Civil Engineering Reference
In-Depth Information
Although we can calculate values for the pore pressure
u
around the slip circles
we cannot, at present, calculate the normal stresses
σ
. Thus the simple analysis
which served for undrained loading for which
τ
=
s
u
cannot be used for drained
loading.
The approach adopted for the method of slices is to subdivide the mechanism into
a number of approximately equal vertical slices and examine the statical equilibrium
of the slices and, by summation, of the whole mechanism. Figure 20.10(a) shows the
mechanism of Fig. 20.9 divided into four slices, of which a typical slice FGHJ is shown
in Fig. 20.10(b). The total forces on the slice shown in Fig. 20.10(b) are its weight
W
, and total normal and shear forces
N
and
T
on the base FJ, and forces
F
1
and
F
2
from adjacent slices. The interslice forces
F
1
and
F
2
are not necessarily equal and
opposite, and their resultant
F
acts at a height
a
above the centre of the base of the
slice and at an angle
to the horizontal. The total normal and shear forces on the base
of the slice are related by
θ
φ
T
=
(
N
−
U
) tan
(20.18)
Figure 20.10
Slip circle method for drained loading - method of slices.
