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• Analysis of a full scale "in situ" test simulating repository conditions

  A. Gens, A. García-Molina, S. Olivella, E. Alonso, F. Huertas

  International Journal for Numerical and Analytical Methods in Geomechanics (1998). Vol. 22 (7), pp. 515-548

  
  

  Abstract

  Coupled thermo‐hydro‐mechanical (THM) analyses have been used to examine the interacting phenomena associated with the simultaneous heating and hydration of an engineered bentonite barrier placed in a drift excavated in granite. The specific problem studied is an in situ test being carried out in the underground laboratory at Grimsel (Switzerland). After describing the test and the theoretical formulation, the results of a coupled THM analysis using the best parameter estimation currently available are presented and discussed. The effect of various features of analysis are explored by means of additional analyses in which each of those features are varied, one at the time. Finally, sensitivity analyses have been carried out to examine some critical aspects of the in situ test design. Performance of coupled THM analyses has led to a better understanding of the various inter‐related phenomena occurring during heating and hydration of the engineered clay barrier.

  Abstract
  Coupled thermo‐hydro‐mechanical (THM) analyses have been used to examine the interacting phenomena associated with the simultaneous heating and hydration of an engineered [...]

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• Porosity variations in saline media caused by temperature gradients coupled to multiphase flow and dissolution/precipitation

  S. Olivella, J. Carrera, A. Gens, E. Alonso

  Transport in Porous Media (1996). Vol. 25 (1), pp. 1-25

  
  

  Abstract

  We present a theoretical-numerical investigation of porosity variations induced by temperature gradients in unsaturated saline media. It is known that temperature variations cause humidity variations which lead to liquid flow towards and vapour flow away from the hot source. When this phenomenon occurs in saline media, the liquid is salt saturated brine, so that evaporation causes salt precipitation and an ensuing porosity reduction. Condensation of water causes salt dissolution and porosity increase. This process may be important in the case of heat generating waste because it suggests that selfsealing may take place near the waste. On the other hand, salt mass balance will lead to porosity increases in other zones.

  Abstract
  We present a theoretical-numerical investigation of porosity variations induced by temperature gradients in unsaturated saline media. It is known that temperature variations [...]

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• Coupled analysis of a backfill hydration test

  E. Alonso, A. Lloret, C. Delahaye, A. Gens, G. Volckaert

  International Journal for Numerical and Analytical Methods in Geomechanics (1998). Vol. 1 (1), pp. 10-25

  
  

  Abstract

  BACCHUS2 in situ isothermal wetting experiment has been analysed by means of a coupled flow-deformation approach. Backfill material, a mixture of Boom clay powder and high density pellets, has been extensively tested in the laboratory in order to determine its hydraulic and mechanical properties. Parameters of constitutive equations were derived from this experimental data base. Two mechanical constitutive models have been used in the simulation of the 'in situ' experiment: a state surface approach and an elastoplastic model. Calculations have shown several features of the hydration process which help to understand the behaviour of expansive clay barriers. Predictions using both models have been compared with each other and with actual measurement records. This has allowed a discussion of the comparative mertis of both approaches and the identiÞcation of some critical parameters of backfill behaviour. Overall agreement between calculations and field measurements is encouraging and shows the potential of the methods developed to model the behaviour of engineered clay barriers in the context of nuclear waste disposal.

  Abstract
  BACCHUS2 in situ isothermal wetting experiment has been analysed by means of a coupled flow-deformation approach. Backfill material, a mixture of Boom clay powder and high [...]

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• Evaluation of safety factors in discontinuous rock

  E. Alonso, I. Carol, C. Delahaye, A. Gens, P. Prat

  International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts (1996). Vol. 33 (5), pp. 513-537

  
  

  Abstract

  Safety factors for kinematically admissible failure mechanisms in jointed rock masses have been defined with linear and nonlinear failure criteria for rock discontinuities. Data required to compute these safety factors are obtained by means of two finite element anlayses of the effects of selfweight and external (structural) loading, respectively. Both types of analysis are closely linked since they share a common geometry. Joint elements are used to simulate the behaviour of rock discontinuities. If kinematically admissible mechanisms are possible under field conditions, the finite element mesh should also allow them to develop. Different aspects of the methodology have been illustrated through the safety evaluation of a 150 m high arch dam and its foundation in fractured cretaceous limestone. Special attention has been paid to the modelling of a realistic geometry including three-dimensional rock blocks and discontinuities. The paper discusses the effect of initial state of stress, the evolution of safety as the external load increases and the relation between the defined safety factors. It also provides practical guidelines for conducting this type of analysis in complex situations.

  Abstract
  Safety factors for kinematically admissible failure mechanisms in jointed rock masses have been defined with linear and nonlinear failure criteria for rock discontinuities. [...]

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• Estimation of parameters in geotechnical backanalysis. I- Maximum likehood approach

  A. LEDESMA, A. Gens, E. Alonso

  Computers and Geotechnics (1996). Vol. 18 (2), pp. 1-27

  
  

  Abstract

  The estimation of soil and rock parameters based on field instrumentation data is a common procedure in geomechanics. The use of system identification and optimization techniques allows the performance of this type of analyses in a more rational and objective manner. In this paper a probabilistic formulation for the backanalysis problem is presented. The procedure described involves the evaluation of the measurement covariance matrices, which are derived for some geotechnical instruments used in field instrumentation. The algorithm used to solve the mathematical problem of optimization is also presented, as well as its coupling to a finite element code. The algorithm requires the computation of the sensitivity matrix, which can be evaluated “exactly” in terms of the finite element method. Finally, a synthetic example, based on the excavation of a tunnel, is presented in which the elastic modulus E and the K0 parameter of the material are identified from measured displacements. The effect of the number of measurements and their error structure is also discussed.

  Abstract
  The estimation of soil and rock parameters based on field instrumentation data is a common procedure in geomechanics. The use of system identification and optimization [...]

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• Estimation of parameters in geotechnical backanalysis. II- Application to a tunnel excavation problem

  A. Gens, A. Ledesma, E. Alonso

  Computers and Geotechnics (1996). Vol. 18 (1), pp. 29-46

  
  

  Abstract

  A general statistical framework to perform backanalysis in geotechnical problems from field instrumentation has been presented in a companion paper. Here, an application to a real case involving the excavation of a tunnel in stiff overconsolidated clay is described. Both, extensometer and inclinometer measurements are used as input data and elastic moduli of the ground and the value of the K0 coefficient are estimated. The finite element method is used as the computational procedure to solve the direct problem, and has been coupled to the identification algorithm as described in the companion paper. In addition, a discussion on the reliability of the parameters identified is presented.

  Abstract
  A general statistical framework to perform backanalysis in geotechnical problems from field instrumentation has been presented in a companion paper. Here, an application [...]

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• Hidratación de un relleno compactado que contiene agregados de alta intensidad

  E. Alonso, A. Lloret, A. Gens, C. Delahaye, J. Facio

  Boletin de la Sociedad Española de Mecánica de Suelos y Cimentaciones (1996). Vol. 121, pp. 3-7

  
  

  Abstract

  Se han investigado, mediante ensayos de laboratorio, las características de hidratación de mezclas expansivas constituídas por una matriz de polvo arcilloso y agregados (pastillas de pequeño tamaño) de arcilla de alta densidad. Estas mezclas se han propuesto como material de relleno para aislar residuos potencialmente contaminantes del medio geológico que los rodea. Se ha simulado el comportamiento de este material de relleno bajo las condiciones existentes "in situ" mediante un analisis hidro-mecánico acoplado. En el análisis, el relleno se caracterizó de acuerdo con los resultados del programa de ensayos de laboratorio. Hydration characteristics of an expansive mixture of compacted clay matrix and high density pellets ha ve been investigated by means of laboratory tests. These mixtures are proposed as a suitable barrier material to isolate hazardous wastes from the surrounding geological environment. The behaviour of this material under conditions prevailing "in situ" has been simulated by means of a coupled flow-deformation analysis. The backfill was characterized according to the results of the laboratory tests program.

  Abstract
  Se han investigado, mediante ensayos de laboratorio, las características de hidratación de mezclas expansivas constituídas por una matriz de polvo arcilloso [...]

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• Parameter and variance estimation in geotechnical backanalysis using prior information

  A. LEDESMA, A. Gens, E. Alonso

  International Journal for Numerical and Analytical Methods in Geomechanics (1996). Vol. 20 (2), pp. 119-141

  
  

  Abstract

  A probabilistic framework to perform inverse analysis of geotechnical problems is presented. The formulation allows the incorporation of existing prior information on the parameters in a consistent way. The method is based on the maximum likelihood approach that allows a straightforward introduction of the error structure of field measurements and prior information. The difficulty of ascribing definite values to the uncertainties associated with the various types of observations is overcome by including the corresponding variances in the set of parameters to be identified. The inverse analysis results in a minimization problem that is solved by coupling the optimization technique to the finite element method. Two examples are presented to illustrate the performance of the method. The first one corresponds to a synthetic case simulating the excavation of a tunnel. Young's modulus, K0 value and measurements variances are identified. The second case concerns the excavation of a large underground cavern in which again Young's modulus and K0 are identified. It is shown that introduction of prior information permits the estimation of parameters more consistent with all available informations that include not only monitored displacements but also results from in situ tests carried out during the site investigation stage.

  Abstract
  A probabilistic framework to perform inverse analysis of geotechnical problems is presented. The formulation allows the incorporation of existing prior information on the [...]

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• Numerical formulation for a simulator (CODE_BRIGHT) for the coupled analysis of saline media

  E. Alonso, S. Olivella, A. Gens, J. Carrera

  Transport in Porous Media (1994). Vol. 15 (3), pp. 87-112

  
  

  Abstract

  Presents numerical aspects of the program CODE_BRIGHT, which is a simulator for COupled DEformation, BRIne, Gas and Heat transport problems. It solves the equations of mass and energy balance and stress equilibrium and, originally, it was developed for saline media. The governing equations also include a set of constitutive laws and equilibrium conditions. The main peculiarities of saline media are in the dissolution/precipitation phenomena, presence of brine inclusions in the solid salt and creep deformation of the solid matrix.

  Abstract
  Presents numerical aspects of the program CODE_BRIGHT, which is a simulator for COupled DEformation, BRIne, Gas and Heat transport problems. It solves the equations [...]

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• Nonisothermal multiphase flow of brine and gas through saline media

  S. Olivella, J. Carrera, A. Gens, E. Alonso

  Transport in Porous Media (1994). Vol. 15 (3), pp. 271-293

  
  

  Abstract

  We propose a general formulation for nonisothermal multiphase flow of brine and gas through saline media. The balance equations include mass balance (three species), equilibrium of stresses and energy balance (total internal energy). Salt, water and air mass balance equations are established. The balance of salt allows the establishment of the equation for porosity evolution due to solid skeleton deformation, dissolution/precipitation of salt and migration of brine inclusions. Water and air mass balance equations are also obtained. Two equations are required for water: total water in the medium and water present in solid phase brine inclusions. The mechanical problem is formulated through the equation of stress equilibrium. Finally, the balance of internal energy is established assuming thermal equilibrium between phases. Some general aspects of the constitutive theory are also presented.

  Abstract
  We propose a general formulation for nonisothermal multiphase flow of brine and gas through saline media. The balance equations include mass balance (three species), equilibrium [...]

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