Index and strength properties of clay gravel mixtures

This paper reports the results of fall-cone tests and quasi-static cone penetration tests undertaken on mixtures of fine gravel and a highly plastic clay. The influence of the gravel on the plasticity of the misture is quantified through the derivation of liquid and plastic limits obtained by direct testing of the mixture. The plastic limit used in this study is defined as the moisture content corresponding to a hundredfold increase in the liquid limit undrained strength, and is denoted PL100. Liquid limits were determined from conventional fall-cone tests. The range of plasticity of the mixture is defined through the Plasticity Index derived from the fall-cone liquid limit and the PL100 index. The study demonstrates that the plasticity of mixtures of fine and coarse grained soil can be quantified without the requirement to remove the coarser fraction. The influence of the coarse fraction on both the undrained shear strength and index properties was observed to be significant for gravel volume fractions greater than about 50%. This gravel volume fraction is associated with the maximum void ratio of the gravel, and suggests that interaction between the gravel particles and high gravel contents results in a frictional contribution to the mixture behaviour.