Shear Tests Classification Based on Drainage Conditions
Before considering different methods of performing shearing strength tests on a soil, it’s important to think about the potential drainage conditions before and during the tests because they have a big impact on the results.
Shearing strength can be determined in both dry and saturated conditions for a cohesionless or coarse-grained soil. In order to analyse a cohesive or fine-grained soil, it must be saturated. Shear tests on such saturated soils are categorized depending on whether drainage is allowed before and during the test.
There are mainly three shear tests based on drainage conditions which are as follows :
- Unconsolidated Undrained Test (UU Test)
- Consolidated Undrained Test (CU Test)
- Consolidated Drained Test (CD Test)
Also Read : Unconfined Compression Test【IS 2720(Part 10):1991 PDF】
Also Read : Triaxial Shear Test for Shear Strength of Soil
Also Read : Direct Shear Test for Shear Strength of Soil
Also Read : Vane Shear Test [ IS 2720 (Part XXX) – 1980 ]
Unconsolidated Undrained Test Condition (UU Test)
Drainage is not allowed at any point of the test, including before the test when the usual stress is applied and during the test when the shear stress is applied. As a result, no time is required for pore water pressure to dissipate and the soil to consolidate, and no major volume changes are anticipated.
Because of the short drainage course, 5 to 10 minutes may be sufficient for the entire test. Undrained samples, on the other hand are often conducted only on low-permeability soils. It is also known as the ‘Quick test’ since only a short amount of time is required for testing before failure.
Consolidated Undrained Test Condition (CU Test)
During the application of the normal stress, absolute drainage is permitted, but no drainage is permitted during the application of the shear stress. As a result, no volume changes occur during shear and excess pore pressure grows.
After the soil has settled to the desired degree under normal stress, 5 to 10 minutes may be sufficient for the test.The ‘consolidated quick test’ is another name for this test. In geotechnical engineering, these conditions are also general.
Consolidated Drained Test Condition (CD Test)
Drainage is allowed fully before and during the test at all times. The soil is consolidated under normal stress and shear is measured by slowly applying shear stress to the soil, with drainage allowed at all times. At no point does there appear to be any excess pore pressure, and volume changes occur.
In the case of cohesive soils, it can take 4 to 6 weeks to complete a single test of this kind, while in the case of cohesionless soils, less time is needed because the latter drain off quickly. Except for research purposes, this test is rarely performed on cohesive soils. The ‘Slow Test’ or ‘consolidated slow test’ is another name for it.
The type of soil and the reality of the situation will determine which of these tests should be used. UU-tests are useful for determining the short-term stability of foundations, excavations, and earth dams etc. Depending on the drainage conditions in the area, either CU-tests or CD-tests are suitable for long-term stability issues.