Except for simple sliding motions, the stability of a slope does not depend only on the resistance of the basal failure surface. It is affected by the internal distortion of the moving mass, which plays an important role on the stability and post-failure behaviour of a landslide. The paper examines the stability conditions and the post-failure behaviour of a compound landslide whose geometry is inspired by one of the representative cross-sections of Vajont landslide. The brittleness of the mobilized rock mass was described by a strain-softening Mohr-Coulomb model, whose parameters were derived from previous contributions. The analysis was performed by means of a MPM computer code, which is capable of modelling the whole instability procedure in a unified calculation. The gravity action has been applied to initialize the stress state. This step mobilizes part of the strength along a shearing band located just above the kink of the basal surface, leading to the formation a kinematically admissible mechanism. The overall instability is triggered by an increase of water level. The increase of pore water pressures reduces the effective stresses within the slope and it leads to a progressive failure mechanism developing along an internal shearing band which controls the stability of the compound slope. The effect of the basal shearing resistance has been analysed during the post-failure stage. If no shearing strength is considered (as predicted by a thermal pressurization analysis), the model predicts a response similar to actual observations, namely a maximum sliding velocity of 25 m/s and a run-out close to 500 m.
Except for simple sliding motions, the stability of a slope does not depend only on the resistance of the basal failure surface. It is affected by the internal distortion of the moving mass, which plays an important role on the stability and post-failure behaviour of a landslide. The paper examines the stability conditions and the post-failure behaviour of a compound landslide whose geometry is inspired by one of the representative cross-sections of Vajont landslide. The brittleness of the mobilized rock mass was described by a strain-softening Mohr–Coulomb model, whose parameters were derived from previous contributions. The analysis was performed by means of a MPM computer code, which is capable of modelling the whole instability procedure in a unified calculation. The gravity action has been applied to initialize the stress state. This step mobilizes part of the strength along a shearing band located just above the kink of the basal surface, leading to the formation a kinematically admissible mechanism. The overall instability is triggered by an increase of water level. The increase of pore water pressures reduces the effective stresses within the slope and it leads to a progressive failure mechanism developing along an internal shearing band which controls the stability of the compound slope. The effect of the basal shearing resistance has been analysed during the post-failure stage. If no shearing strength is considered (as predicted by a thermal pressurization analysis), the model predicts a response similar to actual observations, namely a maximum sliding velocity of 25 m/s and a run-out close to 500 m.
This discussion a) reviews the geological model adopted for the landslide analysis and argues that there is nothing new in the reference paper, b) examines the conditions for fast catastrophic sliding and demonstrates that conditions for such a phenomenon may be present in the case of Canelles slide, against the opinion of the authors, and c) justifies the corrective measures adopted to stabilize the landslide.
Rain induced instabilities are a common geotechnical problem specially in partially saturated soils. A large number of steep, natural or cut slopes remain stable for a long time and then fail during heavy rainstorms or after a long period of rain. In this work, the Material Point Method (MPM) is used to model the whole instability process. The
MPM theoretical framework regarding a "single layer" of material points is extended to take into account interactions between solid gains and fluids within the pores (liquid and gas). A dynamic coupled hydro-mechanical formulation is considered hence the modelling of unsaturated soils is allowed. The paper presents the validation of the coupled formulation by means of an infiltration problem. Finally, the analysis of a theoretical slope failure induced by a heavy rain, which is inspired in real surface ruptures experienced by some embankment slopes is presented. A constitutive Mohr-Coulomb elastoplastic model is formulated in terms of net stress and suction in order to simulate the evolution of the material with the evolving suction.
Catastrophic landslides are often preceded by a stage of creeping behavior during a certain time. The paper focusses on the transitional behavior from slow creeping motions to rapid events by evoking two phenomena: the strain rate effects on friction (which explain the creeping motion) and the thermo-mechanical interaction at the
scale of the sliding surface and its vicinity. A coupled formulation for the relevant physical phenomena is presented. It was formulated in dimensionless terms with the purpose of generalizing the results beyond specific cases. A few dimensionless parameters were found to control the entire deformation process. A sensitivity analysis provides
considerable insight into the evolution of the sliding velocity and its eventual blow-up. The blow-up takes place when thermal pressurization dominates the slide motion. It may occur for a combination of different factors mainly related to specific properties of the shear band. It depends also on the current straining rate of the landslide which is a consequence of the unbalanced forces. The formulation developed is applied to planar slides. The effect of the
kinematics of the landslide and of relevant parameters is discussed in terms of the evolution of the motion.
Actual deformation mechanisms during sliding are a consequence of several interacting aspects: the geometry of the basal sliding surface, the degree of heterogeneity of the moving mass, the presence of internal weak or hard zones and the stress-strain behaviour of the involved materials. Except in simple cases (pure rotational or planar motions) the development of an admissible mechanism leads to internal shearing. As a result, extensive damage may result, especially in brittle materials. The paper examines the stability conditions and the post-failure behaviour of a compound landslide whose geometry is inspired by one of the representative cross sections of Vajont landslide. The effect of the rock brittleness and stiffness will be presented. A Mohr-Coulomb elasto-plastic strain softening constitutive model defines the rock behaviour. Strength parameters are made dependent of the accumulated deviatoric plastic deformation. The cases discussed were solved by the Material Point model which is well adapted to deal with large deformations and displacements within a general dynamic formulation.
In a preliminary laboratory test, a homogeneous sand slope loaded on top was destabilized by increasing the base slope, the motion of the system was recorded by means of high speed camera. An image processing software was used to analyze the laboratory data. The test was simulated using a MPM code and others numerical tools. Numerical results and laboratory measurements were compared. Failure loads in simulations were verified against the one applied in the laboratory. The onset of the failure and evolution of the movement were discussed for different simulations comparing MPM against laboratory behavior. A sensitivity analysis on some constitutive parameters and on discretization details is also part of this study. Numerical results showed a strong dependence of the failure geometry
on the strength parameters. Different combinations of cohesion, friction angle and dilatancy leads to failures occurring with significant different final geometries and consequently involving different run-out.
One of the factors causing the acceleration of landslides is the loss of strength of the soil involved in the potential unstable mechanism. The travelled distance and the landslide velocity, a key factor in risk analysis, will be determined by the loss of resistant forces. Brittle behaviour, commonly associated with cemented soils, overconsolidated plastic clay formations and sensitive clays, lead to the progressive failure phenomenon explained by the reduction of the strength with increasing strain. In the present study, this phenomenon has been analysed in the case of a saturated slope which becomes unstable by increasing the boundary pore water pressure. A Mohr-Coulomb model with strain softening behaviour induced by increasing deviatoric plastic strain is used. The paper focusses not only on the stability of the slope but also on the post failure behaviour (run-out and sliding velocity). A coupled hydro-mechanical formulation of the material point method has been used to simulate the whole instability process. The influence of the brittleness of the material on the triggering of instability and run-out is evaluated by means of a parametric study varying peak and residual strength. The onset of the failure and the failure geometry are controlled by both peak and residual values. Good correlations between run-outs and brittleness are found. The decay of the strength determines the acceleration of the landslides and the travelled distance.
Les esllavissades representen un dels problemes més destacats en el camp de la geotècnia ja que cada any causen danys importants arreu. La comprensió de la mecànica de tot el procés és de particular importància en l'avaluació de riscos.En primer lloc, és important determinar quines zones poden ser susceptibles a lliscaments o inestabilitats. A més a més, també és essencial estimar la velocitat i la distància recorreguda per la massa inestable. El desenvolupament de tècniquesnumèriques capaces de simular de forma unificada des de l'inici de la trencada fins a l'estabilització final són claus en problemes d'estabilitat de talussos però també en altres anàlisis geotècnics. Per exemple, en el disseny de preses, túnels,canonades, fonamentacions o terraplens.La predicció d'aquest tipus d'episodis catastròfics presenta diversos reptes a causa de la complexitat del comportament real del sòl. A més, la implementació de formulacions hidro-mecàniques és vital per tal de tenir en compte l'efecte de fluids(líquid i/o gas) dins la matriu porosa del sòl.Els anàlisis geotècnics tradicionals, com ara els mètodes d'equilibri límit (MEL) i la formulació clàssica del mètode dels elements finits (MEF) són molt útils per estudiar l'inici de la trencada, però proporcionen informació molt limitada de la posttrencadai del comportament de la massa mobilitzada.Actualment, s'estan desenvolupant mètodes numèrics capaços de simular de forma unificada tot el procés (trencada i posttrencada), com per exemple el Mètode del Punt Material (MPM) que ofereix una alternativa interessant. El MPM discretitza elmedi continu mitjançant un conjunt de punts lagrangians que es mouen units al material (punts materials) i transporten les propietats d'aquest. Per altra banda, les equacions de govern es resolen de forma incremental als nodes d'una mallacomputacional que roman fix durant tot el càlcul. Aquesta doble discretització evita els problemes de distorsió de malla típics en el MEF.Aquesta tesi es centra en la simulació d'esllavissades i inestabilitats de talussos, analitzant les condicions estàtiques inicials, la formació de la trencada i el comportament post-trencada. Es descriuen diferents aspectes rellevants per a lainterpretació de les esllavissades: el mecanisme de trencada progressiva, el paper exercit per la degradació interna del material i l'efecte de la fragilitat del material en la trencada i en el desplaçament final.Es presenten diferents casos. En primer lloc, es simula l'experiment de Selborne. Aquest cas, ben identificat mitjançant dades de laboratori, ha estat una oportunitat per dur a terme una validació de la formulació MPM. Una geometria simplificadade l'esllavissada de Vajont també s'analitza en un segon model. S'ha demostrat que un mecanisme de trencada cinemàticament admissible requereix el cisallament i la degradació interna de la massa mobilitzada depenent de la geometria de la superfície basal de lliscament. A més, per mitjà d'un estudi paramètric variant les resistències pic i residual,s'ha determinat que l'abast està directament relacionat amb la fragilitat del material.Finalment, s'ha aconseguit fer un pas endavant en l'aplicació del MPM en problemes multi-fàsics en medis porosos. Per tal de simular el comportament del sòls no saturats, la formulació MPM s'ha ampliat mitjançant una formulació MPM acobladaamb 3 fases. D'aquesta manera, es té en compte la interacció de sòlid, líquid i gas en cada punt material. Aquest enfocament s'ha validat mitjançant un problema d'infiltració. Finalment, es presenta la inestabilitat d¿un terraplè, en terrenyno saturat, degut a fortes pluges.En les diferents aplicacions presentades, s'utilitzen dos models constitutius: un model fràgil amb reblaniment per sòls saturats, i un model elastoplàstic de Mohr-Coulomb formulat en tensió neta i succió.
The scope of the paper encompasses planar and compound sliding motions, which may exhibit creeping behaviour during a certain period but may evolve to a very rapid motion. Thermo-mechanical interactions, at the scale of the sliding surface, are accepted as a critical aspect to explain these motion phases and their relationship. The sliding kinetics and global equilibrium are described at a large scale and the evolving shearing strength at the sliding surface derives from the local analysis of the shearing band and its vicinity. Pore pressures, temperatures and related variables are calculated by resolving a set of balance equations. The paper describes the transition from creeping motions to a rapid event. Results are found in terms of dimensionless numbers. Calculation of the slide evolution requires special numerical techniques described in the paper. Band permeability is found to be the dominant property controlling the triggering of fast motions. The creeping stage and the eventual slide blow-up are intimately linked. This relationship is explored in the paper. The models presented can be readily used to back-analyse relevant case histories or, in principle, even to carry out predictive modelling, provided an adequate calibration is available for the material parameters.
Alonso, E.; Yerro, A.; Pinyol, N.M. IOP conference series: earth and environment Vol. 26, num. conference 1, p. 012003- DOI: 10.1088/1755-1315/26/1/012003 Data de publicació: 2015-09 Article en revista
The paper describes first the theoretical framework of a "single layer" three phase material. The formulation is general and particular cases are dry and fully saturated soils. The formulation and discretization of the motion and balance equations is presented. Two constitutive equations are used in the applications described: A brittle model for saturated soils and a Mohr-Coulomb elastoplastic soil formulated in terms of net stress and suction. Three aspects of the behaviour of landslides are discussed: first time failures in over-consolidated clays; internal shearing in deep seated landslides and rain induced failures in unsaturated slopes. The discussion is supported by three real cases which are described and analyzed in detail.
In 2001 a storm induced the failure of four caissons recently built for the construction of a dyke in a new entrance to the Barcelona Harbor. Understanding the failure required the analysis of the construction history and the wave action. The caissons were founded on a thick layer of soft silty clays potentially liquefactable. The main properties of foundation soils are discussed based on laboratory and field data. A back-analysis of the failure is developed using an analytical and a numerical procedure. The analytical procedure is based on well-known solutions and concepts in Soil Mechanics. It involves the calculation of pore water pressure distributions on the foundation, its dissipation, the induced increase of the undrained strength, and the caisson stability during construction stages. An analysis of liquefaction, which explains the collapse observed, is also described. The problem is also solved numerically using a commercial finite difference code. This second analysis allowed a cross-checking of the analytical solution and the quantification of some simplifications introduced that can be properly accounted for in the numerical analysis. In the final part of the paper an alternative stable design of the caisson dyke is presented.
The 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.
The paper describes a three-phase single-point material point method formulation of coupled flow (water and air) for hydro-mechanical analysis of geotechnical problems involving unsaturated soils. The governing balance and dynamic momentum equations are discretised and adapted to material point method characteristics: an Eulerian computational mesh and a Lagrangian analysis of material points. General mathematical expressions for the terms of the set of governing equations are given. A suction-dependent elastoplastic Mohr-Coulomb model, expressed in terms of net stress and suction variables is implemented. The instability of a slope subjected to rain infiltration, inspired from a real case, is solved and discussed. The model shows the development of the initial failure surface in a region of deviatoric strain localisation, the evolution of stress and suction states in some characteristic locations, the progressive large strain deformation of the slope and the dynamics of the motion characterised by the history of displacement, velocity and acceleration of the unstable mass.
Las propiedades de los suelos compactados se relacionan habitualmente con sus condiciones de compactación de acuerdo con los índices propuestos por Proctor (densidad seca y humedad). Sin embargo, una misma densidad puede estar asociada a distintas distribuciones de poros. Compactar por el lado seco conduce a una agrupación de las partículas finas en agregados de mayor tamaño y, en consecuencia, a una distribución de poros con dos tamaños
preferentes. Por el contrario, compactar por el lado húmedo resulta en una estructura más homogénea y un único tamaño dominante de poros. Ello afecta al comportamiento del suelo. Por ejemplo, un mismo suelo compactado a igual densidad pero a diferentes humedades, llevado a un mismo estado inicial, presenta diferente permeabilidad, rigidez o deformación de colapso por mojado. En este artículo se interpretan las condiciones de compactación incluyendo la distribución del tamaño de poros, que se cuantifica a partir de una variable de
estado que se puede determinar mediante ensayos de porosimetría. El resultado es un marco teórico, más preciso que el tradicional, en el que la densidad y humedad de compactación se expresan en términos de tensión aparente de preconsolidación, succión y una variable de estado asociada a la distribución de poros.
Los deslizamientos en embalses plantean riesgos en las laderas
del vaso o incluso en la propia presa de materiales sueltos. El daño está frecuentemente asociado al alcance y a la velocidad de la masa deslizada, que puede provocar olas destructivas. El estudio completo de un deslizamiento es un proceso complejo, que incluye la pre-rotura y la post-rotura. En este contexto, el Método del Punto de Material (MPM) es una herramienta prometedora. En el MPM el dominio se discretiza en un conjunto de puntos materiales que se mueven
a través de una malla de cálculo fija. Esta doble descripción evita los problemas de distorsión de malla que sufre el método de los elementos finitos (FEM). En este trabajo se plantea una formulación dinámica hidromecánica, adecuada para resolver problemas en suelos saturados. También se ha implementado una ley constitutiva elastoplástica con reblandecimiento, con el fin de analizar la rotura de materiales frágiles, sujetos a rotura progresiva, como margas,
argilitas y arcillas sobreconsolidadas. El análisis se centra en el estudio de un talud sometido a un aumento del nivel freático. Se determinan las velocidades y los alcances de la masa movilizada considerando distintas fragilidades del material.
A 100 m high gravity dam (Castrovido dam, Burgos, Spain) is founded on a soft Weald red claystone with inter-bedded sandstone strata. Claystone weathering was identified in vertical and horizontal profiles of water content and dry density. Peak and residual strength of rock matrix is given in the paper. Clay and sandstone layers dip 15–20
in the upstream direction. Dam stability is essentially controlled by the available shear strength at the sedimentation planes. Block samples were recovered for each of the contact types mentioned. They were tested in high capacity shear box machine (30x30 cm). In addition, large scale shear tests (80x80 cm) were performed in situ. The roughness of the sheared plane was determined after each one of the tests. Additional information includes some direct shear tests on joints
recovered in borings and the results of residual strength tests performed on clay fillings on a ring shear apparatus. The results of all the tests performed are compared in the paper showing the effect of
shearing surface roughness. In general, strength envelopes determined ‘‘in situ’’ exhibit a marked nonlinearity at the low range of confining stresses (0–0.2 MPa). Laboratory tests on smaller specimens
show linear Mohr–Coulomb envelopes. Existing relationships between joint roughness and strength envelopes were also included in the comparison. The paper discusses the procedure followed to select an appropriate strength envelope for design purposes on the basis of all the available information. The case points out the difficulty to select unambiguously a suitable design strength for the critical sedimentation surfaces despite the efforts displayed in the field and in the laboratory.
In this paper a slope stability problem due to an increase of the pore pressure is analysed by means of the material point method (MPM). A strain softening model is implemented and the progressive failure is examined in terms of mobilized shear strain. A parametric study is performed varying the strength decay but maintaining a common peak envelope. The influence of the brittleness of the material, evaluated in
terms of the brittleness index (IB) proposed by Bishop (1967), in run-out and velocity of unstable slopes is examined. The initiation of motion in the defined slope is observed for values of IB>0.5 and a well correlation between run-outs and brittleness is found. It can be concluded that MPM is able to simulate both the initiation of the failure and the post-failure stages.
The paper discusses first the effect of rainfall on the stability of slopes and it focusses on the role of permeability. Existing or induced discontinuities are shown to have a remarkable effect on the slope safety. Also, the relevance of changes in hydraulic properties in weathered profiles is discussed through a real case. The performance of modern “Lagrangian particle” methods and their capabilities to deal with large displacements is presented. The cases solved refer to brittle materials, a common situation in overconsolidated high plasticity clays.
A benchmark was designed with the aim of evaluating the capabilities of current modelling techniques and computational codes to reproduce slope failures. Laboratory tests and three instrumented small scale experiments of slopes initially unsaturated and subjected to a controlled rainfall up to failure were performed. The objective of the benchmark was to predict one of the slope failures knowing the rest of the data. The paper presents the modelling strategy and the results obtained using the finite element code Code_Bright and the Barcelona Basic Model as the constitutive model for the unsaturated soils.
In this work, the progressive failure phenomenon is studied using the Material Point Method (MPM). MPM is categorized as a method in between the classical Finite Element Method (FEM) and the truly particle methods. It is capable of modeling large deformations, displacements and contact between different bodies. Two different geotechnical applications are presented: the pull-out of a pile; and the failure and run-out of a slope due to a rise of the phreatic level. In both cases a strain-softening Mohr-Coulomb constitutive model is used to simulate the brittleness of the materials.
Tunnel in difficult soils may require procedures to prevent tunnel face failures. Face stabilization can be achieved by the installation of some structural elements. This paper presents an analysis of face stability of shallow tunnels in undrained soils reinforced by an umbrella of subhorizontal micropiles. Upper bound solutions for two dimensional plane strain conditions are given including the effect of micropiles. The micropile umbrella is embedded in the soil and supported on the tunnel lining. The kinematically admissible collapse mechanism defined to calculate the upper bound solution includes the action provided by a subhorizontal micropile at limiting conditions. The solutions are given in practical dimensionless charts which are useful to quantify easily the effect of the umbrella of micropiles. The plots provide a simple procedure to design the umbrella. The most relevant properties defining the umbrella are grouped into a single dimensionless coefficient which includes the yielding conditions and the geometry of the micropiles as well as the distance between them.
In recent years, the Material Point Method (MPM) has been applied to a number of geotechnical problems and has been extended to solve coupled flow-deformation problems. The dynamic formulation and the dual description of the media (lagrangian material points and an eulerian numerical mesh) provide the MPM the capabilities of handling problems involving large displacements and deformations. The paper presents four examples with the aim of highlighting the dynamic formulation and the capability of the method to analyze in a unified mathematical framework the static-dynamic transition of a slope failure.
The selection of constitutive variables has been extensively discussed when modeling unsaturated soils. In general, suction and degree of saturation have been involved in the definition of constitutive variables. Recently, microstructural features have also been included due to their relevant effects on the mechanical and hydraulic soil response. This is the case of a model recently published by the same authors which includes a state variable in the definition of the constitutive variables to take into account microstructural effects. Details of the performance of this model, which has a relatively simple elastoplastic formulation, are presented. The simulation of isotropic stress paths illustrates the capabilities of the model and the effect of the microstructural state variable.
Un aspecto crítico a controlar en la gestión de embalses es la velocidad de descenso de nivel de agua debido al riesgo de inestabilidades en las laderas que lo forman. Este artículo presenta el estudio realizado para conocer las causas del desarrollo de un gran
deslizamiento en la ladera de un embalse. Para ello se realizó en primer lugar un reconocimiento geológico y geotécnico con el fin de definir la tipología del movimiento, su geometría y materiales involucrados mediante la realización de sondeos profundos y la
instalación de inclinómetros y piezómetros. Se determinó que se trataba de la reactivación de un antiguo deslizamiento traslacional de 40 Mm3. Mediante la observación detallada de los testigos y la información proporcionada por las medidas inclinómetricas, se determinó la posición de la superficie de deslizamiento. Ésta se
localizó en un estrato arcilloso de edad Garumniense y de alta plasticidad. Se analizaron en el laboratorio muestras extraídas de esta arcilla y se evaluaron sus propiedades geotécnicas.
La causa más probable del movimiento se asignó al descenso rápido del nivel del embalse que alcanzó velocidades de 1.2 m/s. El análisis se realizó para una sección representativa y mediante un programa de elementos finitos capaz de resolver de forma acoplada la respuesta mecánica e hidráulica. Se modeló la variación del nivel del embalse durante los 4 años previos a la rotura. La distribución de presión de agua calculada, determinante en el análisis de estabilidad, se validó mediante la comparación de los resultados con los valores medidos en los piezómetros instalados alrededor de la superficie de deslizamiento después de la rotura.
The paper explores the behaviour of compacted soils throughout the (dry density-water content) compaction plane by means of a conceptual framework that incorporates microstructural information. The engineering properties of compacted soils are described by an initial state in terms of a yielding stress, soil suction and a microstructural state variable. Microstructure is defined by the ratio of microvoid volume to total void volume. The pattern of variation of the microstructural parameter within the compaction plane has been determined, for some compacted soils, by analysing mercury intrusion porosimetry data. The microstructure of wet and dry compaction conditions can then be quantified. To ensure consistency, the framework is cast in the form of a constitutive model defined in terms of an effective suction and a constitutive stress that incorporate the microstructural variable...
Predicting the pore pressure distribution in a slope after rapid drawdown conditions requires the solution of a coupled flow‐deformation analysis in a saturated‐unsaturated porous media. A fully coupled finite element code (Code_Bright), able to handle in a consistent manner the drawdown conditions, is used to simulate the pore water pressure measured in the upstream slope of an earth dam (Glen Shira Dam, Scotland) subjected to a controlled rapid
drawdown event. A comparison of some calculation alternatives is then given and compared with field pressure records. The paper describes also the analysis performed to interpret a recent case of a major landslide triggered by a rapid drawdown in a reservoir. A key aspect of the case is the correct characterization of permeability of representative soil profiles. This is achieved by combining laboratory test results and a back analysis of pore water pressure time records during a period of reservoir level fluctuations.
The selection of constitutive variables has been extensively discussed when modeling unsaturated soils. In general, suction and degree of saturation have been involved in the definition of constitutive variables. Recently, microstructural features have also been included due to their relevant effects on the mechanical and hydraulic soil response. This is the case of a model recently published by the same authors which includes a state variable in the definition of the constitutive variables to take into account microstructural effects. Details of the performance of this model, which has a relatively simple elastoplastic formulation, are presented. The simulation of isotropic stress paths illustrates the capabilities of the model and the effect of the microstructural state variable
The stability of tunnel faces supported by an umbrella of subhorizontal micropiles is analyzed by means of the upper bound theorem of plasticity. The micropiles are considered beams subjected to the kinematic motion imposed by the assumed failure mechanism. The limiting resistance of the supporting beams is first addressed. The failure mechanism imposes a displacement pattern on the beam, which
is similar to the response of vertical piles subjected to horizontal load. The mechanism of beam deformation was included into the overall
instability mechanism for the tunnel face stability. Limiting conditions of the beam under undrained conditions are used. A general minimization
process associated with the upper bound formulation is developed. Stability conditions are described in dimensionless parameters and plotted in ready-to-use design charts. In particular, a dimensionless micropile coefficient, which includes all the relevant design parameters
of the umbrella, could be isolated and plotted in undrained soil strength and tunnel geometry. A real face failure is finally described, and the required supporting umbrella is designed on the basis of the developed procedure.
The calculation of pore water pressure distribution in a slope partially submerged and subjected to a drawdown is analyzed in this chapter. Drawdown phenomenon is a critical situation for the stability of slopes of earth dams and bank rivers affected by variations of water level. The prediction of pore water pressure in two earth dams subjected to a drawdown event under different calculation hypothesis highlight that classical flow methods of analysis may overestimate or underestimate pore water pressures depending on several external condition and material properties. In particular, an instrumented dam is simulated and calculated results can be compared with field measurements. Finally, in the chapter, an incipient landslide (40 Mm3) in the left margin of a reservoir triggered by a drawdown is presented and
Lechago dam (Teruel, Spain) is a 40 m high zoned earth and rockfill dam sitting on soft continental deltaic deposits. A relatively narrow central clay core is stabilised by wide rockfill shoulders. The dam was well instrumented and continuous records of stress development, pore-water pressures and vertical displacements are available for the construction period. Compaction conditions were followed by means of laboratory and in situ control tests. Core clay material was investigated by means of tests performed on compacted specimens of tertiary clays. Rockfill samples were excavated in outcrops of highly fractured Cambrian quartzitic shale. A testing programme on compacted rockfill gravels was conducted under relative humidity control in a large-diameter oedometer and triaxial cells. A coupled finite-element model has been developed to analyse the tests performed and dam behaviour during construction. Model predictions, essentially based on laboratory tests, are compared with measurements during construction. The predicted response of the dam under an assumed programme of impounding is also given. In the future, once impounding occurs, it will be possible to compare these predictions with actual dam performance. The paper provides an integrated description of the dam design, construction and early behaviour. It presents a procedure to interpret available data (laboratory as well as in situ data) on compacted materials from the perspective of modern constitutive models. It also provides an evaluation of the capabilities of advanced numerical tools to reproduce the measured dam behaviour.