Shear Strength of Soils - Elements of Soil Mechanics - page 105

Fig. 4.16 Stresses in the triaxial test.
Fig. 4.17 Typical triaxial test results.
Principal stresses
The intermediate principal stress, σ 2 , and the minor principal stress, σ 3 , are equal and are the radial stresses
caused by the cell pressure, p c . The major principal stress, σ 1 , consists of two parts: the cell water pressure
acting on the ends of the sample and the additional axial stress from the load transducer, q. To ensure
that the cell pressure acts over the whole area of the end cap, the bottom of the plunger is drilled so that
the pressure can act on the ball seating.
From this we see that the triaxial test can be considered as happening in two stages (Fig. 4.16) , the
first being the application of the cell water pressure (p c , i.e. σ 3 ), while the second is the application of a
deviator stress (q, i.e. [ σ 1 − σ 3 ]).
A set of at least three samples is tested. The deviator stress is plotted against vertical strain and the
point of failure of each sample is obtained. The Mohr circles for each sample are then drawn and the best
common tangent to the circles is taken as the strength envelope (Fig. 4.17) . A small curvature occurs in
the strength envelope of most soils, but this effect is slight and for all practical work the envelope can be
taken as a straight line.
Types of failure
Not all soil samples will fail in pure shear; there are generally some barrelling effects as well. In a sample
that fails completely by barrelling there is no definite failure point, the deviator stress simply increasing
slightly with strain. In this case an arbitrary value of the failure stress is taken as the stress value at 20%
strain (see Fig. 4.18) .
Note: In the past, soil laboratories made use of dial gauges to measure displacement, and proving rings
to measure applied loads. Some laboratories still use such equipment, and any reader interested in an
explanation and examples of their use is guided to the 6th, or earlier, editions of this topic.