Impact of sand shape and stress level on the volumetric strain of highly plastic clay–sand mixtures

In the design of clay sand liners, the characteristics of sand play a vital role in defining the compressibility and the hydraulic conductivity properties. The clay may fill up the voids within the sand skeleton entirely or partially. The void ratio can be examined to figure out the optimum clay content needed to block moisture flow. The compressibility is dependent on the sand morphology, shape, texture, and void ratio. These parameters can affect the overall carrying capacity of the layer. The edge-to-edge contact and interlocking are provided by sand. This study addresses three types of sand concerning gradation: fine, medium, and coarse of two shapes representing the angularity (AGS; Angular-grained sand and RGS; Rounded grained sand). Compressibility tests were done using one-dimensional consolidation and a multidimensional flexible wall permeameter cell. Two stress levels were considered for comparison: 60 kPa and 400 kPa to show the volumetric strain variations. These are relevant when the liner is located at the bottom of a landfill or placed on the top as a cover layer. This study viewed the compressibility for three different clay contents: 10, 20 and 30% by dry weight of sand. An empirically measured parameter is introduced as reference strain and taken as 50% of the volumetric strain. This is correlated to the axial strain obtained in standard oedometer tests. Volumetric strain prediction equations are given to use the measured axial strain to find the compressibility. The volumetric strain measured for clay-sand mixtures of different morphologies at 400 kPa confinement was found in the range of (22-32.5%), while for 60 kPa confinement, the volumetric strain was found in the range of (5–10%). These results are observed trends for angular and rounded shaped sands in general.