Analysis of the Stability and Response of Deep-Seated Landslides by Monitoring their Basal Temperature

Deep-seated landslides are known as large slides involving millions of cubic meters that move as a rigid block on top of a deep (below the roots of the trees and the groundwater level) basal layer of heavily deformed minerals. This kind of landslides geometrically shears as translational/rotational (depending on the stratigraphy of the area), with very low velocities (cm/year) during long periods (years to tens of years). However, their collapse is usually very sudden, happening within minutes and without previous warning, reaching high velocities up to 120m/s (as the 1963 Vaiont landslide in Italy, \cite{Muller1964}). The catastrophic and fast collapse of this kind of landslides makes the evacuation of the area that is going to be affected almost impossible, thereby possibly causing fatalities and infrastructure damages. Moreover, the lack of understanding of the physical processes behind the mechanisms of failure of this kind of landslides makes the development of reliable early warning systems (or tools/protocols to stop the acceleration of the landslide) challenging, therefore potentially causing significant damages to civil infrastructures. The landslide-prone areas are widespread around the world, having a detrimental fatality rate of tens of thousands. Hence, landslides are a globally threatening natural hazard with disproportional consequences.

This thesis focuses on the understanding of the mechanism of the fast collapse of large deep-seated landslides and provides the first-stage tool for the early warning system. First, is described the Vardoulakis Forecasting Model (VFM), which is a heat energy-based mathematical model that considers that the temperature of the shear band material is critical in the behavior and stability of the landslide. The model contemplates the external and internal factors of this kind of landslide. The external factors of a landslide are considered as the loading conditions, such as groundwater level. And the internal factors of the landslide are focused on the thin shear band, such as the reduction of the friction coefficient of the material, hence, the loss of resistance of the basal material due to continuous friction and cycles of loading-unloading of external forces such as the groundwater. Moreover, the constitutive law used in the VFM theoretically implies that the material of the shear band (usually clay or clay-like material) is rate (velocity) hardening and thermal softening \cite{Vardoulakis2002}, but this assumption has never been tested experimentally. This model has been applied previously by \cite{Veveakis2007} for the case of the famous Vaiont landslide, which collapsed catastrophically in 1963 causing over 2000 fatalities. However, the study did not consider a time-dependence of the loading conditions, and the parameters of the basal material were taken from the literature. This thesis thus presents an extension of the work that the late Professor Vardoulakis and Professor Veveakis started from 2002 until 2007, by implementing the VFM to other case studies with time-dependent loading conditions. Moreover, the present thesis proves the theory that the temperature plays a critical role in the behavior of deep-seated landslides by instrumenting an active deep-seated landslide for the first time, called El Forn landslide (Andorra), with a thermometer in the shear band. The log-samples of this landslide have been studied in the laboratory in different ways, firstly in the triaxial machine to test the theoretical constitutive law of Vardoulakis that the clay material inside the shear band is rate hardening and thermal softening. The tests performed in the triaxial machine have validated for the first time that, indeed, the basal material (as a clay-like material) behaves as Vardoulakis postulated. Furthermore, micro-scale tests, such as X-Ray diffraction, SEM-EDS, MicroCT, and Plasticity Index have been performed to understand the effect of this behavior. Hence, mineralogical, textural, porosity, and plasticity results have been obtained for the samples, and, indeed exists a correlation of why the basal material is velocity and thermal sensitive.

Field data of the El Forn landslide has been obtained, such as the shear band's temperature, groundwater pressure, and displacement of the landslide. The data has demonstrated that, indeed, the temperature of the material of the shear band varies when the pressure changes, and then the landslide accelerates. The field data has shown that for this case study, the material is thermal sensitive when the water pressure varies, not when the landslide accelerates and, due to friction, the material heats.

The VFM model has been applied to four different cases, Vaiont (Italy), Shuping (Three Gorges Dam, China), Mud Creek (California, USA), and the El Forn (Andorra) landslides. The first three landslides have been implemented in the model by using literature data, and the model has reproduced with accuracy the behavior of the three landslides. Finally, the El Forn landslide has been applied to the VFM by implementing field and experimental data, thus reducing the uncertainty of the mathematical model, which accurately reproduces its behavior as well.

The VFM allows to forecast and control deep-seated landslides by using the heat-energy based mathematical model, and the constitutive law. This model works in a dimensionless form of the parameters, to avoid complications in the model by working with so many parameters. Furthermore, this unique model allows accounting in it the external loading and several parameters of the material of the shear band. By taking the heat-diffusion equation in dimensionless form, allows working with only a single dimensionless parameter, that includes the material parameters and the external loading. The single dimensionless parameter is then plotted against the temperature of the shear band (calculated by the model) and is, thus, mapped in the phase space. The phase-space is a curve calculated by the heat equation in the dimensionless form at a steady-state. It is a generic curve for all materials and allows to map the behavior of the landslide with the single dimensionless parameter against the temperature. This mapping allows to locate the creeping stage of the landslide and see if the landslide is close to collapse. Hence, the VFM can become a very useful tool to control and forecast the behavior of a deep-seated landslide and take remediation measures in time.