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

Numerical Modelling of Multi-directional Earthquake Loading and Its Effect on Sand Liquefaction

Earthquakes generate multi-directional ground motions, two components in the horizontal direction and one in the vertical. Nevertheless, the effect of vertical motion on site response analysis has not been the object of extensive research. The 2010/2011 Canterbury sequence of seismic events in New Zealand is a prime example among other earlier field observations strongly corroborating that the vertical acceleration may have a detrimental effect on soil liquefaction. Consequently, this study aims to provide insight into the influence of the input vertical motion on sand liquefaction. For this reason, two ground motions, with very different frequency contents, are used as the input excitations. Non-linear elasto-plastic plane strain fully coupled effective stress-based finite element analyses are conducted to investigate the occurrence of liquefaction in a hypothetical fully saturated Fraser River Sand deposit. The results indicate that the frequency content of the input motion is of utmost importance for the response of sands to liquefaction when the vertical loading is considered

On the assessment of energy dissipated through hysteresis in finite element analysis

The accurate reproduction of the hysteretic behaviour exhibited by soils under cyclic loading is a crucial aspect of dynamic finite element analyses and is typically described using the concept of damping ratio. In this paper, a general algorithm is presented for assessing the damping ratio simulated by any constitutive model based on the registered behaviour in three-dimensional stress-strain space. A cyclic nonlinear elastic model capable of accurately reproducing a wide range of features of soil behaviour, including the variation of damping ratio with deformation level, is chosen to illustrate the capabilities of the proposed algorithm. The constitutive model is described and subsequently employed in two sets of finite element analyses, one involving the dynamic response of a sand deposit subjected to different types of motion and another focussing on the simulation of a footing subjected to cyclic vertical loading. The application of the presented algorithm provides insight into the processes through which energy is dissipated through hysteresis

Accounting for partial material factors in numerical analysis

The concept of a safety factor in the design of geotechnical structures has traditionally been developed within the framework of classical soil mechanics, where the analysis methods for its calculation involve simple limit equilibrium or limit analysis approaches. Therefore the inclusion of a safety factor within an advanced analysis method, such as finite elements or finite differences, is a more complex issue. In particular, the problem arises with design codes, such as Eurocode 7, in which partial factors on soil strength (or partial material factors) must be accounted for. Eurocode 7 implies that a numerical analysis should be performed accounting for a characteristic strength, which is reduced by partial factors. There are two ways in which such partial factors can be included in numerical analysis: one in which the strength is reduced at the beginning of the analysis, and the other in which this is done during the analysis. Eurocode 7 gives no guidance as to which one of these two approaches is more appropriate to apply. More importantly, there is no guidance on the appropriate numerical procedure that should be implemented in any software in order to perform the required strength reduction during the analysis in the latter approach. Therefore different software programs account for this in different ways, and mostly only for simple constitutive models. This paper presents, first, a consistent methodology for accounting for partial material factors in finite-element analysis, which can be applied to any constitutive model. It then demonstrates the implications of the two ways the partial material factors can be introduced in any analysis, using the example of a bearing capacity problem and employing constitutive models of increasing complexity. The paper shows that the two approaches for accounting for partial material factors may lead to different results, and that it is therefore necessary to develop a rational set of guidelines for their inclusion in advanced numerical analysis. </jats:p

Blinding struts under long-term loading

The term ‘blinding' is used to describe the thin layer of unreinforced over-site concrete used to protect the base of excavations during construction. Blinding is not generally considered a structural element even though it clearly provides some temporary lateral support to the retaining walls of excavations. The authors have previously shown that enhanced blinding can be used to prop retaining walls in cut-and-cover excavations during construction prior to the completion of the base slab. This paper describes a series of laboratory tests carried out to investigate the effect of concrete creep on the axial resistance of blinding struts. The tests show that blinding can creep to failure if the sustained load exceeds a critical value that depends on the imperfection amplitude and profile as well as the strut thickness. The test results are used to validate a numerical model which is then used to carry out a series of parametric studies on full-scale blinding struts. </jats:p

The use of kinematic hardening models for predicting tunnelling-induced ground movements in London Clay

The use of a kinematic hardening soil model for predicting short- and long-term ground movements due to tunnelling in London Clay is investigated. The model is calibrated against oedometer and triaxial tests on intact samples from different units of the London Clay. The calibrated model is then used in finite-element analysis to simulate the field response at St James's Park during excavation of the Jubilee Line Extension tunnels. The finite-element predictions compare well with the available field monitoring data. The importance of using consistent initial conditions for this complex boundary value problem in conjunction with the model parameters selected is highlighted. The stiffness response of different regions of the finite-element mesh indicates that the rate at which the stiffness degrades and the stiffness response further away from the tunnel boundary affect the short-term predictions significantly. The long-term predictions confirm that the compression characteristics of the soil control the magnitude of the consolidation settlements and its permeability the shape of the long-term settlement profiles

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