Complex conductivity monitoring of uniaxial shear failure in soils

Abstract:

Soils are prone to failure under a variety of natural and/or anthropogenic conditions (e.g., increases in moisture content due to rainfall or irrigation). Failure in soils typically occurs in shear form, posing major risks to infrastructure. Shear failure is the main cause of landslides, posing a natural hazard to society. Monitoring for deformation prior to shear failure is usually performed by sparse in-situ or ex-situ point measurements. Field-scale geophysical methods might provide rich spatiotemporal information on deformation unobtainable from sparse point measurements. Autonomous, geophysical monitoring systems could be deployed in the field, serving as an early warning system for shear failure, thereby eliminating the need for labor intensive point measurements.

In this study, we induced shear failure in a soil sample whilst monitoring its complex conductivity signature during deformation and up to the point of failure. The soil sample consisted of a homogeneous mixture of sand (98 %) and clay (2 %), saturated below the liquid limit and packed into a flexible cylindrical sleeve equipped with four electrodes. A uniaxial vertical load (σ_v) was applied to the soil column placed inside a load frame by continuously compressing the sample at a constant rate. Changes in complex conductivity, vertical stress and strain as well as effluent volume of the soil were continually monitored. A very slow compression speed was used to facilitate measurement of a broad frequency spectrum during complex conductivity monitoring, and measurements extended beyond the shear failure point.

Our findings indicate that both real and quadrature conductivity components of the complex conductivity track the relative increase of vertical stress of the sample under the applied vertical load. Both real and quadrature conductivity respond to sample failure, with the quadrature conductivity showing a sharp decrease, similar to the drop in stress, at the failure point. Our results provide evidence that complex conductivity can be used to monitor soils under shear stress and might be developed to predict failure before it occurs.