This research aims to investigate the hydraulic and mechanical characteristics of an unsaturated completely decomposed tuff (CDT) in response to drying-wetting cycles. The investigation consists of laboratory investigation and field monitoring....[ Read more ]

This research aims to investigate the hydraulic and mechanical characteristics of an unsaturated completely decomposed tuff (CDT) in response to drying-wetting cycles. The investigation consists of laboratory investigation and field monitoring.

The laboratory investigation included 19 direct shear tests to explore the influence of drying-wetting cycles on hydraulic and mechanical behavior prior at different suction levels and strain rates to and during shearing. The specimens were subjected to different drying-wetting history and sufficiently long duration was allowed to ensure suction equalization. The investigation of the cyclic effect was repeated at suction levels, 1kPa, 20kPa, 100kPa, and 400kPa. It was observed that the hydraulic and mechanical behavior demonstrated by the specimens was highly path and suction dependent. Volumetric deformation changed from contractive to dilative as suction increased. Suction generally increased the brittleness and the magnitude of negative dilatancy of an unsaturated soil. The higher the suction, the greater the difference between peak shear strengths attained by the specimens subjected to drying and wetting. At 400kPa, the difference was as high as 21% for the first drying-wetting cycle. As drying-wetting cycle progressed, the differences in behavior (e.g. dilatancy, peak shear strength, and total volume change) exhibited by specimens subjected to drying and wetting decreased at a given suction. In addition, a higher strain rate resulted in a higher peak shear strength and less dilative deformation exhibited at suction of 100kPa. At a given horizontal displacement, there was a greater increase in volumetric water content when a specimen was sheared at a lower strain rate.

A field monitoring was conducted in Tung Chung to determine in-situ stress dependent soil water characteristic curves (SDSWCCs) and permeability and monitor their corresponding responses to artificially applied wetting-drying history by the instantaneous profile method. It was observed that the hysteresis loop size decreased with depth while the air-entry value increased with depth. For depths at which air-entry value was exceeded during the monitoring, the desorption rate was higher than the adsorption rate. Generally, there was no major difference between SDSWCCs for the first and second wetting-drying cycle. To determine in-situ permeability, the equation of continuity and Darcy’s law were applied. Two methods, namely Method 1 (M1) and Method 2 (M2), were adopted to assess the permeabiltiy. The rock stratum at 4m was assumed to be impervious in M1 while this assumption was not needed in M2. Overall agreement was demonstrated by estimates obtained from the two methods during drying processes but not wetting processes. The range of variation in permeability can be up to 2 orders of magnitude over suction range of around 1 – 10kPa. This may be related to in-situ features like cracks, fissures and rootlets.