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

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.

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