Triaxial Compression Test: Apparatus and Procedure | Soil Engineering

In this article we will discuss about:- 1. Apparatus for Triaxial Compression Test 2. Preparation of Soil Specimen for Triaxial Compression Test 3. Assembly of the Apparatus 4. Test Procedure 5. Calculation of Principal Stresses 6. Determination of Shear Strength Parameters 7. Types of Shear Tests Based on Drainage Conditions 8. Merits 9. Demerits 10. Coefficient of Earth Pressure at Rest for Sands and Other Details.

Apparatus for Triaxial Compression Test:

The main apparatus for triaxial compression test is the triaxial cell that is shown in Fig. 13.19 with all its accessories. The triaxial cell is a high-pressure cylindrical cell made of Perspex or other transparent material fitted between the base and the top cap. The triaxial cell assembly with all accessories is shown schematically in Fig. 13.20.

The soil specimen is cylindrical in shape with 38 mm diameter and 76-mm height, enclosed in a rubber mem­brane between solid or porous discs at top and bottom. The triaxial cell is filled with a cell fluid through the cell fluid inlet, during which the air-release valve is kept open. Pressure is increased in the cell fluid to cause 3D consol­idation of the soil specimen.

The pore water from the soil specimen escapes through the top and bottom drainage tubes during consolidation under cell pressure. The O-rings prevent entry of cell fluid into the voids of the soil specimen. The additional axial load is applied through the loading ram. The rubber sealing ring prevents escape of cell fluid from the triaxial cell and helps to maintain constant cell pressure.

Preparation of Soil Specimen for Triaxial Compression Test:

Undisturbed soil specimen of 38 mm diameter and 76-mm height (2:1 height-to-diameter ratio) may be obtained using a split mold from an undisturbed soil sampler. It is possible to collect three identical soil specimens from the same level of a 100-mm-diameter undisturbed soil sampler using a three-member mold, welded to a central vertical axis. Each specimen is extracted from the mold using the sample extractor.

Alternately, compacted soil specimens at the required water content and dry density may also be prepared by static or dynamic compaction to determine the shear strength of the compacted fills.

Assembly of the Apparatus for Triaxial Compression Test:

The soil specimen is enclosed in a rubber membrane with filter paper and solid or porous discs at the top and bottom. Solids discs are used to conduct undrained test. The soil specimen is enclosed in the rubber membrane to prevent entry of cell fluid into the voids of the soil specimen. The soil specimen in rubber membrane is mounted on the pedestal of the triaxial cell and fixed in position with the help of O-rings.

The triaxial cell is fixed to its bottom. The cell fluid inlet and air-release valve are opened and the triaxial cell is completely filled with the cell fluid through the cell fluid inlet. Oil or water may be used as a cell fluid. The air-release valve and the cell fluid inlet are then closed. The schematic diagram of the triaxial test setup is shown in Fig. 13.20.

The triaxial cell is mounted on the loading platform of triaxial testing machine. The loading ram is brought in contact with the top of the specimen. The proving ring and the axial deformation dial gauges are fixed in position.

Test Procedure for Triaxial Compression:

The pressure of the cell fluid is increased to the desired value by a lateral pressure assembly apparatus. Either oil- or water-based lateral pressure assembly, is used for applying the cell pressure on the soil speci­men. For undrained test, the soil specimen can be sheared under additional axial load immediately after application of the cell pressure, keeping the drainage valve closed.

In case of a drained test, the soil specimen is allowed to consolidate under the cell pressure before the applica­tion of additional axial load, keeping the drainage valve open during the application of cell pressure and the deviator stress. The additional axial load is applied at a constant rate of axial strain until the load reaches a peak value and then decreases.

In case pore pressure is required to be measured during the undrained test, the drainage valves are connected to the Bishop’s pore pressure apparatus, during the application of both cell pressure and deviator stress. The Bishop’s pore pressure apparatus.

When the soil specimen fails under a maximum deviator stress, the additional axial load is removed and the cell pressure is reduced to zero. The air-release valve is opened and the triaxial cell is completely emptied of the cell fluid. The specimen assembly is dismantled, the soil specimen is removed, and its final water content is deter­mined. The mode of failure of soil specimen either by plastic or by brittle failure is noted. If the specimen fails by brittle failure, the angle of failure plane with horizontal is noted. The test is repeated on identical soil specimens at different cell pressures of 100, 200, and 400 kN/m².

Thus, there are the following two stages of application of stresses on the soil specimen in a triaxial compression test:

1. Consolidation – Application of Cell Pressure:

During this stage –

Horizontal stress = Vertical stress = Cell pressure

2. Shearing – Application of Deviator Stress:

During this stage –

Horizontal stress = Cell pressure = Minor principal stress Þ σ_h = σ₃

Vertical stress = Cell pressure + Deviator stress = Major principal stress Þ σ_v = σ₃ + σ_d = σ₁

Calculation of Principal Stresses for Triaxial Compression Test:

The direction of principal stresses is known in the triaxial compression test. The minor principal stress is horizontal, equal to cell pressure, and major principal stress is vertical. The magnitude of cell pressure is directly indicated by the dial gauge of the lateral pressure assembly apparatus. So the minor principal stress –

σ₃ = cell pressure

and the major principal stress –

σ₁ = σ₃ + σ_d …(13.20)

The deviator stress at failure is calculated from –

σ_d = Deviator load at faliur/A_f …(13.21)

The area at failure depends on the whether the test is a drained or an undrained test.

Area at the Time of Failure in Drained Test:

In a drained test, where the soil specimen is consolidated under the cell pressure, the volume change is measured with the help of a burette. The volume of the soil specimen at any stage of consolidation may be obtained from –

V_oV_i ± ΔV …(13.22)

The area at failure under deviator load is determined from the relation –

where V_i is the initial volume of the specimen at the beginning of the test, l_i the initial length of the specimen at the beginning of the test, ԑ_v the volumetric strain, and ԑ₁ the axial strain during the application of the deviator load.

Area at the Time of Failure in Undrained Test:

In undrained test, the soil specimen does not undergo any volume change during the application of cell pressure. The area at failure is calculated from the relation –

A_f = A_o/(1 – ԑ₁) …(13.27)

where ԑ₁ is the axial strain during the application of the deviator load.

Determination of Shear Strength Parameters for Triaxial Compression Test:

The set of principal stresses on three or four identical soil specimens are used to draw the Mohr stress circles. A common tangent is drawn to these Mohr stress circles, which is the failure envelope for the soil. The y-intercept and slope of the failure envelope give the cohesion and angle of shearing resistance for the soil, respectively.

Alternately, the shear strength parameters may also be obtained by substituting the principal stresses in the Bell’s equation.

Types of Shear Tests Based on Drainage Conditions:

Three types of shear tests are commonly used in the triaxial compression test, depending on the drainage conditions in the field.

They are:

1. Unconsolidated undrained (UU) test.

2. Consolidated undrained (CU) test.

3. Consolidated drained (CD) test.

We know that there are two stages of loading in triaxial compression test – (a) Consolidation, during which cell pressure is applied and (b) shearing, during which deviator stress is applied.

In all three types of tests based on drainage condition listed above, the first letter (C or U) refers to the drainage condition during consolidation. If drainage is allowed during consolidation stage, the test is called “consolidated (C),” as in test types 2 (CU) and 3 (CD). If drainage is not allowed during consolidation stage, the test is called “Unconsolidated (U),” as in test type-1 (UU).

Thus, it may be seen that drainage is not allowed in the UU test during consolidation stage (during application of the cell pressure) and is allowed in the CU and CD tests.

The second letter (U or D) refers to the drainage condition during shearing stage. If drainage is not allowed during shearing stage, the test is called “Undrained (U),” as in test types 1 and 2. If drainage is allowed during shearing stage, the test is called “Drained (D),” as in test type-3.

Thus, it may be seen that drainage is not allowed in the UU and CU tests during the shearing stage (during application of deviator stress) and is allowed in the CD test.

1. Unconsolidated Undrained Test:

In this test, no drainage is allowed during the application of cell pressure and deviator stress. This is done simply by keeping the drainage valve closed throughout the test. Pore pressures are developed in the soil specimen during the application of cell pressure and deviator stress. Solid disks are used on the either side of soil specimen (instead of porous disks), if no pore pressure measurement is done. However, if it is desired to measure the pore pressures, porous disks are used and the triaxial cell is connected to Bishop’s pore pressure apparatus.

Thus, both the total and effective principal stresses can be determined and hence the corresponding shear parameters. The shear parameters obtained from total stresses are known as undrained shear parameters (c_u and ɸ_u ) and those obtained from effective stresses are known as effective shear parameters (c’ and ɸ’). As the test takes short time, UU test is also called as Quick test or Q-test.

2. Consolidated Undrained Test:

In this test, the soil specimen is allowed to consolidate under the cell pressure. Porous discs and filter papers are used on the either side of the soil specimen and the drainage valve is kept open to allow drainage of the pore water during the application of cell pressure.

The volume change during consolidation is measured by a burette or a sensitive volume change gauge. When the consolidation of the specimen under the cell pressure is completed, as indicated by negligible rate of volume change, the drainage valve is closed and the additional axial (deviator) load is applied. The soil specimen fails in shear under undrained conditions at a failure deviator load.

The pore pressure developed in the soil specimen during the application of deviator load may be measured by connecting Bishop’s pore pressure apparatus to the triaxial cell. The test takes more time than UU test for consolidation but shearing stage is completed in a short time. The CU test is also referred to as Rapid test or R-test.

3. Consolidated Drained Test:

This test is similar to the CU test, except that drainage of the soil specimen is permitted during both the consolida­tion stage and shearing stage (application of deviator stress). The consolidation of the soil specimen is completed as in a CU test. The drainage valve is kept open and the deviator load is applied at such a slow rate that no pore pressures are developed in the soil specimen during any stage of shear.

The effective principal stresses are equal to the total stresses, since the drainage is allowed all through the test. The shear parameters obtained are the effective shear parameters (c’ and ɸ’) also called the drained shear parame­ters (c_d and ɸ_d). The CD test takes considerably long time and hence is also referred to as slow test or S-test.

Merits of Triaxial Test:

i. The failure of the soil specimen occurs along the weakest plane.

ii. The stress distribution on the failure plane is uniform.

iii. There is complete control over drainage conditions of the soil specimen. Tests can be done for all types of drain­age conditions.

iv. Pore pressures and volumetric changes can be measured directly.

v. The state of stress at all intermediate stages up to failure is known. The Mohr stress circles can be drawn at any stage of shear.

vi. The apparatus is adaptable to special requirements such as extension tests and tests for different stress paths.

Demerits of Triaxial Test:

i. The apparatus is elaborate, bulky and costly.

ii. The assembly of the specimen and the triaxial cell is rather cumbersome.

iii. Skilled personnel are required to conduct the test.

iv. The drained tests take considerably long periods extending more than a week.

v. It is not possible to determine the cross-sectional area of the soil specimen at large strains, as the assumption that the soil specimen is cylindrical does not hold good.

vi. The consolidation of the soil specimen in the test is isotropic, whereas it is anisotropic in the field.

vii. The test simulates only axisymmetrical problems. In field, the problems are generally three-dimensional.

Despite the above limitations, the triaxial test is the most reliable for the accurate determination of shear parameters of the soil.

Relation between Principal Stresses and Shear Parameters of the Soils:

Figure 13.23 shows the Mohr’s stress circle with failure envelope.

We know that –

OA = σ₃ and OB = σ₁

Radius of Mohr’s circle will be –

CD = (σ₁ – σ₃)/2

Substituting the values of FC from Eq. (13.32) and CD in Eq. (13.29), we get –

Equation (13.33) is known as Bell’s equation that relates the principal stresses and shear parameters of the soil.

Coefficient of Earth Pressure at Rest for Sands:

It is common to test soils in a triaxial test, by first consolidating the sample at some cell pressure. Anisotropic con­solidation is found to produce better strength and stress-strain data. The consolidation of the sample back to its in situ state is desirable. The ratio of the lateral-to-vertical stress at rest (zero strain) is –

K_O = σ_h/σ_v = σ_h/γh …(13.37)

where σ_v = γh is the in situ vertical stress from the soil at depth h. This state can be obtained in a consolidation test by applying a vertical load to produce σ_v = γh, since the consolidation ring prevents any lateral strain. When the vertical settlement is completed, the sample is K_O consolidated.

The coefficient of earth pressure at rest for sand is given by the following equation –

K_O = 1-sin ɸ’ …(13.38)

Shear Strength Characteristics of Saturated Cohesive Soils:

The shear strength of cohesive soils depends on the degree of saturation, effective stress, stress history, and loading and drainage conditions. In order to understand their behavior, it is customary to treat cohesive soils in a saturated state, since this is the worst condition that can prevail in the field. Thus, the parameters estimated will be generally conservative.

Consolidated Drained Test Behavior:

The slow rate of loading during consolidation and shearing in a CD triaxial test does not allow excess pore water pressure to build-up during the test. Hence, the total stresses are the same as effective stresses.

Figure 13.29 shows the failure envelope for the CD triaxial test for normally consolidated clay. The shear strength equation for CD test is expressed as –

τ = c_d +σ’ tan ɸ_d …(13.39)

where c_d = 0 for normally consolidated clay and 0_d is the effective angle of shearing resistance for drained tests.

Figure 13.25 shows variation of deviator stress with axial strain for over-consolidated clays. For normally consol­idated clays, the volume of the specimen gradually decreases during application of deviator stress. Over-consolidated clays show increase in volume, after some initial adjustment, during the application of deviator stress.

In the case of over-consolidated clays, peak shear stress reaches at very low axial strain, after which the stress decreases rapidly with further increase of axial strain. In case of normally consolidated clays, the deviator stress continues to increase with increase in axial strain, and at some strain, the stress becomes constant with further increase in strain.

From Fig. 13.25, it may be seen that for any clay, the deviator stress reaches a constant value at very large strains. The shear strength of clays at very large strains is referred to as residual shear strength. It has been proved that the residual shear strength of a given soil is independent of its stress history and can be given by –

τ_r = σ ‘tan ɸ_ult …(13.40)

where τ_r is the residual shear strength and ɸ_ult the ultimate angle of shearing resistance given by –

Table 13.5 presents typical values of friction angle for normally consolidated clays.

It is found that sinɸ_d has linear correlation with the plasticity index (log₁₀I_P) of the soil, although there is some scatter.

Use of Consolidated Drained Strength in Engineering Practice:

CD tests are considerably more time consuming than CU or UU test and are, therefore, not common during rou­tine investigations. In clay soils, drained tests are sometimes used for investigating the long-term stability of earth dam, embankment, or a retaining wall at considerable time after construction. It may also be used for determining strength of clay embankments in which drainage channels are embedded.