A. Lloret

Temperature effects on the hydraulic behaviour of an unsaturated clay

The influence of temperature on the hydraulic properties of unsaturated clays is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. This paper presents an experimental study centred on the investigation of the influence of temperature on soil hydraulic properties related to water retention and permeability. Laboratory tests were conducted on artificially prepared unsaturated fabrics obtained from a natural kaolinitic-illitic clay. Special attention is given to the testing procedures involving controlled suction and temperature oedometer cells and the application of the vapour equilibrium technique at high temperatures. Retention curves at different temperatures show that total suction tends to reduce with increasing temperatures at constant water content. Temperature influence on water permeability is more relevant at low matric suctions corresponding to bulk water preponderance (inter-aggregate zone). Below a degree of saturation of 75% no clear effect is detected. Experimental data show that temperature dependence on permeability at constant degree of saturation and constant void ratio is smaller than what could be expected from the thermal change in water viscosity. This behaviour suggests that phenomena such as porosity redistribution and thermo-chemical interactions, which alter clay fabric and pore fluid, can be relevant.

Measurement of landslide displacements using a wire extensometer

A continuous recording of landslide displacements is often required in order to better understand the complex relationship between the triggering factors and the dynamics of the movement. In this paper, we discuss the performance of the borehole wire extensometer and the interpretation of its results. The analysis for the case of a translational slide shows that the displacements measured with the wire extensometer are systematically smaller than the movements observed at the ground surface. A relationship between the wire readings and the horizontal component of the landslide movement has been established by means of three equations representing different stages of the wire displacement within the borehole. The applicability of these equations and the interpretation of the wire extensometer readings have been successfully checked at two landslide sites: Vallcebre in the eastern Pyrenees and Alvera in the Dolomites.

Backanalysis of thermohydraulic bentonite properties from laboratory tests

Abstract The paper presents a general methodology to perform backanalysis of laboratory tests involving thermohydraulic behaviour of bentonite in a systematic manner. The procedure is based on a maximum likelihood approach that defines a probabilistic framework in which error measurements and the reliability of the parameters identified can be estimated. The method is applied to the identification of some thermal and hydraulic properties of a bentonite specimen, using temperature and water content measurements as input data. Three basic cases have been considered: (a) thermal case, identifying the global thermal conductivity, λ e , and the global specific heat, c e ; (b) hydraulic case, identifying the tortuosity factor, τ , and the exponent of the unsaturated permeability law, n ; and (c) thermohydraulic case, in which τ , n , and the saturated thermal conductivity, λ sat , have been estimated. A numerical code that solves the coupled thermohydraulic equations has been used as main computational tool. A detailed description of the experimental device and a discussion on the tests results and on the parameters identified are also included. The values obtained are within the normal range of these parameters, but the method provides a systematic and consistent procedure to find the best parameters that reproduce the measurements for the selected models. Also the method gives an insight into the model structure, and allows detecting dependence and coupling between parameters.

Gas flow to a vertical gas extraction well in deformable MSW landfills.

Active gas control systems are commonly used in municipal solid waste (MSW) landfills and the design of such systems requires thorough understanding of the gas flow pattern. A model is developed to predict the two-dimensional radial transient gas flow to a vertical gas extraction well in deformable MSW landfills. Variations of gas storage include time-dependent compression of the refuse, dissolution of gas components and porosity enlargement due to organic matter degradation. Mechanical compression of solid skeleton is coupled with gas pressure using K-H rheological model which is capable of reproduce the evolution of settlement for MSW landfills. The new analytical solution obtained in Laplace transform domain can be used to determine excess gas pressure fields, gas fluxes in the well and through the top cover as well as landfill settlements. The solution is validated by comparison with field measurements and numerical simulations. It demonstrates that the gas storage variation term becomes predominant only during early times. Long-term gas flow is controlled by the gas generation rate and the quasi-steady solution is valid. Parametric studies indicate that the solution given in this paper is useful for the prediction of gas fluxes, for the choice of the optimum spacing between wells, and for the determination of the final cover properties as well as appropriate vacuum pressure imposed in the extraction well.

Numerical Analysis of an Instrumented Steel-Reinforced Soil Wall

AbstractThe paper describes the results and lessons learned using a FEM model to simulate quantitative performance features of the Minnow Creek steel-strip reinforced soil wall structure located in the United States. The Minnow Creek Wall structure was constructed and instrumented in 1999. It is a unique case study because of the comprehensive measurements that were taken to record a wide range of wall performance features. Two different constitutive models for the soil were used (a linear-elastic Mohr-Coulomb model and hardening soil model with a Mohr-Coulomb failure criterion), and numerical outcomes were compared with physical measurements. The numerical results were shown to be sensitive to boundary conditions assumed at the toe of the wall. The generally encouraging agreement between physical and numerically predicted results gives confidence that commercial FEM software packages can be useful for the analysis and design of these types of structures, provided that care is taken in the selection of in...

Vertical-Facing Loads in Steel-Reinforced Soil Walls

The paper investigates the influence of backfill soil, foundation soil, and horizontal joint vertical compressibility on the magnitude of vertical loads developed in steel-reinforced soil concrete panel retaining walls at the end of construction. Measurements of toe loads recorded from instrumented field walls are reviewed and demonstrate that vertical toe loads can be much larger than the self-weight of the facing. In extreme cases, these loads can result in panel-to-panel contact leading to concrete spalling at the front of the wall. Vertical loads in excess of panel self-weight have been ascribed to relative movement between the backfill soil and the panels that can develop panel-soil interface shear and downdrag loads at the connections between the panels and the steel-reinforcement elements. A two-dimensional finite-element model is developed to systematically investigate the influence of backfill soil, foundation soil, bearing pad stiffness, and panel-soil interaction on vertical loads in the panel facing. The results show that an appropriately selected number and type of compressible bearing pads can be effective in reducing vertical compression loads in these structures and at the same time ensure an acceptable vertical gap between concrete panels. The parametric analyses have been restricted to a single wall height (16.7 m) and embedment depth of 1.5 m, matching a well-documented field case. However, the observations reported in the paper are applicable to other similar structures. The general numerical approach can be used by engineers to optimize the design of the bearing pads for similar steel-reinforced soil wall structures using available commercial finite-element model packages together with simple constitutive models.

Assessment of the use of the vapour equilibrium technique in controlled-suction tests

This paper presents a study on suction control methodology using the vapour equilibrium technique. To reduce the time needed to reach equilibrium, humid air is forced to flow through the sample or on its boundaries. An analysis of the physical processes that occur in the tested soil samples using a numerical approach is included. The test is numerically simulated and the key parameters of the process calibrated. A sensitivity analysis of the unmeasured parameters and variables is performed. It could be observed that forcing humid air to flow through the sample reduces the equalization time, but the results from the numerical simulations highlight that this flow must be carefully applied to avoid reaching, under steady-state conditions, a different suction than that wished for. When the circulation of air along the boundaries is slow, an increase in air velocity tends to increase the rate of suction changes in the sample. However, if the circulation of air increases over some limit, the flow conditions ins...

Numerical study of the influence of foundation compressibility and reinforcement stiffness on the behavior of reinforced soil walls

Abstract Most geosynthetic and metallic reinforced soil walls are designed assuming that the wall foundation is rigid and/or does not influence the magnitude and distribution of reinforcement loads under operational conditions. This assumption may not apply to walls constructed over compliant (compressible) foundations. This paper describes the results of a series of numerical simulations that were carried out on idealized 3·6, 6, and 9 m-high modular block walls seated on foundations having four different compressibility values. The walls were constructed with two reinforcement materials having very different stiffness values but the same tensile strength. The results of simulations show that as foundation stiffness decreases, reinforcement loads increase. However, for the two reinforcement materials in this study, the influence of axial stiffness of the reinforcement had a greater effect on wall performance than the foundation stiffness for walls subjected to operational (working stress) conditions at end of construction.

Influence of Water Chemistry on the Swelling Capacity of a High-Density Bentonite

The paper presents the results of a systematic experimental investigation under oedometer conditions aimed at understanding the chemical influence of the water chemistry on the swelling capacity of a statically compacted Ca-bentonite. Specimens were soaked under constant stress (0.02, 0.5 and 2.0 MPa) using distilled water, and NaCl and CaCl 2 aqueous solutions with concentrations of 0.5, 2.0 and 5.5 M. test results showed important mechanical changes induced by the effect of the hydration fluids. The swelling strains of compacted samples on soaking were systematically reduced with increasing solute concentrations. This observation had an important consequence on the development of swelling pressures, which are a key design parameter of the engineered barrier system.

COUPLED ANALYSIS OF A BACKFILL HYDRATION TEST

SUMMARY BACCHUS2 in situ isothermal wetting experiment has been analysed by means of a coupled flowdeformation 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 identification 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. ( 1998 by John Wiley & Sons, Ltd.