Direct shear test is simple and faster to operate. As thinner specimens are used in shear box, they facilitate drainage of pore water from a saturated sample in less time. This test is also useful to study friction between two materials � one material in lower half of box and another material in the upper half of box.

From Wikipedia, the free encyclopedia. A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses.

The testing procedure can be summarized in the following stages:

1. Weigh the initial mass of the soil.
2. Measure the height and diameter of the shear box.
3. Place the shear box inside the shearing device.
4. Stabilize the bottom part of the shear box by fixing the 2 screws.

3. Advantages and Disadvantages of Direct Shear Tests. The continual decrease in the contact surface of the two halves during the test produces a small error on the shear and normal stress and affects the Mohr-Coulomb failure envelope. This error is generally ignored.

The direct shear test has the following merits and demerits as compared to the triaxial compression test. The sample preperation is easy. The test is simple and convenient. As the thickness of the sample is relatively small, the drainage is quick and the pore pressure dissipates very rapidly.

The triaxial test has many advantages over the direct shear test: The soil samples are subjected to uniform stresses and strains. Different combinations of confining and axial stresses can be applied. Drained and undrained tests can be carried out.

A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil (e.g., sand, clay) and rock, and other granular materials or powders. There are several variations on the test.

 The test is called “triaxial” because the three principal stresses are assumed to be known and are controlled. 6. (can take up to 2 weeks)  Since there is no excess pore pressure total stresses will equal effective stresses.  This test simulates long term shear strength for cohesive soils.

Shear box test is a testing method used to determine the shear strength of a soil sample. Since granular soils cannot be retrieved undisturbed, the soil is re-compacted inside a shear box. The soil’s friction angle is then determined by direct shear and the tests are carried out under saturated conditions.

Carefully transfer the sample into shear box τf = c’ + σ’f tan ø’ Where c’ = apparent cohesion of soil in terms of effective stresses σ’f = effective normal stress on failure plane ø’ = angle of shearing resistance of soil in terms of effective stresses σ’f = σf – uf uf = pore water pressure on failure plane Page 4 4.

In direct shear device the normal and shear stresses on failure plane are calculated directly from acting forces. In the triaxial device the intermediate and minor principal stresses are equal and are normally specified constant, the major principal stress σ 1 is increased incrementally until the sample fails.

Like Cone Penetration Testing, Triaxial Testing is used to measure soils’ properties and be used on more solid materials like rock. Typically, Triaxial Testing is used to solve stability problems by determining the shear strength and stiffness of soil when retaining reservoirs of water.

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.

There are 3 main types of triaxial tests:

• Unconsolidated – undrained (UU)
• Consolidated – undrained (CU)
• Consolidated – drained (CD)

The CU test gives c’ and Φ’ for effective stresses and c and Φ for total stresses. The total stress parameters c and Φ are of smaller values as compared to effective stress parameters (c’ and Φ’). A is the pore pressure parameter due to the shear deformation of the soil specimen.

The main difference between Unconfined compression test and triaxil compression test is that in this test the confining cell pressure is kept zero during the test, in fact it is a special case of triaxial test.

The unconsolidated-drained test is meaningless because the shear strength test can be performed only after consolidation takes place. Water cannot be drained from an unconsolidated soil mass.

2.2 Deviator Stress (Principal Stress Difference)–Deviator stress is the difference between the major and minor principal stresses in a triaxial test, which is equal to the axial load applied to the specimen divided by the cross-sectional area of the specimen, as prescribed in the section on calculations.

It is important that laboratory tests on soils saturated in situ be performed on fully saturated specimens. Use of back pressure causes the air to be dissolved in the water and eliminates the initial compression commonly found in time-compression curves from standard tests. …

Discussion :: Soil Mechanics and Foundation Engineering – Section 2 (Q. No. 4)

2. In ‘Drained Triaxial Shear Test’, drainage is permitted under a specified all-round pressure until consolidation is complete. The principal stress difference is then applied with no drainage being permitted.

The Unconfined Compression Test is a laboratory test used to derive the Unconfirmed Compressive Strength (UCS) of a rock specimen. Unconfirmed Compressive Strength (UCS) stands for the maximum axial compressive stress that a specimen can bear under zero confining stress.

Which of the following shear test is developed based on drainage conditions? iii) Drained test. Explanation: Since the shearing strain is made to increase at a constant rate in a direct shear test, and hence the test is called the Strain controlled shear box test.

What is the relationship between permeability and viscosity of water? It is found that permeability is directly proportional to the unit weight of water and inversely proportional to its viscosity.

Which of the following tubes are used in the constant head test? Explanation: The constant head test apparatus consists of three tubes: the inlet tube, the overflow tube and outlet tube.

The shear force P in the shear plane is equal to tension force F. The average shear stress in the plane is τave= F/A. This joint is said to be in double shear. To determine the average shear stress in each shear plane, free-body diagrams of rivet and of the portion of rivet located between the two planes are drawn.

Single shear means a cross section having unbalanced force on its either side and it is ineffective to take that effective load,then it fails in single shear. For double shear an unbalanced load acting on it’s both side and whole section is failed is called double or punching shear.

Double shear test is used to determine the shear strength (ultimate shear stress) of the mild steel specimen. Double shear is load applied in one plane that would result in the fastener being cut into three pieces, while single shear would result in two fastener pieces.