ry GEOTTCHnIICS 1 CIV3lO

Stabilify of Slopes: When a difiference of level exists between two surfaces gravitationalforces tend to cause movement from the higher to ttre lower level. Unless the shearingresistance within the material is sirfficient to witlstand these forces, the movement takesplace.Movements of sloped soil masses canbe classified into broad categories, depending on, ,

the t5pe of motion relative to the adjagent or underlying soil.Slide refers to the occrurence where the moving mass is rather well'defined and

separated from the undetlying and adjacent soil by a plane where slippage result.The stippage plane or zone rgpresents the continuous surface where thE maximtrm shear

strargth of the soil has been reached, with the resultthat large displacement occurs.Slides are classified as rotational or translational in accord with the shape anddirectional movdment ofthe soil.Rotational slides are associated with natural slopes and. constructed embanlanents ofhomogenous materialspossessing eohesion; tre faihne srface is surve{ with the failedmass slumped inthe toe area of the original slope.Translational slides me associated with slopes of layered materials where the mechanism ,

of slippage occurs along a weak plane ot "d*

that fossess a downward. dip and incohesio:r less soil slopes,where a change in condition occurs.

;,1; IJnlike thgrotationalslide,,whosemov,ement,tends torcEaqewhenthemass reaches thei]'..s1u*pedpositioatne#iitio"*sIiae.H.coufiueovera1cing'dice....l'

Fundamentals of S lop e Stability AnalysisConventional approach: Conventional slope stability analysis investigates.the equilibriumof a mass of soil bounded below by an assumed potentiai slip surface and above by thesurface of the slope.Forces and moments tending to cause instability of the mass are compared to thosetending to resist instability.Suggested assumptions are made regarding the potential slip surface unJil the mostcritical surface- lowest factor of safety, is found.If the shear resistance of the soil along the slip surface exceeds that necessary to provideequilibrium, the mass is stable. ffthe shem resistance is inadequate, the mass is unstable.The stability or instability of the mass depends on:

o Its weightr The extemal forces acting on it such as surcharges or accelerations caused. by

dynamic loads.o The shear sFengths

. . Pore water pressrues along the slip surfacer The strength of any internal reinforcement crossing potential slip surface.

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The factor of safety: Conventional analysis procedures characterize the stability of aslope by calculating a factor of safety. The factor of safety is defined with respect to theshear strength ofthe soil as ratio of the available shear strength (s) to the shear strength

required for equilibrium (t) i.e. F:available shear strengtb/equilibrium shear stress:

If the shear strengt! is defined interms of effective sEesses, the factor of safety is

expressed as: F: c+(o1t)P0

For total sfresses, the factor of safety is expressed using the shear sfrength parameters in

terms of total stresses, i.s.p:cloffid '

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Limit of Equilibrium Method; the factor of safety is calculared usjng one or more of theequations of static equilibrium applied to a soil mass bounded by an assumed potentialslip surface and the surface of the slope.Slopes on Dry Cohesion Less Sand: a slope underlainby clean tky sand is stable

regardless of its heigh! provided the angle 9 S O , 6p=ft where p is the actralwp "'i ' 1:

slopeangleexistingorplanned- : , : , : , ' . : " : ':,

Typical method of slices

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{&) Scour by rivers and streams

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13.4 SOME CAUSES OF SLOPE FAILURE 569

Rainfall

(c) Rainfall fills crack and introducesseepage forces in the thin, weaksoil layer

Overloadlng at crest of slope

C + Critical state line for compression

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Crest

(a) Steepening of slope by erosion

{.r) Gravity and earlhquake lorces.

ffi#(e) Geological feature---toil stratification (fl Overloading at the crest ol the slope

(II RE!!rvolr stresser

During rapid drawdown the restralnlngwater force ls removed

F|GURE13.3 Some causes of slope failure.

(g) Excavation at toe ol the slope

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0) Rapid dtawdown 0J Groundvrater seepage

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e (v ?( o--a' 13.3 SOME TYPES OF SLOPE FAILURE 567

(a) Movemenl ol soil mass along a thinlayer of weak soil

(d) Slop€ slide ..

(e) Flow slide (0 Block slide

F|GUBEl3.2 Some common types of slope failure.

Slope failures depend on the soil {ype, soil stratification, gnrundwater, seepage, and the slopegcometry. We will introduce a few typcs of slope failure that are common in soils, Failure of aslope along a weak zone of soil is called a translationai slide (Fig. 13.2a). The sliding rnass can travellong distances before coming to rest. Translational slides are common in coarse-grainetl soils.

A common type of failure in homogeneous fine-grained soils is a rotational slide that has itspoint of rotation on an imaginary axis parallel to the slope. Three types of rotational failure oftenoccur. One type, called a base slide, occurs by an arc engulfing the whole slope. A soft soil layerresting on a stiff layer of soil is prone to base failure (Fig. 13.2b). The second type of rotationalfailurc is tlte toc slide, whcreby the failure surface passes through the toe of tlre slopc (Fig. 13.2c).l'he tltird type of rotational failure is the stope slicle, whereby the failure surface passes through theslope (Fig. 13.2d).

A flow slide occurs when internal and external conditions brce a soil to bbhave like a viscousfluid and flow down even shallow slopes, spreading out in several directions (Fig. i3.2e). The failuresurface is ill defined in flow slides. Multiple failure surfaces usually occur and change continuously asflow proceeds. Flow slides can occur in dry and wet soils.

Block or wedge slides occur when a soil mass is shattered along joints, seams, fissures, and weakzones by forces emanating from adjacent soils. The shattered mass moves as blocks and wedges downthe slope (Fig. 13.2f).

(b) Base slide

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7GEOTECHNICS 1 CIV 310

Stability of Earth Slopes

Many civil engineering projects are located on or near sloping ground, and thus arepotentially subject to various kinds of slope instability such as slides, flows, and falls.Slope failwes often produce extensive property damage, and occasionally resultin losg oflife.

civil errgineer$ u$e seveml q>ecial ternr when dericribing eirth slopes. Thesc i'clude the'faltowing (as sbown in Figrrrc f+.tji ..'" ""='

' f;[liffiA:re rho$e nradeby:&n exc'v'rion, T.hey e.'Fose na$rar grounrr dur was

c'EiItslbttes are,t'hose m-adp by plaeirrg a filt.' Nantruil slope' flrer iLq. fte nrr-rne impfiur, p*t of the nntunat ropogrnprry.' Ttie slitpe rqrro clcscribe* irs *tu"pn"s*, ;l k;r"yr;'-d#H horizonral;verlieal.F'or exarnpl*. a "ihrce ro one*.slo'pe (3:l j is inctinerr -rrhl;;;;rrrat ,o one verrieirt;'slopq sEeFer dr;an l:r. are ae*"rib,J *i"q .rr"*,ioo;.'..,.Lffi #. Trris norarion can.be ,esnf4sips ro ensineers used ; ;;Tii"q ;ifil;;;it:#; dre qusrjo'.ariry. $:1t:t-o-"9 i-n

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:r, ori zon tat ).- r'B xsF a!.rtope \I,:1 of tfpe-are .he poinrs wherel.riUfers€cm fla{gr groqn{.' ,Tl.re {, efirce js tge g.round,.iurfaceber*:*,n*oe .. ;f;ffifi#toe,of,{opa. . , :, : : , :' Ile'rlape hcig:ia"lr, is'tne a;r'rercn""iJli*uo,ion berweqn riie iii'orsrope arrcr [oeofslope'(i' e.' me as u rcd yrri.c*lrvr r; ; ds;;;*ut ;"G ffi ;'# oiour* *rop"r.' ff;fi:"r.|.}i,1il:-

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