Page 139 - Numerical Analysis and Modelling in Geomechanics

120 C.W.W.NG AND Q.SHI

Figure 4.7 Factor of safety vs rainfall intensity.
result of the reduction of matrix suction in the unsaturated zone (refer to
Figure 4.3).
Figure 4.10 shows the variation of the factor of safety with duration of
rainfall. The factor of safety decreases with an increase in duration of rainfall, until
a “critical duration” is reached. The initial fall in the factor of safety (≥ f ) is
1
attributed to the rise of the main groundwater table as the duration increases.
However, if the rainfall lasts longer than the critical duration, the rise of the main
water table actually decreases as the duration increases. This is because the
average rate of rainfall drops significantly with duration (see Figure 4.8).
Based on the analysed results, a critical rainfall duration exists, which leads to
the lowest factor of safety. For the present investigation, the critical rainfall
duration is found to lie between 3 and 7 days. The concept of the existence of a
critical duration is in fact consistent with field measurements of groundwater
response at the Mid-levels to rainfalls (GCO [22]). It was found that a critical
duration of storm could be identified in both CDG and colluvium to characterise
the pore water pressure response to a rainstorm for a given location. Typically
the critical duration was found to lie between 2 and 7 days. Before reaching the
critical duration, infiltration of rain-water is continuing to improve the initially
low permeability of unsaturated soils and it results in a gradual rise of the main
water table, until the main water table reaches its maximum at the critical
duration. As the average rainfall intensity decreases rapidly with time (see
Figure 4.8), the main water table will not rise further for rainfalls with duration
which is longer than the critical value. Infiltrated groundwater will be drained
away by soils with sufficient high permeability after “soaking”. The concept and
existence of a critical duration can be further justified by considering landslides