Shallow Geophysical tools to Investigate the Potential Extent of Subsidence Anomalies Associated with Hydrothermally Altered and Compressible Clays

Abstract

Identifying the potential extent of future subsidence associated with deeply buried compressible clay-rich materials of hydrothermal origin, is a useful objective for planning and hazard assessment purposes. Geotechnical assessments of foundation conditions are often undertaken in advance of construction and infrastructure projects, but these typically focus on detailed shallow investigations using tools such as cone penetrometers and shallow core drilling. Geophysical methods, however, can offer a more costeffective means of covering larger areas and penetrating to greater depths where subsidence and ground deformation effects might become an issue, especially for buildings, in the future. This study reviews the application of a variety of shallow geophysical tools to help map the extent and depth of buried hydrothermally altered clay-rich materials that might compact when subjected to pore pressure decline (e.g. a drop in groundwater level) or an increase in surface loading. Methods include ground penetrating radar (GPR), electro-magnetic soundings (TEM), resistivity mapping, electro-kinetic soundings (EKS), seismic refraction and reflection, and seismic attenuation. The results are presented of trial GPR, TEM, and EKS surveys at Crown Park, Taup?, across an area of known subsidence anomalies. Previous measurements here revealed changes in subsidence rates over time, and nearby core-drilling has revealed the anomalous physical properties of some highly compressible clay-rich materials.