The impact of water repellency on soil mechanics

Contrary to a prevalling assumption in geotechnical engineering, soils can exhibit low wettability. Water repellent behaviour has been identified in contaminated, engineered and natural soils, and waste. These include oil contaminated soils (e.g. oil spills in refineries), carbon rich waste (e.g. lignitic mine waste), carbon engineered soils (e.g. soils amended with biochar), soils subjected to wildfires and some agricultural and forest soils, and sulphur blocks in mineral exploration. Despite research on the hydrological implications (reduced infiltration, enhancement of soil erodibility by wind and water, post fire debris flows), very little is known on the influence of water repellency on soil engineering properties. This proposal will address this knowledge gap by investigating the relationship between wettability and the engineering properties of unsaturated soils (with air and water in the pores). It will test the hypothesis that soils with a lower affinity for water exhibit poorer performance in terms of their engineering behaviour (higher compressibility and lower shear strength). This will be achieved by combining experimental techniques from Soil Science, Chemistry and Soil Mechanics. The results will provide a first insight into the influence of wettability on the engineering behaviour of soils.

The data obtained will provide a first insight into the influence of wettability on the mechanical behaviour of granular media. In particular: (1) the work described in the proposal will provide basic understanding of the physical aspects of soils needed to define soil conservation strategies in the UK; (2) it will elucidate the physical mechanisms acting in water repellent soil slopes and their potential linkage to the initiation of debris flows which constitutes a frequent hazard in Southern Europe, western U.S. states and South Australia; (3) it will provide a basic understanding of the hydro-mechanical behaviour of water repellent soils (deformation, shear strength, water retention) needed for the construction and mining industry. To promote this work the communication of results will use a network of contacts made by the collaborators in this project who are active in the organization of meetings, special editions, and societies. The collaborators fields cover soil science, geotechnical engineering, chemistry, and geomorphology. To maximize the impact of the results, the data will be published in the different fields through journal publications and presentations in conferences. It is expected that this cross-disciplinary collaboration between the Schools of Earth and Ocean Sciences, Chemistry, and Engineering at Cardiff University and the Schools of the Environment and Society, and Engineering at Swansea University will continue and will help to consolidate a research network in Wales looking at fundamental and applied Earth Surface Processes.