Analysis of hydrological processes in unstable clayey slopes

Abstract

In slope stability research a ground water level increase is often the critical factor for failure. High ground water levels (or more properly stated: high pore water pressures) reduce the internal strength of the slope. It is recognised for quite some time that fast infiltration of precipitation towards the (perched) ground water table can induce high pore pressures and thus instability within formerly stable slopes. However, deeper located landslides are affected on the long term by a slow but distinct ground water table rise, which may take place a significant time after the occurrence of rainfall. From the above it is clear that hydrological insight in natural slopes susceptible to landsliding, is more than appropriate for the long-term risk assessment in case of land use or climate change. The objective of this thesis is to analyse and quantify the hydrological processes that play a role in landslides within clayey slopes. The following questions are addressed: - What can hydro- and geochemistry add to our knowledge of a hydrological system in a clayey slope? - What processes dominate the ground water recharge in unstable clayey slopes and how can the ground water recharge be quantified? - What is the consequence of a land use or climatic change with respect to unstable clayey slopes?

The study of the potential use of hydro- and geochemistry in refining the hydrological knowledge of a site is described in the second part (chapter 3). Analyses have been made to cation exchange capacity and exchangeable cation compostion with NH4Ac and NaCl method. It is concluded that geochemistry is a potentially valuable technique for e.g. landslide reserarch, but is recognised that still a lot of work has to be done before this technique can be applied in engineering practice. The hydrological analyses of the Beline study area at Salins-les-Bains shows the importance of the role of the unsaturated zone in deep-seated landslides. A methodology is presented to model state-dependent ground water recharge. This is basically a precipitation time series that is rescaled as a function of the state of the unsaturated zone. The latter can be calculated on basis of the results of an unsaturated zone model, soil moisture content or even be approximated using a sinusoidal function. The results of the Beline case study showed that clear improvement of the modelling of ground water level fluctuations can be obtained with this focus on the input series. It was made plausible that the Beline slope suffers from creep displacement process. The changing ground water conditions as a result of changes in climate or land use have two important consequences for the creep displacement at the Beline slope. Acceleration of the displacement will occur because of an increased ground water level. Probably more significant is the impact of prolonged periods of high ground water level. This prolonged period of movement is for slopes, which encounter creep features, probably the most important factor for catastrophic failure. In that respect a long-term change in vegetation pattern has much more influence on a clayey slope system than a short-term change in climate input.