5 Slope Stability

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Chapter 5

Slope Stability

FKAAS, UTHM

Dr. CM Chan


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5.1 Natural and artificial slopes

(Whitlow 2001)


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5.2 Soil moements ! Slope insta"ilit#

Falls

Topples

Slides- rotational, compound, translational

Lateral spreads

Flows

Complex or compound failure


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5.2.1 $#pes of soil moements

(Whitlow 2001)


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5.% Causes of insta"ilit#

Main categories-te causes of slope insta!ilit"#

$ %round conditions- weatered, fissures, etc&

$ %eomorpological processes- tectonic

acti'ities, wa'e erosion, etc&

$ ("sical processes- rainfall, tidal effects,etc&

$ Man-madeprocesses- exca'ation, drawdown

of water le'el )reser'oirs*, mining, +uarr"ing,

etc&


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Commoncauses of slope insta!ilit"#

$ eateringeffect

$ Canges of slope geometr"

$ ecrease in effecti'e stressof soil mass

$ .xcess loading)e+ui'alent to increase in

eigt of slope*

$ (oor design and/or construction- e&g&exca'ation near toe of existing slope


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5.& Slope sta"ilit# anal#sis

Metods of anal"sis#

$ 0nfinite slope metod- plane translational

slide

$ .ffecti'e and total stress approac )usuall"

u1 2o*

$ Fellenius or Swedis or ordinar" slice

metod

$ Metod of slices )effecti'e stress*- 3isop4s

metod, 3isop4s modified metod


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$ Ta"lor4s metod- undrained anal"sis

$ 3isop-Morgenstern4s metod

$ 5an!u4s metod- non-circular slip surfaces

$ edge metod- single plane

Focus:

$ Fellenius or Swedish or ordinary slice method

$ Taylors method- undrained analysis


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5.5 'actor of safet# ('S)

6elies on sound engineering 7udgement,

consideration of 'arious contri!uting factors and

appropriate metod of anal"sis&

Factors to !e considered#

$ Conse+uencesof failure

$ Confidencein te a'aila!le information and

data


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Higfactor of safet" )FS*#$ ris8 of loss of li'es and ad7acent structures-

ultimate limit state

$ possi!ilit" tat slope could !e prone tpdeformations wic could affect ad7acent

structures, e'en if it as sufficient sta!ilit"-

ser'ice limit state


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Lowerfactor of safet" )FS*#

$ period of exposure to factors causing

insta!ilit" is sort- e&g& temporar" wor8s

$ do notaffect li'es or structures

$ localised- simple remedial measures )wen

re+uired*


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9arious factors to !e considered in coosinga

suita!le FS#

$ (ast or local experience- presence or pre-

existing slip planes, future canges to soil

profile, water ta!le le'els, etc&

$ Complexit" of ground conditions

$ ata from site in'estigationreports

$ Confidence in design parameters)searstrengt, pore pressures, etc&*


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5. $a#lors method

6efers to Ta"lor4s cur'es )Ta"lor4s # undrained

anal"sis sta!ilit" cart*&

Ta"lor ):;, depends on slope inclination

angle, ?, and dept factor, , @ is dept of stiff

stratum&


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Ta"lor4s undrained anal"sis sta!ilit" cart&


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F 1 te lowest FS o!tained from a circular arc

anal"sisof a omogeneousslope wit undrainedsear strengt, cu, !ul8 unit weigt, , and eigt,

H u1 2o&

>s depends on te slope angle )or*, and

dept factor , were H )or @* 1 dept to stiff

stratum&

See Taylors curves (Fig. 12.10, pg.

!1"#arnes$.

%&ample 12.' (#arnes$

30


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Hard la"er

.xample#

3ased on te a!o'e figure, @ 1 :B&2 m&

1 :B/:2 1 :&B

From Ta"lor4s cart, wit 1 :&B and ? 1 B2, terefore > 1 2&:B&>s 1 cu/ FH

FS 1 )


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5.* +rdinar# slice ! Swedish ! 'ellenius

method

Assumption# te inter-slice forcesare e+ualand

opposite- teir resultants are @ero&

.: 1 .B and E: 1 EB&


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6esol'ing te forces acting normalto te !ase

of eac slice# ) *cos - ul

*here,

u ) whw

hw) height to the water ta+le a+ove the

+ase o the slice or related to the

nearest euipotential line i a low netis drawn


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For a omogeneousslope wit c and constantalong te slip surface#

F 1 cLA3 tan)cos - ul*

sin*here,

$ *, and u are determined or each slice

and ta+ulated.$ can +e positive or negative.

-


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Te formula can !e rewritten as#

F 1 Gc4L )cos - rusec* tan4

sin

3" ta8ing ! 1 Lcos,

u 1 ru

Lcos

u 1 ru sec

L


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For a stead" state seepage troug slopes, te

pore pressure must !e ta8en into account-effecti'esear strengt parameters are used&

0f pore pressure is assumed to !e @ero#

F 1 GcL costan

sin

%&amples .2a and .2+ (otes +y F /ing$


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.xample =&Ba (otes +y F /ing$

ivenTe slope and soil properties, as sown in

Fig& =&:IFind FS 'ia te Fellenius metod of slices for te

trial circle sown

:2;&Io

J

:2 m

6 1 : m

B&I m


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(rocedure#

:& Te slope was plotted to a con'enient scale&

B& For te assumed failure circle and te slices

selected, te widt of eac slice is measured as

B&I m&


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e&g& Te calculations for slice :are as follow#

: 1 )area* x )* 1 !/B

1 )B&I x : 1 :cos: 1 =


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Ta!le =&Ba

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5 Slope Stability