On the Volume Changes of Normally-Consolidated Clays

Abstract

Changes in volume of soils are a function of changes both in normal stresses and shearing stresses. Development of mathematical expressions for the change in volume would be of aid for the understanding of the failure criteria for clays and would permit more reliable estimates for the settlement of structures founded on massive clay deposits.This paper describes an investigation of the stress-volume change characteristics of a normally-consolidated clay. The test procedure was similar to that in the case of drained compression test, except that the loading was arranged so that the mean principal effective stresses might be maintained constant throughout the test.The nomenclature used in this paper is as follows: C: coefficient of compressibility. D: coefficient of dilatancy. V0: initial volume corresponding to σ0'. ΔV: total volume change. ΔVc: change in volume due to the change in mean principal stress. ΔVd: change in volume due to the change in shearing stress. σ1' σ3': principal effective stress. σm': mean principal effective stress. σ0': pre-consolidation pressure. σc: critical stress, below which dilatancy is zero. σm'-const.-test: drained compression test in which the mean principal effective stress is maintained constant.The analysis of the test data shows that the volume change behaviour can be adequately described by the following conclusions:1) The volume of normally-consolidated clays decreases when the mean principal effective stress is increased, and vice versa. The virgin branch of a semi-logarithmic plot of the consolidation diagram is usually straight (Fig. 1) and can be expressed by the equationΔVc/V0=C·logσm'/σ0'2) The volume decreases when normally-consolidated clays are subjected to an increase in shearing stress. Fig. 6 shows that the volume change during a σm'-const.-test ΔVd/(V0-ΔVc) is a unique function of (σ1-σ3)/σm' and correlations between them can be established.ΔVd/(V0-ΔVc)=D{(σ1-σ3)-σc/σm'}3) The total volume change ΔV of normally-consolidated clays is ΔV=ΔVc+ΔVd, then ΔV is given by the expressionΔV/V0=C·logσm'/σ0'+D(1-C·logσm'/σ0'){(σ1-σ3)-σc/σm'}