CONSOLIDATION TEST
ASTM D2435, BS EN ISO 17892
1. Scope*
1.1 These test methods cover procedures for determining
the magnitude and rate of consolidation of soil when it is
restrained laterally and drained axially while subjected to
incrementally applied controlled-stress loading. Two alternative
procedures are provided as follows:
1.1.1 Test Method A—This test method is performed with
constant load increment duration of 24 h, or multiples thereof.
Time-deformation readings are required on a minimum of two
load increments. This test method provides only the compression
curve of the specimen and the results combine both
primary consolidation and secondary compression deformations.
1.1.2 Test Method B—Time-deformation readings are required
on all load increments. Successive load increments are
applied after 100 % primary consolidation is reached, or at
constant time increments as described in Test Method A. This
test method provides the compression curve with explicit data
to account for secondary compression, the coefficient of
consolidation for saturated materials, and the rate of secondary
compression.
NOTE 1—The determination of the rate and magnitude of consolidation
of soil when it is subjected to controlled-strain loading is covered by Test
Method D4186.
1.2 These test methods are most commonly performed on
saturated intact samples of fine grained soils naturally sedimented
in water, however, the basic test procedure is
applicable, as well, to specimens of compacted soils and intact
samples of soils formed by other processes such as weathering
or chemical alteration. Evaluation techniques specified in these
test methods assume the pore space is fully saturated and are
generally applicable to soils naturally sedimented in water.
Tests performed on other unsaturated materials such as compacted
and residual (weathered or chemically altered) soils
may require special evaluation techniques. In particular, the
rate of consolidation (interpretation of the time curves) is only
applicable to fully saturated specimens.
1.3 It shall be the responsibility of the agency requesting
this test to specify the magnitude and sequence of each load
increment, including the location of a rebound cycle, if
required, and, for Test Method A, the load increments for
which time-deformation readings are desired. The required
maximum stress level depends on the purpose of the test and
must be agreed on with the requesting agency. In the absence
of specific instructions, Section 11 provides the default load
increment and load duration schedule for a standard test.
1.4 These test methods do not address the use of a back
pressure to saturate the specimen. Equipment is available to
perform consolidation tests using back pressure saturation. The
addition of back pressure saturation does not constitute nonconformance
to these test methods.
4. Summary of Test Methods
4.1 In these test methods a soil specimen is restrained
laterally and loaded axially with total stress increments. Each
stress increment is maintained until excess pore water pressures
are essentially dissipated. Pore pressure is assumed to be
dissipated based on interpretation of the time deformation
under constant total stress. This interpretation is founded on the
assumption that the soil is 100% saturated. Measurements are
made of change in the specimen height and these data are used
to determine the relationship between the effective axial stress
and void ratio or strain. When time deformation readings are
taken throughout an increment, the rate of consolidation is
evaluated with the coefficient of consolidation.
5. Significance and Use
5.1 The data from the consolidation test are used to estimate
the magnitude and rate of both differential and total settlement
of a structure or earthfill. Estimates of this type are of key
importance in the design of engineered structures and the
evaluation of their performance.
5.2 The test results can be greatly affected by sample
disturbance. Careful selection and preparation of test specimens
is required to reduce the potential of disturbance effects.
11. Procedure
11.1 Preparation of the porous disks and other apparatus
will depend on the material being tested. The consolidometer
must be assembled in such a manner as to prevent a change in
water content or swelling of the specimen. Dry porous disks
and filters must be used with dry, highly expansive soils and
may be used for all other soils. Damp disks may be used for
partially saturated soils. Saturated disks may be used only
when the specimen is saturated and known to have a low
affinity for water. The disks should be prepared using the test
water. Assemble the ring with specimen, porous disks, filter
screens (when needed) in the consolidometer. If the specimen
will not be inundated shortly after application of the seating
load (see 11.2), enclose the consolidometer in a loose fitting
plastic or rubber membrane to prevent change in specimen
volume due to evaporation.
NOTE 15—In order to meet the stated objectives of this test method, the
specimen must not be allowed to swell in excess of its initial height prior
to being loaded beyond its preconsolidation stress. Detailed procedures for
the determination of one-dimensional swell or settlement potential of
cohesive soils is covered by Test Method D4546.
11.2 Place the consolidometer in the loading device and
apply a seating load that results in a total axial stress of about
5 kPa [100 lbf/ft2]. Immediately after application of the seating
load, adjust the deformation indicator and record the initial
deformation reading, do. If necessary, add additional load to
keep the specimen from swelling. Conversely, if it is anticipated
that a total axial stress of 5 kPa [100 lbf/ft2] will cause
significant consolidation of the specimen, reduce the seating
load to produce a total axial stress of about 3 kPa [50 lbf/ft2] or
less. If necessary, allow time for the consolidometer temperature
to reach the test temperature range (6 2 °C).
11.3 If the test is performed on an intact specimen that was
either saturated under field conditions or obtained below the
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water table, inundate with the test water shortly after application
of the seating load. As inundation and specimen wetting
occur, quickly increase the load as required to prevent swelling.
Record the applied load required to prevent swelling and
the resulting deformation reading. If specimen inundation is to
be delayed to simulate specific conditions, then inundation
must occur at a total axial stress that is sufficiently large to
prevent swell. In such cases, apply the required load and
inundate the specimen. Take deformation readings during the
inundation period as specified in 11.5. In such cases, note in the
test report the total axial stress at inundation and the resulting
axial deformation.
NOTE 16—Inundation is necessary to eliminate the air water interface at
the soil boundary which can cause negative pore pressures to exist in the
pore space. Inundation will not significantly increase the degree of
saturation of the test specimen and should not be used as the basis to claim
a specimen is fully saturated.
11.4 The specimen is to be subjected to load increments of
constant total axial stress. The duration of each load increment
shall conform to guidelines specified in 11.5. The specific
loading schedule will depend on the purpose of the test, but
should conform to the following guidelines.
11.4.1 The standard loading schedule shall consist of a load
increment ratio (LIR) of one which is obtained by approximately
doubling the total axial stress on the soil to obtain
values of about 12, 25, 50, 100, 200, etc. kPa [250, 500, 1000,
2000, 4000, etc. lbf/ft 2].
11.4.2 If the slope and the shape of the virgin compression
curve or determination of the preconsolidation stress is
required, the maximum total axial stress shall be sufficiently
high to provide either a) three points which define a straight
line when plotted in log stress space, b) three points which
define a concave up curve when plotted in log stress space or
c) a stress level which is eight times the estimated preconsolidation
stress. In other circumstances, the maximum total axial
stress should be agreed on with the requesting agency.
11.4.3 The standard unloading (or rebound) schedule should
be selected by approximately halving the total axial stress on
the soil (that is, use the same stress levels as 11.4.1, but in
reverse order). However, if desired, each successive stress level
can be only one-fourth as large as the preceding stress level,
that is, skip every other stress level.
11.4.4 In the case of overconsolidated clays, a better evaluation
of recompression parameters may be obtained by imposing
an unload-reload cycle once the preconsolidation stress has
been exceeded. Specification of the stress level and the
magnitude of an unload-reload cycle is the option of the agency
requesting the test (see 1.3), however, unloading shall always
include at least two decrements of total axial stress.
11.4.5 An alternative loading, unloading, or reloading
schedule may be employed that reproduces the construction
stress changes or allows better definition of some part of the
stress-strain (compression) curve, or aids in interpreting the
field behavior of the soil, or is specified by the requesting
agency.
11.5 Before each load increment is applied, record the
height or change in height, df , of the specimen. Two alternative
procedures are available that specify the time sequence of
readings during the load increment and the required minimum
load increment duration. Longer durations are often required
during specific load increments to define the slope of the
characteristic straight line secondary compression portion of
the axial deformation versus log of time graph. For such
increments, sufficient readings should be taken near the end of
the load increment to define this straight line portion. It is not
necessary to increase the duration of other load increments
during the test.