Page 145 - Numerical Analysis and Modelling in Geomechanics
P. 145
126 C.W.W.NG AND Q.SHI
Figure 4.12 Factor of safety vs water permeability.
the matrix suction. For parametric studies, two anisotropic cases are considered:
(k /k )=2 and 4, keeping k =4.8×10 −5 m/sec unchanged but reducing k . The
y
x
x
y
effects of anisotropic permeability on the initial (i.e. Q=0 in equation (4.8))
groundwater table are shown in Figure 4.13. It can be seen that permeability
anisotropy has a significant influence on the position of the initial groundwater
table. Reduction of permeability in the vertical direction (k ) limits the amount of
y
water flowing in the y-direction for a given slope and time. Since the hydraulic
boundaries of the hillside slope are fixed, to maintain the continuity of flow, the
main groundwater table has to rise and the rising of the main groundwater table
is higher in the lower part than the one in the upper part of the natural hillside
slope. This results in the “divergence” of the main groundwater tables for the
two anisotropic cases, k /k =1 and 2, from the upper to the lower slope. For the
y
x
case of permeability anisotropy k /k =4, the main groundwater table rises
x
y
substantially on the upper part of the hillside slope. The sloping surface of the
cut slope behaves as a low permeability membrane to cause a substantial rise of
the main groundwater table.
Figures 4.14a and b show the pore water pressure distribution with depth
during a rainfall intensity of 267 mm/day for the two anisotropic soil
permeabilities, k / k =2 and k /k =4 respectively. Comparing with the pore water
y
x
y
x
pressure distribution for the case of isotropic water permeability shown in
Figure 4.6, a substantial rise of the main groundwater table (more than 1 m) is
observed for the three cross-sections considered in the soil with k /k =2. This is
x
y
mainly attributed to the increase in the ratio Q/k y as due to the reduction of k y
(refer to equation (4.8)). However, a negative pore water pressure (unsaturated)
zone still exists in the slope.
