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

Chapter 6 SLOPE STABILITY
6.3 GENERAL PROCEDURE FOR ANALYSIS
In general, analysis of slope stability would involves three basic parts:
a) Obtaining subsurface information
b) Determining appropriate soil shear strengths and
c) Determining a potential slide failure surface which provides the minimum safety factor
against failure under the various conditions
6.3.1 Obtaining Subsurface Information
Previous works carried out at the site of interest generally can provide some subsurface information
which are usually indicated in the design report or construction plans. The bore logs obtained may
or may not be located close to the site and the engineer must determine if additional subsurface
information is required. Additional boring(s) at the site are generally preferable. Other completed
work in the nearby vicinity may also provide useful information. Soil type, thickness of each soil
zone, depth to bedrock, and groundwater conditions must be known to proceed with a slope
stability analysis. Reader can refer to Volume 6 Part 2 for further information on this matter.
Before any analysis being carried out, it is always advisable to carry out geomorphological mapping
of the project area. The observations during the mapping works can sometimes help significantly in
deciding the types of tests, site investigation works and strengthening measures. The tell tale signs
observed during the mapping works i.e., water seepages, ground saturation, erosion; mode of
failure (deep seated or shallow slip) can be the references in the analysis and design stage. These
geomorphologic features are always tie up with the estimation of the design parameter i.e., ground
water condition, drainage adequacy and inherent properties (existence of discontinuities) which are
difficult to retrieve from site investigation works.
6.3.2 Determining of Soil Shear Strengths
The shear strength parameters of the embankment soil are normally defined in terms of a friction
component (φ ) and a cohesion component (c). Shear strengths are usually determined from
laboratory tests performed on specimens prepared by compaction in the laboratory or undisturbed
samples obtained from exploratory soil borings. The laboratory test data may be supplemented
with in situ field tests and correlations between shear strength parameters and other soil properties
such as grain size, plasticity, and Standard Penetration Resistance (N) values. For a more detail
discussion, reader can refer to Item. 3.3 of this Part.
In general, for drained shear parameters for effective stress analysis, consolidated undrained (CU)
can be used to obtained the effective soil strength parameter i.e., effective frictional angle φ‘ and
effective cohesion c’. Shear box test can also be used in determining the strength parameter. The
shear box sample shall be soaked in water for saturation and the shear rate shall be low to avoid
misleading results. High cohesion (sometimes as high as 10kPa) and low frictional angle are the
common error obtained from such tests if the saturation procedure is omitted.
6.3.3 Determining a Potential Slide Failure Surface
All of the limit equilibrium methods require that a potential slip surface to be assumed in order to
calculate the factor of safety. Circular slip surfaces can be assumed if the soil conditions are
revealed to be relatively homogeneous. If the soil conditions are not homogeneous or if geologic
anomalies appear, slope failures may occur on non-circular slip surfaces. The shape of the failure
surface will depend on the problem geometry and stratigraphy, material characteristics (especially
anisotropy), and the capabilities of the analysis procedure used. Commercially available computer
March 2009 6-3