Study on the Secondary Compression of Fully-Saturated Clay by Means of Triaxial Tests

Abstract

The famous Terzaghi's theory on the consolidation of clay is based on the assumption that the flow of the pore water is vertically one-dimensional under isotropic pressure. The settlement of a clay layer actually occurs in general under the condition of no lateral strain, the lateral pressure decreasing gradually with the progress of consolidation. This is recognized in the consolidation test made with a triaxial apparatus. Thus the shear stress arises within a clay sample because of the decrease in lateral pressure in spite of constant vertical pressure. This shear stress may be considered to affect the secondary compression to a great extent. In order to investigate the character of secondary compression, therefore, we have to study the behaviors of clay from the view-point not only of the deformation, but of the actually working stress.This paper treats the results of the one-dimensional consolidation tests performed for a long term on a fully-saturated clay in the triaxial apparatus under various conditions, from the point of view that the intergranular displacement causes a creep by the occurence of shear stress during the one-dimensional consolidation.The conclusions obtained from a series of experiments are as follows:(1) The lateral stress σ3 decreases with the dissipation of pore pressure from the initial value σ1 up to the final value about 0.5σ1. This variation in the lateral stress should be taken into consideration in the discussion of the secondary compression.(2) The principal stress ratio K=σ3/σ1 decreases in proportion to the degree of consolidation. It may be considered, therefore, that the gradient of the secondary compression-time curve is a linear function of the K-value.(3) During the secondary compression, excess pore pressure appears to keep the value of 0.01-0.02kg/cm2.(4) In the case where specimens are laterally confined with a rigid metal ring, the secondary compression ratio increases with the decrease in the thickness of specimens. Therefore, one tends to over-estimate the amount of the secondary compression, unless the sufficient thickness of specimens is taken.(5) There exists no significant difference in the test results between the two cases of lateral drainage and vertical drainage. The secondary compression ratio can be assumed as about 10-15% of the total compression for the ordinary alluvial marine clay.