Numerical interpretation of the coupled hydromechanical behaviour of expansive clays in constant volume column tests

© The authors and ICE Publishing: All rights reserved, 2015.Experimental and numerical studies of the behaviour of expansive clays have been attracting increasing interest, due to their good sealing properties, which render them ideal to be used as engineered barriers (buffers) in both active (e.g. nuclear) and non-active waste disposal facilities. Both large scale and laboratory scaled experiments indicate that the sealing capabilities of the buffer are fundamentally governed by its volumetric behaviour when wetted. In this paper, a constant volume column infiltration test, performed under isothermal conditions on compacted MX80 bentonite, is modelled numerically using the Imperial College Finite Element Program (ICFEP). A modified version of the Barcelona Basic Model is used to simulate the behaviour of the buffer, which is inherently partly saturated. The numerical results agree well with the observed experimental data, especially with regard to the advancement of the wetting front. A detailed interpretation of the computed evolutions with time of stress state, suction and void ratio at different elevations along the samples axis is carried out, providing insight into the complex hydro-mechanical response of the buffer during the experiment. Indeed, even though the overall volume of the sample was kept constant, a region of localised dilation, which induced the contraction of other zones of the material, was observed to advance simultaneously with the wetting front along the height of the soil column

Swelling behaviour of an expansive clay at high suction

Deep geological disposal designs for nuclear waste often include an engineered barrier to protect the waste canisters and prevent leakage . The long - term safety of the re pository relies on studies of the buffer material . Oedometer tests provide values of de sign parameters for numerical simulations. A newly - developed oedometer with automated suction control is presented to assist in the investigation of the coupled hydro - me- chanical - volumetric behaviour of an expansive clay, namely a natural sodium bentonite. The displacement - controlled device was developed to apply suction over a range of 1 0 MPa to 300 MPa using a divided - flow humidity - generator. The device allows the application of combined stress and suction states, and continuous stress paths of constant vo lume, stress or suction. The development of the new oedometer is described. Results obtained during the preliminary tests are evaluated through comparison with experimental data from similar tests found in the literature. The current method benefits from c ontinuous control of suction with servo - control of relative humidity using calibrated capacitance hygrometers. The system self - compensates for minor temper- ature changes and therefore the requirement for thermal insulation is not as crucial as in vapour equ ilibrium methods

Investigating soil-water retention characteristics at high suctions using Relative Humidity control

A technique for controlling relative humidity (RH) is presented, which involves supplying a sealed chamber with a continuous flow of air at a computer-regulated RH. The desired value of RH is achieved by mixing dry and wet air at appropriate volumes and is measured for servo-control at three locations in the chamber with capacitive RH sensors and checked with a sensitive VAISALA sensor. The setup is capable of controlling RH steadily and continuously with a deviation of less than 0.2% RH. The technique was adopted to determine wetting soil-water retention curves (SWRC) of statically compacted London Clay, under both free-swelling and constant volume conditions. The RH within the chamber was increased in a step-wise fashion, with each step maintained until vapour equilibrium between the chamber atmosphere and the soil samples was established. Independent filter paper measurements further validate the method, while the obtained retention curves complement those available in the literature for lower ranges of suction

Temperature-controlled oedometer testing on compacted bentonite

A new temperature - controlled oedometer h as been designed at Imperial College London and commissioned to investig ate the thermo - hydro - mechanical behaviour of soils. Temperature control is achieved by submerging the s pecimen in a water bath. The water temperature is regulated by heaters positioned radially around the s pecimen , or by an external unit. T he temp erature can be varied between 5°C and 85 °C . The temperature gradient across the s pecimen is minimised by circulating water beneath the s pecimen through a hollow plate. A thermo - mechanical, elas tic, finite element model of the equipment has been produced using the Imperial College Finite Element Program (ICFEP). The experimental results are used to develop and validate the numerical model. The model is then used to inform and improve the experime ntal testing programme. The accuracy of temperature control has already been established. The testing programme includes heating tests at constant applied stress, and loading tests at discrete temperature values. Of particular interest is thermally - induced overconsolidation behaviour . The experimental results are used to verify the existing numerical framework and to establi sh the effect of temperature on the behaviour o f saturated soil

Soil-atmosphere interaction in unsaturated cut slopes

Interaction between atmosphere and soil has only recently attracted significant interest. Soil-atmosphere interaction takes place under dynamic climatic conditions, which vary throughout the year and are expected to suffer considerable alterations due to climate change. However, Geotechnical Analysis has traditionally been limited to simplistic approaches, where winter and summer pore water pressure profiles are prescribed. Geotechnical Structures, such as cut slopes, are known to be prone to large irreversible displacements under the combined effect of water uptake by a complex vegetation root system and precipitation. If such processes take place in an unsaturated material the complexity of the problem renders the use of numerical analysis essential. In this paper soil-atmosphere interaction in cut slopes is studied using advanced, fully coupled partially saturated finite element analyses. The effect of rainfall and evapotranspiration is modelled through sophisticated boundary conditions, applying actual meteorological data on a monthly basis. Stages of low and high water demand vegetation are considered for a period of several years, before simulating the effect of vegetation removal. The analysis results are presented with regard to the serviceability and stability of the cut slope

Coupled consolidation in unsaturated soils: an alternative approach to deriving the Governing Equations

The equations governing coupled consolidation in unsaturated soils are known to contain additional parameters when compared to the equations for saturated soils. Nonetheless, the variation of these parameters with suction or degree of saturation is not generally agreed upon. The paper introduces a novel approach to deriving general equations for each of these parameters and their variation, and explains that, for consistency with the constitutive and soil-water retention curve models adopted, these general equations need to be transformed into case-specific expressions. Finally, a conceptual model is presented highlighting how the behaviour of unsaturated soil reflects aspects of its water content

Investigating the effect of tunnelling on existing tunnels

A major research project investigating the effect of tunnelling on existing tunnels has been completed at Imperial College London. This subject is always of great concern during the planning and execution of underground tunnelling works in the urban environment. Many cities already have extensive existing tunnel networks and so it is necessary to construct new tunnels at a level beneath them. The associated deformations that take place during tunnelling have to be carefully assessed and their impact on the existing tunnels estimated. Of particular concern is the serviceability of tunnels used for underground trains where the kinematic envelope must not be impinged upon. The new Crossrail transport line under construction in London passes beneath numerous tunnels including a number of those forming part of the London Underground networ

A ghost-stabilised material point method for large deformation geotechnical analysis

The Material Point Method (MPM) is advertised as the method for large deformation analysis of geotechnical problems. However, the method suffers from several instabilities which are widely documented in the literature, such as: material points crossing between elements, different number of points when projecting quantities between the grid and points, etc. A key issue that has received relatively little attention in the literature is the conditioning of the linear system of equations due to the arbitrary nature of the interaction between the physical body (represented by material points) and the background grid (used to solve the governing equations). This arbitrary interaction can cause significant issues when solving the linear system, making some systems unsolvable or causing them to predict spurious results. This paper presents a cut-FEM (Finite Element Method) inspired ghost-stabilised MPM that removes this issue

An open-source Julia code for geotechnical MPM

There is considerable interest in the Material Point Method (MPM) in the computational geotechnics community since it can model problems involving large deformations, e.g. landslides, collapses etc. without being too far from the standard finite element method, which can struggle with large deformation problems. The open-source code AMPLE developed at Durham University in recent years is a compact set of MATLAB functions that “address the severe learning curve for researchers wishing to understand, and start using, the MPM”. It is well known that MATLAB can be very slow hence limiting its utility for major studies of large problems, so here we introduce an MPM code with the same aims as AMPLE but written in the relatively new language Julia, specifically for fast runtimes. We highlight areas where MATLAB code constructs are inefficient if just transferred to Julia and show that to unlock large speed gains with Julia, one needs to code in a different way and we demonstrate this on a geotechnical problem. While this paper is concerned with the MPM, the advice regarding coding using Julia is transferable to other computational geotechnics methods and tools

Numerical characterisation of the rotational behaviour of grey cast iron tunnel joints

The structural assessment of segmental grey cast iron (GCI) tunnel linings to nearby construction is challenging due to the presence of the joints affecting the stiffness of the tunnel lining. This paper presents an extensive investigation, using 3D finite element (FE) analyses, into the bending moment-rotation (M-θ) behaviour of two GCI tunnel joint geometries. These two geometries correspond to standard running and station tunnels of the London Underground network. The contribution of this study is two-fold. i) The novel characterisation of the M-θ response enables the development of new models for simulating the mechanical response of GCI tunnel joints with structural elements which can be used in simplified, 2D geotechnical analysis for tunnel safety assessments. ii) The analyses provide insight into the behaviour of GCI tunnel linings that would be difficult to achieve through experimental and field observations alone. More specifically, the analyses show that when the bolts are removed from the joints the possibility of tensile failure can be disregarded; that the initial bolt preload influences the rotational stiffness only after some rotation has taken place and does not alter the bending moment of opening; and that the out-of-plane displacement restraint has little influence on the joint response