Characterisation of shear stress properties of compacted, unsaturated heaving soils

Abstract

The shear strength of soil is a fundamental property that governs the stability of soil material under structural loads. As such, it is required to determine the shear strength parameters and stiffness of soil support for foundation design. The literature review revealed that research on the influence of swelling on the shear strength of expansive soils is limited. The swelling stress influence on the shear strength of expansive soils is generally ignored in engineering practice. The measurement of unsaturated shear strength is challenging and time-consuming. Geotechnical predictive models can be utilised as an alternative to assess the geotechnical properties of these soils. The formulation of a predictive shear strength constitutive relation and associated models for compacted unsaturated expansive soils were summarised in this thesis. The geotechnical properties of soil samples were determined using laboratory tests. The tested soils portray a range of liquid limits from 40 % to 79 %, plastic limits from 19 % to 57 %, a free swell index ranging from 36 % to 133 %, and a free swell ratio range varying between 1.17 to 2.50. A total suction ranging from 110 kPa to 11283 kPa, a matric suction of 80 kPa to 8745 kPa, and an air entry value ranging from 6 kPa to 20 kPa. The reported unsaturated shear strength ( ) values range from 119 kPa to 1512 kPa, while saturated shear strength ( ) values range from 94 kPa to 941 kPa. The values are 1.1 to 1.78 times higher than the . The friction angle values range from 22º to 53º, the angle related to matric suction ranges from 1.41º to 10º, the angle indicating the rate of reduction in shear strength with respect to a variation of swell stress ranges from 16.96º to 69º, and the cohesion ( ) ranges from 35 kPa to 78 kPa. The findings indicated that soils BFS, WIS, and WES belong to the high plastic clay (CH) category, while BES falls under the low plastic clay (CL) category. The X-ray diffraction test quantified the mineralogical constituents controlling the swelling behaviour of identified soils and validated the soil classification tests. The SWRC Fit (Soil water retention curves software) was used to establish the soil-water characteristic curves (SWCCs) of tested soils based on models by Seki (BL), Durner (DB), Van Genuchten (VG), Fredlund and Xing (FX), and Kosugi (LN). Among the models, BL and DB portray the best-fitting SWCCs. Multivariate regression analysis was performed using NCSS 12 (Statistical analysis software for researchers) on data obtained from 60 tested soil specimens to design the geotechnical predictive models. The modified-unsaturated shear strength ( ) was established from the equilibrium of three stress variables: the effective normal stress ( − ), matric suction ( − ), and swelling stress ( ). The reported values fall within the range of 31 kPa to 1162 kPa at the dry optimum. Nonetheless, exhibits marginal values beyond the dry optimum. The assessment of was conducted by comparing with three precedent constitutive relations of established by Fredlund and Rahardjo (1993), Garven and Vanapalli (2006), and Bulolo and Leong (2019). Previous unsaturated shear strength constitutive relations may overestimate the shear strength of compacted unsaturated expansive soils because the stress state induced by the swelling stress within these soils is ignored in design. A novel approach, applicable to unsaturated compacted expansive soils, was established by bridging classical soil mechanics (CSM) and unsaturated soil mechanics (USM) through mathematical relations. This approach aims to determine the unsaturated shear strength characteristics using basic geotechnical index properties. Geotechnical predictive models were developed for soil mineralogy, soil suction, compaction features, and unsaturated shear strength parameters. The predicted values were found to exhibit an acceptable level of accuracy. Lastly, the research work clearly illustrates that the modified-unsaturated shear strength constitutive relation proposed in this study is an efficient tool to characterise the shear strength of expansive unsaturated soils. The influence of the swelling stress on the shear strength of expansive soils should not be overlooked in design. Taking into account the influence of swelling stress on shear strength of expansive soils can enhance the durability of constructions. The geotechnical predictive models designed in this study can predict the geotechnical parameters and reduce the cost and time required for laboratory tests. The developments explored in this research project contribute to the advances in unsaturated soil mechanics and design enhancement.