Civil Engineering Reference
In-Depth Information
Figure 21.21
Stability of a simple slope - Examples 21.1 and 21.2.
Example 21.2: Drained slope stability
For drained loading of the slope in Fig. 21.21,
from Eq. (21.45),
F
s
=
m
−
nr
u
20
◦
the gradient is 2.75:1 and, from Fig. 21.19, for
φ
=
30
◦
we have
m
For
i
=
≈
1.6
and
n
≈
1.8. Taking a characteristic value for
r
u
=
0.3,
F
s
=
1.6
−
(0.3
×
1.8)
=
1.06
Near the toe of the slope the flowlines will be approximately parallel to the slope and
the phreatic surface is close to ground level. From Eq. (21.30), replacing tan
φ
c
with
φ
c
/
F
s
,
tan
1
tan
φ
c
tan
i
−
γ
w
γ
F
s
=
The calculated factor of safety is
1
tan 30
◦
tan 20
◦
=
10
20
F
s
=
−
0.80
and local instability will occur near the toe. In order to stabilize the slope the drain in
Fig. 20.19 should be lowered to reduce the pore pressures.
References
Bishop, A. W. and N. R. Morgenstern (1960) 'Stability coefficients for earth slopes',
Geotechnique,
10
, 129-150.
Taylor, D. W. (1948)
Fundamentals of Soil Mechanics,
Wiley, New York.
Further reading
Atkinson, J. H. (1981)
Foundations and Slopes,
McGraw-Hill, London.
Bromhead, E. N. (1992)
The Stability of Slopes,
Taylor and Francis, London.
Chandler, R. J. (ed) (1991) Slope stability engineering,
Proc. Int. Conf, Slope Stability,
I. C. E.,
London.
