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To conduct un-drained test, which of the following is used?
Slope grids
Perforated grids
Plain grids
All of the mentioned
Plain grids
To conduct an unconsolidated-undrained (UU) triaxial test or similar shear strength tests where pore water pressure must not dissipate, plain solid grids (or caps) are used. These grids prevent the drainage of pore water from the soil specimen, ensuring that the test conditions remain undrained throughout the shearing process.
To conduct an unconsolidated-undrained (UU) triaxial test or similar shear strength tests where pore water pressure must not dissipate, plain solid grids (or caps) are used. These grids prevent the drainage of pore water from the soil specimen, ensuring that the test conditions remain undrained throughout the shearing process.
╬Фu=B[╬Ф╧Г3тАЛ+A(╬Ф╧Г1тАЛтИТ╬Ф╧Г3тАЛ)] тАФ Skempton's pore pressure equation for undrained loading
╧ДfтАЛ=cuтАЛ+╧Гtan(╧ХuтАЛ) тАФ Shear strength equation in terms of total stress
In an undrained test, the objective is to prevent the dissipation of excess pore water pressure generated due to shearing. Plain (non-perforated) plates or grids seal the top and bottom of the specimen, isolating it from the drainage line, thereby maintaining a constant volume state during the application of the deviator stress.
Plain grids are used in UU tests to seal the soil specimen.
Perforated grids are specifically used for drained (CD) tests to allow water to escape.
The undrained test assumes that no volume change occurs in the soil specimen.
In saturated soil, the internal angle of friction (╧ХuтАЛ) is often taken as 0┬░ in the total stress analysis.
Simulates rapid loading conditions in the field (e.g., sudden construction on clay).
Faster test duration compared to drained tests.
Does not provide information on effective stress parameters.
Results are highly sensitive to the saturation degree of the specimen.
Stability analysis of embankments built rapidly.
Bearing capacity of foundations on saturated clays.
UU test is also known as the Quick Test.
Option B (Perforated grids) is used for Drained (CD) or Consolidated-Undrained (CU) tests where drainage is required.
C is correct тАФ Plain grids are used to ensure the specimen remains in an undrained state during testing.
Always remember: Perforated = Drainage (Drained Test); Plain/Solid = No Drainage (Undrained Test).