Consolidation Test - Soil Properties: Testing, Measurement, and Evaluation - page 293

pressure. In the case of overconsolidated clays, a better evaluation
of recompression parameters may be obtained by imposing an
unload-reload cycle after the preconsolidation pressure has been
defined. Details regarding location and extent of an unload-reload
cycle are the option of the agency requesting the test; however,
unloading shall always span at least two decrements of pressure.
(4.1) The standard loading schedule shall consist of a load incre-
ment ratio (LIR) of one which is obtained by doubling the pressure
on the soil to obtain values of approximately 12, 25, 50, 100, 200,
etc. kPa (250, 500, 1000, 2000, 4000, etc. lb/ft 2 ).
(4.2) The standard rebound or unloading schedule should be se-
lected by halving the pressure on the soil (that is, use the same
increments of (4.1), but in reverse order). However, if desired, each
successive load can be only one-fourth as large as the preceding
load; that is, skip a decrement.
(4.3) An alternative loading, unloading, or reloading schedule may
be employed that reproduces the construction stress changes or
obtains better definition of some part of the stress deformation (com-
pression) curve, or aids in interpreting the field behavior of the soil.
Note 5 —Small increments may be desirable on highly com-
pressible specimens or when it is desirable to determine the
preconsolidation pressure with more precision. It should be
cautioned, however, that load increment ratios less than 0.7
and load increments very close to the preconsolidation pres-
sure may preclude evaluation for the coefficient of consolida-
tion, c v , and the end-of-primary consolidation.
(5) Before each pressure increment is applied, record the height or
change in height, d f , of the specimen. Two alternative procedures
are available that specify the time sequence of readings and the re-
quired minimum load duration. Longer durations are often required
during specific load increments to define the slope of the character-
istic straight line secondary compression portion of the deformation
versus log of time graph. For such increments, sufficient readings
should be taken near the end of the pressure increment to define
this straight-line portion. It is not necessary to increase the dura-
tion of other pressure increments during the test.
(5.1) Test Method A —The standard load increment duration shall
be 24 h. For at least two load increments, including at least one load
increment after the preconsolidation pressure has been exceeded,
record the height or change in height, d , at time intervals of
approximately 0.1, 0.25, 0.5, 1, 2, 4, 8, 15 and 30 min, and 1, 2, 4, 8,
and 24 h, measured from the time of each incremental pressure
application. Take sufficient readings near the end of the pressure
increment period to verify that primary consolidation is completed.
For some soils, a period of more than 24 h may be required to reach
the end-of-primary consolidation. In such cases, load increment