torofsafetyvariesli
earlywithγua
dca
thusbeexpressedas
Fm
γu
910
where，ma
d
arethestabilitycoefficie
ts．Thecoefficie
ts，ma
d
arefu
ctio
sofβυ’，thedime
sio
less
umbercγa
dthedepthfactorDUsi
gtheFelle
iusmethodofslices，determi
ethefactorofsafety，i
termsofeffectivestress，oftheslopeshow
i
Fig96forthegive
failuresurface．Theu
itweightofthesoil，bothabovea
dbelowthewatertable，is20kN／m3a
dthereleva
tshearstre
gthparametersarec’10kNm2a
dυ’29°Thefactorofsafetyisgive
byEquatio
94Thesoilmassisdividedi
toslicesl5mwideTheweightWofeachsliceisgive
byWγbh20×15×h30hkN／mTheheighthforeachsliceissetoffbelowthece
treofthebasea
dthe
ormala
dta
ge
tialcompo
e
tshcosαa
dhsi
αrespectivelyaredetermi
edgraphically，asshow
i
Fig96The
Wcosα30hcosαWsi
α30hsi
αTheporewaterpressureatthece
treofthebaseofeachsliceistake
tobeγwzw，wherezwistheverticaldista
ceofthece
trepoi
tbelowthewatertableasshow
i
figure．Thisprocedureslightlyoverestimatestheporewaterpressurewhichstrictlyshouldbeγwze，wherezeistheverticaldista
cebelowthepoi
tofi
tersectio
ofthewatertablea
dtheequipote
tialthroughthece
treoftheslicebase．Theerrori
volvediso
thesafeside．Thearcle
gthLaiscalculatedas1435mm．Theresultsaregive
i
Table91
∑Wcosα30×1750525kN／m∑Wsi
α30×845254kN／m∑wcosαul525132393kN／m
fItisassumedthatthepote
tialfailuresurfaceisparalleltothesurfaceoftheslopea
disatadepththatissmallcomparedwiththele
gthoftheslopeTheslopeca
the
beco
sideredasbei
gofi
fi
itele
gth，withe
deffectsbei
gig
ored．Theslopeisi
cli
edata
gleβtothehorizo
tala
dthedepthofthefailurepla
eisz．asshow
i
sectio
i
Fig97Thewatertableistake
tobeparalleltotheslopeataheightofmz0m1abovethefailurepla
e．Steadyseepageisassumedtobetaki
gplacei
adirectio
paralleltotheslope．forceso
thesidesofa
yverticalsliceareequala
doppositea
dthestressco
ditio
sarethesameTheateverypoi
to
thefailurepla
e．
I
termsofeffectivestress，theshearstre
gthofthesoilalo
gthefailurepla
eis
fa
dthefactorofsafetyis
Theexpressio
sforστa
dμare
Thefollowi
gspecialcasesareofi
terest．Ifc’0a
dm0iethesoilbetwee
thesurfacea
dthefailurepla
eis
otfullysaturatedthe

911
Ifc’0a
dm1iethewatertablecoi
cideswiththesurfaceoftheslope，the
：
912
Itshouldbe
otedthatwhe
c’0thefactorofsafetyisi
depe
de
tofthedepthz．Ifc’isgreatertha
zero，thefactorofsafetyisafu
ctio
ofza
dβmayexceedυ’providedzislesstha
acriticalvalue．Foratotalstressa
alysisther