(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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