Which of the following is the appropriate triaxial test to assess the immediate stability of an unloading
problem, such as an excavation of a clay slope?
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A.
CU test
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B.
Unconsolidated drained test
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C.
UU test
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D.
CD test
Correct Answer:
C. UU test
Explanation:
The appropriate triaxial test to assess the immediate stability of an unloading problem, such as the excavation of a clay slope, is the Consolidated Undrained (CU) test.
In an excavation scenario, the clay has already undergone consolidation under its original in-situ lithostatic pressure over a long period. When the excavation occurs rapidly, the change in stress happens under undrained conditions because the low permeability of the clay prevents the instantaneous dissipation of pore water pressure. Therefore, the test must first involve a consolidation stage to replicate the initial field stresses, followed by an undrained shearing stage to simulate the rapid unloading. While some quick assessments use the Unconsolidated Undrained (UU) test for short-term loading, the CU test is the most accurate for assessing the immediate stability of an unloading problem in previously consolidated clay.
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