Hydrological and erosional processes in synthetic water repellent soils

Abstract

Water repellent soils are granular materials that show a low affinity for water, with delayed water infiltration. Due to their distinctive hydraulic properties, interest has been growing in their potential as infiltration barriers. The current research aims to explore the hydrological and erosional behavior of synthetic water repellent soils, with a view to facilitating their utilization in the built environment. The specific objectives of this research are to (1) assess the influence of induced or synthetic soil water repellency on water content measurement; (2) investigate the effect of water repellency on the hydrological response of a sloping surface; (3) evaluate the interaction effect between water repellency and soil type on soil erosion; and (4) explore the potential application of water repellent soils as capillary barriers.

Moisture sensors were tested in silane-treated granular materials to reveal the influence of silane treatment on soil water content determination. The results showed that the saline treatment increased the output of the moisture sensors, by increasing the electrical conductivity. This influence showed a positive correlation with the silane concentration and soil water content, regardless of soil and sensor type. The results therefore suggested that soil-specific calibration should be conducted for soil moisture sensors in silane-treated soils.

The influence of synthetic soil water repellency on the hydrological response was investigated by carrying out twenty-four flume tests on model slopes under artificial rainfall. Overall, an increase in soil water repellency reduced infiltration and shortened the time for runoff generation, with the effects amplified for high rainfall intensity. Comparatively, the slope angle and relative compaction had only a minor contribution to the slope hydrology. The subcritical water repellent soils (contact angle, CA ~ 90°) sustained infiltration for longer than both the wettable (CA < 90°) and water repellent (CA > 90°) soils, which presented an added advantage if they are to be used in the built environment.

The isolated impact of synthetic soil water repellency on soil erosional behavior, including splash erosion and rill processes, was evaluated. The results revealed that water repellency did not necessarily lead to increased soil erosion yield. Its impact was dependent on grain size. This study indicated that an optimum condition that inhibited the infiltration while controlling the increase in erosion can be achieved.

The potential application of synthetic water repellent soils in capillary barriers, as a landfill cover system, was explored. Four flume tests were conducted on inclined model slopes to compare the performance of a monolithic cover, a conventional capillary barrier and two proposed water repellent capillary barrier cover systems. The capillary barrier effect was significantly strengthened by soil water repellency, including increased surface runoff, promoted lateral diversion, decreased basal percolation and delayed breakthrough. Particularly, the release of accumulated water into the underlying layer is still inhibited even after breakthrough.

This research sheds light on the understanding of the hydrological and erosional behavior of synthetic water repellent soils under rainfall, and therefore provides a basis for using these novel materials in the built environment.