Use of resistivity to measure deterioration of engineered soils

Abstract

Climate change induced seasonal water content variations are likely to influence the soil parameters that determine the stability of engineered slopes. These slopes constitute a large amount of UK transport infrastructure, Repeated fluctuations of water content affect the soil water retention behaviour and hence the suction-dependent shear strength behaviour of engineered soils, resulting in deterioration in both the soil suction and shear strength, which impact the long-term slope stability. Resistivity methods can provide spatial information on the electrical properties of underground soils and therefore has a great potential to be used to monitor subsurface geotechnical property changes by applying geotechnical-geophysical transformation approaches. In order that the resistivity method can be used more effectively in slope monitoring, the interrelationships between soil geotechnical and geophysical properties need to be more fully understood. Therefore, this research focused on establishing an improved understanding of soil deterioration by investigating the effects of repeated drying and wetting cycles on soil property interrelationships and investigating the potential use of resistivity methods to qualitatively and quantitatively describe the property variation of engineered soils subjected to drying and wetting cycles.
In this thesis, three phases of laboratory-based experiment programmes were conducted to investigate the interrelationships between soil electrical and geotechnical properties on samples with different sizes by using different measurement techniques. In the first phase, several point measurement sensors were employed for estimating the soil water content, electrical resistivity, and suction on a cubic glacial till sample during cyclic drying and wetting. In the second phase, soil electrical resistivity, suction and shear strength were determined by a four-electrode resistivity probe, WP4C potential meter and performing Unconsolidated Undrained triaxial test on triaxial specimens with 38mm diameter made of three different types of soils. In the third phase, the 3D time-lapse ERT method was employed to map the in-time resistivity changes in a 1.2m diameter cylindrical lysimeter; point sensors were also used to provide water content and suction measurements on different depths within the lysimeter.
Soil water retention and shear strength behaviour under multiple drying-wetting cycles has been investigated in this research. Result showed that the both the SWRCs and the undrained shear strength-water content relationships changes over time with drying-wetting cycling, indicating a hysteresis between drying and wetting paths and a deterioration over cycling. The hysteresis of SWRC and shear strength-water content relationship was found to be more pronounced in the first drying-wetting cycle and decreased over cycling. Deterioration in soil suction (i.e. negative pore water pressure) were found to be most significant between the first and the second cycle. Similar evolutionary trends were found in the cyclic swelling and shrinkage behaviour of the soils which may be a key factor to the hysteresis and deterioration. Furthermore, the spatial geotechnical information (i.e., the water content distributions) within the large lysimeter sample can be extrapolated from the ERT-derived resistivity data by applying the proxy relationships (i.e., the Waxman-Smits model).