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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