The significance of fragipans to early- Holocene slope failure: application of physically based modelling

Abstract

Interpreting slope failure in the early Holocene has traditionally focused on climatic change. Little research considers the role of soil-profile characteristics, in particular the effect of fragipan occurrence. Fragipan formation has been associated with periglacial processes in northwest Europe, but recent evidence suggests that fully developed fragipans did not exist until 2000-3000 years after the close of the Lateglacial. Fragipans have been widely reported as having reduced permeability and altered soil moisture retention curves. The implications of such hydrological properties for slope stability is considered in this paper, by comparing stability of a soil profile containing a fragipan with one that is freely draining using a physically based soil hydrology-slope stability model. The results suggest differences in angles of limiting stability of 15° between the two soil profiles, while differences resulting from likely climatic variation are only 3-4°. Modelling results of maximum stable slope angles for fragipans are consistent with those reported in the landscape, placing confidence in model outputs. This is used as a basis for suggesting that pedologic factors might be more significant than climatic factors when interpreting early Holocene slope instability. The potential utility of physically based modelling is explored more generally, and suggestions are made for future research to elucidate more fully the role of fragipans in slope stability.