How to Determine the Shear Strength of Soil? | Soil Engineering

The following are the various types of tests available for the determination of shear strength: 1. Direct Shear Test 2. Triaxial Compression Test. 3. Unconfined Compression (UCC) Test. 4. Vane Shear Test. 5. Bore Hole Shear Test.

1. Direct Shear Test:

Direct shear test is a simple and commonly used test performed in a shear box to determine the shear parameters of soils. The direct shear test is also known as shear box test.

The principle of the test is to cause shear failure of a soil specimen, placed in a shear box along a predetermined horizontal plane, under a given normal stress, and to determine the shear stress at failure. The test is repeated on identical soil specimens under different normal stresses and the shear stress at failure under each normal stress is determined. A graph is plotted between the normal stress and the shear stress and the y-intercept and the slope of the failure envelope so obtained are taken as the shear parameters c and ɸ, respectively.

2. Triaxial Compression Test:

Triaxial compression test is used for the determination of shear parameters of all types of soil under any drainage condition.

In this test, a cylindrical soil specimen is subjected to a confining pressure in all directions in a triaxial cell. An additional axial load, known as deviator load, is then applied in vertical direction until the specimen fails (refer to Fig. 13.18). Here σ₃ is the confining pressure, also called cell pressure, or all-round pressure and σ_d is the deviator stress, also called additional axial stress or added axial stress.

The test is repeated on three or more identical soil specimens and the principal stresses, so obtained, are used to draw the Mohr’s circles. The failure envelope is drawn as a common tangent for the Mohr’s circles to determine the shear parameters of the soil.

3. Unconfined Compression Test:

The unconfined compression (UCC) test is a special case of a triaxial compression test in which the confining (cell) pressure is zero. The test can be conducted only on saturated cohesive soils, which can stand unsupported without confining pressure. A cylindrical soil specimen is subjected to axial vertical stress (major principal stress), as shown in Fig. 13.37, until the specimen fails due to shearing along a critical plane of failure.

The merits of the UCC test are that the test is simple, convenient, and quick. It is ideally suited for measuring undrained shear strength of saturated clays.

In its simplest form, the apparatus consists of a load frame fitted with a proving ring to measure the vertical stress, σ₁ applied to the soil specimen. Figure 13.38 shows the test setup for the UCC test. The deformation of the soil specimen during the application of vertical stress is measured by a separate dial gauge. The load versus defor­mation readings during the test are taken and the stress-strain curve is plotted.

The soil specimen may undergo either plastic failure or brittle failure in the UCC test, as shown in Fig. 13.39. When a brittle failure of the soil specimen occurs, the proving ring dial gauge indicates a peak load, which decreases rapidly with further increase of strain. In brittle failure, a distinct failure plane in the form of a crack will occur across the specimen as shown in Fig. 13.39.

In plastic failure, no definite peak load is indicated and the soil specimen bulges laterally. In such a case, the load corresponding to 20% axial strain is taken as the failure load. There will be no distinct failure plan and a number of small and fine cracks occur, distributed throughout the specimen.

Figure 13.40 shows the stress conditions in the soil specimen at the time of failure. The test is essentially an undrained test, if it is assumed that no moisture content is lost from the soil specimen during the test. Thus, for saturated cohesive soils in UCC test.

σ₃ = 0 and ɸ_u = 0

Thus, the Mohr’s circle of stress starts from the origin and the diameter of the Mohr’s circle will be σ₁ = q_u, where q_u is the unconfined compressive strength of the soil. The unconfined compressive strength is obtained by dividing the failure load with the cross-sectional area of the soil specimen at failure and it can be defined as –

q_u = Failure load/A_f …(13.45)

The cross-sectional area of the soil specimen at failure is computed from –

A_f = A_o/1 – ԑ₁ …(13.46)

where A₀ is the original cross-sectional area of the specimen at the commencement of the test and ԑ₁ the axial strain corresponding to failure load.

The Mohr’s-Coulomb failure envelope is horizontal and the undrained cohesion, equal to the undrained shear strength of the clay, is given by the radius of the Mohr’s circle –

c_u = τ_u = σ₁/2 = qu/2 …(13.47)

For soils that undergo brittle failure, the approximate friction angle (ɸ_u) of undrained strength may be estimated from the slope of the failure plane with the major principal plane (horizontal) using the relation –

α = 45 + (ɸ_u/2) …(13.48)

Figure 13.41 shows the Mohr’s circle for soil specimen that fails by brittle failure. The angle of failure plane may be directly measured as the slope of the major crack in the soil specimen with the base. The undrained cohesion may be then computed from –

σ = σ₃ tan² α + 2c_u tan α …(13.49)

where σ₁ = q_u and σ₃ = 0, and substituting the same in Eq. (13.49), we get –

q_u = 2c_u tan α Þ c_u = q_u/2 tanα …(13.50)

4. Vane Shear Test:

The laboratory vane shear test is used for the measurement of undrained shear strength of cohesive soils of low shear strength less than about 0.5 kgf/cm². This test gives the undrained strength of the soil, and the undisturbed and remolded strengths obtained are also used for evaluating the sensitivity of the soil.

5. Bore Hole Shear Test:

It is a field test to determine the in situ undrained strength of soft clays, in which the bore hole shear device is inserted into the bore hole and expanded into the soil using compressed air. The soil is sheared and the corresponding load P_v is determined. The test device operates by applying the direct shear test principle in situ. The results from the bore hole shear test approximates a CU test.