Stability of Slopes - Elements of Soil Mechanics - page 387

Fig. 13.1 Forces involved in a slope of granular material.

GWL

WL

River

(a)

Fig. 13.2 Seepage due to rapid drawdown.

the result 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 for any point within the slope
can be determined.
Assume that a potential failure plane, parallel to the surface of the slope, occurs at a depth of z and
consider an element within the slope of weight W. Let the excess pore water pressure induced by seepage
be u at the mid point of the base of the element.

Normal reaction N = cos β

W cos β W cos

2

β b

Normal stress σ = = since l =

l b cos β

W cos

2

β

′ =

Normal effective stress σ − u

b

γ zb cos

2

β

= − = u γ z cos

2

β − u

b

(Where γ = the average unit weight of the whole slice, it is usually taken that the whole slice is
saturated).

Tangential force = sin β

W sin β

⇒ Tangential shear stress , τ = = γ z sin cos β β

l

= ′

′ =

Ultimate shear strength of soil σ tan φ τ F