Numerical analysis of rapid drawdown: applications in real cases

In this study, rapid drawdown scenarios were analyzed by means of numerical examples as well as modeling of real cases with in situ measurements. The aim of the study was to evaluate different approaches available for calculating pore water pressure distributions during and after a drawdown. To do that, a single slope subjected to a drawdown was first analyzed under different calculation alternatives, and numerical results were discussed. Simple methods, such as undrained analysis and pure flow analysis, implicitly assuming a rigid soil skeleton, lead to significant errors in pore water pressure distributions when compared with coupled flow-deformation analysis. A similar analysis was performed for the upstream slope of the Glen Shira Dam, Scotland, and numerical results were compared with field measurements during a controlled drawdown. Field records indicate that classical undrained calculations are conservative but unrealistic. Then, a recent case of a major landslide triggered by a rapid drawdown in a reservoir was interpreted. A key aspect of the case was the correct characterization of permeability of a representative soil profile. This was achieved by combining laboratory test results and a back analysis of pore water pressure time records during a period of reservoir water level fluctuations. The results highlight the difficulty of predicting whether the pore water pressure is overestimated or underestimated when using simplified approaches, and it is concluded that predicting the pore water pressure distribution in a slope after a rapid drawdown requires a coupled flow-deformation analysis in saturated and unsaturated porous media.Peer ReviewedPostprint (published version

Criteria for rapid sliding I.: a review of Vaiont case

Vaiont slide has been represented by a model of two interacting evolutive wedges. Mass conservation during the motion implies that the upper wedge transfers mass to the lower one through an internal shearing plane. The model respects available in situ observations. It was formulated in dynamics terms. Outcomes of the analysis are the determination of safety factors of the valley before dam impoundment, and the calculation of run-out distance once the motion starts. Rock strength degradation as motion develops has also been included. This degradation, even if it is intense, was unable to explain the very high estimated landslide velocities.Peer ReviewedPostprint (author's final draft

Slope stability in slightly fissured claystones and marls

The final publication is available at Springer via http://dx.doi.org/10.1007/s10346-014-0526-5The paper deals with the behavior of some characteristic soft rocks found in the Iberian Peninsula. In geological terms, they belong to Tertiary basins, the Keuper period and the Jurassic-Cretacic transition. The discussion is organized around the following aspects: (a) the intact material and its brittle behavior; (b) the weathering action of atmospheric events; (c) the persistent discontinuities and scale effects; and (d) the modification of strength after failure. In all cases, instability phenomena are addressed in connection with several case histories. Regarding material brittleness and the initial stress state, two cases of first time failures are discussed. Practical implications concerning the selection of operative strength will be given. Field observations of the relevance of weathering and, also, on the rate of weathering, are given for a Weald claystone. Field observations emphasize the importance of sharp transitions between weathered and intact (or slightly weathered) levels. A recent long-term laboratory investigation on the nature of degradation will be summarized. Macroscopic variables such as stiffness and tensile strength have been found to be uniquely predicted by a degradation law in terms of the accumulated plastic deformations. Persistent discontinuities and, in particular, sedimentation planes play a dominant role to explain slope failures not related to the shallow failures, usually associated with weathered profiles. The strength of discontinuities in a Weald formation was investigated by means of tests performed at two scales. Finally, the evolution in time of residual strength induced by chemical actions, associated with groundwater flow, is highlighted in connection with actual field data of unstable slopes.Peer ReviewedPostprint (author's final draft

Criteria for rapid sliding II.: thermo-hydro-mechanical and scale effects in Vaiont case

Thermally induced excess pore pressures have been included into a two-wedge evolutive model of Vaiont landslide. The problem requires the solution of a system of four coupled balance equations for the shear bands and the surrounding rock as well as the joint equation of motion of the entire slide. The model predicts the high velocities observed and is consistent with other data (slide geometry, residual strength, and conditions on the sliding surface). The interpretation of a sensitivity analysis suggests that there exists a threshold permeability band, in the range 10- 8 to 10- 10 m/s, which separates potentially fast motions from slow motions. This conclusion is maintained if the scale of the landslide is reduced.Peer ReviewedPostprint (author's final draft

Landslides in reservoirs: a coupled thermo-hydromechanical approach

La ocurrencia de deslizamientos es uno de los principales problemas presentes durante el diseño, construcción y explotación de presas y embalses. La inundación del pie de los taludes, así como el desembalse rápido, pueden provocar primeras roturas o reactivar antiguos deslizamientos, existentes con frecuencia en el entorno de los embalses. Esta Tesis se centra en el caso particular de los deslizamientos en las márgenes de embalses en los que una masa de terreno desliza sobre una superficie de corte bien definida sin que se observe una intensa degradación del material movilizado. En la primera parte de la Tesis se discute el desembalse rápido como un problema acoplado de flujo y deformación en condiciones saturadas/no saturadas. Un riesgo adicional aparece cuando el deslizamiento acelera e invade el embalse a gran velocidad, generando una ola. La discusión de los fenómenos responsables de la aceleración de los deslizamientos sigue actualmente activa. Probablemente la falta de casos bien documentados dificulta el avance en su conocimiento. En esta Tesis se considera como fenómeno responsable de la aceleración del deslizamiento el efecto del calor en la banda de corte, lo que induce la generación de presión de agua y, por consiguiente, la reducción de la resistencia friccional. Se definen las ecuaciones del problema (balance de masa y calor y ecuaciones constitutivas) en la banda de corte y se integran junto a la ecuación del movimiento. Con objeto de encontrar criterios prácticos para cuantificar el riesgo de aceleración debido al fenómeno analizado, se ha obtenido una solución analítica para el caso de deslizamientos planos bajo las hipótesis de incompresibilidad del agua, partículas sólidas y medio poroso. La comparación entre la solución analítica y la numérica (sin considerar las hipótesis impuestas en el desarrollo analítico), cuando se varían los parámetros más relevantes dentro de un rango razonable, indica que la solución analítica es suficientemente precisa para aplicaciones prácticas. El desarrollo termo-hidro-mecánico presentado se aplica al caso del deslizamiento de Vaiont. En primer lugar se explica la situación de estabilidad previa a la rotura introduciendo la resistencia de la roca movilizada. El análisis es coherente con la información disponible (geometrías del deslizamiento, resistencia residual, propiedades de los materiales y ensayos de laboratorio). Cuando el mecanismo de generación de presión de agua debido al calor resultante del trabajo friccional se incluye en el análisis dinámico del modelo del deslizamiento de Vaiont, se predice bien la alta velocidad observada. Se han realizado análisis de sensibilidad y de efecto escala tanto para el caso de deslizamiento plano como para la geometría de Vaiont. Se han encontrado tres parámetros relevantes para explicar la aceleración del movimiento: el espesor de la banda de corte, su permeabilidad y su rigidez. De hecho, la permeabilidad y el espesor de la banda de corte están directamente relacionados dado que ambos dependen de la granulometría del material. Los resultados obtenidos indican que la permeabilidad de la banda es el parámetro clave. Un umbral de permeabilidad establecido entre 10-8 y 10-10 m/s, marca la transición entre deslizamientos potencialmente acelerados (cuando la permeabilidad es baja) y deslizamientos seguros (permeabilidades mayores a 10-10 m/s). En el caso de deslizamientos de grandes dimensiones, combinaciones críticas de la permeabilidad y espesor de la banda de corte resultan en un incremento substancial de la temperatura. Los altos valores resultantes de los cálculos (cientos o miles de ºC) invalidan el análisis aquí presentado. Fenómenos como la evaporación o la fusión de la roca deberían ser incluidos en estos casos. En el último Capítulo se analiza el deslizamiento de Canelles ocurrido recientemente. El deslizamiento pone en riesgo el buen funcionamiento de la presa y del embalse. Algunos de los desarrollos descritos en la Tesis, como son la solución del desembalse rápido y el modelo termo-hidro-mecánico para el análisis de la aceleración de los deslizamientos se han aplicado al caso de Canelles. El capítulo describe la metodología adoptada, que puede ser aplicada en casos similares.Landsliding is an important problem when facing the design, construction and operation of dams and reservoirs. Impoundment of the slope toes as well as the rapid drawdown may trigger the movement of first-time landslides or reactivate ancient landslides often located in reservoir sites. This Thesis deals with the particular case of landslides around reservoirs defined as a mobilized mass that slides on a well-defined shearing surface without experiencing a major degradation. In the first part of the Thesis the mechanism of rapid drawdown is discussed as a fully coupled flow-deformation problem for saturated/unsaturated conditions. Additional risk appears when landslide accelerates and is able to enter the reservoir at high speed creating impulsive waves. The discussion on the different phenomena leading to the fast acceleration is today very active. Probably the lack of well-documented cases makes the advancement of knowledge difficult. The mechanism to explain the rapid acceleration of landslides favoured in this Thesis is based on thermal effects on the sliding surface that induce the generation of pore water pressure and therefore, the reduction of the frictional strength. The governing equations (mass and heat balance equations and constitutive equations) formulated in the shear band have been written and integrated together with the motion equation of the slide. With the aim of finding practical criteria to decide the actual risk of slide acceleration due the phenomena analysed, a closed-form solution has been obtained for the case of planar landslides under the hypothesis of incompressible water, solid particles and porous media. For a rational range of the most relevant parameters, comparison between analytical and numerical (relaxing the assumptions introduced in the analytical development) solutions shows a remarkable similarity and reveals that the closed-form solution is accurate enough for practical applications. The thermo-hydro-mechanical approach discussed is applied in the case of Vaiont landslide. The stability of this landslide before the failure is first discussed by means of a simple explanation introducing the internal strength of the mobilized rock. The analysis is consistent with the available data (slide geometry, residual strength, material properties and laboratory tests). When the self-feeding mechanism of pore pressure generation due to heat resulting from the frictional work is introduced in the dynamic analysis of the Vaiont model, the high velocity actually observed is predicted. Sensitivity and scale analysis have been performed for the case of a planar landslide and for the geometry of Vaiont. Three parameters have been found important to explain the acceleration of the motion: the thickness of the shearing band, its permeability and its stiffness. In fact, permeability and thickness can be related since both depend on the particle size distribution. Calculated results indicate that the permeability of the shear band is a key parameter. A threshold of permeability established around 10-8 to 10-10 m/s marks the transition for a potentially risky slide (when the permeability is lower) to a safe one (for higher values of permeability). For very large landslides, critical combinations of band permeability and band thickness result in a substantial increase in temperature. At high calculated values of temperature (hundreds or even thousands of ºC), the analysis presented is not applicable. Evaporation or advance constitutive equations including rock melting should be included. In a final chapter a recent large landslide located in Canelles reservoir is analyzed. The slide is regarded as a potential risk for the operation of the dam and the reservoir. Some of the developments made in the Thesis, namely the solution of rapid drawdown and the thermal coupled model for fast landsliding, are applied to Canelles. The chapter describes the methodology adopted which can be applied in similar cases

Simulación numérica de deslizamientos tipo flujo inducidos por licuefacción

Los deslizamientos de tierras tipo flujo se caracterizan por recorrer grandes distancias y alcanzar grandes velocidades. La simulación numérica de este tipo de problemas que involucran la dinámica de suelo saturado y no saturado, licuefacción y grandes deformaciones es, en la actualidad, un desafío. En este trabajo se presenta un análisis numérico para simular deslizamientos de tipo flujo inducidos por licuefacción estática durante el movimiento. Para ello se elige el método del punto material (MPM) que permite simular problemas de grandes deformaciones y desplazamientos sin las limitaciones del método de los elementos finitos estándar extensamente utilizado en la práctica. La licuefacción se modela mediante un modelo constitutivo avanzado capaz de simular este proceso bajo cargas estáticas y dinámicas implementado en un código MPM. Los desarrollos llevados a cabo se aplican a la simulación de un deslizamiento ocurrido en Catalunya. La comparación de los resultados bajo otras hipótesis confirma la necesidad de adoptar un modelo constitutivo capaz de simular la pérdida de resistencia del material durante el movimiento hasta valores significativamente bajos, típicamente asociados a procesos de licuefacción, para poder explicar el alcance del deslizamiento observado.Los autores agradecen el apoyo financiero a CIMNE proporcionada por el programa CERCA/ Generalitat de Catalunya, la asistencia de la comunidad de investigación MPM y el equipo de desarrollo de software de Anura3D. El segundo autor, profesor Agregado Serra Húnter, agradece la financiación del Departament de Recerca i Universitats de la Generalitat de Catalunya. Esta publicación es parte del proyecto de I+D+i RTI2018-097365-B- financiado por MCIN/ AEI/10.13039/501100011033, “FEDER Una manera de hacer Europa”.Postprint (published version

Constitutive modelling of bonded expansive geomaterials

This paper presents a discussion on existing approaches to derive constitutive models for bonded geomaterials conceived as composite materials. The methodology involves the partitioning of strains, the adoption of constitutive models of constituents and the integration of component stresses into the external stress. These steps are guided by microstructural observations, but are open to alternative formulations which are part of the overall constitutive behaviour.Peer ReviewedPostprint (published version

Slope stability under rapid drawdown conditions

The rapid drawdown condition arises when submerged slopes experience rapid reduction of the external water level. Classical analysis procedures are grouped in two classes: the “stress-based” undrained approach, recommended for impervious materials and the flow approach, which is specified for rigid pervious materials (typically a granular soil). Field conditions often depart significantly from these simplified cases and involve materials of different permeability and compressibility arranged in a complex geometry. The drawdown problem is presented in the paper as a fully coupled flow-deformation problem for saturated/unsaturated conditions. Some fundamental concepts are first discussed in a qualitative manner and, later, explored in more detail through the analysis of two embankment dams. In Shira earthdam pore pressures were recorded at different points inside the embankment as a consequence of a controlled drawdown. Predictions of four calculation procedures (instantaneous drawdown, pure flow, coupled flow-elastic and coupled flow-elastoplastic, all of them for saturated/unsaturated conditions) are compared with measured pressure records. Only the coupled analysis provides a consistent and reasonable solution. The case of a large landslide, immediate to a reservoir, reactivated by a condition of rapid drawdown is also described in the paper.Peer ReviewedPostprint (author’s final draft

Modelling large deformation problems in unsaturated soils

Applications of unsaturated soil mechanics often involve large deformations and displacements. This is the case of collapse behaviour of low density soils or the unrestrained swelling of expansive clays. Rain induced instability of unsaturated slopes is a further example of large displacements that cause important damages around the world every year. Since standard lagrangian Finite Element methods are not well suited to model large deformations, particle-based methods are under development. This is the case of the Material Point Method (MPM), which offers an interesting alternative. Recently, the MPM has been extended to model unsaturated soil problems, where the soil is understood as a unique medium integrated by three distinct phases (solid, liquid and gas). In this paper, the MPM computational cycle for unsaturated soils is described. In addition, a validation of the 3-phase MPM approach is presented by means of the modelling of a one-dimensional infiltration problem. Finally, the applicability of MPM to solve slope instabilities is presented. The simulation of the whole instability process of an embankment subjected to rain infiltration is analysed in detail.Postprint (published version

Landslide motion assessment including rate effects and thermal interactions: revisiting the Canelles landslide

The re-activation of a large (40 Mm3) landslide on the valley slopes of a reservoir motivated a research initiative to estimate the risk of a fast-sliding mass moving into the reservoir. A previous simplified analysis had suggested that a joint consideration of strain rate effects on friction and thermal pressurization phenomena in the sliding surface could provide a rational approach to answer the question raised. The paper describes first the capability of strain rate effects on friction to reproduce long-term creeping records of two real cases. The joint and coupled phenomena of creeping motion and thermal pressurization in shearing bands was incorporated into a material point method computational technique for hydromechanical analysis of porous materials. A representative cross section of the Canelles landslide was then analysed, profiting from previous finite element investigations of the landslide. It was found that a rapid rate of landslide acceleration could be a possibility under extreme external actions. However, it was also found that a moderate strain rate effect on the basal residual friction angle could create conditions that avoid the triggering of a fast motion.Peer ReviewedPostprint (author's final draft