Non-invasive moisture monitoring within an earth embankment - a precursor to failure

An experimental monitoring system is described using electrical resistivity as a proxy for imaging changing moisture content distribution in engineered earthworks. The approach is illustrated using a case history concerning a road embankment constructed of tropical red soil in Kenya. Tropical red soils have highly variable properties, governed by their soil fabric and mineralogy. As earthworks materials, their geotechnical behaviour is extremely sensitive to changes in moisture content and compaction. The relationship between moisture content and electrical resistivity was established in the laboratory on core obtained using a monitored drilling and sampling technique. The non-invasive nature of electrical resistivity surveys was exploited in the design of a monitoring system placed below the pavement in the topmost layers of compacted soil. Monitoring over a period of 18 months is reported, starting prior to the construction of the pavement following the completion of soil compaction. Initially substantial variability in moisture content was inferred from surface monitoring, and even larger changes were seen in corresponding downhole measurements. The moisture content within the body of the embankment stabilised after 6 months, while a moist layer 'trapped' beneath the pavement dissipated over the following 10 months. Two surveys were undertaken during the December rains showed large changes in moisture content had occurred quickly in the surface layers on one side of the embankment. This area subsequently failed as a small landslip and was remediated by additional drainage.