Physical Modelling of Retrogressive Landslides In Sensitive Clay
Sensitive clay landslides represent a significant geohazard in Eastern Canada and Scandinavia, often exhibiting complex lateral spreading or flow-like retrogressive behavior. The upslope extent of this hazard is represented by the retrogression distance, which can be upwards of hundreds of meters from the top of the original slope to the crown of the post-failure slope (e.g. the zone of depletion). The ability to estimate the retrogression distance is essential for risk management and effective risk communication concerning these types of landslides. To understand the site-specific risk, it would be desirable to estimate retrogression distance from soil properties and topological characteristics. Empirical correlations with Taylor’s stability number, Ns, have been developed, however, field data using these relationships shows considerable scatter, which could overestimate or underestimate retrogression distance. The large geotechnical centrifuge at C-CORE in Newfoundland, Canada, was used to conduct several physical model experiments aimed at investigating the relationship between stability number and the observed retrogression distance in a controlled laboratory environment. Before testing physical models, an artificially sensitive material was developed. A mixture of 12% cement and Speswhite kaolin with a design water content of two times the liquid limit was selected for the physical model experiments. Five centrifuge tests were conducted to investigate the change in retrogression behaviour of slopes of varying stability number and ease of flow of the remolded material. This was achieved through designing mixes to vary the peak undrained shear strength in the former, and the addition of a 5-degree slope angle for the remolded material for the latter. The addition of a 5-degree slope angle incited notably larger retrogression distances. Undrained stability analyses were performed for each observed failure stage. These analyses were remarkably accurate in representing the observed failure geometry and factor of safety. The relationship between Taylor’s stability number and the observed retrogression distance increased, as expected. The results of this research indicate that the geometric characteristics and material properties of a retrogressive landslide may be able to explain one component of the scatter when correlating the Taylor stability number with retrogression distance.