CHAPTER
CHAPTER
4.0
4.0
SLOPE STABILITY
SLOPE STABILITY
BFC34402
BFC34402
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
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
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 dististananeses (e$ore o%in to rest.
(e$ore o%in to rest. iii
iii.. Co%Co%%on %on in in oursourseerairained ned soilsoil.. ()
() FlFlo! o! slslidide:e:
i.
i. OOuurs rs !*!*en en ininteternrnal al & e& e+t+terernanall ondition $ore a soil to (e*a'e li"e a ondition $ore a soil to (e*a'e li"e a 'isous ,uid & ,o! do!n.
'isous ,uid & ,o! do!n. ii.
ii. -ul-ultitile le $ai$ailurlure se sur$ur$aeaes us usuasuall/ ll/ oourur
& *ane ontinuousl/ as ,o!
& *ane ontinuousl/ as ,o!
roeeds. roeeds.
4.1 TYPES & CAUSES OF SLOPE FAILURE
4.1.1 Types of slope failure
) Rotational slide:
i. Co%%on in *o%oenous 0nerained
soil.
ii. t/es o$ rotational slides2 1. 3ase slide:
3/ an ar* enul0n t*e !*ole sloe.
A so$t soil restin on sti la/er. 5. Toe slide:
Failure sur$ae asses t*rou*
t*e toe o$ sloe . Sloe slide:
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) 8eoloial $eatures 6e) 9) E+ternal loadin 6$)
) Constrution ati'ities
6)
4.5 STA3ILITY OF I<FI<ITE SLOPES
4.2.1 (actor of safety
8enerall /:
4.5 STA3ILITY OF I<FI<ITE SLOPES
4.5 STA3ILITY OF I<FI<ITE SLOPES
4.2.2 nfinite slope" )ithout seepage
4.5 STA3ILITY OF I<FI<ITE SLOPES
4.5 STA3ILITY OF I<FI<ITE SLOPES
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
4. -ET@O OF SLICES
4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis
odes of failure
4. -ET@O OF SLICES
4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis
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 24.4 k and 43$.2 k respectively. The location of these masses are l 1 5 3.$m and l 2 5 $.3m . 6y using the mass procedure method or %wedish slip circle method
-i, Calculate the total moment of the driving force about point O.
(ii) Determine the factor of safety of the trial circle. (iii) Comment what happens to the calculated factor
4. -ET@O OF SLICES
4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis
4. -ET@O OF SLICES
4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis
0nalysis method 2 ethod of slices"
*xamples 1
4. -ET@O OF SLICES
4.3.1 (inite slope" 'ircular (ailure %urfaces 0nalysis
0nalysis method 2 ethod of slices"
*xamples 2
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4.1 'ut %tability
Cuttins are e+a'ated> !*ereas e%(an"%ents are (uilt.
A derease in total stresses and also a dereasin in ore ressure !*en e+a'ation o$ t*e re%o'al o$ soils.
Seeae rei%e de'elos in t*e sloe a$ter onstrution !*i* it deends
on t*e drainae ondition and t*e er%ea(ilit/ o$ t*e soil.
T*ere$ore> an inre%ent in ore ressure> onstant in total stress (ut $alls steadil/ in eeti'e stress.
3esides> eeti'e nor%al stress dereases too sine itBs a art o$
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4.2 *mbankment stability
3uilt (/ rollin or ot*er!ise o%atin la/ers o$ seleted soil.
Co%ation roess s7uee#es out air> (ut as t*e (uilt u *ei*t inreases> t*e lo!er la/ers e+eriene an inrease in ore ressure. In oarse rained soils> t*e e+ess ore ressures dissiate 7ui"l/.
In 0ne rained soils> t*e e+ess ore ressure is slo! to dissiate and onsolidation %a/ ontinue $or se'eral /ears.
T*e installation o$ *ori#ontal or 'ertial drainae (lan"ets is used to seed u t*is roess.
In t*e ourse o$ ti%e> t*e ore ressure dereases and t*e eeti'e stresses inrease> t*ere$ore t*e s*ear strent* inrease.
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.4 CUT STA3ILITY> E-3A<?-E<T & <ATURAL SLOPE
4.9 SLOPE STA3ILIDATIO< -ET@O
4.$.1
7rainage
rainae is one o$ t*e %ost !idel/ used %et*ods $or i%ro'in sta(ilit/.
Clearl/ sur$ae !ater %ust (e re%o'ed and (uildu o$ !ater ressures
in tension ra"s re'ented. Su(sur$ae drainae %ust (e desined to redue t*e !ater ressures atin on atual or otential sli sur$aes2 in t*is !a/> t*e 'alue o$ t*e ore ressure> u> is redued> t*ere(/ roduin an inrease in t*e $ator o$ sa$et/.
4.9 SLOPE STA3ILIDATIO< -ET@O
4.$.1
7rainage
Se'eral %et*ods e+ist $or drainae s/ste%> inludin *ori#ontal drains and 'ertial drains.
4.9 SLOPE STA3ILIDATIO< -ET@O
4.$.1
7rainage
rainae %a/ also (e a*ie'ed (/ t*e use o$ eletroos%osis and (/ lantin suita(le 'eetation.
4.9 SLOPE STA3ILIDATIO< -ET@O
4.$.2 8estraining %tructure
Restrainin strutures su* as iles> retainin !alls and an*ors %a/ (e used to i%ro'e sta(ilit/. It %ust (e areiated t*at t*e $ores and %o%ents to !*i* t*ese strutures are su(eted %a/ (e 'er/ lare and *ene are$ul desin is essential.
4.9 SLOPE STA3ILIDATIO< -ET@O
4.$.3
odification of %lope 9eometry
C*anin t*e eo%etr/ o$ a sloe to i%ro'e sta(ilit/ an in'ol'e t*e: excavation to unload t*e sloe2 0llin to load t*e sloe2 and reduin t*e o'erall *ei*t o$ t*e sloe.
4.9 SLOPE STA3ILIDATIO< -ET@O
:.$.4
8eplacement
=*ere t*e sli sur$ae is not undul/ dee> re%o'al o$ all 6or art) o$ t*e slied %aterial and relae%ent ro'ides a relati'el/ si%le and strai*t$or!ard re%edial %easure. T*e re%o'ed soil %a/ (e relaed (/ $reedrainin %aterial 6in !*i* ase so%e additional (ene0t %a/ (e a*ie'ed (/ drainae) or (/ li*t struture su* as eo$oa%.
4.9 SLOPE STA3ILIDATIO< -ET@O
4.$.$ 9eotextiles
8eote+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 $ro% un*in into t*e soil (elo! usin a eote+tile
underla/.
Tensile strength: @ori#ontal %e%(ranes an (e used to ro'ide tensile rein$ore%ent and redue settle%ent.
A drainage layer: Eit*er as a !aterondutor or as a 0lter to redue t*e %iration o$ 0ne artiles into a ranular soil drains.
An impermeable barrier: To re'ent or ontrol t*e ,o! o$ onta%inated round!ater $ro% or in land0ll sites.