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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