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Chapter 4- Slope Stability

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CHAPTER

CHAPTER

4.0

4.0

SLOPE STABILITY 

SLOPE STABILITY 

BFC34402

BFC34402

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1.

1. FallFall. This is the detachment of soil. This is the detachment of soil and/or rock fragments that fall down a and/or rock fragments that fall down a slope.

slope. 2.

2. ToppleTopple. This is a forward rotation of. This is a forward rotation of soil and/or rock mass about an axis soil and/or rock mass about an axis  below

 below the the center center of of gravity gravity of of massmass  being displaced.

 being displaced. 3.

3. SlideSlide. This is the downward. This is the downward movement of a soil mass occurring on a movement of a soil mass occurring on a surface of rupture.

surface of rupture. 4.

4. SpreadSpread. This is a form of slide by. This is a form of slide by translation. t occurs by !sudden translation. t occurs by !sudden movement of water"bearing seams of movement of water"bearing seams of sands or silts overlain by clays or sands or silts overlain by clays or

4.

4.11 TYTYPEPES & S & CACAUSUSES ES OF OF SLSLOPOPE FE FAIAILLURUREE

%lope failures ma&or categories

%lope failures ma&or categories

Fall Fall Topple Topple Slide Slide Spread Spread Flow Flow

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1.

1. FallFall. This is the detachment of soil. This is the detachment of soil and/or rock fragments that fall down a and/or rock fragments that fall down a slope.

slope. 2.

2. ToppleTopple. This is a forward rotation of. This is a forward rotation of soil and/or rock mass about an axis soil and/or rock mass about an axis  below

 below the the center center of of gravity gravity of of massmass  being displaced.

 being displaced. 3.

3. SlideSlide. This is the downward. This is the downward movement of a soil mass occurring on a movement of a soil mass occurring on a surface of rupture.

surface of rupture. 4.

4. SpreadSpread. This is a form of slide by. This is a form of slide by translation. t occurs by !sudden translation. t occurs by !sudden movement of water"bearing seams of movement of water"bearing seams of sands or silts overlain by clays or sands or silts overlain by clays or

4.

4.11 TYTYPEPES & S & CACAUSUSES ES OF OF SLSLOPOPE FE FAIAILLURUREE

%lope failures ma&or categories

%lope failures ma&or categories

Fall Fall Topple Topple Slide Slide Spread Spread Flow Flow

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4.

4.11 TYTYPEPES & S & CACAUSUSES ES OF OF SLSLOPOPE FE FAIAILLURUREE

4.1.1

4.1.1 TyTypes opes of slopf slope faie failurelure

a)

a) TTranranslaslatiotional slidnal slide:e:

i.

i. FFaiailulurre ae alolon n a !a !eaea" #" #onone oe o$ s$ soioil.l. ii.

ii. SlidSlidin in %as%ass s an an tratra'el 'el lonlon d dististananeses (e$ore o%in to rest.

(e$ore o%in to rest. iii

iii.. Co%Co%%on %on in in oursourseerairained ned soilsoil.. ()

() FlFlo! o! slslidide:e:

i.

i. OOuurs rs !*!*en en ininteternrnal al & e& e+t+terernanall ondition $ore a soil to (e*a'e li"e a ondition $ore a soil to (e*a'e li"e a 'isous ,uid & ,o! do!n.

'isous ,uid & ,o! do!n. ii.

ii. -ul-ultitile le $ai$ailurlure se sur$ur$aeaes us usuasuall/ ll/ oourur

& *ane ontinuousl/ as ,o!

& *ane ontinuousl/ as ,o!

roeeds. roeeds.

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4.1 TYPES & CAUSES OF SLOPE FAILURE

4.1.1 Types of slope failure

) Rotational slide:

i. Co%%on in *o%oenous 0nerained

soil.

ii.  t/es o$ rotational slides2 1. 3ase slide:

 3/ an ar* enul0n t*e !*ole sloe.

 A so$t soil restin on sti la/er. 5. Toe slide:

 Failure sur$ae asses t*rou*

t*e toe o$ sloe . Sloe slide:

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4.1 TYPES & CAUSES OF SLOPE FAILURE

4.1.2 'auses of slope failure

1) Erosion 6a & () 5) Rain$all 6)

) Eart*7ua"e 6d)

4) 8eoloial $eatures 6e) 9) E+ternal loadin 6$)

) Constrution ati'ities

6)

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4.5 STA3ILITY OF I<FI<ITE SLOPES

4.2.1 (actor of safety

8enerall /:

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4.5 STA3ILITY OF I<FI<ITE SLOPES

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4.5 STA3ILITY OF I<FI<ITE SLOPES

4.2.2 nfinite slope" )ithout seepage

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4.5 STA3ILITY OF I<FI<ITE SLOPES

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4.5 STA3ILITY OF I<FI<ITE SLOPES

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4. -ET@O OF SLICES

4.3.1 (inite slope

)hat the different between finite and infinite slope+

a, )hen the value of the critical height of the slope -

cr 

, approaches the

height of the slope -, the slope generally may be considered finite.

 b, The surface of potential failure can be considered to be curved or plane

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4. -ET@O OF SLICES

4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis

odes of failure

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4. -ET@O OF SLICES

4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis

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4. -ET@O OF SLICES

4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis

0nalysis method 1 ass procedure"

*xamples

0 slope is to be cut to construct an embankment as shown in Figure 1. The mass of )1and )2 are 24.4 k and 43$.2 k respectively. The location of these masses are l 1 5 3.$m and l 2 5 $.3m . 6y using the mass procedure method or %wedish slip circle method

-i, Calculate the total moment of the driving force about point O.

(ii) Determine the factor of safety of the trial circle. (iii) Comment what happens to the calculated factor

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4. -ET@O OF SLICES

4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis

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4. -ET@O OF SLICES

4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis

0nalysis method 2 ethod of slices"

*xamples 1

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4. -ET@O OF SLICES

4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis

0nalysis method 2 ethod of slices"

*xamples 2

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

4.4.1 'ut %tability

 Cuttins are e+a'ated> !*ereas e%(an"%ents are (uilt.

 A derease in total stresses and also a dereasin in ore ressure  !*en e+a'ation o$ t*e re%o'al o$ soils.

 Seeae rei%e de'elos in t*e sloe a$ter onstrution !*i* it deends

on t*e drainae ondition and t*e er%ea(ilit/ o$ t*e soil.

 T*ere$ore> an inre%ent in ore ressure> onstant in total stress (ut  $alls steadil/ in eeti'e stress.

 3esides> eeti'e nor%al stress dereases too sine itBs a art o$

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

4.4.2 *mbankment stability

 3uilt (/ rollin or ot*er!ise o%atin la/ers o$ seleted soil.

   Co%ation roess s7uee#es  out air> (ut as t*e (uilt u *ei*t  inreases> t*e lo!er la/ers e+eriene an inrease in ore ressure. In oarse rained soils> t*e e+ess ore ressures dissiate 7ui"l/.

 In 0ne rained soils> t*e e+ess ore ressure is slo! to dissiate and onsolidation %a/ ontinue $or se'eral /ears.

  T*e installation o$ *ori#ontal or 'ertial drainae (lan"ets is used to seed u t*is roess.

 In t*e ourse o$ ti%e> t*e ore ressure dereases and t*e eeti'e stresses inrease> t*ere$ore t*e s*ear strent* inrease.

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE

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4.9 SLOPE STA3ILIDATIO< -ET@O

4.$.1

7rainage

 rainae is one o$ t*e %ost !idel/ used %et*ods $or i%ro'in sta(ilit/.

 Clearl/ sur$ae !ater %ust (e re%o'ed and (uildu o$ !ater ressures

in tension ra"s re'ented. Su(sur$ae drainae %ust (e desined to redue t*e !ater ressures atin on atual or otential sli sur$aes2 in t*is !a/> t*e 'alue o$ t*e ore ressure> u> is redued> t*ere(/ roduin an inrease in t*e $ator o$ sa$et/.

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4.9 SLOPE STA3ILIDATIO< -ET@O

4.$.1

7rainage

 Se'eral %et*ods e+ist $or drainae s/ste%> inludin *ori#ontal  drains and 'ertial drains.

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4.9 SLOPE STA3ILIDATIO< -ET@O

4.$.1

7rainage

 rainae %a/ also (e a*ie'ed (/ t*e use o$ eletroos%osis  and (/ lantin suita(le 'eetation.

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4.9 SLOPE STA3ILIDATIO< -ET@O

4.$.2 8estraining %tructure

 Restrainin strutures su* as iles> retainin !alls  and an*ors  %a/ (e used to i%ro'e sta(ilit/. It %ust (e areiated t*at t*e $ores  and %o%ents  to !*i* t*ese strutures are su(eted %a/ (e 'er/ lare  and *ene are$ul desin is essential.

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4.9 SLOPE STA3ILIDATIO< -ET@O

4.$.3

odification of %lope 9eometry

 C*anin t*e eo%etr/ o$ a sloe to i%ro'e sta(ilit/ an in'ol'e t*e: excavation  to unload t*e sloe2 0llin  to load t*e sloe2 and reduin t*e o'erall *ei*t o$ t*e sloe.

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4.9 SLOPE STA3ILIDATIO< -ET@O

:.$.4

8eplacement

 =*ere t*e sli sur$ae is not undul/ dee> re%o'al o$ all 6or art) o$ t*e slied %aterial and relae%ent ro'ides a relati'el/ si%le and strai*t$or!ard re%edial %easure. T*e re%o'ed soil %a/ (e relaed (/ $reedrainin %aterial 6in !*i* ase so%e additional (ene0t %a/ (e a*ie'ed (/ drainae) or (/ li*t struture su* as eo$oa%.

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4.9 SLOPE STA3ILIDATIO< -ET@O

4.$.$ 9eotextiles

8eote+tiles an (e used $or:

 Segregation of layers: Ro"0ll la/ on so$t round to $or% a road or e%(an"%ent (ase an (e re'ented $ro% un*in into t*e soil (elo! usin a eote+tile

underla/.

 Tensile strength: @ori#ontal %e%(ranes an (e used to ro'ide tensile rein$ore%ent and redue settle%ent.

 A drainage layer: Eit*er as a !aterondutor or as a 0lter to redue t*e %iration o$ 0ne artiles into a ranular soil drains.

 An impermeable barrier: To re'ent or ontrol t*e ,o! o$ onta%inated round!ater $ro% or in land0ll sites.

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