coefficient of consolidation, cv, is shown in figure 3-7.

H = total thickness of compressible soil

Thus,

the actual time required for the various

Other terms have been previously defined. Secondary

percentages of consolidation to occur in the field can be

compression settlements may be important where

determined by the following formula:

primary consolidation occurs rapidly, soils are highly

plastic or organic, and allowable settlements are

2

=

TH

(5-5)

tf

unusually small.

cv

where

The

=

time for U(%) consolidation in the

tf

permeability of cohesionless soils is usually sufficiently

field stratum

great that consolidation takes place during the

H

=

length of the drainage path in the

construction period. For important projects, estimate

field

settlement using consolidation tests on undisturbed

T

=

time factor corresponding to U(%)

samples or samples remolded at natural density.

consolidation

Alternately, settlements may be estimated from plate

bearing tests described in chapter 4. Design of footings

When settlement occurring during the construction

on cohesionless soils,

based on settlement

period may be of interest, the values of T and U also

considerations using the Standard Penetration Test, is

can be obtained from figure 5-5.

described in chapter 10.

c. Secondary compression.

For refined

estimates and special purposes, settlement resulting

Design techniques for ameliorating

from secondary compression may have to be evaluated.

settlement problems are summarized in table 5-4.

The amount ∆Hs can be calculated as follows:

Differential settlements beneath existing structures can

be corrected by releveling by jacks, grouting (i.e., mud

∆Hs =

jacking) beneath slab foundations, or underpinning.

CαH log

tsc

(5-6)

These techniques are expensive, to varying degrees,

tp

and require specialists.

where

=

tp + time interval during which

tsc

secondary

compression

settlement is to be calculated

=

time

to

complete

primary

tp

consolidation

Integrated Publishing, Inc. |