A case study of loess collapse in a UK field trial in Kent, South-east England

Loess soils undergo collapse due to bond weakening under loading and, especially, wetting, and consequently constitute a major engineering geology hazard. To understand better the relationship between wetting and volume reduction in loess, a field collapse test was conducted at a 'brickearth' quarry, where a 5.0 x 5.0 x 1.5 m deep sample was isolated, flooded in a controlled manner and subjected to a surface stress of up to 210 kPa for 10 days. Geotechnical instrumentation, consisting of piezometers and rod extensometers, was complemented by geophysical instrumentation (resistivity arrays, shear wave transducers and a resistivity probe) to provide evidence of changes in interparticle bonding during the collapse process. Laboratory index and oedometer testing, together with SEM study of samples removed from the site, complemented the site monitoring. The field collapse test eliminated many problems associated with laboratory testing, notably small volumes of material and sample disturbance. This paper presents the geotechnical findings on 'large-scale' loess performance and relates them to the results of shear wave velocity and resistivity monitoring. The different behaviour of two distinct soil strata and the importance of the degree of saturation to soil fabric changes are demonstrated. The results identify how the soil in situ and oedometer samples respond under similar applied stresses.