Effect of Wetting and Drying Cycles on the 3D Hydromechanical Behaviour of Expansive Residual Soils in Auckland

Abstract

Swelling-induced stresses and deformations have significantly influenced the long-term safety of lowrise buildings, pavements, slope instability and retaining structures constructed on and/or adjacent to expansive soils. These materials are commonly subject to seasonal wetting and drying and undergo cyclic swelling and shrinkage in the field. Such variable behaviour of cyclic swelling and shrinkage often results in damage readily seen around the world as well as in Auckland, New Zealand. The principal objectives of this research are to investigate the one-dimensional swelling pressure and strain of compacted expansive clay, to obtain a precise and accurate method for the shrinkage curve determination and soil-water characteristic curve and to investigate the hydromechanical behaviour for Auckland’s expansive residual soils subjected to multiple wetting and drying cycles. A modified oedometer set-up was used to obtain the one-dimensional swelling pressure of compacted soils during inundation and under constant volume condition. In addition, the one-dimensional swelling strain was evaluated using the free swell method for the same tested soil specimens. An experimental programme was developed to evaluate the swelling potential of two expansive soils found in Auckland and prepared at a wide range of initial moisture content and density. Based on the database of two soils, a prediction formula of the swelling pressure, as well as swelling strain, was developed. The formula considers not only the initial soil condition but also implicitly, the amount of active minerals through the free swell index. In addition, the formulas input parameters are easily obtained from simple lab tests. The developed formulas were validated using the database from the other two expansive soils obtained from Auckland and from a data reported in the literature. The proposed formulas were able to predict reasonably well the swelling potential of soils at a wide range of initial moisture contents and densities, considering the range of expansivity in this study. A state-of-the-art technique for measuring the shrinkage curve was established based on a stereo vision system. Soil mass and volume change were measured during drying using a digital balance and a 3D EinScan technique, respectively. The three-dimensional (3D) soil model was constructed via stereo duos using fully automated software. An expansive residual soil found in Auckland, New Zealand was compacted at two different initial densities using three different ring sizes. Volumetric shrinkage strain was calculated based on a developed software, after a 3D model reconstruction, designed by the IVS - intelligent vision systems- lab in conjunction with the Geomechanics laboratory at the University of Auckland. The proposed technique was validated using conventional manual measurements. Furthermore, the error of the volume measurement method was evaluated to be 0.93% through tests on a PVC cylinder. The proposed EinScan technique could be less subjective and less prone to human mistakes in comparison with the manual measurements approach. The soil-water characteristic curves and shrinkage curves were then combined to estimate volume change under different stages of unsaturated conditions. A comprehensive laboratory programme was performed on two compacted unsaturated samples using a modified K0 suction-controlled triaxial apparatus. The modified apparatus was capable of continuously monitoring changes in vertical and horizontal pressures, gravimetric water content, and degree of saturation along multiple cycles of wetting and drying. The experimental results were interpreted within the context of the two exist independent stress state variables, namely the net mean stress, p, and matric suction, s. The unsaturated clay showed complicated hydromechanical behaviour. Within the suction range considered (from 100 to 400 kPa), both vertical and horizontal net normal stresses trends are very similar during wetting and drying cycles. However, the vertical net normal stress was always higher than the corresponding horizontal net normal stress, due to the effect of the microstructure of the clay layers sheeting. The contribution of suction to hydromechanical behaviour is significant for compacted specimens. Reversible swelling-shrinkage behaviour is mainly observed at the early stage of the wetting and drying cycles, followed by irreversible swelling and shrinkage behaviour at the second shrinkage cycle and at the third swelling cycle for two compacted specimens, respectively. The irreversible swelling and shrinkage behaviour difference was attributed due to the effect of different initial soil suction. The third swelling and shrinkage cycle represented the peak value of both swelling and shrinkage net mean stresses for both specimens. The equilibrium stage occurs after the third cycle, regardless of the initial soil conditions for both specimens. Lastly, the mechanical and hydraulic hysteresis phenomena appear during the cycles of wetting and drying.