TM 5-818-1 / AFM 88-3, Chap. 7
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
5-6.
Settlement of cohesionless soils.
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
5-7.
Eliminating, reducing, or coping with
estimates and special purposes, settlement resulting
settlement.
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
5-10
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