(3) Gradual Load Application. If construction time is appreciable
compared to time required for primary consolidation, use the time factors of
Figure 13 (Reference 10, Consolidation Under Time Dependent Loading, by
Olson) to determine consolidation rate during and following construction.
(4) Coefficient of Consolidation From Field Measurements. Where
piezometers are installed to measure pore water pressure under the applied
loads, c+v, is computed as shown in Figure 14.
c. Time Rate of Multi-Layer Consolidation. If a compressible stratum
contains layers of different overall properties, use the procedure of Figure
15 to determine overall settlement time rate.
3.
SECONDARY COMPRESSION.
a. Laboratory e-log p Curve. A laboratory e-log p curve includes an
amount of secondary compression that depends on duration of test loads.
Secondary compression continues exponentially with time without definite
termination. Thus, total or ultimate settlement includes secondary
compression to a specific time following completion of primary
consolidation.
b. Settlement Computation. Compute settlement from secondary
compression following primary consolidation as follows:
t+sec,
H+sec, = C+[alpha], (H+t,) log )))))
t+p,
H+sec, = settlement from secondary compression
C+[alpha], = coefficient of secondary compression
expressed by the strain per log cycle of time
(See Chapter 3)
H+t,
= thickness of the compressible stratum
t+sec, =
useful life of structure or time
for which settlement is significant
See example in Figure 9 for calculating the secondary settlement.
The parameter C can be determined from laboratory consolidation tests
(Chapter 3); for preliminary estimates, the correlations in Figure 16 (after
Reference 2) may be used. This relationship is applicable to a wide range
of soils such as inorganic plastic clays, organic silts, peats, etc.
c. Combining Secondary and Primary Consolidation. If secondary
compression is important, compute the settlement from primary consolidation
separately, using an e-log p curve that includes only compression from
primary consolidation. For each load increment in the consolidation test,
compression is plotted versus time (log scale) (see Chapter 3). The
compression at the end of the primary portion (rather than standard 24
hours) may be used to establish e-log p curve.
7.1-231
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