Monitoring fine-grained soils loading with Time-Domain Reflectometry

Subsurface geophysical investigations have the potential of providing information for the long-term monitoring of geotechnical assets. This research evaluates the suitability of vertically and horizontally orientated, embedded Time Domain Reflectometry (TDR) measurements for monitoring of near-saturated, fine-grained soils under vertical loading conditions. TDR measurements were carried out regularly during vertical loading and unloading of near- and fully-saturated soil mixtures containing fine-sand, kaolinite and bentonite. The results show that TDR probe orientation, in relation to the load direction, affects the values of TDR-measured apparent permittivity (AP) and bulk electrical conductivity (BEC). The relationship between the soil void ratio and AP was found to be clearer when measured in the direction of loading whereas AP and BEC measured normal to the load application appears to reflect changes in pore-water pressure. BEC was found to be more variable and less obvious. It is concluded that monitoring relative changes in temporal AP and BEC using embedded TDR sensors can provide unique and valuable information on how a soil responds to loading under near-saturated conditions.