ADVANCED TRIAXIAL TESTING
Rakesh J Pillai
IIT Palakkad
Google Scholar ID -
https://scholar.google.com/citations?user=MieMHGoAAAAJ&hl=en
OVERVIEW
1 Why Triaxial Test?
2 Mohr-Coulomb Failure Envelop
3 Standard Triaxial Test
4 Stress Paths
5 Cyclic Triaxial Test
WHY TRIAXIAL TEST?
SHEAR STRENGTH OF SOILS
• Stability of slopes: Cuts, embankments, earth dams, hill slopes etc.
• Bearing capacity of shallow foundations
• Stability of retaining walls, sheeting/bracing
• Load carrying capacity of deep foundations (eg. pile foundations)
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SHEAR STRENGTH OF SOILS
Strength of the soil is its capacity to resist stresses acting on it. Soil gets its
strength from three main components.
1. Frictional resistance to sliding and rolling between the soil particles
2. Cohesion between the particles
3. Interlocking between the soil particles
Within the soil mass there will be a large number of soil particles interacting
between one another and it is difficult to delineate the influences of each
component.
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SHEAR STRENGTH OF SOILS
Strength of the soil is its capacity to resist stresses acting on it. Soil gets its
strength from three main components.
1. Frictional resistance to sliding and rolling between the soil particles
2. Cohesion between the particles
3. Interlocking between the soil particles
Within the soil mass there will be a large number of soil particles interacting
between one another and it is difficult to delineate the influences of each
component.
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SOIL - FRICTIONAL MATERIAL
From the basics of friction, we have seen that the maximum shearing
resistance is developed when the angle of obliquity (α) equals to its limiting
value (ie. angle of friction, ϕ)
Analogous to this, it can be assumed that the soil mass will fail when on a
plane the angle of maximum obliquity (α) reaches a limiting value.
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SOIL - FRICTIONAL MATERIAL
In the case of soil this limiting angle is called the angle of internal friction (ϕ).
Therefore in the case of soil the failure plane is not the plane of maximum
shear, rather it is the plane of maximum obliquity.
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MOHR-COULOMB FAILURE ENVELOP
Shear strength can be expressed using two lines passing through the origin
and making angles +ϕ and –ϕ to the horizontal.
These lines are called Mohr - Coulomb strength (or failure) envelope whose
equation is given by τ f = σ f tan(ϕ)
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MOHR-COULOMB FAILURE ENVELOP
MOHR-COULOMB FAILURE ENVELOP
Difference in sand and clay behaviour:
Sand when poured Clays will form lumps
The complete Mohr-Coulomb failure envelope includes Cohesion as shown
below with the equation τ f = C + σ f tan(ϕ) .
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MOHR-COULOMB FAILURE ENVELOP
Importance of Effective Stress
Mohr-Coulomb failure envelope as given by previous equation is written in
terms of total stresses and was found to be only partially successful in
predicting the shear strength of the soil.
Terzaghi introduced the concept of effective stress and proved
experimentally that all the important properties of soils like compressibility,
shear strength etc. are related to the effective stress.
So, the Mohr-Coulomb failure envelope in terms of effective stress is given as:
τ f = c′ + σ′f tan(ϕ′ )
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MOHR-COULOMB FAILURE ENVELOP
Importance of Effective Stress
The shear strength parameters corresponding to the total stresses (c, ϕ) will
be different from those corresponding to effective stresses (c′ , ϕ′ )
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STANDARD TRIAXIAL TEST
STANDARD TRIAXIAL TEST
Standard Triaxial test is a widely used laboratory experiment to determine
the total and effective shear strength parameters for almost all types of soil
samples.
The actual stress conditions in the field can be simulated in this test and the
soil sample is allowed to fail along its natural failure plane (failure plane is
not predefined).
Complete control of the drainage from the soil sample is possible along with
provision to measure the pore water pressure. This is very important to get
the effective strength parameters.
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TRIAXIAL TEST (IS 2720 - PART 11 AND PART 12)
Is this stress state truely triaxial ???
The stress state is a special case of triaxial stress state with intermedi-
ate principal stress equal to minor principal stress (ie. σ2 = σ3 ). This ♣
can be easily idealized as a biaxial stress state.
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TRIAXIAL TEST SETUP
(Taken from "Soil Mechanics Fundamentals", by Muni Budhu (2015))
Test is performed on a cylindrical sample with height twice as that of
diameter kept inside a triaxial cell after wrapping with a rubber membrane.
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TRIAXIAL TEST
Volume change of the sample can be determined by measuring the volume
of water draining out from (or going in to) the sample and the axial load is
applied through a piston at the top of the sample.
The deviatoric stress (σd ) can be calculated from the measured values of the
P
applied axial load (P) as σd = where Ac is the corrected area given by
Ac
following equation for undrained shearing
A
Ac =
1 – ϵa
where, A is the initial area, and ϵa is the axial strain.
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TRIAXIAL TEST
In order to determine the shear strength parameters, we need to perform
triaxial tests under different confining pressures (minimum three trials with
different σ3 values are required).
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TRIAXIAL TEST
Major principal stress at failure can be obtained as
σ11 f = σ13 + σ1d f
Mohr circles corresponding to different effective confining pressures can be
plotted
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TYPES OF TRIAXIAL TESTS BASED ON DRAINAGE CONDITIONS
Based on the drainage conditions prevailing during the triaxial tests, they
can be classified into three
1. Unconsolidated Undrained (UU) test
2. Consolidated Undrained (CU) test
3. Consolidated Drained (CD) test
To understand the differences between these types, we have to first
understand in detail the stages involved in a triaxial test.
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TYPES OF TRIAXIAL TESTS BASED ON DRAINAGE CONDITIONS
Based on the drainage conditions prevailing during the triaxial tests, they
can be classified into three
1. Unconsolidated Undrained (UU) test
2. Consolidated Undrained (CU) test
3. Consolidated Drained (CD) test
To understand the differences between these types, we have to first
understand in detail the stages involved in a triaxial test.
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TYPES OF TRIAXIAL TESTS BASED ON DRAINAGE CONDITIONS
Based on the drainage conditions prevailing during the triaxial tests, they
can be classified into three
1. Unconsolidated Undrained (UU) test
2. Consolidated Undrained (CU) test
3. Consolidated Drained (CD) test
To understand the differences between these types, we have to first
understand in detail the stages involved in a triaxial test.
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STAGES IN A TRIAXIAL TEST
The three main steps involved in triaxial tests are discussed in detail in this
section. The three stages are (a) saturation (b) consolidation and (c) shearing
Saturation stage
In triaxial tests (except in UU test), it is required that the sample is completely
saturated.
Complete saturation of the sample is ensured by applying back pressure
(external pressure is applied into the sample to dissolve the air in the pore
spaces).
To check whether the sample is fully saturated we use a parameter called
Skempton’s B parameter. Theoretically, a fully saturated soil sample will
exhibit a B parameter value of 1.
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STAGES IN A TRIAXIAL TEST
The three main steps involved in triaxial tests are discussed in detail in this
section. The three stages are (a) saturation (b) consolidation and (c) shearing
Saturation stage
In triaxial tests (except in UU test), it is required that the sample is completely
saturated.
Complete saturation of the sample is ensured by applying back pressure
(external pressure is applied into the sample to dissolve the air in the pore
spaces).
To check whether the sample is fully saturated we use a parameter called
Skempton’s B parameter. Theoretically, a fully saturated soil sample will
exhibit a B parameter value of 1.
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STAGES IN A TRIAXIAL TEST
The three main steps involved in triaxial tests are discussed in detail in this
section. The three stages are (a) saturation (b) consolidation and (c) shearing
Saturation stage
In triaxial tests (except in UU test), it is required that the sample is completely
saturated.
Complete saturation of the sample is ensured by applying back pressure
(external pressure is applied into the sample to dissolve the air in the pore
spaces).
To check whether the sample is fully saturated we use a parameter called
Skempton’s B parameter. Theoretically, a fully saturated soil sample will
exhibit a B parameter value of 1.
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STAGES IN A TRIAXIAL TEST
The three main steps involved in triaxial tests are discussed in detail in this
section. The three stages are (a) saturation (b) consolidation and (c) shearing
Saturation stage
In triaxial tests (except in UU test), it is required that the sample is completely
saturated.
Complete saturation of the sample is ensured by applying back pressure
(external pressure is applied into the sample to dissolve the air in the pore
spaces).
To check whether the sample is fully saturated we use a parameter called
Skempton’s B parameter. Theoretically, a fully saturated soil sample will
exhibit a B parameter value of 1.
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STAGES IN A TRIAXIAL TEST
The three main steps involved in triaxial tests are discussed in detail in this
section. The three stages are (a) saturation (b) consolidation and (c) shearing
Saturation stage
In triaxial tests (except in UU test), it is required that the sample is completely
saturated.
Complete saturation of the sample is ensured by applying back pressure
(external pressure is applied into the sample to dissolve the air in the pore
spaces).
To check whether the sample is fully saturated we use a parameter called
Skempton’s B parameter. Theoretically, a fully saturated soil sample will
exhibit a B parameter value of 1.
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STAGES IN A TRIAXIAL TEST
Consolidation stage
The second stage in the triaxial tests (except in UU test) is the consolidation
stage.
The consolidation stage is to ensure that the sample is at equilibrium under
the required effective stress (matching with the field conditions). Eg: if the
required effective confining pressure (σ′3 ) is 50 kPa, difference between the
cell pressure and back pressure is maintained as 50 kPa and the sample is
allowed to consolidate.
The sample should be allowed to consolidate by allowing drainage from the
sample till the whole pore pressure is dissipated (minimum one day should
be allowed for this stage).
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STAGES IN A TRIAXIAL TEST
Shearing stage
The third stage in the triaxial tests is the shearing stage where the axial load
is applied on the top of the sample.
Usually the shearing is performed by making the soil sample to deform at a
constant rate (eg. 0.05 mm/min) - strain controlled loading
The resistance offered by the sample is measured using a proving ring (or a
load transducer), the deformation using a dial gauge (or LVDT), and pore
pressure using a pressure transducer.
During the shearing stage we can allow the drainage from the sample or
prevent the drainage, depending on the requirements.
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UNCONSOLIDATED UNDRAINED (UU) TEST
Though the samples are confined at different confining pressures as shown
by three total stress circles, all the samples have only one effective stress
circle.
Angle of internal friction cannot be determined using a UU test as the
effective normal stress is not changed.
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CONSOLIDATED DRAINED (CD) TEST
Consolidated drained (CD) test is a completely drained test where after
consolidation, drainage is allowed during the shearing stage also.
The shearing stage of the CD test has to be performed very slowly ensuring
that the pore pressure is completely dissipated during the entire duration of
the test (so preferred for coarse grained soils only).
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CONSOLIDATED UNDRAINED (CU) TEST
During the shearing stage drainage is not allowed in the case of CU test by
closing the drainage valve excess pore water pressure generated during the
shearing stage is measured.
Both total and effective strength parameters can be obtained from a CU test.
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SAND BEHAVIOUR IN TRIAXIAL TEST
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CLAY BEHAVIOUR IN TRIAXIAL TEST
Over consolidated
Over consolidated
e (%)
Normally consolidated Normally consolidated
Axial strain, e (%)
(a) (b)
Normally consolidated
(o)
Over consolidated
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CLAY BEHAVIOUR IN TRIAXIAL TEST
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STRESS PATHS
STRESS PATHS
(σ1 + σ3 ) (σ′1 + σ′3 )
s= s′ =
2 2
(σ1 – σ3 ) (σ′1 – σ′3 )
t= t′ =
2 2
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STRESS PATHS
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STRESS PATHS
Based on Octahedral Stresses
(σ1 + 2σ3 ) (σ′1 + 2σ′3 )
p= p′ =
3 3
q = σ1 – σ3 q′ = σ′1 – σ′3
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STRESS PATHS
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NORMALLY CONSOLIDATED (NC) CLAYS
CD TESTS
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NORMALLY CONSOLIDATED (NC) CLAYS
CIU TESTS
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OVER CONSOLIDATED (OC) CLAYS
CD TESTS
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OVER CONSOLIDATED (OC) CLAYS
CIU TESTS
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NORMALLY CONSOLIDATED (NC) CLAYS
FAMILIES OF UNDRAINED TESTS
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NORMALLY CONSOLIDATED (NC) CLAYS
FAMILIES OF DRAINED TESTS
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THE CRITICAL STATE
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STRESS PATH CONTROLLED TRIAXIAL TEST
Anisotropic Consolidation
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STRESS PATH CONTROLLED TRIAXIAL TEST
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CYCLIC TRIAXIAL TEST
SOILS UNDER CYCLIC LOADING
• Soil deposits are subjected to repeated (cyclic) loading due to:
– Earthquakes
– Wave action in case of marine deposits
– Traffic loading
• Wide ranges of frequencies and amplitudes
– Behavior is studied by applying equivalent cyclic loading on
laboratory specimen
– Cyclic triaxial tests
– Cyclic Simple Shear tests
– Resonant Column tests
• Cyclic loading on clays leads to pore pressure build up and cumulative
increase of strains (Strain softening), which may exceed the permissible
values
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SOILS UNDER CYCLIC LOADING
Experimental observations shows that soils subjected to cyclic loading
respond in different ways
• If the load is sufficiently small,
the soil respond elastically and
no permanent strains are
induced.
• If the load is above critical elastic
limit, permanent strains develop
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CYCLIC TRIAXIAL TEST
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SOILS UNDER CYCLIC LOADING
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CYCLIC TRIAXIAL TEST SETUP
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CYCLIC TRIAXIAL TEST RESULTS
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CYCLIC TRIAXIAL TEST RESULTS
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CYCLIC TRIAXIAL TEST - PAVEMENTS
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Thank you for your Attention !!!
Questions?
