OCR = preconsolidation pressure/overburden pressure
(P+c,/P+o,) (See Chapter 3.)
([W-DELTA]H)+oc, = calculated settlement resulting from stress increment
of P+o, to P+c, by procedures outlined in Figure 3,
Section 2.
2.
TIME RATE OF PRIMARY CONSOLIDATION.
a. Application. Settlement time rate must be determined for foundation
treatment involving either acceleration of consolidation or preconsolidation
before construction of structure. Knowledge of settlement rate or percent
consolidation completed at a particular time is important in planning
remedial measures on a structure damaged by settlement.
b. Time Rate of Consolidation. Where pore water drainage is
essentially vertical, the ordinary one dimensional theory of consolidation
defines the time rate of settlement. Using the coefficient of consolidation
the time factor T+v, curves of Figure 9 (upper panel, Reference 7, Soils and
Geology, Procedures for Foundation Design of Buildings and Other Structures
(Except Hydraulic Structures), by the Departments of the Army and Air
Force). For vertical sand drains use Figure 10 (upper panel, Reference 7).
For preliminary estimates, the empirical correlation for c+v, in Chapter 3
may be used.
(1) Effect of Pressure Distribution. Rate of consolidation is
influenced by the distribution of the pressures which occur throughout the
depth of the compressible layer. For cases where the pressures are uniform
or vary linearly with depth, use Figure 9 which includes the most common
pressure distribution. The nomograph in Figure 11 may be used for this
case.
For nonlinear pressure distribution, use Reference 8, Soil
Mechanics in Engineering Practice, by Terzaghi and Peck, to obtain the time
factor.
(2) Accuracy of Prediction. Frequently the predicted settlement
time is longer than that observed in the field for the following reasons:
(a) Theoretical conditions assumed for the consolidation
analysis frequently do not hold in situ because of intermediate lateral
drainage, anisotropy in permeability, time dependency of real loading, and
the variation of soil properties with effective stress. Two or three
dimensional loading increases the time rate of consolidation. Figure 12
(after Reference 9, Stress Deformation and Strength Characteristics, by Ladd
et al.) gives examples of how the width of the loaded area and anistropy in
permeability can affect the consolidation rate substantially. As the ratio
of the thickness of the compressible layer to the width of the loaded area
increases, the theory tends to overestimate the time factor. For deposits
such as some horizontal varved clays where continuous seams of high
permeability are present, consolidation can be expected to be considerably
faster than settlement rates computed based on the assumption of no lateral
drainage.
(b) The coefficient of consolidation, as determined in the
laboratory, decreases with sample disturbance. Predicted settlement time
tends to be greater than actual time (see Chapter 3).
7.1-226
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