Colluvial soils, major materials for natural terrain slopes in Hong Kong and in the Three Gorges Reservoir Zone, are normally widely and gap-graded with high gravel contents and fines contents. This research focuses on (1) the hydraulic properties of such gap-graded coarse soils, which are essential in the investigation of triggering mechanisms of landslides but have seldom been studied to date, and (2) the micro-structures of such gap-graded soils, which are a key to understanding the hydraulic and mechanical behavior of gap-graded soils....[ Read more ]

Colluvial soils, major materials for natural terrain slopes in Hong Kong and in the Three Gorges Reservoir Zone, are normally widely and gap-graded with high gravel contents and fines contents. This research focuses on (1) the hydraulic properties of such gap-graded coarse soils, which are essential in the investigation of triggering mechanisms of landslides but have seldom been studied to date, and (2) the micro-structures of such gap-graded soils, which are a key to understanding the hydraulic and mechanical behavior of gap-graded soils.

The influences of coarse contents and wetting / drying cycles on the micro-porosity structure are investigated using mercury intrusion porosimetry and scanning electron microscopy techniques. The results demonstrate that gap-graded soil with a high coarse fraction (>70%) forms a coarse-controlled, dual-porosity structure during compaction and retains this structure after wetting / drying. The gap-graded soil with a high fines content (>30%) forms a fines-controlled, dual-porosity structure (bimodal pore size distribution) during compaction, which evolves into an intra-aggregate pore structure (unimodal pore size distribution) after saturation and shows significant shrinkage during drying.

One difficulty hindering the measurement of hydraulic properties is the lack of a direct and time-saving technique for measuring the permeability function over wide suction and permeability ranges. A wetting front advancing method is developed to measure the permeability functions rapidly over a wide suction range, in which the wetting process in an unsaturated, large-scale soil column is simulated and monitored. A new analytical interpretation procedure is proposed to calculate the permeability function based on the monitored water contents and suctions. This method is applicable to a wide range of soil suctions and different soil types. Using this method, the permeability function can be measured within a few days, resulting in considerable time-saving as compared to the traditional steady-state methods and the instantaneous profile method. Another difficulty hindering the measurement of hydraulic properties is the lack of a proper technique for measuring the soil-water characteristic curve (SWCC) at extremely low suctions (i.e., 0.01 kPa to 1 kPa). A new device is developed using the axis-translation technique and a water-head control method. A new interpretation equation is proposed to reduce the test error induced by the suction difference along the sample height. Using the new device and the new interpretation equation, the measurable suction range is extended to 0.05 kPa.

The two new techniques are used to measure the hydraulic properties of five soils with different coarse contents. A typical bimodal feature is found in the SWCCs and the permeability functions for gap-graded soils with high coarse fractions. On the contrary, for soils with high fines contents, unimodal SWCCs and permeability functions are common. A new SWCC model is proposed to characterize the bimodal SWCCs. The model uses six parameters with clear physical meaning to describe the drainage process in a dual-porosity structure soil. Regression analysis is conducted on 44 cases of SWCC tests in the literature to relate the six parameters to the soil grain-size distribution and void ratio. A new bimodal permeability function model is also proposed. The bimodal permeability function model uses six meaningful parameters, which include three suction parameters from the bimodal SWCC and three permeability parameters estimated using empirical equations.

To evaluate the risk of failure of colluvial slopes upon rainfall infiltration, the measured hydraulic property functions are used in the numerical simulation of ideal slopes composed of the five soils with different coarse fractions. Simulation results demonstrate that shallow failures in slopes composed of gap-graded soils with high coarse fractions can be easily triggered during a high-intensity rainstorm.