plants/ to young nativespecies by providing shade and thereby reducing moisture loss from the soil. 1rass seeding is oftenconsidered detrimental to tree regeneration, although this need not be the case. For instance, insoutheast (las%a, grass seeding of exposed mineral soil helps establish spruce and hemloc% seedlings byreducing the disruptive influence of frost heave and by retarding alder invasion. 1rass species can also

be selected such that competition with tree species for vital soil moisture during critical growth stages isminimized.

ixtures of at least three plant species is recommended to assure continuous, even protection across aslope. +n addition to factors mentioned at the beginning of this section, other factors to consider inselecting an appropriate mixture include:

- slope stability, angle, aspect, and exposure

- general climatic conditions, including conditions at the time of planting


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- competitive ability of species to be planted in relation to native weed species or desired ultimate

vegetation establishment- susceptibility to foraging by livestoc%, rodents, and game- visual and esthetic considerations- physical and chemical characteristics of the soil.

+t is impossible to recommend specific grass seed mixtures in this document. 2i%ewise, seeding ratesdepend upon the number of live germinant seeds per unit weight and not simply on seed weight. Forexample, one %ilogram of subterranean clover contains $*,666 seeds whereas one %ilogram of timothygrass has )6,666 seeds--a !5.$ fold difference. +n general, !,!66 to !,>66 live pure seed per s'uaremeter !66 to !6 per s'uare foot" are sufficient seed densities for roadside erosion control in temperateclimates =erglund, !)53". +t may be desirable to increase this rate in critical areas--culvert and bridgeinstallations and road fill slopes--and decrease it in less critical or arid areas. =ecause of wide variationsbetween sites and adaptability of individual grass and forb species around the globe, appropriatespecialists should be consulted in each case in order to tailor the seed mixture to site conditions. Thesespecialists include soil scientists, agronomists, ecologists, range conservationists, wildlife biologists, andlandscape architects.

1enerally, a vigorous, fast-spreading legume is included in the seed mixture because of its beneficial

effects in replenishing soil nitrogen. are must be ta%en, however, in ensuring that the chosen legumehas been treated with an innoculant of the associated root bacteria. ( problem associated with mostlegumes is their high palatability to livestoc%, deer, el%, and other game. 1razing animals will trample thesoil and mechanical structures and create a more erosive condition than existed prior to the treatment. +tis therefore recommended that legumes not be included in seed mixtures for sites readily accessible togame animals, cattle, sheep, or goats unless the legume is %nown to be unpalatable to animals (dams,et al., !)3$".

0oad construction oftentimes results in the loss of the very thin mantle of fertile topsoil leaving a relativelyinfertile residual subsoil. Fertilizers are often re'uired to provide young plants with sufficient nutrients.

(gain, variability from site to site re'uires the expertise of a specialist in order to determine properfertilizer selection and application rates. +n general, fertilizer prescriptions are developed on the basis ofthe amount of total nitrogen in the soil. +f a soil test shows total nitrogen to be greater than 6.# percent, no

fertilizer is needed.

Fertilization normally occurs together with seeding either prior to or near the end of the rainy season. Twoapplications--one prior to and one after the rainy season--are extremely effective. 0efertilization may alsobe needed in following years due to reductions in vigor of the crop. +f fertilizer costs are prohibitive orsupplies limited, it may be desirable to concentrate efforts on such %ey areas as large fills and culvert andbridge emplacements.

A!B (ny one of a large group of plants of the pea family 2eguminosae". =ecause of their ability to storeand fix nitrogen, legumes, such as alfalfa, are often used in rotation with other cash crops to restore soilproductivity.

5.2.4 Application Methods

Techni'ues used in establishing grasses include hand-operated cyclone seeders, truc%-mountedbroadcast seeders, seed drills, and hydroseeders. Crilling is best as it places the seed directly in the soilat a controlled depth and seeding rate, but may be impossible on steep cut ban%s and fills. ydroseedingis the application of seed, fertilizer, and mulch in a slurry of some sort of viscous water soluble binder,such as wood fiber, from a truc%-mounted tan%. This method is most suitable for large areas and steepslopes where plastering of materials is necessary to achieve uniform coverage. +t is also expensive andsometimes impractical due to climatic, terrain, or road access conditions. and planting is generallyeffective for small areas and is often the least expensive. overing the seed with at least 6. to !


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centimeter of soil is critical. 0ainfall may help cover it, but ra%ing or dragging seeded areas with tirechains, sections of cyclone fence, or similar ob7ects is the most effective.

&oils which are heavily disturbed or which have little surface organic material to retard water runoff needprotection afforded by any readily available mulching materials. &uch materials include excelsior, straw,shredded logging residue or slash, and slurried wood or ground paper fibers. in" of sift loam over compacted calcareous till. Figure )! outlines a decisionmatrix to use in order to choose the most effective erosion control combination for a given set of site andclimatic conditions.

T)&! *+. cm !3 in" by !.3 m > ft" pegged down every third row.

Surfc! Co(!r

,ut! E-c!&"ior StrStr /A"0&t

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0ill erosion!:! slope

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fill slope

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loamcut slope

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metric tons per hectare6. to 6.> tons per acre". (t higher rates, wood fiber improves erosion control but inhibits plantestablishment. ?hen mulching follows seed and fertilizer application, rather than in combination withseeding and fertilizing as is the case sometimes with hydroseeding", there is a much greater chance thatseed will be in direct contact with mineral soil and will germinate more readily. ydroseeding a fiber-seed-water slurry can entrap >6-56 percent of the seed in the mulch layer.


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5.2.5 Wattling and filter strips

@ftentimes grass cover alone is insufficient to prevent erosion on long, steep slopes. ?attling or filterstrips wor% to brea% the slope into short segments so that the %inetic energy of water flowing over thesurface is dissipated. any different methods and materials can be employed to achieve this ob7ective.eede !)5" successfully used submerged burlap strips $6 cm ! ft" wide, placed vertically into the

ground on the contour 6. to ! meter !. to $ ft" apart from each other to control rilling in semi-desertregions. Filter windrows can be fabricated from slash accumulated during road construction and caneasily be constructed simultaneously along with the road oo% and Ding, !)3$". ( rough estimate ofproduction rates for windrow construction during one demonstration is # m9hour !56 ft9hour" using atrac%-mounted aterpillar #$ hydraulic pull shovel a large bac%hoe". &ediment trapping efficiency wasestimated at between 5 and 3 percent. ?indrows in this demonstration consisted of logs not less than6.* m !3 in" diameter secured against undisturbed stumps, roc%s, or trees on fill slopes immediatelyabove and parallel to the toe of the slope. &lash tops, limbs, and brush not exceeding ! cm > in" indiameter and $.5 m !# ft" in length were then placed above the logs in neat piles also see hapter >.$.$,Figure !!) ".

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

?attling Figure )>" consists of combined mechanical and natural stabilization techni'ues in which sta%esare placed on the contour 6. m !.> ft" apart and at !.# m * ft" intervals between rows. ( trench is thendug #6 cm 3 in" wide and # cm !6 in" deep against or immediately above the contour sta%es. =undlesof live vegetative materials such as &alix spp., =ambusa spp., assia sepium or other locally availablematerial" !$ cm in" in diameter and $ m !6 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 areleft visible. +t is very important that the soil is wor%ed thoroughly into the interstices of the wattles. Curingthe installation wor%ers should wal% on the wattles as much as possible to insure maximum compactionand wor%ing the soil into the bundles.

+ncorrect installation of wattles may actually aid in soil slumping because of collection of water in thewattle trenches. +t is therefore important that soil is thoroughly wor%ed into the wattles and no trench

remains to be filled with water. 2i%ewise firm sta%ing is important particularly in areas where frost heavingis a problem. (n average !6 person crew can treat #66 mG to #6 mG #,666 to #,66 ft#" in a day &heng,!)55b". ore detailed information on wattling procedure and installation can also be found in Draebel!)$>", 1rey and 2eiser !)3#", and &chiechtl !)53, !)36".

T)&! *3.?indrow protective strip widths re'uired below the shouldersA!Bof year oldA#Bforest roadsbuilt on soils derived from basaltA$B, having ) m cross-drain spacingA*B, zero initial obstruction distanceAB,and !66 percent fill slope cover densityA>B.H. &.


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A!B For protective strip widths from centerlines of proposed roads,

increase widths by one-half the proposed road width.A#B +f storage capacity of obstructions is to be renewed when roadsare $ years old, reduce protective strip width by 5 m.A$B +f soil is derived from andesite, increase protective strip width by

$6 cm; from glacial till, increase ! m; from hard sediments, increase#.* m; from granite, increase #. m; from loess, increase 5 m.A*B For each increase in cross-drain spacing beyond ) m, increaseprotective strip width $6 cm.AB For each !. m increase in distance to the initial obstructionbeyond zero or the road shoulder", increase protective strip width

!.# m.A>B For each !6 percent decrease in fill slope cover below a densityof !66 percent, increase protective strip width 6.$6 m.

Fi#ur! $+.Preparation and installation procedure for contour wattling, using live willow sta%es &hown area" stems of cut brush wattles; b" live willow sta%es that have rooted sprouts; c" inert construction sta%es

driven through wattles; d", e", and f" vegetation grasses, shrubs, and trees"after Draebel, !)$>".

5.2.6 Brush ayering

ontour brush layering Figure )5" involves embedding green branches of shrubs or trees on successivehorizontal layers into the slope. =rush layering is different from wattling in that !" branches are placedinto the slope perpendicular to the stri%e instead of parallel creating better resistance to shallow shearfailure, #" sta%ing is not re'uired, $" brush layers and surfaces can be reinforced with wire mesh or othermaterial, *" brush layers can be incorporated into the construction process of a fill. That is, brush layersare laid down, the next lift of soil is placed and compacted, and the process is repeated.

ontour brush layering is comparable to the /reinforced earth/ concept where the cuttings or branches actin the same fashion as the reinforcing strips.


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Fi#ur! $3.=rush layer installation for slope stabilization using rooted plants for cut slope and greenbranches for fill slope stabilization.

(ccording to &chiechtl !)53, !)36" three brush layering techni'ues may be used. The first techni'ueuses brush layers consisting of rooted plants or rooted cuttings only. (pproximately to #6 rootedseedlings per meter are re'uired &ee Figure )5".

The second techni'ue utilizes green cuttings or branches from alder, cottonwood or willow. @n cut slopes,cuttings from 6. to #.6 meters in length are used. @n fill slopes, cutting length can vary from #.6 to .6

meters. This method is particularly suited for use in critical and sensitive areas.

The third techni'ue is a combination of the first two methods where rooted seedlings or cuttings areinstalled together with branches or cuttings. From ! to rooted cuttings per meter are re'uired.

+n all three methods, the material should be placed with the butt ends slightly dipping into the fill #6percent" and the tips protruding a few centimeters. 4ertical spacing of brush layers can vary from 6. to!. meters depending on soil type, erosion hazard, slope angle and length of slope. ( good practice is tovary the vertical spacing on long slopes with short spacings at the bottom and increasing the spacingtowards the upper end of the slope.

( variation to the contour brush layering approach where the layers are positioned along the contours orhorizontally is to arrange the layers at a !6 - *6 percent incline. This variation is called for on wet, heavy

soils or slopes with numerous small springs. ?ater collecting in the berms or brush layers is drained offand does not stagnate and infiltrate into the slope.

+nstallation procedures typically proceed from bottom to top. Fill slope installation is simple. owever, careshould be ta%en that the brush layer dips into the slope at least #6 percent. The next soil layer is placedon top and compacted. ut slope installation re'uires the opening of a ditch or berm. (s with fills, wor%progresses from bottom to top. The excavation of the upper berm is used for filling-in and covering thelower brush layer.


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5.2.!. Mechanical "reat#ent

echanical surface stabilization measures consist of diversion ditches and terraces, serrations, orscarification and can be used in con7unction with vegetative methods discussed above. These methodsgenerally re'uire detailed engineering design and location. T. . &heng !)55a" discusses severaldifferent methods for the construction of bench terraces together with tables providing design information

and costs.

&errations consist of steps of >6 to !#6 cm # to * ft" cut vertically and horizontally along the normal,intended slope gradient. (fter treatment, the slope is seeded, fertilized, and mulched as discussed inhapter .#.$. The steps provide improved seedbeds free of sliding forces normally experienced on steepslopes. &errations are only effective on cut slopes of soft roc% or similar material that will stand verticallyor near vertically for a few years in cut heights of approximately one meter. 2i%ewise, this method is notapplicable to soil types where the rate of slough is so high that vegetative cover is buried and destroyed.+f acceptable slope material is soft, the slope should be allowed to slough before seeding until about one-third of the steps are filled. @therwise, grass may be destroyed by the excessive rate of initial slough.

0oughened or scarified slopes may not be as esthetically pleasing to the eye as smoothly graded cut andfill slopes, but they are far more effective in increasing infiltration and impeding runoff. &carified slopes

also provide small depressions for the retention of seed and also help mulch to better adhere to the slope.0oughening may be accomplished by several means including deep cleated bulldozers traveling up anddown the slope, sheepsfoot rollers, roc% rippers, and brush ra%es mounted on bulldozers. The path of theroughened slope should trend perpendicular to the direction of flow.

5.* M"" Mo(!'!nt Prot!ction

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

!. A(oidnc! M!tod"60elocate road on a more stable area for large, unstable fills probably byfar the most appropriate approach".

#. R!ducin# S!r Str!""6This is achieved through excavation of unacceptable materials. +tcreates a reduction in soil weight and can be accomplished by: a" removal of soil mass at the topof the potential slide, b" flattening of cut slopes above the road, c" benching of cut slopes.

$. Incr!"in# S!r Str!n#t6This is achieved through retaining structures. They can be groupedinto a" roc% buttresses at the toe of fill slope, b" cribs or gravity retaining walls at toe of fill or cut,and c" piling walls, li%ewise at the toe of fills or cuts.


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syphons, or pumps, and electro-osmosis the use of direct current passing between wellpointsand steel rods placed midway between the rods to increase the drainage rate" for drainage of lowpermeability soils.

$. R!"trinin# "tructur!".These include retaining walls, piles, buttresses, counterweight fills,cribs, bin walls, reinforced earth, and pre-stressed or post-tensioned soil or roc% anchors Figure

)3". @rganizations such as highway departments and railroads have developed charts and tablesgiving earth pressures for the design of retaining walls that re'uire a minimum of computation.8early all of these charts and tables are based on the 0an%ine formula which describes earthpressures as a function of unit weight and internal angle of friction of the bac%fill material.

*. Mi"c!&&n!ou" t!cni7u!".1routing can be used to reduce soil permeability, therebypreventing the ingress of groundwater into a failure zone. hemical stabilization, generally in theform of ion exchange methods, is accomplished by high pressure in7ection of specificionexchange solutions into failure zones or into closely spaced pre-drillled holes throughout themovement zone. eating or ba%ing of clay soils can sometimes improve their strength, and,rarely, freezing of soils will help gain temporary stability. 2ocalized electro-osmosis can be used toform in situ anchors or tie-bac%s. &uppression of natural electro-osmosis can be used to reduceunfavorable groundwater pressures. =lasting is sometimes used to disrupt failure surfaces and to

improve drainage.

Fi#ur! $8.Types of retaining walls.


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For correcting cut or fill failures, a detailed investigation into the reason for the failure, particularly theposition and geometry of the failure surface and other potential failure surfaces, is re'uired prior toprescribing ameliorative measures. The neutral line concept, discussed by utchinson !)55" and &idle,et al. !)3", 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 ofsafety. +f the load falls up-slope of the neutral line, the factor of safety will decrease; if it is downslope ofthe neutral line, the factor of safety will decrease.

The use of any of these stabilization techni'ues re'uires extensive site specific investigations into themechanics of soils, groundwater, and bedroc% occurring on the site. +t is advisable to utilize the mostexperienced geotechnical or highway engineer available in order to provide the most effective designpossible. (s can be inferred from the above discussion, any of these techni'ues will be 'uite costly todesign and install. Furthermore, the success of such measures in functioning ade'uately through time ishighly dependent on the s%ill of the design engineer and the degree of maintenance employed after


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construction. ence, avoidance of areas where structural stabilization measures are re'uired will result inconsiderable short term and long term cost savings, and the ma7or opportunity for reducing landslide ris%is at the route planning stage.

The purpose of a retaining structure is to provide stability against sliding or failure and protection againstscour 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.


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For low toe walls or toe-bench structures it is usually possible to use standard designs. &tandard designshave been developed on the basis of soil mechanics and past performance. &tandard designs have beendeveloped by a number of sources and are typically available on re'uest. These sources includemanufacturers of retaining wall systems e.g. gabions, crib walls, welded-wire walls, geotextiles", tradeassociations e.g. (merican ?ood Preservers +nstitute", and state and federal agencies e.g. Forest&ervice, ighway (dministration, local transportation departments". &tandard designs can be used safelyprovided they conform with the local conditions The factors or conditions to consider include maximumwall height, surcharge conditions, strength and finish of structural members, inclination re'uirements,construction re'uirements, bac%fill material, soil conditions at base, and groundwater conditions.

(n example of a standard log crib wall is shown in Figure !66. ( thorough discussion of timber walls isprovided by &chuster et al. !)5$". Timber crib walls and gabion structures can withstand some limited,differential base settlement without a significantly affecting the retaining action. Crainage characteristicsof the bac%fill and crib material is important because of potential water pressure build-up. ost standarddesigns assume free draining sand or gravel fills.

1abions are rectangular containers made of heavy steel wire and filled with cobble-sized roc%s !6 to $6cm in diameter". ( typical gabion retaining wall is shown in Figure !6!. (dvantages of gabion structuresare ease of construction, tolerance of uneven settlement, and good drainage characteristics. 1abion wallsare particularly suited in areas where only small, fragmented roc%s are available. Typically, they can bebuilt without heavy e'uipment. =oth crib and gabion walls lend themselves to incorporation of vegetativesystems to provide additional strength over time as well as providing a more esthetically pleasingappearance.


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1ravity retaining structures utilizing standard designs are typically limited to a height of less than >.6meters. &tructures re'uiring a larger height have to be designed based on site-specific soil mechanicalconditions.

Fi#ur! 199.


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Fi#ur! 191.2ow gabion breast walls showing se'uence of excavation, assembly, and filling.From ?hiteand Fran%s, !)53".

CHAPTER 5:slope

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