Triaxial Compression Test - Soil Properties: Testing, Measurement, and Evaluation - page 355

Figure 22-8 Results of Consolidated Undrained Triaxial Tests on
Normally Consolidated Clay

(It should be noted that is the pore pressure corresponding to the unit

axial load at failure and p .) The strength envelope in this case

is referred to as the effective stress strength envelope .
Results of CU tests are also commonly presented with Mohr circles
plotted in terms of total stress

σ 1 = σ 3 + Δ

. The strength envelope in this case is
referred to as the total stress strength envelope .
Both the effective stress strength envelope and the total stress
strength envelope obtained from a CU test on normally consolidated
clay are shown in Figure 22-8. It will be noted that the Mohr circle has
equal diameters for total stresses and effective stresses but the Mohr
circle for effective stresses is displaced leftward by an amount equal to
the pore pressure at failure µ f .
If several CU tests are performed on the same normally consoli-
dated clay initially consolidated under different lateral pressures

σ

σ 3 , the
total stress strength envelope is approximately a straight line passing
through the origin (see Figure 22-8). Hence, results of CU triaxial tests
on normally consolidated clays can be expressed by Coulomb's equation,

s tan (22-9)

σ CU

where

CU is known as the consolidated undrained angle of internal
friction.

A consolidated undrained triaxial compression test was performed on a
soil sample in the laboratory according to the procedure described. The
data obtained in the laboratory and subsequent computed data are
given in the form on page 361 and 362. (At the end of the chapter, two

NUMERICAL
EXAMPLE