Rainfall-triggered shallow landslides: infiltration dynamics in a physical hillslope model

In this study, we investigate the triggering of shallow landslides through the analysis of physical experiments performed in an artificial hillslope. The physical model consists of a reinforced concrete box containing a soil prism with maximum height of 3.5m, length of 6m, and width of 2m. In order to analyse and examine the factors leading to the failure and the triggering modes, the hillslope is equipped with sensors to monitor the pore water pressure and moisture content response to rainfall in a 60cm thick sand layer overlying a sandy clay soil. Two experiments were performed with different degrees of the sand initial compaction, to investigate the role of porosity on the hydrologic response and the subsequent failure. The experimental results showed that, with initially loose sand, the failure occurred suddenly, without premonitory signs, the soil behaving like a viscous fluid. The collected data showed a rapid increase of the water pressure contextual to the failure of the sand layer. In the second experiment, with initially dense sand, three levels of instability were observed: (i) abundant runoff with limited erosion of the ground surface; (ii) local slip detachments involving a soil thickness of few centimetres; and (iii) a slow advancement of the entire sand layer volume. The hydrologic dynamics observed in the landslide experiments were simulated with numerical solutions of the Richards equation. The results of the simulations agree well with the experiments for the loose sand, while for the dense sand the comparison between experimental and numerical results shows some limitations related to the assumptions of single phase and rigid soil matrix implicit in the Richards equation. Copyright (c) 2016 John Wiley & Sons, Ltd.