TRIAXIAL STRESS AND STRAIN - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 103

to the failure surface, such that its middle (being the centre of the circle) lies on the
horizontal coordinate axis. The construction is shown in Fig 7.9 (aided by the
dotted line that cuts the line T s to DC 1 in half). Laboratory test results (line AB)
reveal some reserve strength before failure would occur. Note that groundwater
table rise by heavy rain would make the circle move to the left creating failure
when touching AB.

kPa

A

q

A'

B'

160

160

B

q

C'

120

120

C

u

80 80

M

40

40

1

p, p'

0.05 0.10 0.15

80 160

A B C

(a) (b)
Figure 7.10 A triaxial test and corresponding total (ABC) and effective (A'B'C') stress
paths

application 7.2
A soil sample is tested in a triaxial cell. The first stage of the test is drained
(consolidated) under an isotropic stress of 180 kPa; the second is undrained under
vertical strain-controlled loading (a constant strain rate is applied). The response, in
terms of deviator stress q and pore pressure u , is continuously measured and
represented as a function of the vertical strain

1 in Fig 7.10a.
At first inspection, it may seem that the sample almost immediately collapses,
but a detailed analysis shows that this is not the case. The development of the
stress-strain state is evaluated at three distinct states (strain levels A, B and C) in a
p-q diagram, see Fig 7.10.

TABLE 7.3

q 1 u 3 3 ' 1 1 ' p p' M

o

kPa % kPa kPa kPa kPa kPa kPa kPa

A

190 1.25 40 180 140 370 330 243 203

B

165 7.5 100 180 80 345 245 235 135 1.22 30.5

C

125 15 120 180 60 305 185 222 102 1.23 30.7

obtained from the graph 3 u q+ 3 1 u % / 3 p u q/p' (7.13)

remark

At state A the effective stresses are still high, and the pore pressure is yet low.
With time, the pore pressure increases during continuing compression, the effective