SOIL INVESTIGATION - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 41

More recent procedures use piston control to obtain the Constant Rate of Strain
(CRS) test, the Constant Rate of Loading (CRL) test, the Constant Gradient test
(CG), the Constant Pore Pressure Ratio test (CPR), and recently the K 0 -CRS
compression test (Fig 3.2), which allows to measure the horizontal stress changes
during the test.
There are various methods to determine the uniaxial deformation parameters
from the measurements. The c v value can be obtained by Taylor's method,
Casagrande's method, Sridharan-Prakash' method, and Asaoka's method (see
Chapter 6). Approximate values for the elasticity modulus E can be obtained from
the CPT value q c , according to

E = aq c (3.1a)

Here, a is < 1 for peat, 1 - 2.5 for (silty) sand, 3 - 6 for clayey sand, or 3 - 8 for
soft clay. An alternative value for E is, incorporating the typically non-linear soil
behaviour

1 5 '

v
C

E

(3.1b)

( n )

c

v ' is an estimate of the vertical effective soils stress, and n the porosity. A
typical value for the compressibility index C c may be 0.005 for sand, 0.1 for clay
and 0.3 for peat.
Some clays are known to heave (swell) when they become saturated, which may
involve considerable forces. Measuring the free swell in the oedometer is called the
simple swelling test, and measuring the vertical load required to prevent swell
during saturation is called the swelling pressure test. The swelling process may
require a considerable time to reach final stage.

Here

N N

T T

T

T

(a) (b)
Figure 3.3 Direct Shear and Direct Simple Shear Box

Shear box test
The shear strength of soil is a crucial parameter for soil stability (failure). It is
related to the friction capacity between soil particles. Several shear tests are
available to evaluate the shear strength, such as under direct shear DS and direct
simple (uniform) shear DSS conditions (Fig 3.3). The normal force N and the shear
force T can be approximated by a uniform normal stress . The

test outcome is presented as an envelope, known as the Mohr-Coulomb failure line,
in the

and shear stress

stress plane, which determines the stability limit in terms of cohesion c
and internal friction angle

'-

. The state of stress in the DS test is clearly highly non-