Contents preface (VII) introduction 1—37

15.4.3. Laboratory Tests for Shear Strength of Compacted Impervious Soils
There are three types of laboratory tests which are commonly used for the determination of shear strength of compacted impervious soils. These methods differ in the method of consolidating the soil sample before the sample is failed in shear. These methods are as follows:
(i) Undrained test,
(ii) Consolidated-undrained test, and (iii) Drained test.

In the undrained test (also known as the ‘quick’, ‘unconsolidated-undrained’ or ‘Q’ test), drainage or dissipation of pore pressure is not allowed at any stage of the test. The relationship between the shear strength and normal pressure, obtained in terms of total stress, is used in the stability analysis of an embankment dam for ‘during’ and ‘immediately after’ construction condition. The soil samples are tested at a moisture content and density which would prevail during or immediately after construction. If the moisture content corresponds to saturation level, it will be found that all soil samples (saturated) have the same shear strength if no consolidation (i.e., drainage) is allowed (14).

EMBANKMENT DAMS

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In consolidated-undrained test (also known as ‘consolidated-quick’ or ‘R’ test) the sample is first allowed to consolidate (with full pore pressure dissipation) under a specified consolidation pressure, and is then failed in shear without permitting drainage. To obtain strength parameters in terms of effective stress (i.e., c′ and φ′), the pore pressure should be measured. These values are used for effective stress method of stability analysis. If the test values are to be used for the total stress method of stability analysis, the sample should be tested (without pore pressure measurement) at a water content which is anticipated in the dam during the period being analysed. It should often be difficult to determine the water content which would prevail in the dam. Hence, it is usual practice to conduct the tests on saturated samples for total stress method of analysis. The results of these tests would be useful for analysing sudden drawdown condition of impervious zones of embankment and foundation (14). The test results would also be useful for analysing the upstream slope during a partial pool condition and the downstream slope during steady seepage (12).

Drained test (also known as the ‘slow’ test or ‘S’ test) permits drainage and complete dissipation of pore pressure at all stages of the test. The strength parameters are determined in terms of effective stress. The results of this test are to be used for freely-draining soils in which pore pressures do not develop (12).