Geotechnical Engineering

DHARM

320 GEOTECHNICAL ENGINEERING

Shear stress

Unstable slope

Mohr-coulomb strength envelope

Stable slope

f

b

s

t

Normal stress

Fig. 9.2 Relation between strength envelope and angle of slope

Rapid Drawdown in a Slope in Cohesionless Soil

When the water level in a river or reservoir recedes, say after floods or after a drawdown,
the water in the slope of the embankment may not fall as rapidly as that in the river or the
reservoir, depending upon the permeability of the soil. This gives rise to a condition commonly
known as “sudden or rapid drawdown”. The effect of this is that seepage occurs from the high
water level in the slope to the lower water level of the river. A flow net can be drawn for this
condition and the excess hydrostatic head at any point within the slope can be determined.
Let us consider an element within the slope as shown in Fig. 9.3.
Let the weight of the element be W. Let the excess pore water pressure induced by
seepage be u at the base of the element. Let the length of the element perpendicular to the
plane of the figure be unity.

Normal stress σn =

Normal component of weight NW

l

,

l being the width of the element parallel to the surface.

∴σn =

W

l

W

b

cosββcos

=

2

, since l = b/cos β

σn =

γβ

γβ

zb

b

z

cos

cos

(^22)
= ...(Eq. 9.2)
b
W.L.
Reservoir or river
z
l
b
(a) Earth slope subjected to rapid drawdown (b) Element of the earth slope
Fig. 9.3 Rapid drawdown is a slope in cohesionless soil