Slope Stab

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CHAPTER 10

Slope Stabilization

This chap ter pertain s to the design of earthslopes as it relates to road construction. Itparticularly concerns slope stability andwhich slopes should be used under average

conditions in cuts and embankm ents. Someof the su bjects covered are geologic featuresthat affect slope stability, soil mechanics, in-dicators of unstable slopes, types of slopefailur es, and slop e stabilization.

Road failures can exert a tremendous im-pact on mission success. It is vital thatpersonnel engaged in road-building activitiesbe aware of the basic principles of slopestability. They mu st und erstand how theseprinciples are applied to construct stable

roads through various geologic materialswith specific cond itions of slope and soil.

Basic slope stability is illustrated by adescription of the balance of forces that existin undisturbed slopes, how these forceschange as loads are applied, and howgroun dwater affects slope stability andcauses road failure.

GEOLOG IC FEATURESThere are certain geologic features that

have a p rofound effect on slope stability andthat can consequently affect road construc-tion in an area. Many of these geologicfeatures can be observed in the field and mayalso be identified on topographic map s andaerial photographs. In some cases, thepresence of these features may be located bycomparing geologic and topographic maps.

The following paragraphs describe geologicfeatures that have a significant effect on slopestability and the techniques that may be usedto identify them:

FaultsThe geologic uplift that accomp anies moun-

tain building is evident in the m ountainousregions throughout the world. Stresses builtup in layers of rock by the warp ing that ac-comp anies up lift is u sually relieved byfracturing. These fractures may extend forgreat d istances both laterally and verticallyand are known as faults. Often the materialon one side of the fault is displaced verticallyrelative to the other side; sometimes igneous

material or serpentine may be intrud ed intofaults. Fau lts are the focal point for stressrelief and for intru sions of igneou s rock andserpentine; therefore, fault zones usually con-tain rock that is fractured , cru shed , or pa rtlymetamorp hosed. It is extremely important torecognize that fault zones are zones ofgeologic weakness and, as such, are critical inroad location. Fau lts often leave topograph icclues to their location. An effort should bemad e to identify any faults in the vicinity of aprop osed road location.

The location of these fault zones is estab-lished by looking for

Sadd les, or low sections in ridges, thatare aligned in the same general di-rection from one drainage to another.

Streams that app ear to deviate fromthe general d irection of the nearbystreams.

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Notice that the proposed locations of the faultzones on the topograp hic map follow sad dlesand drainages in reasonably straight lines.

Aerial photographs should be carefully ex-amined for possible fault zones when neither

geologic maps nor top ographic maps offer anyclues. An important feature of a fault zoneslide that may be detected from aerial photog-raphy is the slick, shiny surface caused by th eintense heat d eveloped by friction on slidingsurfaces within the fault zone.

Field personnel shou ld be alert for on-the-ground evidence of faulting when neithergeologic maps nor topographic maps p rovidedefinite clues to th e location of faults.

Bedding Plane SlopeThere are man y locations wh ere sedi-mentary or metamorp hic rocks have beenwarped or tilted and the bedding planes maybe steeply sloped . If a road is planned for suchan area, it is important to determine the slopeof the bedding p lanes relative to the groundslope. In areas w here bedd ing planes are ap-proximately parallel to the slope of thesidehill, road excavation may remove enough

support to allow large chunks of rock and soilto slide into the road (see Figure 10-1). Ifpreliminary surveys reveal that these condi-tions exist, then the rou te may need to bechanged to the opp osite side of the drainagearea or ridge where the bedd ing planes slope

into the hillside.

SOIL MECHANICSThe two factors that have the greatest ef-

fect on slope stability are-

Slope gradient.Groundwater.

Generally, the greater the slope grad ient andthe more ground water p resent, the less stablewill be a given slope regardless of the geologicmater ial or the soil type. It is absolutely es-

sential that engineers engaged in locating,designing, constructing, and maintainingroads understand why slope gradient andgroundwater are so important to slopestability.

Slope GradientThe effects of slope grad ient on slope

stability can be understood by discussing thestability of pu re, dry san d. Slope stability in

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sand depends entirely on frictional resistanceto sliding. Frictional resistance to sliding, inturn, depends on

The slope gradient that affects theportion of the w eight of an object thatrests on the su rface.

The coefficient of friction.

Normal Force. The fraction of the weight ofan object that r ests on a su rface is known asthe normal force (N) because it acts normal to,or perpendicular to, the surface. The normalforce changes as the slope of the surface chan-ges.

The up per curve inFigure 10-2, page 10-4,show s how the grad ient of a surface changesthe normal force of a 100-pound block restingon the surface. When the slope gradient iszero, the entire w eight of the 100-pou nd blockrests on the surface and the normal force is100 pound s. When the sur face is vertical,there is no weight on the surface and the nor-mal force is zero. The coefficient of frictionconverts the norm al force to frictional resis-tance to sliding (F). An average value for thecoefficient of friction for sand is about 0.7.This means that the force required to slide ablock of sand along a su rface is equal to 0.7times the normal force.

The lower curve in Figure 10-2, page 10-4show s how the frictional resistance to slidingchanges with slope gradient. The lower curvewas d eveloped by mu ltiplying the values ofpoints on the u pp er curve by 0.7. Therefore,wh en the slope gradient is zero, the normalforce is 100 pounds, and 70 pounds of force isrequired to slide the block along the surface.When the slope gradient is 100 percent, thenorm al force is 71 pounds (from point 3 on theup per curve), and 50 pound s (or 71 poun ds x0.7) is required to slide the block along thesurface (from point 3 on the lower curve).

Dow nslope Force. The portion of the weightthat acts downslope provides some of the forceto overcome frictional resistance to sliding.The downslope force, sometimes known as thed riving force, also depend s on the slopegradient and increases as the gradient in-creases (see Figure 10-3 , page 10-5).

Obviously, when the slope grad ient becomessteep enough, the driving force exceeds thefrictional resistance to sliding an d the blockslides.

Figure 10-4, page 10-6, shows the curve forfrictional resistance to sliding (fromFig-ure 10-2, page 10-4)superimp osed on thecurve of the dr iving force (fromFigure 10-3,

page 10-5). These two curves intersect at 70percent (35 degree) slope gradient. In this ex-ample, this means that for slope gradientsless than 70 percent, the frictional resistanceto sliding is greater than the downslope com-ponent of the weight of the block and the blockremains in p lace on the su rface. For slopegrad ients greater than 70 percent, the blockslides because th e d riving force is greaterthan the frictional resistance to sliding. This

discussion has been confined to the case ofpu re, dry sand , a case which is seldom foundin soils, but the principles of the effects ofslope grad ient and frictional resistance tosliding apply to any dry soil.

Shear Strength. A block of uniform soilfails, or slides, by shearing. That is, one por-tion of the block moves past another portionin a p arallel d irection. The su rface alongwhich this shearing action takes p lace iscalled the shear plane, or the plane of failure.The resistance to shearing is often referred to

as shear strength. Pure sand develops shearstrength by frictional resistance to sliding;how ever, pure clay is a sticky substan ce thatdevelops shear strength because the in-divid ual p articles are cohesive. The p resenceof clay in soils increases the shear streng th ofthe soil over that of a pu re sand because of thecohesive na ture of the clay.

A dry clay has considerable shear strengthas demonstrated by the great force requiredto crush a clod with th e fingers. However, as

a d ry clay absorbs water, its shear strengthdecreases because water films separate theclay particles and reduce its cohesivestrength. The structure of the clay particledetermines how mu ch water w ill be absorbedand , consequ ently, how mu ch the shearstrength will decrease upon saturation.There are som e clays, such as illite and


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kaolinite, that provide relative stability tosoils even when saturated However, asaturated montmorillonite clay causes a sig-nificant decrease in slope stability.Saturated illite and kaolinite clays haveabout 44 percent of their total volum e oc-

cupied by water comp ared to abou t 97 percentfor a saturated mon tmor illonite clay. Thisexplains why montrnorillonite clay has such ahigh shrink-swell potential (large change involume from wet to d ry) and saturated claysof this type have a low shear strength . Thu s,the type of clay in a soil has a significant effecton slope stability.

Granitoid rocks tend to weather to sandysoils as the weathering process destroys thegrain-to-grain contact that holds the mineralcrystals together. If these soils remain inplace long enough, they eventually develop asignificant amount of clay. If erosion removesthe weathered material at a rapid rate, theresulting soil is coarse-textured and behavesas a san d for p ur poses of slope stabilityanalysis. Many soils with a significant claycontent have developed from granitoidmaterial. They have greater shear strengthand support steeper cut faces than agranitoid-derived soil with little clay.

The relative stability among soils depends

on a comparison of their shear strength andthe dow nslope component of the weight of thesoil. For two soils developed from the samegeologic material, the soil with th e higher p er-centage of illite or kaolinite clay has greatershear strength than a soil with a significantamount of montmorillonite clay.

GroundwaterA common observation is that a hillslope or

the side slopes of a drainageway may be per-fectly stable during the summer but may slideafter the winter rains begin. This seasonal

change in stability is due mainly to thechange in the amou nt of water i n the pores ofthe soil. The effect of groun dw ater on slopestability can best be understood by again con-sidering the block of pu re sand . Frictionalresistance to sliding in dry sand is developedas the produ ct of the coefficient of friction andthe normal force acting on th e surface of the

failure plane. A closeup view of this situationshows that the ind ividu al sand g-rains are in-terlocked, or jammed together, by the weightof the sand . The greater th e force that causesthis interlocking of sand grains, the greater isthe ability to resist the shear force that is

caused by the dow nslope component of thesoil weight. As groun dw ater rises in thesand, the w ater redu ces the n ormal force be-cause of the buoyant force exerted on eachsand grain as it becomes subm erged.

Uplift ForceThe uplift force of the ground water r edu ces

the interlocking force on the soil particles,wh ich redu ces the frictional resistance to slid-ing. The uplift force of groun dw ater is equalto 62.4 pou nd s per foot of water in th e soil.The effective normal force is equal to the

weight of the soil resting on the su rface minusthe uplift force of the groundwater.

The following example illustrates the cal-culation of the effective normal force.

If 100 pounds of sand rests on ahorizontal surface and contains 3 in-ches (or 0.25 foot) of groundwater,then the effective normal force is 100- (62.4 x 0.25) = 100 - 15.6 or 84.4pounds. This shows how groundwaterreduces frictional resistance to slid-

ing.The frictional resistance to slid ingwith this groundwater condition is84.4 x 0.7 (average coefficient of fric-tion for sand), which equals 59.1pounds.As a comparison, for 100 pou nd s ofdry sand on a horizontal surface thefrictional resistance to sliding is 100 x0.7 or 70 pounds.

The following examples further emphasize

how the presence of ground water candecrease slope stability by reducing the fric-tional resistance to sliding:

First, a layer of dry sand 5 feet thickis assumed to weigh 100 pounds perfoot of depth. The downslope com-ponent of the dry weight and the fric-tional resistance to sliding for dry


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sand was calcu lated for various slopegradients as in Figure 10-2, page 10-4,andFigure 10-3, page 10-5.Next, 6 inches of ground water is as-sumed to be present and the frictionalresistance to sliding is recalculated,

taking into account the uplift force ofthe ground water. The results of thesecalculations are show n in Figure 10-5.

N ote: In a d ry con dition , slidin g occursw hen the slope grad ient exceeds 70 per-cent. With 6 inches of ground w ater, thesoil sl ides wh en th e slope gradient ex-ceeds 65 percent.

For a compar ison, assume a d ry sandlayer only 2 feet thick that weighs100 pounds per foot of depth . Again,

assume 6 inches of groundw ater andrecalculate the downslope componentof the soil weight and the frictionalresistance to sliding with and withoutthe ground water (seeFigure 10-6).

Note: With 6 inches of ground water,this thin layer of soil slides wh en theslope grad ient exceed s 58 percent.

These examples demon strate that the thin-ner soil mantle has a greater potential forsliding und er the same ground water condi-

tions than a thicker soil mantle. The 6 inchesof groun dw ater is a greater proportion of thetotal soil thickness for th e 2-foot soil than forthe 5-foot soil, and the ratio of up lift force tothe frictional resistance to sliding is greaterfor the 2-foot soil. A pu re sand was used inthese examp les for the sake of simp licity, butthe p rinciples still apply to soils that containvarying am ounts of silt and clay together withsand.

Although add ing soil may d ecrease the ef-

fect of up lift force on the frictional resistanceto sliding, it is dangerous to conclude thatslopes can be made stable solely throu gh thisapproach. The added soil reduces uplift force,but it may increase another factor that inturn decreases frictional resistance, resultingin a slope failure. Decreasing the uplift forceof water can be best achieved throu gh a

properly d esigned ground water control sys-tem.

Seep age ForceThere is still another way that ground-

water contributes to slope instability, and

that is the seepage force of groun dw ater as itmoves downslope. The seepage force is thedrag force that moving water exerts on eachindividu al soil particle in its path. Therefore,the seepage force contributes to the drivingforce that tend s to move masses of soildow nslope. The concept of the seepage forcemay be visualized by noting how easily por-tions of a coarse-textured soil may bedislodged from a road cut bank w hen the soilis conducting a relatively high volume ofgroundwater.

SLOPE FAILURESlope failure includes a ll mass soil move-

ments on

Man-made slopes (such as road cutsand fills).

Natural slopes (in clear-cut areas orundisturbed forest).

A classification of slope failure is useful be-cause it provides a common terminology, andit offers clues to the type of slope stability

problem that is likely to be encountered.Types of slope and road failures areremarkably consistent with soils, geologicmaterial, and top ography. For example, fast-moving debris avalanches or slides develop inshallow, coarse-textured soils on steephillsides; large, rotational slumps occur indeep, saturated soils on gentle to moderateslopes.

Rockfalls and Rockslid esRockfalls and rockslides u sually originate

in bedded sediments, such as massivesandstone, where the beds are und ercut bystream erosion or road excavation. Stabilityis maintained by the

Comp etence of the rock.Frictional resistancethe bedding planes.

to sliding along


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These factors are par ticularly imp ortantwhere the bedding planes dip down slopetoward a road or stream. Rockslides occursud denly, slide w ith great speed, and some-times extend entirely across the valleybottom. Slide debris consists of fractured

rock and may include some exceptionallylarge blocks. Road locations through areaswith a potential for rockslides should be ex-amined by specialists who can evaluate thecomp etence of the rock and determine the dipof the bedding p lanes.

Debris Avalanches and Debris FlowsThese two closely related types of slope

failure usually originate on shallow soils thatare relatively low in clay content on slopesover 65 percent. In sou theast Alaska, the US

Forest Service has found that debrisavalanches d evelop on slopes greater than 65percent on shallow, gravelly soils and thatthis type of slope failure is especially frequenton slopes over 75 percent.

Debris avalanches are the rapid downslopeflowage of masses of loose soil, rock, andforest debris containing varying amounts ofwater. They are like shallow land slidesresulting from frictional failure along a slipsurface that is essentially parallel to thetopographic surface, formed where the ac-cumulated stresses exceed the resistance toshear. The detached soil mantle slidesdow nslope above an imp ermeable bound arywithin the loose debris or at the u nweatheredbedrock surface and forms a disarrangeddep osit at the b ase. Down slope, a d ebrisavalanche frequently becomes a debris flowbecause of substantial increases in water con-tent. They are caused most frequently whena sud den influx of water red uces the shearstrength of earth material on a steep slope,and they typically follow heavy rainfall.

There are two situations where these typesof slope failure occur in areas with shallowsoil, steep slopes, and heavy seasonal rainfall.

The first situation is an area w here streamd evelopm ent and geologic erosion hav eformed high ridges w ith long slopes and

steep, V-shaped drainages, usually in bedd edsedimentary rock. The grad ient of many ofthese streams increases sharply from themain str eam to the ridge; erosion has createdheadwalls in the upper reaches. The bowl-shaped headw all region is often the junction

for tw o or m ore intermittent stream channelsthat begin at the ridgetop. This leads to aquick rise in grou nd w ater levels du ringseasonal rains. Past debris avalanches mayhave scoured round-bottom chutes, ortroughs, into the relatively hard bedrock.The headwall region may be covered withonly a shallow soil mantle of precariousstability, and it may show exposed bedrock,wh ich is often dark w ith ground waterseepage.

The second situation with a high potentialfor debris avalanches and flows is wh ere ex-cavated material is sidecast onto slopesgreater than 65 percent. The sidecastmaterial next to the slope maintains stabilityby frictional resistance to sliding a nd bymechanical sup port from brush an d stum ps.As more material is sidecast, the bru sh andstump s are bu ried and stability is maintainedsolely by frictional resistance to sliding .Since there is very little bonding of thismaterial to the underlying rock, the entireslope is said to be overloaded. It is quite com-mon for new road fills on steep overloadedslopes to fail when the seasonal rainssaturate this loose, unconsolidated material,causing d ebris avalanches and flows. Und erthese circumstances, the road fill, togetherwith a p ortion of the und erlying naturalslope, may form the debris avalanche (seeFigure 10-7).

Debris avalanches and debris flows occursuddenly, often with little advance warning.There is practically nothing that can be don e

to stabilize a slope that shows signs of an im-pending debris avalanche. The best possibletechnique to use to prevent these types ofslope failures is to avoid

Areas with a high potential for d ebrisavalanches.Overloading steep slopes with exces-sive sidecast.


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Engineers should learn the vegetative andsoil indicators of this type of unstable terrain,especially for those areas w ith high seasonalgroundwater levels.

If un stable terrain mu st be crossed byroads, then rad ical changes in road grade androad w idth m ay be required to minimize sitedisturbance. Excavated material may needto be hauled away to keep overloading of un-

stable slopes to an absolute minimum. Thelocation of safe d isposal sites for th is materialmay be a serious problem in steep terrainwith sharp ridges. Site selection w ill require

just as much attention to the principles ofslope stability as to the location and construc-tion of the rem ainder of the road.

Slumps and EarthflowsSlumps and earthflows usually occur in

deep , mod erately fine- or fine-textured soilsthat contain a significant amount of silt

and / or clay. In this case, shear strength is acombination of cohesive shear strength andfrictional resistance to sliding. As n oted ear-lier, ground water not only reduces frictionalresistance to shear, but it also sharplyreduces cohesive shear strength. Slumps areslope failures where one or m ore blocks of soil

hav e failed on a hem ispher ical, or bow l-shap ed, slip su rface. They may show varyingamoun ts of backward rotation into the hill inaddition to downslope movement (see Figure10-8, page 10-12). The lower part of a typicalslump is displaced upward and outward like abulbous toe. The rotation of the slum p blockusu ally leaves a dep ression at the base of themain scarp. If this depression fills with waterduring the rainy season, then this feature is

known as sag pond . Another feature of largeslum ps is the hum mocky terrain, composedof many depressions and uneven ground thatis the result of continu ed earth flow after theoriginal slump . Some areas that are und er-lain by particularly incompetent material,deeply weathered and subject to heavy winterrainfall, show a characteristically hum mockyappearance over the entire landscape. This

jumbled and rumpled appearance of the landis known as melange terrain.

Depressions and sag ponds allow winterrains to enter the ground water reservoir,reduce the stability of the toe of the slump,and promote further downslope movement ofthe entire mass. The m ature timber th atusually covers old slumps often containsjackstrawed , or crazy, trees that lean at


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many different angles within the stand. Thisindicates unstable soils and actively movingslopes (see Figure 10-9).

There are several factors affecting slumpsthat need to be examined in d etail to und er-stand how to prevent or remedy this type ofslope failure. The block of soil that is subjectto slump ing can be considered to be resting ona potential failure surface of hemisphericalshape (seeFigure 10-10). The block is moststable when its center of gravity is at itslowest p osition on this failur e surface. Whenthe block fails, its center of gravity is shiftedto a lower, more stable position as a result ofthe failure. Added weight, such as a road fill,at the head of a slump shifts the center ofgravity of the block to a higher, more unstableposition and tends to increase the potentialfor rotation. Similarly, removing weight fromthe toe of the slump, as in excavating for aroad , also shifts the center of gravity of theblock to a higher position on the failure sur-

face. Therefore, load ing the h ead of a slumpand / or unloading the toe will increase thepotential for further slumping on short slopes(seeFigure 10-11). The chance of slumpingcan be reduced by shifting the center ofgravity of a potential slump block to a lowerposition by following the rule: Unload thehead and load the toe.

If it is absolutely n ecessary to locate a roadthrough terrain w ith a potential for slump-ing, there are several techniqu es that may beconsidered to help prevent slum ps andearthflows. They are

Improve the surface d rainage.Lower the ground water level.Use rock riprap, or buttresses, to pro-vide support.Install an interceptor drain.Compact fills.

Surface Drainage. Improving the surfacedrainage is one of the least expensive andmost effective techniques, but it is often over-looked. Sag ponds and depressions can beconnected to the nearest stream channel withditches excavated by a bulldozer or a grader.Figure 10-12, page 10-14, shows the theoreti-cal effect on the grou nd water reservoir of a

surface d rainage project. Improved drainageremoves surface water quickly, lowers thegroundwater level, and helps stabilize theslump.

Groundw ater Level. Low er the ground -water level by means of a perforated pipe thatis augered into the slope at a slight u pw ard


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angle. These drains are usually installed inroad cutbanks to stabilize areas above an ex-isting road or below roads to stabilize fills.Installing p erforated p ipe is relatively expen-sive, and there is a risk that slight shifts inthe slump mass m ay render th e pipe ineffec-tive. In addition, periodic cleaning of thesepipes is necessary to pr event blockage byalgae, soil, or iron deposits.

Rock Riprap, or Buttresses. Stabilize ex-

isting slumps and prevent potential slumpsby using rock riprap, or buttresses, to providesup port for road cuts or fills (see Figure 10-13,

page 10-15). Heavy rock riprap replacesthe stabilizing weight that is by excavationdu ring road construction (see Figure 10-11).Another feature of riprap is that it is porousand allows ground water to drain out of the


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slump material while providing supp ort forthe cut slope.

Interceptor Drain. Install an interceptordrain to collect groundwater that is movinglaterally downslope and und er the road,saturating the road fill. A backhoe can beused to install interceptor drains in the d itchalong an existing road .Figure 10-14 shows asamp le installation.

Fills. Comp act fills to redu ce the risk of roadfailure when crossing small drainages. Comp-action increases the density of the material,reduces the pore space, and thereby reducesthe adverse effect of ground water. The foun-dation material under the proposed fill


should be evaluated as part of the designprocess to determine if this material will sup-port a compacted fill without failure. Roadsmay often be built across gentle slopes of in-comp etent material with a high ground watertable by overexcavating the material, placinga thick blanket of coarse material, then build-ing the road on the blanket (seeFigure 10-15,

page 10-16). The coarse rock blanket d is-tributes the w eight of the roadw ay over alarger area an d provides better drainage forgroundwater under the road.

Soil CreepMany of these slope failures may be

preceded and followed by soil creep, a rela-tively slow-moving type of slope failure. Soil

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creep m ay be a continuou s movement on theorder of less than 1 foot per decade. The in-d icators of soil creep ma y be subtle, but youmust be aware of the significance of this typeof slope failure. Soil creep, at an y moment,may be immeasurable; however, when the ef-fect is cumulated over many years, it cancreate stresses within the soil mantle that

may ap proach the limit of frictional resis-tance to sliding and/ or the cohesive shearstrength along a potential shear failure sur-face.

Soil creep is particularly treacherou s inconjunction with d ebris avalanches. Thebalance between stability and failure may beapproached grad ually over a nu mber of yearsuntil only a heavy seasonal rain or a minordisturbance is necessary to trigger acatastrophic slope failure. Soil creep alsobuilds up stresses in p otential slum ps so th at

even moderate rainfall may start a slowearthflow on a p ortion of the slope. Depend -ing on the particular cond itions, minormovem ent may temp orarily relieve thesestresses and create sags or bu lges i n the slope;or it may slightly steepen the slope and in-crease the potential for a major slump duringthe next heavy rain. The point to rememberis that soil creep is the process that slowlychanges th e balance of forces on slopes. Eventhough an area may be stable enough towithstand high seasonal groun dw ater levelsthis year, it may not be able to 5 years fromnow.

STABLE SLOPE CON STRUCTIO NIN BEDDED SEDIMENTS

Construction of stable roads requires notonly a basic understanding of regional geol-ogy and soil mechanics but also specific,detailed inform ation on th e characteristics of

soils, ground water, and geology where theroad is to be built. This following paragraphspresent techniques on road location an d thesoil and vegetative indicators of slope in-stability and high grou nd water levels.

Bedd ed sediments vary from soft siltstone

to hard, massive sandstone. These differentgeologic ma terials, together w ith geologicprocesses and the effect of climate acting overlong periods of time, determine slopegrad ient, soil, and the rate of erosion. Thesefactors also determine the particular type ofslope stability problem that is likely to be en-countered. There are four slope stabilityproblems associated with d istinctive siteswithin the bedd ed sediments. They are

Sand stone - Type I.Sandstone - Type II.Deeply weathered siltstone.

Sandstone adjoining ridges of igneousrock.

Sandstone - Type IType I sites are characterized by sh arp

ridges with steep slopes that may show auniform gradient from near the ridgetop tothe valley bottom. The landscape is sharplyd issected by nu merou s stream chann elsthat may become extremely steep asthey approach the ridgetop. Headwalls(bowl-shaped areas with slope grad ients often

100 percent or greater) may be p resent in theup per r eaches of the drainage. The head wallis usually the junction for several intermit-tent streams tha t can cause sharp rises in thegroun dw ater levels in th e soil mantle du ringwinter storms. It is quite comm on to n otegroun dw ater seepage on exposed bedrock inthe headwall even du ring the summer. Fig-ure 10-16 shows a block diagram that


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illustrates the features of Type I sites. Thisarea was taken from the topograp hic mapof the upper Smith River in Oregon (Figure10-17, page 10-18).

The soils on th e most critical por tions of

Type I sites are coarse-textured and shallow(less than 20 inches to bedrock). These soilsare considered to be un stable on slopesgreater than 80 percent. In areas wheregroundwater is present, these soils are con-sidered to be unstable on slopes considerablyless than 80 percent.

Debris avalanches and debris flows ar e themost common slope failure on Type I sites,and the headwall region is the most likelypoint of origin for these failures. Road con-struction through headwalls causesun avoidable sidecast. The p robability is highfor even minimum amou nts of sidecast to

overload slopes with marginal stability and tocause these slopes to fail. Observers oftencomment on the stability of full bench roadsbuilt through h eadw alls without r ealizingthat debris avalanches may have occurredduring construction before any traffic movedover the road.


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Unstable-Slope Indicators. There are cer-tain indicators of unstable slopes in Type Isites that may be used du ring road location.They are

Pistol-but ted trees (see .Figure 10-18).Slid ing soil or d ebris or active soil

creep caused these trees to tipdow nslope while they were small. Asthe tree grew, the top regained a ver-tical posture. Pistol-butted trees area good ind icator of slope instabilityfor areas w here rain is the major com-ponent of winter precipitation; how-ever, deep, heavy snow p acks at highelevations may also cause this samedeformation.Tipped trees (see Figure 10-19).These trees have a sharp angle in thestem.

This indicates that the treegrew straight for a number of yearsuntil a small shift in the soil mantletipped the tree. The angled stem isthe result of the recovery of verticalgrowth.

Tension cracks (seeFigure 10-20).Soil creep builds u p stresses in thesoil man tle that are sometimes re-lieved by tension cracks. These fea-tures may be hidd en by vegetation,but they d efinitely indicate active soilmovement.

Road-Location Techniques. Techniquesfor proper road location on Type I sites in-clud e the following:

Avoid headwall regions. Ridgetop lo-cations are preferred rather thancrossing through headwall regions.Roll the road grade. Avoid head wallsor other unstable areas by rolling theroad grad e. Short, steep pitches ofadverse and favorable grade may beincluded,

Construction Techniques. Consider thefollowing techniques when roads must be con-structed across long, steep slopes or aboveheadwall regions where sidecast must beheldto a minimum:


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Redu ce the road width . This mayrequire a small tractor w ith a m orenarrow blade (for example, a D6) forconstruction. A U-shaped blade re-sults in less sidecast than a straightblade, possibly because of better con-trol of loose material.Control blasting techniques. Thesetechniques may be used to redu ce over-breakage of rock and reduce the

amou nt of fractured material that isthrown out of the road right-of-wayand into stream channels.Remove material. Hauling excavatedmaterial away from the steepestslopes may be necessary to avoidoverloading the lower slopes.

Select safe disposal sites. Disposalsites for excavated material should bechosen w ith care to avoid overloadinga natu ral bench or spur ridge, causingslope failure (see Figure 10-21, page10-20). The closest safe disposal sitemay be a long distance from the con-struction site but the additional haul-ing costs must be w eighed against thedamage caused by failure of a closerdisposal site with a higher probabilityof failure.Fill sadd les. Narrow sadd les may beused to hold excess material by firstexcavating bench road s below and oneach side of the saddle. The saddlemay th en be flattened and the loose

Figure 10-20. Tension cracks .


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material that rolls down slope w ill becaught by the benches. Excavatedmaterial may be comp acted on th eflattened ridge to buildup the grade.Choose the correct culvert size. A cul-vert should carry the maximu m es-timated flow volume for the d esignstorm.Protect slopes. Culverts must not dis-charge drainage water onto the baseof a fill slope. Culverts should eitherbe designed to carry water on the

natural grade at the bottom of the fillor dow nspouts or half-round culvertsshould be used to condu ct water fromthe end of shorter culverts down thefill slope to th e natu ral channel.

Sand stone - Type IIType II sites have slopes w ith grad ients

that ran ge from less than 10 percent up to 70or 80 percent. The longer slopes may bebroken by benches and have roun der ridges,fewer drainages, and gentler slope grad ients

than those on Type I sites. Headwalls arerare, and small patches of exposed bedrockare only occasionally found on the steeperslopes.

The soils on the gentle slopes developedover many centuries and are deep (often


greater than 40 inches), with a clay content ashigh as 50 to 70 percent. The soils on thesteeper slopes may be as deep as 40 inches,but the bedrock is fractured and weathered sothere is a gradual transition from the soil intothe massive bedrock. It is these factors ofdeeper soils, higher clay conten t, gentler

slopes, and a gradu al transition to bedrockthat m akes this terrain more stable than theterrain on Type I sites.

The factors th at characterize Type II sitesalso cause this terrain to have m ore slopefailures du e to slump s and earthflows. Themost unstable portions of Type II sites are thesteep, concave slopes at the heads ofdrainages, the edge of benches, or the loca-tions where ground water tends toaccumulate. Road failures frequently involve

poorly consolidated or poorly dr ained roadfills and em bankm ents greater th an 12 to 15feet on any of the red clayey soils. Soil creepalso creates tension w ithin the clayey soils atthe convex ridge nose where slope gradientsmay be on fy 50 percent. Excavation for roadsat these points of sharp slope convexity some-times causes failure of the emban kmen t.

Unstable-Slope Indicators. Vegetative in-dicators of unstable portions of the landscapeinclud e matu re trees that tip or lean as a

result of minor earth flow or soil creep on thesteeper slopes. Tipped or leaning trees mayalso be found on poorly drained soils adjacentto the stream chann els. Actively movingslopes may sh ow ten sion cracks, particularlyon the steeper slopes.

There are several good ind icators that m aybe used to determine the height thatground water may rise in the soil and roughlyhow long during the year that the soilremains saturated. Iron compounds withinthe soil profile oxidize and turn rusty red or

bright orange and give the soil a mottled ap-pearance when the groundwater rises andfalls intermittently during the winter. Thedepth below the soil surface where these mot-tles first occur indicates the averagemaximum height that this fluctuating watertable rises in the soil. At locations w here the

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water table remains for long periods d uringthe year, the iron comp oun ds are chem icallyreduced and give the soil profile a gray orbluish-gray appearance. The occurrence ofthese gleyed soils indicates a soil that issaturated for m uch of the year. Occasionally,mottles may ap pear above a gleyed su bsoil,

wh ich ind icates a seasonally fluctuatingwater table above a subsoil that is subject toprolonged saturation. Engineers should beaware of the significance of mottled andgleyed soils that are exposed during road con-struction. These ind icators give clues to theneed for d rainage or extra attention concern-ing the suitability of a subsoil for found ationmaterial.

R oad-Location Techniques. Techniquesfor locating stable road s on Type II sites in-clud e the following

Avoid steep concave basins. Do notlocate roads through these areaswh ere stability is questionable, as in-dicated by vegetation an d topography.Ridgetop locations are preferred.Choose stable benches. Benches mayoffer an opportunity for location ofroads and landings, but these benchesshould be examined carefully to seethat they are supported by rock andare not ancient, weathered slum pswith marginal stability.Avoid cracked soil. Avoid locating aroad around convex ridge noses orbelow the edge of benches where ten-sion cracks or catsteps ind icate a h ighprobability for embankment failure.

Construction Techniques. Certain designand construction practices should be con-sidered wh en building roads in this terrain.

Avoid overconstru ction. If it is neces-sary to build a road across steepdr ainages, avoid overconstruction and

haul excess material away to avoidoverloading the slopes.Avoid high cut embankm ents. An en-gineer or soil scientist may be able tosuggest a maximu m h eight at theditch line for the particu lar soil andsituation. A rule-of-thumb estimate

for maximu m height for a cut bank indeep, clayey soils is 12 to 15 feet.Pay special attention to fills. Fills ofclayey mater ial over steep stream cross-ings may fail if the material is notcompacted and if groundwater sat-urates the base of the fill. Fill failures

in this wet, clayey material on steepslopes tend to move initially as aslump, then may change to a mudflowd own the dr ainage. To avoid this,compact the fill to accepted engineer-ing standards, paying special atten-tion to proper lift thicknesses, mois-ture content, and foun da tion condi-tions. Also, design drainage features,where necessary, to control ground-water in the base of a fill. For ex-ample, consider either a perforatedpipe encased in a crushed rock filteror a blanket of crushed rock un derthe entire fill.

Deep ly Weathered SiltstoneThis stability problem originates in

siltstone that is basically incompetent andeasily weathered. Slumps and earthflows,both large and small, are very common whenthis material is subjected to heavy winterrainfall. The land scape may exhibit a benchyor hu mm ocky appearance. Slopes withgradients as low as 24 percent may be con-sidered unstable in deeply weatheredsiltstone with abun dant water.

Unstable-Slope Indicators. Vegetativeand topograp hic indicators of slope instabilityare numerous. Large patches of plants asso-ciated with set soils indicate high ground-water levels and impeded drainage. Conifersmay tip or lean du e to earthflow or soil creep.Slumps cause numerous benches, some ofwhich show sag ponds. Blocks of soil may sagand leave large cracks, which gradu ally fill in

with d ebris and living vegetation. The sharpcontours of these features soften in time untilthe cracks appear as blind drainages or sec-tions of stream channel that are blocked atboth end s. The cracks collect w ater, keep th egroundwater reservoir charged, and con-tribute to active soil movement.


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Road-Location Techniques. Techniquesfor locating stable roads th rough terrain thathas been derived from deeply weatheredsilt-stone include the following

Check for ind icators of ground water.Avoid locating roads through areaswhere groundwater levels are high

and where slope stability is likely tobe at its w orst. Such locations m ay beindicated by hydrophytes, tipped orleaning trees, and mottled or gleyedsoils.Consider r idges. Ridgetop locationsmay be best because ground waterdrainage is better there. Also, the un-derlying rock may be harder and m ayprovide more stable roadbuildingmaterial than weathered siltstone.Ensure adequate reconnaissance.

Take pains to scout the terrain awayfrom the p roposed road location,using aerial photos and ground recon-naissance to be sure that the line d oesnot run through or under an ancientslump that may become unstable dueto the road construction.

Construction Techniques. Special roaddesign and construction techniques for thistype of terrain may include the following

Drainage ditches. Every effort shou ld

be mad e to improve d rainage, bothsurface and subsu rface, since groun d-water is the major factor contributingto slope instability for this material.Sag pond s and bogs may be d rainedwith ditches excavated by tractor orwith ditching dynamite.Culverts. Extra culverts should beused to prevent water from pendingabove the road and saturating theroad p rism and adjacent slopes.Road d itches. They should be careful-

ly graded to provide plenty of fall tokeep w ater moving. A special effortshould be made to keep ditches andculverts clean following con-struction.


Sandson e Adjoinin g Ridgesof Igneous Rock

This slope stability problem in bedd ed sed i-ments is caused by remnants of sandstoneadjoining r idges of igneous rock. As a generalrule, any contact zone between sedimentarymaterial and igneous material is likely tohave slope stability problems.

Unst able-Slope Indicators. The igneousrock may have caused fracturing and partialmetamorphosis of the sedimentary rock atthe time of intrusion. Also, water is usuallyabundant at the contact zone because the igneous material is relatively impermeablecompared to the sediments; therefore, thesedimentary rock may be deeply weathered.

R oad-Locat ion Techniques. Special roadlocation techniques for this type of slopestability problem includ e the following

Pay attention to the contact zone.Examine the terrain carefully on theground and on aerial photos todetermine if the mass of sandstone islarge or small relative to the igneousrock mass. If the sand stone is in theform of a relatively large spur ridge,then th e contact zone d eserves specialattention. The contact zone should becrossed as high as possible wh eregroundwater accumulation is at aminimum. Elsewhere on the r idge of

sandstone, the stabilty problems arethe same as for Type I or Type IIsandstone.Consider an alternative location. Ifthe remnant of sandstone is relativelysmall, such as a ridge nose, then theentire mass of sandstone may becreeping rapidly enough to be con-sidered u nstable and the road shouldbe located above this material in themore stable igneous rock.

Construction Techniques. Design andconstruction techniques to be considered areas follows:

Avoid high embankments. The sedi-mentary rock in the contact zone is

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likely to be fractured and may besomewhat metamorphosed as a resultof the intrusion of igneous rock. In ad -dition, the accumu lation of ground -water is likely to have caused ex-tensive weathering of this material.The road cut height at the d itch line

should be kept as low as p ossiblethrough this zone. Sup port by rock

riprap may be necessary if the cutembankment m ust be high.Ensure good d rainage. It is good apractice to put a culvert at the contactzone with good gradient on theditches to keep the contact zone welldrained. Other drainage measures,

such as drain tile or perforated pipe,may be necessary.

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