Disadvantages of Direct Shear Test

 Disadvantages of Direct Shear Test in Geotechnical Engineering

The direct shear test is one of the widely used techniques to ascertain soil strength. Even though the test is widely used, there exist various disadvantages related to the application of the direct shear test. 

The understanding of these disadvantages is essential for engineers and geotechnicians to determine when and how the test should be applied. For that purpose, some of the major disadvantages have been summarized as follows:

1. Uniform Deformations and Stresses

In fact, one of the major drawbacks of the direct shear test is that it assumes deformation and stress are uniformly distributed along a specimen. The reality, however, maybe a little more varied than that, especially when one considers non-homogeneous soils or soils with strong structural features. This may create an assay where results from the measurement of soil strength are merely approximate due to the natural conditions not being used as the basis of actual soil behavior.

2. No Detailed Information Regarding the Stress-Strain Behaviour

The direct shear test is not designed to provide detailed stress-strain behavior about soils. However, in other tests, it is conceivable to have an entire stress-strain curve that appears in such tests; the direct shear test, on the other hand, determines shear strength at a given deformation state. This limitation means that engineers can not obtain an in-depth understanding of how the soil will behave under changing levels of stress and strain, which may become very significant at times for designing structures and for computing probable behavior of the soil under load.

 3. Estimated Stresses May Not Represent Real Conditions

Such stresses determined in the direct shear test are not necessarily the field stresses imposed on the shear plane. There could be reasons, arising from the simplifications and assumptions inherent with the setup of a test, that might result in application of stresses not actually representative of real conditions when the soil will be subjected to such. Hence, shear strengths determined from such tests may not reflect the real conditions that should occur when soils are put into practice, thereby bringing forth a possibility of discrepancy between test results and field performance.

4. No Estimation of Pore Pressures

There is no provision for determining pore pressure in the soil sample during the direct shear test. Because pore pressure cannot be ascertained, there will be no effective stresses obtained from any tests conducted undrained. Effective stress is the main parameter of measurement for arriving at soil behavior under various types of loading; and without its measurement, the application of the test for certain types of soil and loading is restricted.

5. Unreliable Undrained Strengths

For the undrained condition, the results coming from a direct shear test will likely not be very reliable. This is due to the difficulty posed by the problem of preventing localized drainage during shearing. The undrained strength of soil cannot properly be represented unless very high shearing rates are achieved or other means to minimize drainage are employed. In cohesive soils, this has been particularized as a cause for major concern because the undrained strength is a parameter in the design analyses of stability.

 6. Quick and Crude Estimates

Usually, shear box tests are used to obtain rapid and approximate values of failure parameters in practice. This is very useful in preliminary analyses but brings with it a lack of precision and detail needed for more detailed analysis. The use of quick estimates, which is based upon an oversimplification of soil behavior, may not highlight other complexities of the behavior of soil.

Conclusion

Although the direct shear test is performed with all its practical advantages, it must be recalled that a test of this sort has certain limitations. These include the assumptions of uniform deformations and stresses, inability to determine the detailed stress-strain behavior, and problems in the estimation of pore pressures and undrained strength. These disadvantages only mean that the direct shear tests will be determined very carefully and practically by engineers and geotechnicians so that the actual time for their application will be chosen in comparison to proper context in the case of any project.