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.
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