Volume Change Characteristics and Shear Behavior of Mixed Clayey Soils

In some geotechnical engineering projects, such as core of earth fill dams, landfill liners, etc, to achieve lower values of hydraulic conductivity it requires to compact clayey soils at wet of optimum water content. Shear strength of clayey soils in general is relatively low and when they are subject to seasonal drying, loss of water occurs due to desiccation that alters their properties, including reduction in soil plasticity, possible cracking, and increasing of hydraulic conductivity. Therefore, some modifications to their properties are often required to satisfy the design criteria. Mixing of clayey soils with granular materials (e.g. sand and gravel) is one of the methods to reduce the swell and shrinkage potential as well as improvement strength. These soils with dominant part of clay are called mixed clayey soils. Geotechnical engineers are usually encountered with mixed soils in nature, either naturally or artificially, and their behavior is not known as well as pure cohesive or granular soils. The review of previous studies reveals that the effects of granular material content, grain size and clay type on the behavior of mixed clayey soils have not studied precisely. In addition, the effects of granular material content and size of grains on the volume change characteristics of these soils are unknown. For this purpose, a number of compaction, shrinkage and direct shear tests are carried out on the specimens made of pure clay and clay-granular material mixtures. Soil mixtures were prepared by mixing different amounts of the clay and sand/gravel (by weight) (i.e. 0%, 20%, 40% and 60% sand/gravel). In order to evaluate the effect of grain sizes on the behavior of soil mixtures, the granular soil retrieved from a barrow pit washed and sieved, and the grains retained between two frequent sieves selected as granular part of mixtures. These sieved materials were named as G, S1, and S2 with average grain size of 5.54, 2.805, and 1.44, respectively. Major part of cohesive materials obtained from the Azarshahr clay deposits near Azerbaijan Shahid Madani University site. This cohesive soil has index properties of LL=33%, PI=12%, and GS=2.698 and denoted with AC. To investigate the effect of clay plasticity on the characteristics of soil mixtures, another clay with index properties of LL=48%, PI=22%, and GS=2.64 (characterized by ACB) was used. The results indicate that inclusion of grains to clay reduces the values of swelling and shrinkage; while for the specimens of 60% sand/gravel, the volume change due to saturation and desiccation approaches to zero, and potential of cracking diminishes in the specimens. In addition, the results of direct shear tests indicate that the shear strength of mixtures depends on the content and size of aggregates. So that adding of grains to clay matrix up to a threshold value is approximately ineffective on shear strength, and even for the specimens of 20% sand/gravel, the shear strength of mixtures decreases as compared with pure clay. This trend is verified for friction angle. Results of these tests suggest threshold values for granular materials which are dependent on the grain sizes and clay plasticity. Also, to improve the properties of mixed clayey soils, the size of added grains should not be very fine; at least it must be larger than coarse sand.