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

Chapter 3 FUNDAMENTAL PRINCIPLES
M-C Failure Envelope
M-C Failure Envelope
Figure 3.7 Mohr-Coulomb’s Circles and Failure Envelopes
The physical meaning of the M-C failure envelope may be explained as follows:
• Every point on the M-C failure envelope represents a combination of normal and shear stress
that results in failure of the soil, i.e. the limiting state of stress for equilibrium.
• If the state of stress is represented by a point below the M-C failure envelope then the soil
will be stable for that state of stress.
• States of stress beyond the M-C failure envelope cannot exist since failure would have
occurred before that point could be reached.
3.4.2 Effective Versus Total Stress Analysis
It is important to note that the properties of soil and its shear strength in the vicinity of construction
facility could change with time. As explained in Item 3.2, when the stress in the soil is suddenly
changed (e.g. due to applied load), the additional stress is initially carried by the pore water
pressure resulting to what is known as excess pore water pressure. If a foundation consolidates
slowly, relative to the rate of construction, a substantial portion of the applied load will be carried
by the pore water, which has no shear strength, and the available shearing resistance is limited to
the in-situ shear strength. In this case, analysis are carried out using the total stress (undrained)
analysis.
3-8 March 2009