Preferential Water Infiltration Path in a Slow-Moving Clayey Earthslide Evidenced by Cross-Correlation of Hydrometeorological Time Series (Charlaix Landslide, French Western Alps)

Grégory Bièvre,Agnès Joseph,Catherine Bertrand

doi:10.1155/2018/9593267

Abstract

Slow-moving clayey earthslides frequently exhibit seasonal activity suggesting that deformation processes do not only depend on slope and intrinsic geomechanical parameters. On the contrary, seasonal motion patterns are frequently observed with acceleration during the wet season and deceleration during the dry season. Within landslides, it is mainly the phreatic water table which is monitored. However, in the case of deep-seated landslides made of heterogeneous lithological units and with several slip surfaces, the characterization of the phreatic water table does not allow to relate satisfactorily the activity of the landslide with environmental parameters such as rainfall and subsequent water infiltration at depth. This paper presents a seasonal analysis of water infiltration within a slow-moving clayey landslide. Results of an extensive geotechnical and geophysical prospect are first exposed. Then, rainfall and water table level time series are analysed for two water tables using the cross-correlation technique: the phreatic water table located a few metres deep and a water table located above a shear surface located 12 m deep. Results show that water infiltrates faster down to the deepest water table. Then, time series were split between “dry” and “wet” seasons and the effective rainfall was computed from the original rainfall time series. Cross-correlation results show that the phreatic water table responds identically to rainfall in both seasons. On the contrary, the water table located above the shear surface has a very contrasting behaviour between summer (mainly drainage) and winter (behaviour similar to the phreatic water table with storage of water during a few weeks). This difference in behaviour is in agreement with the landslide kinematics.

Highlights

Slow-moving clayey earthslides frequently exhibit seasonal activity suggesting that deformation processes do depend on slope and intrinsic geomechanical parameters
The results show several significant peaks with a one-year-long periodicity, suggesting that the two water tables are both controlled by the same environmental parameters
The water table monitored by p2 shows a regular decrease in CC just after the first peak, and nonsignificant CC values are reached for a lag of around 42 days (Figure 9(d)). These results suggest that contrarily to the phreatic water table which behaves like a short-term reservoir, the water is drained directly after reaching the water table located above the shear surface at 12 m depth

Summary

Introduction

Slow-moving clayey earthslides frequently exhibit seasonal activity suggesting that deformation processes do depend on slope and intrinsic geomechanical parameters. Deceleration phases are related to water drainage, lowering of the water table level, and subsequent decrease in pore water pressure during dry seasons This might be related to potential dilation of fine-grained material in the vicinity of the shear zone [8, 9], or to liquefaction/fluidization of water-saturated sediments close to the rupture zone [10,11,12,13]. These relationships were further applied to the build-up of early-warning systems [22, 23] and/or to landslide forecasting [24, 25] These approaches are devoted to shallow landslides, with disrupted mass a very few metres thick and with a relatively homogeneous motion for water infiltration. Even if rainfall and subsequent water infiltration are known to be a major driving mechanism for landslide acceleration (e.g., [31]), the experimental observation of water infiltration deeper than a few metres remains poorly reported

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Conclusion