The triaxial apparatus is the most widely used laboratory device for measuring soil behaviour in shear, and is suitable for all types of soil. The test has the advantages that drainage conditions can be controlled, enabling saturated soils of low permeability to be consolidated, if required, as part of the test procedure, and pore water pressure measurements can be made. A cylindrical speci- men, generally having a length/diameter ratio of 2, is used in the test; this sits within a chamber of pressurised water. The sample is stressed axially by a loading ram and radially by the confining fluid pressure under conditions of axial symmetry in the manner shown in Figure 5.9. The most common test, triaxial compression, involves applying shear to the soil by holding the confining pressure in the chamber constant and applying compressive axial load through the loading ram.
The main features of the apparatus are also shown in Figure 5.9. The circular base has a central pedestal on which the specimen is placed, there being access through the pedestal for drainage and for the measurement of pore water pressure. A Perspex cylinder, sealed between a ring and the circular cell top, forms the body of the cell. The cell top has a central bush through which the loading ram passes. The cylinder and cell top clamp onto the base, a seal being made by means of an O-ring. In some devices (such as that shown in Figure 5.9(b)), the sample and pedestal are pushed up from beneath and reacted against a load cell, though the applied total stresses on the soil are the same.
The specimen is placed on either a porous or a solid disc on the pedestal of the apparatus.
Typical specimen diameters (in the UK) are 38 and 100 mm. A loading cap is placed on top of the specimen, and the specimen is then sealed in a rubber membrane, O-rings under tension being used to seal the membrane to the pedestal and the loading cap to make these connections watertight. In the case of sands, the specimen must be prepared in a rubber membrane inside
Figure 5.9 The triaxial apparatus: (a) schematic, (b) a standard triaxial cell (image cour- tesy of Impact Test Equipment Ltd.).
Soil behaviour in shear a rigid former which fits around the pedestal. A small negative pressure is applied to the pore water to maintain the stability of the specimen while the former is removed prior to the applica- tion of the all-round confining pressure. A connection may also be made through the loading cap to the top of the specimen, a flexible plastic tube leading from the loading cap to the base of the cell; this connection is normally used for the application of back pressure (as described later in this section). Both the top of the loading cap and the lower end of the loading ram have coned seatings, the load being transmitted through a steel ball. The specimen is subjected to an all-round fluid pressure in the cell, consolidation is allowed to take place, if appropriate, and then the axial stress is gradually increased by the application of compressive load through the ram until failure of the specimen takes place, usually on a diagonal plane through the sample (see Figure 5.6). The load is measured by means of a load ring or by an electronic load trans- ducer fitted to the loading ram either inside or outside the cell. The system for applying the all-round pressure must be capable of compensating for pressure changes due to cell leakage or specimen volume change.
Prior to triaxial compression, sample consolidation may be permitted under equal increments of total stress normal to the end and circumferential surfaces of the specimen, i.e. by increasing the confining fluid pressure within the triaxial cell. Lateral strain in the specimen is not equal to zero during consolidation under these conditions (unlike in the oedometer test described in Section 4.2). This is known as isotropic consolidation. Dissipation of excess pore water pressure takes place due to drainage through the porous disc at the bottom (or top, or both) of the speci- men. The drainage connection leads to an external volume gauge, enabling the volume of water expelled from the specimen to be measured. Filter paper drains, in contact with the end porous disc, are sometimes placed around the circumference of the specimen; both vertical and radial drainage then take place (as in Section 4.11) and the rate of dissipation of excess pore water pres- sure is increased to reduce test time for this stage.
The pore water pressure within a triaxial specimen can usually be measured at all stages of the test, enabling the results to be expressed in terms of effective stresses within the sample, rather than just the known applied total stresses. Pore water pressure is normally measured by means of an electronic pressure transducer. In principle, the response measured during triaxial compression can range from fully undrained to fully drained by varying the loading rate of the ram. The drain- age conditions achieved can be verified using the pore water pressure measurements to ensure negligible excess pore water pressure (ue = 0) in a drained test or by sample volume measurements to ensure undrained conditions (ΔV = 0). For triaxial testing, Gibson and Henkel (1954) suggested that the test rates required for a given amount of drainage could be determined using Equations 5.17 and 5.18, where for one-way drainage (e.g. to the bottom of the sample) η = 0.75, while for two-way drainage (i.e. for a top cap containing an additional drain), η = 3. When there is radial drainage from filter paper around the sample, η = 32 and ch is used in place of cv in Equation 5.18.
Consolidation curves (similar to those in Figure 4.15 for the oedometer) are shown in Figure 5.10.
It can be seen that in the triaxial cell, there is a definable maximum test duration, faster than which the test will be undrained. This can be determined by setting Ut = 0 in Equation 5.17 and solving for t. As Equation 5.17 is asymptotic to Ut = 1, the minimum duration for a drained test may be estimated by setting Ut based on a tolerably small amount of consolidation being incom- plete (e.g. Ut = 0.95) and solving for t.
An alternative approach for achieving undrained conditions in the triaxial cell is to close the drains from the sample prior to shearing.
If the specimen is partially saturated, a fine porous ceramic disc must be sealed into the pedes- tal of the cell if the correct pore water pressure is to be measured. Depending on the pore size of
the ceramic, only pore water can flow through the disc, provided the difference between the pore air and pore water pressures is below a certain value, known as the air entry value of the disc.
Under undrained conditions the ceramic disc will remain fully saturated with water, provided the air entry value is high enough, enabling the correct pore water pressure to be measured. The use of a coarse porous disc, as normally used for a fully saturated soil, would result in the measure- ment of the pore air pressure in a partially saturated soil.
Test limitations and corrections
The average cross-sectional area (A) of the specimen does not remain constant throughout the test, and this must be taken into account when interpreting stress data from the axial ram load measurements. If the original cross-sectional area of the specimen is A0, the original length is l0
and the original volume is V0, then, if the volume of the specimen decreases during the test, A A= -
01- 1
e eva
(5.19) where εv is the volumetric strain (ΔV/V0) and εa is the axial strain (Δl/l0). If the volume of the spec- imen increases during a drained test, the sign of ΔV will change and the numerator in Equation 5.19 becomes (1+εv). If required, the radial strain (εr) could be obtained from the equation
ev= ea+2er (5.20)
In addition, the strain conditions in the specimen are not uniform due to frictional restraint pro- duced by the loading cap and pedestal disc; this results in dead zones at each end of the specimen, which becomes barrel-shaped as the test proceeds. Non-uniform deformation of the specimen can be largely eliminated by lubrication of the end surfaces. It has been shown, however, that non-uniform deformation has no significant effect on the measured strength of the soil, provided the length/diameter ratio of the specimen is not less than 2. The compliance of the rubber mem- brane must also be accounted for.
Figure 5.10 Relationships between degree of consolidation and time factor in the tri- axial cell.
Soil behaviour in shear
Interpretation of triaxial test data: strength
Triaxial data may be presented in the form of Mohr circles at failure; it is more straightforward, however, to present it in terms of stress invariants, such that a given set of effective stress condi- tions can be represented by a single point instead of a circle. Under 2-D stress conditions, the state of stress represented in Figure 5.6 could also be defined by the radius and centre of the Mohr cir- cle. The radius is usually denoted by t = ½(s1¢ - ¢s3), with the centre point denoted by s′ = ½(s1′ + ′s3).
These quantities (t and s′) also represent the maximum shear stress within the element and the average (mean) principal effective stress, respectively. The stress state could also be expressed in terms of total stress. It should be noted that
1 2
1
1¢ - ¢3 2 1 3
(
s s)
=(
s -s)
i.e. parameter t, like shear stress τ is independent of u. This parameter is known as the deviatoric stress invariant for 2-D stress conditions, and is analogous to shear stress τ acting on a shear plane (alternatively, τ may be thought of as the deviatoric stress invariant in direct shear). Parameter s′ is also known as the mean stress invariant, and is analogous to the normal effective stress acting on a shear plane (i.e. causing volumetric change but no shear). Substituting Terzaghi’s Principle into the definition of s′, it is apparent that
1 2
1
1¢ + ¢3 2 1 3
(
s s)
=(
s +s)
-uor, re-written, s′ = s–u, i.e. Terzaghi’s Principle rewritten in terms of the 2-D stress invariants.
The stress conditions and Mohr circle for a 3-D element of soil under a general distribution of stresses is shown in Figure 5.11. Unlike 2-D conditions when there are three unique stress com- ponents (s¢x, sz¢, τzx), in 3-D there are six stress components (s¢x, s¢y, sz¢, τxy, τyz, τzx). These can, however, be reduced to a set of three principal stresses, s1¢, s¢2 and s3¢. As before, s1¢ and s3¢ are the major (largest) and minor (smallest) principal stresses, respectively; s¢2 is known as the intermedi- ate principal stress. For the general case where the three principal stress components are different (s1¢ > ¢ > ¢s2 s3, also described as true triaxial conditions), a set of three Mohr circles can be drawn as shown in Figure 5.11.
As, in the case of 2-D stress conditions, it was possible to describe the stress state in terms of a mean and deviatoric invariant (s′ and t respectively), so it is possible in 3-D conditions. To distin- guish between 2-D and 3-D cases, the triaxial mean invariant is denoted p′ (effective stress) or p (total stress), and the deviatoric invariant by q. As before, the mean stress invariant causes only vol- umetric change (does not induce shear), and is the average of the three principal stress components:
¢ = ¢ + ¢ + ¢ p s1 s2 s3
3 (5.21)
Equation 5.21 may also be written in terms of total stresses. Similarly, q, as the deviatoric invari- ant, induces shearing within the sample and is independent of the pore fluid pressure, u:
q= 1 éë
(
-)
+(
-)
+(
-)
ùû2 1 2
2
2 3
2
3 1
2 1
s s s s s s 2 (5.22)
A full derivation of Equation 5.22 may be found in Atkinson and Bransby (1978). In the tri- axial cell, the stress conditions are simpler than the general case shown in Figure 5.11. During a
standard compression test, σ1 = σa, and due to the axial symmetry, σ2 = σ3 = σr, so that Equations 5.21 and 5.22 reduce to:
p= sa+2sr and p¢ = sa¢ + s¢r
3
2
3 (5.23)
q= ¢ - ¢ =sa sr sa-sr (5.24)
The confining fluid pressure within the cell (σr) is the minor principal stress in the standard tri- axial compression test. The sum of the confining pressure and the applied axial stress from the loading ram is the major principal stress (σa), on the basis that there are no shear stresses applied to the surfaces of the specimen. The applied axial stress component from the loading ram alone is thus equal to the deviatoric stress q, also referred to as the principal stress difference. As the intermediate principal stress is equal to the minor principal stress, the stress conditions at failure Figure 5.11 Mohr circles for triaxial stress conditions.
Soil behaviour in shear can be represented by a single Mohr circle, as shown in Figure 5.6 with s1¢ = ¢sa and s3¢ = ¢sr. If a number of specimens are tested, each under a different value of confining pressure, the failure envelope can be drawn and the shear strength parameters for the soil determined.
Because of the axial symmetry in the triaxial test, both 2-D and 3-D invariants are in common usage, with stress points represented by s′, t or p′, q respectively. The parameters defining the strength of the soil (ϕ′ and c′) are not affected by the invariants used; however, the interpretation of the data to find these properties does vary with the set of invariants used. Figure 5.12 shows the Mohr–Coulomb failure envelope for an element of soil plotted in terms of direct shear (σ′, τ), 2-D (s′, t) and 3-D/triaxial (p′, q) stress invariants. Under direct shear conditions, it has already been described in Section 5.3 that the gradient of the failure envelope τ/σ′ is equal to the tangent of the angle of shearing resistance and the intercept is equal to c′ (Figure 5.12(a)). For 2-D condi- tions, Equation 5.16a gives:
¢ - ¢
( )
=(
¢ + ¢)
¢ + ¢ ¢= ¢ ¢ + ¢ ¢
= ¢
s s s s f f
f f
1 3 1 3 2
2 2 2
sin cos
sin cos
s
c
t s c
t s iinf¢ + ¢c cosf¢
(5.25)
Therefore, if the points of ultimate failure in triaxial tests are plotted in terms of s′ and t, a straight-line failure envelope will be obtained – the gradient of this line is equal to the sine of the angle of shearing resistance and the intercept = c′cosϕ′ (Figure 5.12(b)).
Under triaxial conditions with σ2 = σ3, from the definitions of p′ and q (Equations 5.23 and 5.24) it is clear that:
¢ + ¢
(
s1 s3)
=(
s¢ + ¢s)
= 6 ¢ +a r p3 q
(5.26)
Figure 5.12 Interpretation of strength parameters c′ and ϕ′ using stress invariants.
and
¢ - ¢
(
s1 s3)
=(
sa¢ - ¢sr)
=q (5.27)The gradient of the failure envelope in triaxial conditions is described by the parameter M = q/p′, (Figure 5.12(c)) so that from Equation 5.16a:
′ − ′
( )
= ′ + ′( )
′ + ′ ′= ′ +
′ + ′
s s s s f f
f
1 3 1 3 2
6
3 2
sin cos
sin co
c
q p q c ss
sin sin
cos sin
′
= ′
− ′
′ + ′ ′
− ′
f f
f f
q 6 p c f
3
6 3
(5.28)
Equation 5.28 represents a straight line when plotted in terms of p′ and q with a gradient given by
M q
p
M M
= ¢ = ¢
- ¢
\ ¢ =
+ 6 3
3 6
sin sin sin
f f f
(5.29)
Equations 5.28 and 5.29 apply only to triaxial compression, where σa>σr. For samples under tri- axial extension, σr>σa, the cell pressure becomes the major principal stress. This does not affect Equation 5.26, but Equation 5.27 becomes
¢ - ¢
(
s1 s3)
=(
s sr¢ - ¢a)
= -q (5.30)giving:
sin ¢ = . f 3-
6 M
M (5.31)
As the friction angle of the soil must be the same whether it is measured in triaxial compression or extension, this implies that different values of M will be observed in compression and extension.
Interpretation of triaxial test data: stiffness
Triaxial test data can also be used to determine the stiffness properties (principally the shear mod- ulus, G) of a soil. Equation 5.5 may be simplified in terms of principal stresses and strains to give:
e e
e n
n n
n n
n n
s s s
1 2 3
1
1 2
2 1 1
1 1 é
ë êê ê
ù û úú ú
=
(
+)
- -
- -
- -
é ë êê ê
ù û úú ú
¢
¢ G ¢
33
é ë êê ê
ù û úú ú
(5.32)
The deviatoric shear strain εs within a triaxial cell (i.e. the strain induced by the application of the deviatoric stress, q, also known as the triaxial shear strain) is given by:
es= 23
(
e1-e3)
(5.33)Soil behaviour in shear Substituting for ε1 and ε3 from Equation 5.32 and using Equation 5.6, Equation 5.33 reduces to:
es= 1
3Gq (5.34)
This implies that on a plot of q versus εs (i.e. deviatoric stress versus deviatoric strain), the gradient of the curve prior to failure is equal to three times the shear modulus.
Generally, in order to determine εs within a triaxial test, it is necessary to measure both the axial strain εa (= ε1 for triaxial compression) and the radial strain εr (= ε3). While the former is routinely measured, direct measurement of the latter parameter requires sophisticated sensors to be attached directly to the sample, though the volume change during drained shearing may also be used to infer εr using Equation 5.20. If, however, the test is conducted under undrained condi- tions, then there will be no volume change (εv = 0) and hence from Equation 5.20:
er= -1ea
2 (5.35)
From Equation 5.33 it is then clear that for undrained conditions, εs = εa. A plot of q versus εa for an undrained test will thus have a gradient equal to 3G. Undrained triaxial testing is therefore extremely useful for determining shear modulus, using measurements which can be made on even the most basic triaxial cells. As G is independent of the drainage conditions within the soil, the value obtained applies equally well for subsequent analysis of the soil under drained loadings. In addition to reducing instrumentation requirements, undrained testing is also much faster than drained testing (Figure 5.10), particularly for saturated clays of low permeability.
If drained triaxial tests are conducted in which volume change is permitted, radial strain mea- surements should be made such that G can be determined from a plot of q versus εs. Under these test conditions the drained Poisson’s ratio (ν′) can also be determined, being:
¢ = -
n e
e
r a
(5.36) Under undrained conditions it is not necessary to measure Poisson’s ratio (νu), as, comparing Equations 5.35 and 5.36, it is clear that νu = 0.5 for there to be no volume change.
Testing under back pressure
Testing under back pressure involves raising the pore water pressure within the sample artificially by connecting a source of constant fluid pressure through a porous disc to one end of a triaxial specimen. In a drained test this connection remains open throughout the test, drainage taking place against the back pressure; the back pressure is then the datum for excess pore water pressure measurement. In a consolidated-undrained test (described later) the connection to the back pres- sure source is closed at the end of the consolidation stage, before the application of the principal stress difference is commenced.
The object of applying a back pressure is to ensure full saturation of the specimen or to simulate in-situ pore water pressure conditions. During sampling, the degree of saturation of a fine-grained soil may fall below 100% owing to swelling on the release of in-situ stresses. Compacted speci- mens will also have a degree of saturation below 100%. In both cases, a back pressure is applied which is high enough to drive the pore air into solution in the pore water.
It is essential to ensure that the back pressure does not by itself change the effective stresses in the specimen. It is necessary, therefore, to raise the cell pressure simultaneously with the
application of the back pressure and by an equal increment. Consider an element of soil, of vol- ume V and porosity n, in equilibrium under total principal stresses σ1, σ2 and σ3, as shown in Figure 5.13, the pore pressure being u0. The element is subjected to equal increases in confining pressure Δσ3 in each direction i.e. an isotropic increase in stress, accompanied by an increase Δu3 in pore pressure.
The increase in effective stress in each direction = Δσ3–Δu3 Reduction in volume of the soil skeleton = CsV(Δσ3–Δu3)
where Cs is the compressibility of the soil skeleton under an isotropic effective stress increment.
Reduction in volume of the pore space = CvnVΔu3
where Cv is the compressibility of the pore fluid under an isotropic pressure increment.
If the soil particles are assumed to be incompressible and if no drainage of pore fluid takes place, then the reduction in volume of the soil skeleton must equal the reduction in volume of the pore space, i.e.
C Vs
(
D - Ds3 u3)
=C nV uv D 3Therefore,
∆u ∆
n C C
3 3 1
= 1
+
( )
s
v s
Writing 1/[1+n(Cv/Cs)] = B, defined as a pore pressure coefficient,
D = Du3 B s3 (5.37)
In fully saturated soils the compressibility of the pore fluid (water only) is considered negligible compared with that of the soil skeleton, and therefore Cv/Cs→0 and B→1. In partially saturated soils the compressibility of the pore fluid is high due to the presence of pore air, and therefore Cv/Cs> 0 and B < 1. The variation of B with degree of saturation for a particular soil is shown in Figure 5.14.
The value of B can be measured in the triaxial apparatus (Skempton, 1954). A specimen is set up under any value of all-round pressure and the pore water pressure measured. Under undrained conditions the cell pressure is then increased (or reduced) by an amount Δσ3 and the change in Figure 5.13 Soil element under isotropic stress increment.