Page 131 - Numerical Analysis and Modelling in Geomechanics

112 C.W.W.NG AND Q.SHI
The mid-levels on Hong Kong island
Hong Kong is located at the mouth of the Pearl River on the south east coast of
China. The Mid-levels considered in this study is located on the northern slopes
of Hong Kong Island. The geology of Hong Kong has been reported by many
researchers (Davis [21]). The predominant rock types in Hong Kong are granitic,
volcanic and sedimentary rocks. Chemical and physical weathering processes
have resulted in the formation of mantles of saprolitic and residual soils, which
are up to 50 m thick over the granite, with a saturated water permeability varying
−6
−6
between 1.5×10 m/sec and 8.0–10 m/sec (Premchitt et al. [3]). The average
−7
saturated permeability of the underlying bedrock is 2×10 m/sec.
One important issue relating to the slope stability problems in Hong Kong is
the presence of colluvium (clayey, sandy silt) which covers many of the
footslopes. Colluvium is a mixture of soil and decomposed rock debris produced
from landslips migrating progressively downslope. Many slope failures are
associated with the collapse of colluvium during rainstorms. Colluvium on the
upper slopes of the Mid-levels is an interlocked boulder scree with little or no
matrix. However, this progressively changes downslope to a sequence of cobbles
and boulders in a matrix of clayey, sandy silt (Leach and Herbert [10]). The
thickness of the colluvium varies from place to place, ranging from 1 m to 36 m
thick in the Seymour area, with saturated permeability varying between 5×10 −6
m/sec and 3.5×10 −4 m/sec (GCO [22]). Since the thickness of each soil layer
varies from place to place, it was decided to idealise the complex hillside
geology as a single soil layer together with a relatively impermeable rock stratum
for parametric studies presented in this chapter (see Figure 4.4). The practical
range of the saturated water permeability considered is given in Table 4.1.
Rainfalls in Hong Kong originate from two different processes, either slow-
moving low pressure troughs or tropical cyclonic storms (Lumb [8]). Very often
the troughs bring continuous rain over several days, i.e., trough to succeed
another trough after a few days of dry weather. The storms are always associated
with strong winds, which may reach typhoon strength, and can produce very
intensive rainfall over several hours. However, these rainstorms rarely cause
more than two or three days of continuous rainfall.
The average annual rainfall in Hong Kong is 2225 mm, with 80% of this
occurring between May and September each year (Premchitt et al. [3]). During
the rainstorms, hourly rainfall intensities can exceed 100 mm/hr and those in
excess of 50 mm/hr are not uncommon. Based on the actual rainfall record, a 10-
year return period spanning from 1980 to 1990 (Lam and Leung [23]) has been
selected for parametric study in this chapter (refer to Table 4.1). In fact, the
amount of rainfall can vary substantially with time and location. However, for
the scope of the parametric study, the exact magnitude of rainfall is not
particularly important and so any slight discrepancies will not affect any
conclusions arising from this study.