Consolidation of Slightly Overconsolidated Soils

Abstract

An oedometer suitable for studying the amount and rates of settlements, and the rates of dissipation of the boundary pore-water pressures of the layers of a multi-layer soil deposit, has been developed and tested. The oedometer has been in nearly continuous use for two years and has performed to the author's satisfaction. Test results prove that a short resting period induces a quasi-preconsolidation pressure in the clays tested. Furthermore, creep or secondary effects were observed to occur during primary consolidation. As a result of the effect of the quasi-preconsolidation pressure, the coefficient of consolidation should not be estimated from rates of dissipation of pore-water pressure. Also, the calculated value of the coefficient of permeability obtained from using Terzaghi's theory is likely to be too small. Terzaghi's theory has been extended to the solution of the problem of the rate of consolidation of an overconsolidated soil loaded to a pressure greater than its preconsolidation load. It is shown that this extension of Terzaghi's theory is most helpful, and in the author's opinion, must probable explanation of already published consolidation tests results in which pore-water pressures have been measured. The theoretical results suggest that the role of consolidation of sightly overconsolidated soils may be estimated as follows: (1) The amount of settlement may be estimated from e minus effective stress graph in the usual manner; (2) the rate of settlement will be governed by the coefficient of consolidation of the soil wholly within the normally consolidated range; and (3) the pore-water pressures will dissipate rapidly until the critical value is reached. This dissipation below the critical value may be approximately estimated assuming the soil to be normally consolidated and subject to a pressure increment equal to the critical value given in equation (total pressure equals effective stress sub coefficient of consolidation plus pore water pressure sub coefficient of consolidation).