Geotechnical Engineering

DHARM

STABILITY OF EARTH SLOPES 333

the sliding moment while it reduces the shear resistance mobilised by decreasing the effective

stress. The effect of rapid drawdown on slope stability depends very much on the opportunity

for drainage at the base. If the base material is pervious the flow pattern tends to be down-

wards, which is conducive to stability; otherwise, the seepage forces may create more unfa-

vourable conditions with respect to stability. The pore water pressure along the slip surface

can be determined from the flow net. Referring to Fig. 9.17, the pore water pressure u may be

written as follows:

hw h¢

h

Water level before drawdown

Equipotential

before

drawdown Trial

surface

Fig. 9.17 Upstream slope subjected to rapid drawdown

u = uo + ∆u

uo = γw (hw + h – h′) ...(Eq. 9.24)

∆u = B. ∆σ 1 = B. (– γwhw), B being defined as

∆

∆σ

u

1

.

Here B is commonly assumed as unity.
∴∆u = – γwhw ...(Eq. 9.25)
∴ u = γw(h – h′) ...(Eq. 9.26)
Equation 9.22 may now be used to determine the factor of safety, F. If a flow net is not
available, the approximate value of F may be got from Eq. 9.23.

Immediately after Construction

When an earth dam or an embankment is constructed rather rapidly, excess pore pres-
sures are likely to develop which affect the factor of safety. Assuming the initial pore pressure
to be negligible, the pore pressure at any stage is the change in pore pressure, ∆u.

But ∆u = B [∆σ 3 + A (∆σ 1 – ∆σ 3 )]

from Skempton’s concept of the pore pressure parameters,

∴

∆

∆σ

u

1

= BB=+−A

F

HG

I

KJ

L

N

M

M

O

Q

P

P

∆σ

∆σ

∆σ

∆σ

3

1

3

1

1 ...(Eq. 9.27)

ru = u/γ z

= ∆u/γ z

=

B

z

.∆σ 1

γ

∆σ 1 may be taken to be nearly equal to the weight of the material above the point, or γz.