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

Chapter 6 SLOPE STABILITY
6.5.4 Required Safety Factors
Appropriate factors of safety are required to ensure adequate performance of embankments
throughout their design lives. Two of the most important considerations that determine appropriate
magnitudes for factor of safety are uncertainties in the conditions being analyzed, including shear
strengths and consequences of failure (both economic loss and loss of life) or unacceptable
performance.
The values of factor of safety listed in Table 6.3 provide a guidance and are not prescribed for
slopes of embankment dams. Higher or lower values might be warranted in respect of the degree of
uncertainties in the conditions being analyzed, economic loss and loss of life.
Type of slopes
6.5.5 Cut Slope in Clay
Table 6.3 Recommended Factors Of Safety
End of construction
(short-term)
Long-term (steadystage
seepage)
Rapid
drawdown 3
1. Embankment and
Natural Slope 1 1.3 1.4 1.1 – 1.2 4
2. Cut or Excavated Slope 2 1.3 1.4 1.1 - 1.2 4
Notes
1. Applicable to filling for river bank, water retention facilities, levees, sea wall, stockpiles, earth
retaining works. It also includes natural slopes such as river bank and valley slopes.
2. Applicable to excavated slope including foundation excavation, excavated river and retention
facilities, sea wall and other earth retaining works.
3. Rapid drawdown occurs when it is assumed that drawdown is very fast, and no drainage
occurs in materials with low permeability; thus the term “sudden” drawdown.
4. For submerged or partially submerged slopes, the possibility of low water events and rapid
drawdown should be considered. FOS of 1.1 to 1.2 for rapid drawdown recommended here are for
cases where rapid drawdown represents an infrequent loading condition. In cases where rapid
drawdown represents a frequent loading condition, as in river bank subjected fluctuations in water
level and pumped storage projects, the factor of safety should be higher.
For cut slope, the effective stress reduces with time owing to the stress relief after removal of load.
This reduction will allow the clay to expand and absorb water, which will lead to a decrease in the
clay strength with time. For this reason, the factor of safety of a cut slope in clay may decrease
with time. Cut slopes in clay should be designed by using effective strength parameters and the
effective stresses that will exist in the soil after the pore pressures have come into equilibrium
under steady seepage condition.
These changes in the values of total stress and pore pressure with time are shown here in Figure
6.5(a).
March 2009 6-7