The soils in the field are mostly unsaturated, and after weathering processes, soils formed from the same parent rock material and deposited to similar OCR can be subjected to varying environmental conditions such that similar soils are deposited with a distinct difference in soil structural features including pore structure. Many theoretical and experimental studies have demonstrated that unsaturated soil behaviour is strongly affected by suction and the structure of the soils. However, there is no clear distinct evaluation of the effects of pore structure or the rate of aggregation from a soil compacted to similar density and subjected to hydraulic and mechanical loads. There have been some partial studies mostly on clays. Silt and silty sand soils are rarely studied. The hydro-mecha...[ Read more ]

The soils in the field are mostly unsaturated, and after weathering processes, soils formed from the same parent rock material and deposited to similar OCR can be subjected to varying environmental conditions such that similar soils are deposited with a distinct difference in soil structural features including pore structure. Many theoretical and experimental studies have demonstrated that unsaturated soil behaviour is strongly affected by suction and the structure of the soils. However, there is no clear distinct evaluation of the effects of pore structure or the rate of aggregation from a soil compacted to similar density and subjected to hydraulic and mechanical loads. There have been some partial studies mostly on clays. Silt and silty sand soils are rarely studied. The hydro-mechanical behaviour of silty soils compacted at similar densities and subjected to different rates of aggregation, and stress has not been well understood.

The principal objectives of this study are to investigate and evaluate the effects of pore structure and rate of aggregation on the soil water retention properties, suction controlled isotropic compressibility, very small strain shear stiffness and shear stiffness anisotropy of a low plastic silty soil compacted to similar density with different pore structures. Five series of tests including soil water retention, suction-controlled isotropic compression and very small strain shear stiffness were carried out to investigate the hydro-mechanical behaviour of a low plastic silty soil compacted to similar density but with different pore structures under different stress conditions. To interpret the results, microstructural investigations through scanning electron microscopy, mercury intrusion porosimetry and micro x-ray computed tomography were coupled to describe the pore structure and its evolution.

Experimental results from the water retention tests show that the highly aggregated (larger aggregate sizes) low plastic silty soil shows a higher degree of hysteresis, which results in a lower water retention behaviour along the wetting path. The difference in average degree of hysteresis between the highly aggregated soil and the unaggregated soil (smaller aggregate sizes) is between 70% to 80%. Since the dependence of shear strength on the degree of saturation has been reported in the literature, this shows significant importance to pore water distribution and slope stability.

Through numerical seepage and slope stability analysis, it was observed that without considering aggregation, the critical duration for slope failure can be over-estimated which will be detrimental to a city that is prone to shallow rainfall-induced landslides like Hong Kong. From the suction controlled isotropic compressibility test results, a new parameter related to the suction and saturation history, concerning the compacted state suction and degree of saturation (saturation history index ΔSr), shows that the compressibility of unsaturated highly aggregated low plastic silty soils can be regarded as an extension of the compressibility of unsaturated unaggregated low plastic silty soils. This suggests the dependence of suction-controlled compressibility on the rate of aggregation and pore structure of unsaturated low plastic soils.

From the very small strain stiffness measurements, the stiffness of highly aggregated soils is higher with about a 250% increase when suction is increased from 0 to 300 kPa. On the other hand, the stiffness of the unaggregated soil increases by 140 %. Generally, the hydro-mechanical behaviour of compacted unsaturated silty sand shows significant dependence on the aggregation and pore structure. The results also confirmed that suction is anisotropic and induces anisotropic strains resulting in the evolution of anisotropic shear stiffness, for a specimen subjected to isotropic stress states.

In conclusion, void ratio alone cannot be used to describe the effects of stress and suction on the hydromechanical behaviour of a compacted low plastic silty soil when subjected to hydro-mechanical loading.