Characterization and hydraulic conductivity of tropical soils and bentonite mixtures for CCL purposes

• Tropical soil (lateritic, transitional and non-lateritic) samples and bentonite 3, 6, 9 and 12%
• Variation of the geotechnical properties with the addition of bentonite
• Reduction of the hydraulic conductivity with the addition of bentonite
• Kozeny–Carman derived equations using mesoplastic point (MP)
• K–C equation which gave good correlations with the k measured in the laboratory.

Tropical soils have great potential to be used as compacted clay liners (CCLs) for waste disposal facilities, considering their wide distribution over the globe. However, to be used as liners, several technical requirements must be correctly addressed, including low hydraulic conductivity (k), high contaminant retention capacity and suitable long-term performance (compatibility). The use of bentonite as an amendment for reducing k of soils and increasing the cation exchange capacity, in turn, can be an alternative to fulfill these requirements. This paper involves a laboratory-based investigation on the characterization and k determination of compacted mixtures of three tropical soil samples and bentonite at different proportions (3, 6, 9 and 12%, dry weight basis). These tropical soils were classified as lateritic, transitional and non-lateritic. The experimental program consisted of mineralogical, chemical, physicochemical and geotechnical characterization and k determination using a triaxial cell. Additionally, Kozeny–Carman derived equations were proposed to be after correlated with the k measured in the laboratory. The bentonite addition considerably reduced k of the soil samples. For instance, under a confining stress of 80 kPa, the addition of 12% (dry weight basis) resulted in k values of 1.5 × 10− 8 cm/s, 1.2 × 10− 9 cm/s and 6.3 × 10− 10 cm/s, for the lateritic, transitional and non-lateritic soil samples, respectively; these values correspond respectively to k reduction factors of 225.2, 661.5 and 290.0. Additionally, lateritic soil required greater bentonite content to reach the same k value when compared to transitional and non-lateritic soils. The mesoplastic point (MP) successfully replaced the specific surface (So) and the liquid limit (ωL) to represent the plasticity of fine grained soil in the “Kozeny–Carman” modified equation, which resulted in very good correlations with the k measured in the laboratory for these soils.