SOIL MECHANICS II - Triaxial cell

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Jul 18, 2012 (5 years and 28 days ago)

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ACEG220 SOIL MECHANICS II

Triaxial cell

Objective

In this lecture we will learn about another method of testing the shear strength of a
soil in the laboratory other than the shear box test.

Introduction

So far, you have learned how to measure the shear strength of a soil using the shear
box. There is another method, called a triaxial test – so called because the sample of
soil is loaded on 3 axes. The apparatus is called a triaxial cell
.

The soil is not sheared directly like in a shear box but is sheared by shear stresses
generated in the sample by the difference between vertical and horizontal normal
stresses applied to the sample.

The triaxial test is a more advanced method of testing which allows the soil to be
tested in many different ways.

Triaxial cell apparatus

A cylindrical (column-shaped) sample of soil is contained in a rubber membrane and
placed inside the clear Perspex triaxial cell (Figure 1). The cell is filled with water and
the water is given a pressure (the cell pressure
). The cell pressure applies a normal
stress all around the sample of soil. This pressure represents the in-situ stresses that
are present in the ground due to the weight of soil above.



Figure 1: Triaxial cell apparatus and soil sample

Test procedure

At the start of the test, the cell pressure all around the soil sample is the same. The
deviator stress, q
(= σ1 – σ3) is the extra vertical stress in the sample caused by the
vertical load, Q. At the start of the test Q = 0 but it is increased while the horizontal
stresses σ2 and σ3 stay the same.
D = 38mm
or 100mm
L = 2D
σ3
σ2
σ1
Soil sample

vertical load, Q
cell filled with water
clear Perspex cell wall
soil sample contained
in rubber membrane
cell pressure
sample drainage
drained undrained

2
The vertical load Q continues to increase until the soil sample fails (Figure 2).

Figure 2: Shear failure of soil sample at end of test


In a triaxial cell, either drained or undrained tests can be carried out by opening or
closing a tap that allows water to flow into or out of the sample. In drained tests
volume changes of the soil sample can also be measured. In the shear box tests you
measured the vertical movement of the lid to estimate the volume change (dilation or
compression). In a triaxial test, the volume of water flowing into the sample (dilation)
and the volume of water of flowing out of the sample (compression) can be measured
directly.

Triaxial testing on sands as part of a typical site investigation is unusual. Most triaxial
testing is done on clays and silts. There three basic types of triaxial test:

i) Unconsolidated undrained test (UUT)

The most basic type and is used to measure the undrained shear strength cu of a
clay. Each sample of clay is placed in the triaxial cell, a cell pressure is applied and
the vertical load Q increased to failure. The tap to the sample is always closed so
there is no consolidation of the sample and the shear failure is undrained.
It is a quick and cheap method but the disadvantages are that the results are highly
variable and inaccurate and the test does not simulate soil conditions in the ground.

ii) Consolidated undrained test (CUT)

In this test the soil sample is saturated and consolidated back to its in-situ state in the
ground before the vertical load Q is increased. Once the sample is in equilibrium, the
tap to the sample is closed and the deviator stress is increased until undrained failure
occurs.

iii) Consolidated drained test (CDT)

The soil sample is brought to equilibrium as in the CUT but the tap to the sample is
left open as the vertical load Q is increased. Depending on the permeability of the
sample, the vertical load Q must increase very slowly so that drained shear failure
occurs. Volume change can also be measured directly from the volume of water
entering (dilation) or leaving (compression) the sample.

vertical load, Q
shear failure
axial displacement, a
y

3
A quick way to measure the effective stress strength (φ′) properties of a clay

Imagine you need to find the effective stress (drained) friction angle φ′ of a clay. To
do a drained triaxial test on a clay could take days and would be expensive. What
would be a quicker and cheaper way?

If we measured the pore pressure, u
in a consolidated undrained test, we would be
able to calculate the effective stresses inside the sample and hence the friction angle
φ′, even though the soil failed in an undrained way. This type of triaxial test is called a
“consolidated undrained test with measurement of pore pressure
”.


Calculation of soil shear strength φ′ or c
u in a triaxial test

You may remember that in a shear box test the shear strength of a soil is obtained by
measuring the shear force (T) at failure under at least three different normal forces
(N). These results are then plotted and either a friction angle φ′ (drained test) or an
undrained shear strength cu is obtained (Figure 3).
Figure 3: Reminder of shear strengths obtained from shear box tests

Soil shear strengths are measured in a similar way in a triaxial test. The deviator
stress q at failure is calculated from triaxial tests on samples of soil with different cell
pressures.

Figure 4: Typical graph of deviator stress during a triaxial test on a stiff clay

The deviator stress q = Q/A where is A is the cross-sectional area of the sample.
However, A changes during the test due to compression of the sample and this is
taken into account by using the equation q = (Q/A0)(1 – εa)/(1 – εvol) where A0 is the
cross-sectional area of the sample at the start of the test.

Since the soil shears within the sample without any shear force being applied directly
or being measured, we must calculate the shear stress in the soil from Mohr’s circles.
This process will be explained by example, but the Mohr’s circles you plot will look
something like those shown below.
Normal effective
stress σ

(kPa)
φ′
Friction angle
(strength) of soil
Shear stress
τ (kPa)
Normal total
stress σ

(kPa)
Undrained shear
strength cu
Shear stress
τ (kN)
A
xial strain, εa
Deviator stress,
q = (σ1 - σ3)
stiff clay
Deviator stress
at failure

4
Effective stress test
(CDT or CUT with measurement of pore pressure)
Calculate σ′1 and σ′3 at failure for each test. Then draw the Mohr’s circles at failure for
each test by plotting the σ′1 and σ′3 values along the horizontal axis of the graph
shown below (where each axis has THE SAME SCALE
). The friction angle φ′ is then
found by drawing a failure line that touches all three circles and passes as close to
the origin as possible – compare with the failure line in Figure 3.

Reading from the failure line on the graph, slope = (140 – 8)/(230 – 0).
φ′ = tan
-1 (slope) = tan
-1 (132/230)

Therefore, φ′ = 29.9°, c′ = 8kPa

Total stress test
(UUT)
Calculate σ1 and σ3 at failure for each test. Then draw the Mohr’s circles at failure for
each test by plotting the σ1 and σ3 values along the horizontal axis of the graph
shown below (where each axis has THE SAME SCALE
). The undrained shear
strength cu is then found by drawing a horizontal failure line that touches the tops of
all three circles. The point where the failure line crosses the vertical axis is the cu
value. Compare with the undrained failure line in Figure 3.

Reading from the failure line on the graph, the undrained shear strength of the clay
cu = 80kPa.

Advantages of the triaxial test

There are several advantages of the triaxial test over the shear box test for
measuring the shear strength of a soil. These include:
i) direct measurement of volume change of saturated soils
ii) shear failure occurs on preferred planes in the soil rather than forced on a
particular plane as in a shear box
iii) more control of parameters, such as pore pressure and normal stress, is possible
iv) pore pressure can be measured.
0
50
100
150
050100150200250300350400
normal total stress
σ
(kPa)
shear stress
(kPa)
test 2
test 1
test 3
σ1
σ3
σ3
σ3
σ1
σ
1

failure line
c
u
0
50
100
150
050100150200250300350400
normal effective stress
σ
' (kPa)
shear stress
(kPa)
φ′
test 2
test 1
test 3
σ′1
σ′3
σ′3
σ′3
σ′1
σ′1
failure line τ = c′ + σ′ tanφ′
c′