RHEOLOGICAL ASSESSMENTS OF SHEAR STRENGTH AND SETTLEMENT

ABSTRACT

Shear strength rheological assessments play a pivotal role in the determination of bearing capacity of soils. Tropical residual red earth soils have unique characteristics in-between the popular clay and cohesionless soils for which most of the published works and theories for shear strength and settlements were based. The presence of negative pore water pressures in unsaturated soils against positive pore water pressures in saturated soils limits the use of saturated soils’ principles in evaluating rheological characteristics of unsaturated soils. The common knowledge is that tropical residual soils are unsaturated soils. It therefore becomes necessary to evaluate tropical red earth soils’ shearing strength and deformation characteristics using both saturated and unsaturated soil’s principles to determine models that adequately describe the characteristics of tropical residual red earth. Reconstituted tropical residual soils were used to simulate tropical soils from different regions encompassing low, medium and high plasticity tropical residual red earth soil. Shear strength and bearing capacity of reconstituted tropical red earth soils were evaluated using saturated and unsaturated principle. Terzaghi’s and Vesic’s bearing capacity factors were used to assess the safe bearing capacity of structures installed in a tropical red earth soil. Unsaturated soil’s principle incorporating the effects of matric suction in the behaviour of unsaturated soil was also used to evaluate the shearing strength properties of tropical residual soil. One dimensional Oedometer assessments were performed on each reconstituted soil to determine the compressibility and settlement characteristics of various reconstituted tropical soils. Rheological properties of tropical residual red earth were further analysed for saturated and unsaturated conditions using an elastic and elastic-plastic finite element code models. The results of the analyses were compared with those obtained from in-situ Plate load test for reliability assessments. The results show that tropical residual soils can be evaluated using both saturated and unsaturated principles depending on the fines content and confining pressures. At lower fines content below the threshold of about 30%, unsaturated soil’s principles most adequately describe the shear strength and deformation characteristics and the coarse matrix defines the behaviour of the soil. Above the threshold, the fines fraction defines the behaviour of the soil and saturated principles best address the shear strength and deformation characteristics of tropical red earth soil especially at high confining pressures of over 300 kN/m2 . Shear strength values obtained using saturated soil’s principle range between 80 and 250 kPa at the peaks for various groups of the soils ranging from low to high plasticity characteristics. For unsaturated vii condition, the values for peak shear strength range between 80 and 130 kPa at optimum matric suction of about 1000 kPa but values increased to between 80 and 250 kPa when the fitting Parameter “a” of Soil Water Characteristic Curve was modified by 100. Adopting unsaturated soil’s principle resulted in higher allowable bearing capacities as the anticipated settlement in some loading conditions reduced from 400mm to 100mm and settlement reduction to the tune of 60% was recorded. Empirical models utilizing fines content presented in this study present a quick guide to evaluating shear strength and deformation characteristics using basic soil parameters such as fines content.