Durable synthetic water repellent soils for sustainable ground infrastructure

Hong Kong is vulnerable to extreme weather events due to its rugged topography and dense urban development. Ground stabilization measures are intrusive and use concrete, steel, polymers and glass fibres as reinforcement elements, so more sustainable approaches that require fewer raw materials are strongly recommended. This project will explore the use of synthetic water repellent soils in ground infrastructure. New laboratory research by our team has demonstrated that wetting in soils can be controlled by inducing and adjusting water repellency. Existing soils can be treated on a construction site to become water repellent; once treated, they are non-intrusive, integrate with the surrounding environment and are mechanically stable (due to their non-wettable behaviour). However, research on the durability of water repellency is required to determine if it is a temporary or permanent technique and to elucidate applications. We hypothesise that whilst synthetic water repellent soils will degrade over time, through contamination by wettable fine particles deposition, growth of vegetation (roots), soil microbial biomass and organic matter, they have sufficient longevity for engineering applications. In natural water repellent soils, water repellency can last years, so it should be possible to construct very long-lasting synthetic water repellent soils for ground infrastructure. Therefore, the timescales of man-made water repellency systems need to be established, as do the causal agents of its decay. The first step in the development of durable synthetic water repellency is to identify time-stable water repellent treatments. Three approaches will be examined: impregnating an initially wettable soil with organic coatings such as dissolved organic carbon; changing the surface chemistry with silanes; and enhancing water repellency by altering the soil particle attributes (shape and size). In the second step, the spatial and temporal evolution of the treated soils will be tracked by monitoring their response to forced degradation and to microbiology and vegetation scenarios similar to those they would be exposed to in the field. This will identify the limit conditions under which water repellency may revert to its original (wettable) state. This research will provide Hong Kong with a novel technology for constructing water-tight barriers and fills that is both inexpensive and reliant on an abundant local resource – soils. Our research will verify its stability, so that perceived risks to practical implementation are identified and alleviated.