Shear strength: Types of test

Types of test

Many variations of test procedure are possible with the triaxial apparatus but the three principal types of test are as follows:

1        Unconsolidated-Undrained. 

The  specimen  is subjected  to  a specified all-round pressure and then the principal stress difference is applied immediately, with no drainage being permitted at any stage of the test.

2 Consolidated-Undrained: Drainage of the specimen is permitted under a specified all-round pressure until consolidation is complete; the principal stress difference is then applied with no drainage being permitted. Pore water pressure measurements may be made during the undrained part of the test.

3    Drained:   Drainage   of  the  specimen  is  permitted under  a  specified all round pressure  until consolidation is complete;  with drainage    still being permitted,      the principal stress difference is then applied at a rate slow enough to ensure that the excess pore water pressure is maintained at zero.

Shear strength parameters determined by means of the above test procedures are relevant only in situations where the field drainage conditions correspond to the test conditions. The shear strength of a soil under undrained conditions is different from that under drained conditions. The undrained strength can be expressed in terms of total stress in the case of fully saturated. soils of low permeability, the shear strength parameters being denoted by cu and .The drained strength is expressed in terms of the effective stress parameters cu and

The vital consideration in practice is the rate at which the changes in total stress (due to construction operations) are applied in relation to the rate of dissipation of excess pore water pressure, which in turn is related to the permeability of the soil. Undrained conditions apply if there has been no significant dissipation during the period of total stress change; this would be the case in soils of low permeability such as clays immediately after the completion of construction. Drained conditions apply in situations where the excess pore water pressure is zero; this would be the case in soils of low permeability after consolidation is complete and would represent the situation a long time, perhaps many years, after the completion of construction. The drained condition would also be relevant if the rate of dissipation were to keep pace with the rate of change of total stress; this would be the case in soils of high permeability such as sands. The drained condition is therefore relevant for sands both immediately after construction and in the long term. Only if there were extremely rapid changes in total stress (e.g. as the result of an explosion or an earthquake) would the undrained condition be relevant for a sand. In some situations, partially drained conditions may apply at the end of construction, perhaps due to a very long construction period or to the soil in question being of intermediate permeability. In such cases the excess pore water pressure would have to be estimated and the shear strength would then be calculated in terms of effective stress.