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