Section 4.

PRIMARY AND SECONDARY SETTLEMENTS

1.

PRIMARY CONSOLIDATION.

a. Consolidation Settlement. For conditions where excess pore

pressures are developed during the application of load and if

preconsolidation stress is determined reliably, total settlement can be

predicted with reasonable accuracy. The percentage error is greatest for

settlement from recompression only. In this case an overestimate may result

unless high quality undisturbed samples are used for consolidation tests.

(1) Typical Loading Cycle. See Figure 3 for loading sequence in

building construction. Foundation excavation can cause swell and heave.

Application of a structural load recompresses subsoil and may extend

consolidation into the virgin compression range. Stress changes are plotted

on a semilogarithmic pressure-void ratio e-log p curve similar to that shown

in Figure 3.

(2) Pressure-Void Ratio Diagram. Determine the appropriate e-log p

curve to represent average properties of compressible stratum from

consolidation tests. The e-log p curve may be interpreted from straight

line virgin compression and recompression slopes intersecting at the

preconsolidation stress. Draw e-log p curve to conform to these straight

lines as shown in Figure 3.

(3) Magnitude of Consolidation Settlement. Compute settlement

magnitude from change in void ratio corresponding to change in stress from

initial to final conditions, obtained from the e-log p curve (Figure 3). To

improve the accuracy of computations divide the clay layer into a number of

sublayers for computing settlement. Changes in compressibility of the

stratum and existing and applied stresses can be dealt with more accurately

by considering each sublayer independently and then finding their combined

effect.

(4) Preliminary estimates of C+c, can be made using the correlations

in Table 3.

b. Corrections to Magnitude of Consolidation Settlements. Settlements

computed for overconsolidated clays by the above procedures may give an

overestimate of the settlement. Correct consolidation settlement estimate

as follows:

H+c, =[alpha] ([W-DELTA]H)+oc,

[alpha] = function of overconsolidation ratio (OCR)

and the width of loaded area and thickness of

compressible stratum (See Figure 8 for values and

Reference 6, Estimating Consolidation Settlements

of Shallow Foundation on Overconsolidated Clay, by

Leonards.)

7.1-223

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