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 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

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

Modeling swelling soils for disposal barriers

A fully coupled advanced formulation for heat, liquid and gas transfer in unsaturated deforming soils is presented. Balance equations are rooted in the theory of mixtures. A double porosity model, which is believed to be especially relevant when simulating expansive soils, is proposed to characterize the water transfer mechanism. Another relevant feature of the fra- mework developed, is the incorporation of a consistent elastoplastic model to describe stress± strain relationships in unsaturated soils. A ¯exible FE computer program was developed, including an array of alternative numerical strategies and constitutive equations to enhance its capabilities. The experimental results of a hydration and heating experiment performed on con®ned expansive bentonite were satisfactorily compared with model simulations.Peer Reviewe

Hydromechanical analysis in geotechnical engineering using the material point method

The explicit version of the Material Point Method [1] has been extended in order to model coupled hydromechanical saturated problems. MPM discretizes the continuum, which is considered as a saturated soilfluid mixture, by dividing it into particles or material points. The discrete movement equations are not solved at the material points. Instead a support mesh, built to cover the domain of the problem, is used. In this paper it is assumed that particles carry all the variables needed to represent the state of the continuum including the pore pressure as a variable associated with each particle. The particle pore pressure increment is calculated explicitly using the equation of fluid mass balance, from the particle volumetric deformation and the fluid velocity relative to the soil skeleton, at the particle location. The shape functions used for the mesh elements are usually the same bilinear functions of the Finite Element Method and therefore the background mesh elements suffer the same drawbacks. These drawbacks include: volumetric locking for quasiincompressible materials when four particles per cell are used, which is equivalent to four integration points in the finite element method, pressure instability for quasiincompressible and low permeability materials and the generation of zero energy modes when one particle per cell is used, which corresponds to reduced integration in the finite element method. The MPM original version has also the disadvantage of generating "noise" in the solution [2] when a particle pass from one cell to another. A simple procedure that can be used to reduce instabilities is to consider constant stress at each cell equal to the stress average of the particles which are in the cell at the instant k. In this case the internal forces are obtained in the same way as in the finite element method when one point of integration is used, using the gradient of the shape functions calculated in the cell center. In this work, to avoid volumetric locking and simultaneously achieve a stable behavior, internal forces and pressure increments at the nodes are calculated using the gradients calculated at the cell center. The procedure is completely explicit and has proved to be stable for the low permeability values used to model the foundation of Aznalcollar dam. The simulation of Aznalcollar dam progressive failure is presented as an example [3]

Inicio y evolución de deslizamientos: parámetros de cálculo

El artículo aborda el estudio del desplazamiento y la velocidad, posteriores a la rotura, de deslizamientos. Se describe inicialmente el fenómeno de rotura progresiva que, cuando se alcanza la inestabilidad, conduce a un movimiento acelerado. Se presenta un análisis de un caso real: la rotura inducida por sobrepresión intersticial del talud de Selborne, en arcillas plásticas rígidas. El caso se resuelve con la ayuda del Método del Punto Material (MPM). El fenómeno de reptación, entendido como un movimiento lento, se explica por el incremento de la fricción en la superficie de rotura con la velocidad de la deformación de corte. Se explican las bases teóricas de esta dependencia que, en la práctica, es equivalente a introducir una resistencia “viscosa” al corte. En el otro extremo, los movimientos rápidos que se conocen requieren una resistencia al corte prácticamente nula en la superficie de rotura. Esta situación se puede explicar por una presurización térmica del agua intersticial debido al calor generado por el movimiento. Se describe el fenómeno físico y la formulación del problema en el marco del análisis termo-hidro-mecánico de materiales porosos saturados. Se presentan resultados de este análisis en el caso de deslizamientos compuestos por dos bloques que intercambian masa durante el movimiento y, también, el caso general, utilizando el MPM. Finalmente se plantea el caso más habitual de la posible evolución de un movimiento previo de reptación hacia una rotura rápida y se analiza el caso real de un gran deslizamiento activo en la margen de un embalse.Postprint (published version

Riesgos geológicos asociados a las avenidas y su previsión

En esta comunicación se describen procedimientos de previsión de deslizamientos del terreno asociados a las avenidas. Dos son las causas básicas que desencadenan estos fenómenos: la elevación de niveles piezométricos hasta valores excepcionales como consecuencia de lluvias internas y las erosiones de pie en laderas inmediatas a cursos de agua. La metodología de la previsión pasa por diversas actuaciones escalonadas: la identificación de zonas de riesgo en la que se utilizan criterios históricos, geológicos, geomorfológicos y, a escala mas reducida, la instrumentación de campo y la implantación propiamente dicha de métodos de previsión y control en perfiles o zonas de riesgo elevado. Existen procedimientos de previsión que hacen uso de las medidas de desplazamiento a lo largo del tiempo; otros pueden desarrollarse a partir de medidas de presión intersticial e incluso se conocen relaciones directas entre parámetros relacionados con la precipitación y los riesgos de deslizamientos. En el artículo se describen estos procedimientos y se ilustran con algunos ejemplos.Postprint (published version

Protecting sensitive constructions from tunnelling: the case of World Heritage buildings in Barcelona

Permission is granted by ICE Publishing to print one copy for personal use. Any other use of these PDF files is subject to reprint fees.Construction of the tunnel for the high-speed Madrid–Barcelona–France railway link across central Barcelona became a major technical and social challenge due to the impact of the tunnel on nearby historic buildings (two of them, the Sagrada Familia basilica and Casa Milà, being United Nations Educational, Scientific and Cultural Organization (Unesco) World Heritage structures). Protection of sensitive buildings from tunnelling-induced movements relied on the construction of a stiff pile wall, separating the tunnel from historic sites. This paper first presents a simplified procedure to analyse the wall–tunnel interaction in a straightforward manner. The main features of the tunnel, excavated by means of an earth pressure balance machine in tertiary clays and sands below the water table, are then described. Details of the design of the wall that was finally built are presented. Issues that were particularly important include the groundwater flow constraints and the use of small-strain soil stiffness properties to obtain realistic settlements. General criteria to design the protection wall are also presented. The good performance of the wall resulted in negligible tunnelling impact on the sensitive structures. The measured and predicted displacements are compared, suggesting that this type of solution is adequate to protect historic structures from tunnelling.Peer ReviewedPostprint (published version

Yielding of rockfill in relative humidity-controlled triaxial experiments

The final publication is available at Springer via http://dx.doi.org/10.1007/s11440-016-0437-9The paper reports the results of suction controlled triaxial tests performed on compacted samples of two well graded granular materials in the range of coarse sand-medium gravel particle sizes: a quartzitic slate and a hard limestone. The evolution of grain size distributions is discussed. Dilatancy rules were investigated. Dilatancy could be described in terms of stress ratio, plastic work input and average confining stress. The shape of the yield locus in a triaxial plane was established by different experimental techniques. Yielding loci in both types of lithology is well represented by approximate elliptic shapes whose major axis follows approximately the Ko line. Relative humidity was found to affect in a significant way the evolution of grain size distribution, the deviatoric stress-strain response and the dilatancy rules.Peer ReviewedPostprint (author's final draft