Shear strength of tropical soils and bentonite mixtures for barrier design.

Compacted clayey tropical soils have great potential to be used as barriers in waste disposal facilities, considering that some technical requirements are fulfilled (e.g. hydraulic conductivity; compatibility after disposal; shear strength; swelling/cracking, etc). In turn, the bentonite addition is relevant for the cases where the hydraulic conductivity must be lowered, and therefore investigations on the changes of the mechanical parameters of tropical soils induced by the bentonite addition are of great interest. This paper presents the results of a laboratory investigation on the shear strength of different samples of tropical soils and their mixtures with bentonite in the proportions of 0, 3, 6, 9 and 12% (dry weight basis). The tropical soil samples were collected considering the lateritic, transitional and non-lateritic behavior according to the MCT-M (Modified-Miniature-Compacted-Tropical) classification. The laboratory tests consisted of CU (Consolidated- Undrained) triaxial tests under the confining stresses of 100, 200 and 400 kPa. The results have showed that the addition of bentonite produced a significant increase in the plasticity of the tropical soil samples (PI increases 4 to 6 times), considerably reduced their friction angle (by as much as 11o) and stiffness at peak (modulus reductions of 2.5 to 4 MPa) and gave rise to a slight increase in their cohesion (up to 12 kPa). These facts must be considered in the barrier stability analyses when heavy loads are applied. The important role of the shear strength on barrier design is highlighted