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

Chapter 6 SLOPE STABILITY
Table 6.1 Undrained Shear Strength and Consistency of Cohesive Soils
Consistency
Undrained Shear
Strength, S u (kPa)
Visual Identification
Very soft < 12 Thumb can penetrate more than 25 mm
Soft 12 – 25 Thumb can penetrate about 25 mm
Medium 25 -50
Thumb can penetrate with moderate
effort
Stiff 50 – 100 Thumb will indent soil about 8 mm
Very stiff 100 – 200
Thumb will not indent but readily indent
with thumbnail
(After Terzaghi & Peck and ASTM D2488-90)
6.4.2.2 Long-term
Long-term stability analysis is normally carried out using effective stress analysis with drained shear
strength parameters. For cohesive or clayey soil, total stress analysis (for short-term) in addition to
the effective stress analysis (for long-term) are carried out to determine the most critical factor of
safety. As granular or sandy soils are more permeable than cohesive or clayey soils, drainage of
excess pore pressure in sandy soil occurs much more rapidly. Hence, only effective stress analysis is
usually required.
Effective stress analysis requires the estimation of the drained strength parameters c’, φ’ and pore
pressures. For pure free draining sands, φ = φ’ and c = 0. Under conditions of steady seepage, the
phreatic line can be obtained from the flow net.
Some common drained strength parameters, φ' and c’ adopted in the slope analysis are as follows:-
Table 6.2 Typical Drained Parameters For Effective Stress Analysis
Soil type Effective friction angle φ‘ Effective cohesion c’
Well compacted soil 28 o – 30 o 2 – 5 kPa
Residual soil grade V to VI 30 o – 32 o 5 – 10kPa
Residual soil grade IV to V 32 o – 35 o 10 – 15kPa
Note:-
• The values above are just for references. Test shall be carried out before any
analysis is carried out. It is advisable to limit the cohesion to not more than
15kPa even with lab test results. The cohesion shows in test are sometimes
apparent and the changes are subjected to external factors i.e., weathering
process etc
• Description of grade of residual soil:
Grade VI = residual soil : Grade V = completely weathered rock ; Grade IV =
highly weathered
6.5 CIRCULAR ARC ANALYSIS
6.5.1 General Principles
Figure 6.4 shows a potential slide mass defined by a predetermined circular arc slip surface. If the
shear resistance of the soil along the slip surface exceeds that necessary to provide equilibrium, the
mass is stable. If the shear resistance is insufficient, the mass is unstable. Thus, the stability or
instability of the mass depends on its weight, the external forces acting on it, the shear strengths
March 2009 6-5