Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 6 SLOPE STABILITY
a
σ’
σ
u
Increase in pore pressure
Excavation/cut
Time
b
decrease in pore pressure
σ’
σ
u
Construction/fill
Time
During slope cutting, frequent inspections and mapping shall be carried out by experience geologist
to ensure no adverse “inherent” geological features i.e., soil bedding, relicts and rock discontinuities
(if rock cutting). If these adverse features are found on slope outcrop, strengthening measures
such as soil nailing can be specified to improve the stability of the slope. Horizontal drains can be
installed at areas where water seepages are found during cutting to lower the ground water table.
Always avoid cutting slope with large catchment behind the slope. Area with large catchment
always associated with high ground water table. If it is unavoidable, Horizontal drains and deep
trench drains shall be included in the design to lower the ground water table
6.5.6 Filled Slope/Embankment on Clay
Excess pore water pressures are created when fills are placed on clay or silt. Provided the applied
loads do not cause the undrained shear strength of the clay or silt to be exceeded, as the excess
pore water pressure dissipates consolidation will occur, and the shear strength increases with time
as illustrated in Figure 6.5(b). For this reason, the factor of safety increases with time under the
load of the fill. Hence, the most critical state for the stability of an filled embankment is normally
the short-term or end-of-construction condition where total stress analysis with undrained shear
parameters are required.
6.5.7 Effects of Water
Figure 6.5 Relationship Of Total Stress, Pore Pressure And Time
Besides gravity, water (both surface and ground water) is a major factor in slope instability. In
addition, ground water table induced failure is always deep seated and catastrophic. Ground water
table is one of the most difficult parameter to be assumed or estimated. Hence, if necessary
standpipes or piezometers can be installed to monitori and ascertain the fluatuation and worst
ground water levels to be used either in design or verification of design.
If the slope is subjected to inundation and changes in the water levels such as dam, pond, or river
subjected to tidal effects, the designer should consider the possible effects of rapid draw down of
water levels in the stability analysis. For rapid drawdown analysis of soils with low permeability (less
than 10 -4 cm/sec), it is assumed that the drop in water level is so fast that no drainage can occur in
the soil. For this prupose, drained strengths with appropriate phreatic line are used for stability
analysis.
6-8 March 2009