Shear Strength of Soils - Elements of Soil Mechanics - page 109

Solution:
The Mohr circle diagram is shown in Fig. 4.22. The circles are drawn first and then, by
constructing the best common tangent to these circles, the strength envelope is
obtained.
In this case it is seen that the soil is cohesionless as there is no cohesive intercept.
By measurement, φ ′ = 29°.
Fig. 4.22 Example 4.5.
ii) The consolidated undrained test
This is the most common form of triaxial test used in soils laboratories to determine c ′ and φ ′ . It has the
advantage that the shear part of the test can be carried out in only two to three hours.
The sample is consolidated exactly as for the drained test, but at this stage the drainage connection is
shut off and the sample is sheared under undrained conditions. The application of the deviator stress
induces pore water pressures (which are measured), and the effective deviator stress is then simply the
total deviator stress less the pore water pressure.
Although the sample is sheared undrained, the rate of shear must be slow enough to allow the induced
pore water pressures to distribute themselves evenly throughout the sample. For most soils a strain rate
of 0.05 mm/min is satisfactory, which means that the majority of samples can be sheared in under three
hours.
Note: With respect to total stress, the strength parameter is c u (because φ = 0) while with respect to
effective stresses the strength parameters are c ′ and φ ′ .
Testing with back pressures
It should be noted that, with some soils, the reduction of the pore water pressure to atmospheric during
the consolidation stage of a triaxial test on a saturated soil sample can cause air dissolved in the water
to come out of solution. If this happens, the sample is no longer fully saturated and this can affect the
results obtained during the shearing part of the test.
To maintain a state of occlusion in the pore water, i.e. the state where air can no longer exist in a free
state but only in the form of bubbles, its pressure can be increased by applying a pressure (known as a
back pressure) to the water in the drainage line (the soil water can still drain from the sample). The back