TM 5-818-1 / AFM 88-3, Chap. 7
sure distribution in soil caused by load
Cv decreases abruptly at the preconsolidation pressure.
(6) Coefficient of secondary compression.
H = length of longest drainage path (lab or field)
t = time at which the time factor is T for the
Disturbance decreases the coefficient of secondary
degree of consolidation that has occurred
compression in the range of virgin compression.
(generally, use t50 for T = 0.197 and 50
3-6.
Swelling, shrinkage, and collapsibility.
percent consolidation)
a. The swelling potential is an index property
and equals the percent swell of a laterally confined soil
Correlation between Cv and LL are shown in figure 3-11
sample that has soaked under a surcharge of 1 pound
for undisturbed and remolded soil.
per square inch after being compacted to the maximum
g. Coefficient of secondary compression. The
density at optimum water content according to the
coefficient of secondary compression, C∝, is strain εz =
standard compaction test method. Correlation between
∆H/H0, which occurs during one log cycle of time
swelling potential and PI for natural soils compacted at
optimum water content to standard maximum density is
following completion of primary consolidation (fig 3-7).
shown in figure 3-12.
The coefficient of secondary compression is computed
b. The amount of swelling and shrinkage
as
depends on the initial water content. If the soil is wetter
∆H
than the shrinkage limit (SL), the maximum possible
∆εz
C∝ =
Hf
(3-12)
=
shrinkage will be related to the difference between the
log t_
log _t_
actual water content and the SL. Similarly, little swell
tp
tp
will occur after-the water content has reached some
value above the plastic limit.
Hf is total thickness of compressible soil at time tp. Soils
with high compressibilities as determined by the
compression index of virgin compression ratio will
generally also have high values of C∝. Highly sensitive
clays and soils with high organic contents usually exhibit
high rates of secondary compression. Overconsolidation
can markedly decrease secondary compression.
Depending on the degree of overconsolidation, the value
of C∝ is typically about one-half to one-third as large for
pressures below the preconsolidation pressure as it is for
the pressures above the preconsolidation pressures. For
many soils, the value of C∝ approximately equals
0.00015w, with w in percent.
h. Effects of remolding.
Remolding or
disturbance has the following effects relative to
undisturbed soil:
(1) e-log p curve. Disturbance lowers the
void ratio reached under applied stresses in the vicinity
of the preconsolidation stress and reduces the distinct
break in the curve at the preconsolidation pressure (fig 3-
7). At stresses well above the preconsolidation stress,
the e-log p curve approaches closely that for good
undisturbed samples.
(2) Preconsolidation stress. Disturbance
lowers the apparent preconsolidation stress.
(3) Virgin compression.
Disturbance
lowers the value of the compression index, but the effect
may not be severe.
(Courtesy of H. B. Seed, J. Woodward. J and
(4) Swelling
and
recompression.
Lundgren, R., "Predication of Swelling Potential for
Disturbance increases the swelling and recompression
Swelling Clay, " Journal, Soil Mechanics and
indices.
Foundations Division, Vol 88, No. SM3, Part I. 1962,
(5) Coefficient
of
consolidation.
pp 53-87. Reprinted by permission of American
Disturbance decreases the coefficient of consolidation
Society of Civil Engineers, New York.)
for both virgin compression and recompression (fig 3-11)
in the vicinity of initial overburden and preconsolidation
Figure 3-12. Predicted relationship between swelling
stresses. For good undisturbed samples, the value of
potential and plasticity index for compacted soils.
3-15
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