Consolidation during construction
A sufficiently accurate solution is generally achieved by assuming that the entire foundation load is applied halfway through the construction period. For large constructions, spread over some years, it is sometimes useful to know the amount of consolidation that will have taken place by the end of construction, the problem being that whilst consolidating the clay is subjected to an increasing load.
Figure 10.9 illustrates the loading diagram during and after construction. While excavation is proceeding swelling may occur (see Example 9.7) such as that which took place in the course of excavation for the piers of Chelsea Bridge, which involved the removal of about 9 m of London Clay and resulted in a heave of 6 mm (Skempton, Peck and MacDonald, 1955). If the coefficient of swelling, cvs, is known it would be fairly straightforward to obtain a solution, first as the pore pressures increase (swelling) and then as they decrease (consolidation), but the assumption is usually made that once the construction weight equals the weight of soil excavated (time ti in Fig. 10.9) heave is eliminated and consolidation commences. The treatment of the problem has been discussed by Taylor (1948), who gave a graphical solution, and Lumb (1963), who prepared a theoretical solution for the case of a thin consolidating layer.
By plotting the loadtime relationship the time t can be found (Fig. 10.9), the time t2 being taken as the time in which the net foundation load is applied. The settlement curve, assuming instantaneous application of the load at time t], is now plotted and a correction is made to the curve by assuming that
Construction period
Construction period
Time
Excav perioc
Excav perioc
Instantaneous curve
Fig. 10.9 Consolidation during construction.
Instantaneous curve
Fig. 10.9 Consolidation during construction.
the actual consolidation settlement at the end of time t2 has the same value as the settlement on the instantaneous curve at time t2/2. Point A, corresponding to t2/2, is obtained on the instantaneous curve, and point B is established on the corrected curve by drawing a horizontal from A to meet the ordinate of time t2 at point B. To establish other points on the corrected curve the procedure is to:
(ii) determine the settlement on the instantaneous curve for t/2 (point C);
(iii) draw a horizontal from C to meet the ordinate for t2 at D, and
Where OD cuts the ordinate for time t gives the point E on the corrected curve, the procedure being repeated with different values of t until sufficient points are established for the curve to be drawn. Points beyond B on the corrected curve are displaced horizontally by the distance AB from the corresponding points on the instantaneous curve.
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Carol Eubanks1 year ago
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