COLLECTIONS

Documents published in Scipedia

• Efficiency of a borehole seal by means of pre-compacted bentonite blocks

  M. Geet, G. Volckaert, W. Bastiaens, N. Maes, E. Weetjens, X. Sillen, B. Vallejan, A. Gens

  Physics and Chemistry of the Earth (2007). Vol. 32 (1-7), pp. 123-134

  
  

  Abstract

  The backfilling and sealing of shafts and galleries is an essential part of the design of underground repositories for high-level radioactive waste. Part of the EC funded project RESEAL studied the feasibility of sealing off a borehole in plastic Boom Clay by means of pre-compacted bentonite blocks. Two bentonites, namely the FoCa and Serrata clay, have been used. Based on laboratory tests, the bentonite blocks had an initial dry density of about 1.8 g/cm3 to obtain a swelling pressure of about 4.4 MPa, corresponding to the in situ lithostatic stress, at full saturation. The set-up was equipped with several sensors to follow-up the behaviour of the seal and the surrounding host rock during hydration. Full saturation was reached after five months and was mainly reached by natural hydration. Swelling pressure was lower than originally foreseen due to the slow reconsolidation of the host rock. Later on, the efficiency of the seal with respect to water, gas and radionuclide migration was tested. The in situ measured permeability of the seals was about 5 × 10-13 m/s. A gas breakthrough experiment did not show any preferential gas migration through the seal. No evidences of a preferential pathway could be detected from 125I tracer test results

  Abstract
  The backfilling and sealing of shafts and galleries is an essential part of the design of underground repositories for high-level radioactive waste. Part of the EC funded [...]

  • 2
  •  read

• On constitutive modelling of unsaturated soils

  A. Gens, M. Sánchez, D. Sloan

  Acta Geotechnica (2006). Vol. 1 (3), pp. 137-147

  
  

  Abstract

  The paper presents a review of constitutive modelling of unsaturated soils. After a brief historical perspective, a number of existing constitutive models are classified and discussed according to the type of stress variables adopted in their formulation. Afterwards, attention is given to recent developments in the proposal of coupled hydraulic-mechanical models and the possibility of casting them in a sound thermodynamical framework. Finally, a double structure model for expansive soils is described. The incorporation of microstructural considerations and its use as a platform for incorporating the influence of new variables are highlighted.

  Abstract
  The paper presents a review of constitutive modelling of unsaturated soils. After a brief historical perspective, a number of existing constitutive models are classified and [...]

  • 2
  •  read

• THM and reactive transport analysis of expansive clay barrier in radioactive waste isolation

  L. Guimarães, A. Gens, M. Sanchez, S. Olivella

  Communications in numerical methods in engineering (2006). Vol. 22 (8), pp. 849-859

  
  

  Abstract

  A fully coupled formulation combining reactive transport and an existing thermo‐hydro‐mechanical (THM) code is briefly described. Special attention has been given to phenomena likely to be encountered in clay barriers used as part of containment systems of nuclear waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically controlled and equilibrium‐controlled reactions have been incorporated. The formulation has been implemented in the finite element code CODE_BRIGHT. An application is presented concerning the performance of a large scale in situ heating test simulating high‐level nuclear waste repository conditions.

  Abstract
  A fully coupled formulation combining reactive transport and an existing thermo‐hydro‐mechanical (THM) code is briefly described. Special attention has been given to phenomena [...]

  • 2
  •  read

• Coupled thermo-hydro-mechanical analysis of engineered barriers for high level radioactive waste

  A. Gens, S. Olivella

  Chinese journal of rock mechanics and engineering (2006). Vol. 25 (4), pp. 670-680

  
  

  Abstract

  A general formulation for the performance of coupled thermo-hydro-mechanical (THM) analysis is presented. A multi-phase and multi-species approach has been adopted for the development of the formulation. The analysis considers a porous material with three phases (solid, liquid and gas) and three species (mineral, water and air). The formulation is presented in terms of governing equations, constitutive laws and equilibrium restrictions. An extensive programme of coupled THM analyses simulating the behaviour of the near field in a scheme for the underground disposal of high level radioactive waste is presented. The analyses are able to take into account the main interactions between the various processes that occur in the engineered barrier and immediately adjacent rock associated with heating and hydration phenomena. Examination of the results leads to a series of significant conclusions concerning the thermal, hydraulic and mechanical aspects of the problem.

  Abstract
  A general formulation for the performance of coupled thermo-hydro-mechanical (THM) analysis is presented. A multi-phase and multi-species approach has been adopted for the [...]

  • 4
  •  read

• Aznalcóllar dam failure. Part 3: dynamics of the motion

  E. Alonso, A. Gens

  Géotechniques (2006). Vol. 56 (3), pp. 203-210

  
  

  Abstract

  The motion of the Aznalcóllar dam slide, after the initiation of the failure, is examined in the paper. The moving mass remained essentially rigid, and Newton's second law provides the basic equation to determine the resulting dam displacement, velocity and acceleration. Resisting forces have been derived from a previous analysis of the conditions leading to failure. Pore water pressures induced by the forward motion of the dam are approximated as an undrained loading of the clay. Driving forces and their evolution in time derive from the liquefaction of tailings and the condition of constant volume of liquefied tailings pushing the moving dam forward. The stable ‘cliff’ left by the motion inside the tailings lagoon supports this hypothesis. The model is able to reproduce closely the distance travelled by the dam. It shows a low sensitivity with respect to reasonable changes of the main parameters. The model computes that the slide motion lasted about 15 s, and that the maximum acceleration experienced by the dam was 0·14g.

  Abstract
  The motion of the Aznalcóllar dam slide, after the initiation of the failure, is examined in the paper. The moving mass remained essentially rigid, and Newton's [...]

  • 3
  •  read

• Aznalcóllar dam failure. Part 2: stability conditions and failure mechanisms

  A. Gens, E. Alonso

  Géotechniques (2006). Vol. 56 (3), pp. 185-201

  
  

  Abstract

  An analysis of the conditions that led to the failure of the Aznalcóllar dam is made in the paper. Pore water pressures prevailing in the foundation clay at the time of the failure have been established on the basis of piezometric observations made after the failure. A simplified consolidation analysis is first presented to interpret pore water pressure generation and dissipation and to evaluate the evolution of shear stresses along the potential failure plane. Both limit equilibrium and finite element analysis are used to back-analyse the failure and to derive representative strength parameters along the failure surface. Three-dimensional effects introduced by the layered nature of the foundation clay are then described. They provide an explanation for the location of the slide, which affected only the south-eastern portion of the embankment. The failure is examined in the final part of the paper from the perspective offered by the current knowledge of the strength behaviour of brittle clays. The significance of progressive failure is discussed, together with other factors that played a significant role in this case: the homogeneous nature and very low permeability of the foundation clay and the natural state of the clay, probably affected by some initial damage, which reduced the available strength along bedding planes.

  Abstract
  An analysis of the conditions that led to the failure of the Aznalcóllar dam is made in the paper. Pore water pressures prevailing in the foundation clay at the time [...]

  • 2
  •  read

• Aznalcóllar dam failure. Part 1: field observations and material properties

  E. Alonso, A. Gens

  Géotechniques (2006). Vol. 56 (3), pp. 165-183

  
  

  Abstract

  The paper describes the failure of Aznalcóllar dam, in southern Spain, in April 1998. The rockfill dam slid forward and released a flow of acid-saturated tailings. The geology and geomorphology of the site are described. The results of a detailed laboratory testing programme on the tailings material and on the overconsolidated high-plasticity foundation clay are given. Special attention is paid to the identification of cementation of the tailings and to the strength and consolidation properties of the foundation clay. Failure features are interpreted on the basis of the field evidence, taking into account the measured geotechnical properties of the materials involved.

  Abstract
  The paper describes the failure of Aznalcóllar dam, in southern Spain, in April 1998. The rockfill dam slid forward and released a flow of acid-saturated tailings. [...]

  • 7
  •  read

• Analysis of the hydration of a bentonite seal in a deep radioactive waste repository

  J. Vaunat, A. Gens

  Engineering Geology (2005). Vol. 81 (3), pp. 317-328

  
  

  Abstract

  A deep geological repository for nuclear waste requires the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. The paper presents the coupled hydromechanical analyses of an in situ test carried out in the HADES underground laboratory in Mol, Belgium. The test examines the effectiveness of an expansive clay seal in a horizontal borehole specifically drilled for this purpose. The analysis covers the phase of seal hydration up to saturation and subsequent pore pressure equilibration. Hydraulic and mechanical constitutive laws suited to expansive clay materials have been chosen for the analyses with all the parameters determined independently of the in situ test. A quite good agreement has been found between test observations and computed results suggesting that the numerical formulation employed is able to reproduce the main features of a real sealing system

  Abstract
  A deep geological repository for nuclear waste requires the backfilling and sealing of shafts and galleries to block any preferential path for radioactive contaminants. The [...]

  • 2
  •  read

• A hydro-geochemical analysis of the saturation process with salt water of a bentonite crushed granite rock mixture in an engineered nuclear barrier

  C. Mata, L. Guimarães, A. LEDESMA, A. Gens, S. Olivella

  Engineering Geology (2005). Vol. 81 (3), pp. 227-245

  
  

  Abstract

  A MX-80 sodium bentonite crushed granite rock mixture is being saturated in the Äspö Hard Rock Laboratory managed by SKB (the Swedish Company of nuclear waste management) as part of the “Backfill and Plug Test Project”. The Äspö Hard Rock Laboratory is placed in the Swedish Island of Äspö, an underground full-scale laboratory where different testing construction procedures and handling techniques are being studied. The groundwater of Äspö has a variable salt concentration according to the zone considered. The added water during the mixing process of both materials had an average salt concentration of 6 g/L. However, backfill is being saturated with salt water containing higher salt content (up to 16 g/L, 50 / 50 of NaCl and CaCl2 by mass) to speed up the saturation process. The mixture swelling capacity is small due to the low backfill bentonite content, but its activity is still large if compared with natural clayey soils. A coupled hydro-chemical approach was used to simulate the backfill hydration process, comparing the results with in situ measurements. An intrinsic permeability law, depending on salt concentration in the liquid phase, was incorporated into the model. The retention curve was also determined taking into account the effect of the chemical species on backfill behaviour. The simulation of the saturation process shows the importance of studying these problems with a HC (hydro-chemical) formulation, especially if long term behaviour of such mixtures is going to be reproduced.

  Abstract
  A MX-80 sodium bentonite crushed granite rock mixture is being saturated in the Äspö Hard Rock Laboratory managed by SKB (the Swedish Company of nuclear waste management) [...]

  • 6
  •  read

• A double generalized plasticity model for expansive materials

  M. Sánchez, A. Gens, L. Guimarães, S. Olivella

  International Journal for Numerical and Analytical Methods in Geomechanics (2005). Vol. 29 (8), pp. 751-787

  
  

  Abstract

  The constitutive model presented in this work is built on a conceptual approach for unsaturated expansive soils in which the fundamental characteristic is the explicit consideration of two pore levels. The distinction between the macro- and microstructure provides the opportunity to take into account the dominant phenomena that affect the behaviour of each structural level and the main interactions between them. The microstructure is associated with the active clay minerals, while the macrostructure accounts for the larger-scale structure of the material. The model has been formulated considering concepts of classical and generalized plasticity theories. The generalized stress–strain rate equations are derived within a framework of multidissipative materials, which provides a consistent and formal approach when there are several sources of energy dissipation. The model is formulated in the space of stresses, suction and temperature; and has been implemented in a finite element code. The approach has been applied to explaining and reproducing the behaviour of expansive soils in a variety of problems for which experimental data are available. Three application cases are presented in this paper. Of particular interest is the modelling of an accidental overheating, that took place in a large-scale heating test. This test allows the capabilities of the model to be checked when a complex thermo-hydro-mechanical (THM) path is followed.

  Abstract
  The constitutive model presented in this work is built on a conceptual approach for unsaturated expansive soils in which the fundamental characteristic is the explicit consideration [...]

  • 2
  •  read