a frictional component (I,I') and a non-frictional
component, or cohesion (c, c’). Figure 1 below
graphically depicts the Mohr-Coulomb failure criterion.
Figure 1: Mohr-Coulomb failure criterion.
Mohr-Coulomb failure envelopes can be developed
through the use of Mohr’s circles of stress representing
the stress states at failure for a series of tests (often
three tests). As seen in Figure 1, both total and
effective stress failure envelopes can be developed.
Failure envelopes for soils are often curved. However,
for mathematical simplicity, an analyst will
approximate the failure envelope as linear over the
normal stress range of interest for the analysis. The
Mohr-Coulomb failure criterion is, therefore,
represented by a straight line (failure envelope) with a
slope designated as the friction angle (I,I') and an
intercept called the cohesion (c, c’). The normal (or
vertical) stress (ı,ı’) acting on soil in an embankment
at a given depth, is represented by the horizontal axis,
and can be either total stress or effective stress. Note
that the greater the normal stress, the greater the
frictional component and overall shear strength.
Shear strength tests are performed on soils using a
range of consolidation pressures to develop the
strength envelope from Mohr-Coulomb plots. The
following section will discuss various laboratory tests
used to evaluate the shear strength of soils, and which
laboratory tests are appropriate for drained and
Laboratory Shear Strength Tests
The five types of laboratory tests most widely used to
estimate shear strength in soils are:direct shear,
unconfined compression,triaxial shear,direct simple
shear, and torsional ring shear. These five tests are
described in detail below and are performed in
accordance with ASTM standards. The ASTM standard
for each test outlines sample preparation, failure
criterion, and procedures for saturation, consolidation,
applicable).
Direct Shear Test
The oldest and simplest shear strength test is the
direct shear (DS) test. DS testing is performed as
described in ASTM D3080. In the DS test, a thin soil
sample is placed in a shear box that is split horizontally
into halves. A normal force (Pv) is applied to the top of
from 0 to 150 pounds per square inch (psi). The lower
half of the box is fixed, while a shear force (Ph) is
applied to the upper half, thereby moving the upper
half parallel to the lower half and forcing the soil
specimen to fail along a horizontal shear plane. A
schematic of the test apparatus is shown in Figure 2.
Figure 2: Direct shear test apparatus.
The DS test is performed in a strain-controlled
(deformation-controlled) manner per ASTM D3080. In
the strain-controlled test, a constant rate of shear
displacement is applied to the top half of the box by a
motor that acts through gears. Shear displacement
('H) of the top half of the box is measured by a
horizontal dial gauge or displacement transducer. The
resisting shear force of the soil can be measured by a
horizontal proving ring or load cell. A dial gauge or