Vadose Zone Characterisation for Hydrogeological and Geotechnical Applications

Abstract

Rapid urbanization is resulting in increased vertical development and use of anthropogenic materials. Geotechnical site investigation is well established in assessing ground conditions, and moisture specifically is a standard descriptor in soil profile logging and is addressed through a variety of laboratory tests. However, changing moisture conditions, occurring in the vadose zone between land surface and the groundwater table, results in highly variable conditions. Noting the presence and variability in moisture is not sufficient to ensure longevity of engineering structures and protection of water resources. Water at partial saturation is proposed to impact the infrastructure and the vadose zone moisture budget as: (A) perching above lower permeability (lower-k) materials; (B) perching as waterlogged lower-k materials above capillary barriered higher-k materials; both resulting in (C) possible imbibition into less saturated low-k materials; or, under further wetting, resulting in (D) lateral interflow under a hydraulic gradient; (E) gravity-driven percolation breaching capillary barriers and resulting in translatory downward flow; or (F) unsaturated fracture flow. All these mechanisms combine to result in complex moisture implications on infrastructure during project lifecycle, as well as on recharge and contaminant transport rates above the phreatic surface. These are further exacerbated by anthropogenic materials (e.g. made ground) replacing natural materials and infringing on the natural and pre-development subsurface water cycle, as well as climate change, and more elaborate engineering development. Contrary to saturated systems, unsaturated systems result in alternating wetting-drying cycles causing continuous changes in effective stress and redox conditions. The paper addresses some key findings and examples from experiments and case studies.