|dc.contributor.author||Fraser Harris, Andrew Peter||en
|dc.description.abstract||The management of radioactive wastes is a significant environmental issue facing the international
nuclear community today. The current international consensus is for disposal of
higher activity waste from a variety of sources in deep geological disposal facilities (GDFs).
Hydraulic seals, often planned to consist of compacted bentonite-sand blocks, are an important
part of the closure phase of a GDF. As such, an understanding of the hydro-mechanical
(HM) behaviour of these seals, and the ability to model and predict their behaviour is fundamental
to support many planned safety cases and licence applications. Bentonite is well
suited for use as a hydraulic seal due to its high swelling capacity that enables it to swell into
voids while maintaining a low permeability sealed barrier to advective flow, and to provide
structural support by generating a swelling pressure on the excavation walls.
The hydro-mechanical process of bentonite hydration is a highly non-linear problem. As
such, coupled process models that are able to account for the strong inter-dependence of the
hydraulic and mechanical processes are employed to simulate the behaviour of bentonite under
repository conditions. This thesis reports the development of an HM coupled model in the
open source finite element code OpenGeoSys (OGS), and its application to the simulation of a
range of hydraulic seal test conditions.
The developed model couples Richards’ equation for unsaturated flow to a new strain
dependent non-linear elastic mechanical model that incorporates a Lagrangian moving finite
element mesh to inform the material non-linearity. Stress and volumetric dependent water
retention behaviour are incorporated through the implementation of the Dueck suction concept
extended to take into account non-recoverable strains during consolidation. A number of
permeability functions are implemented and tested against experimental data.
The mechanical model is extended to account for wetting-induced collapse behaviour by
the definition of a failure curve derived from experimental results. Similar in definition to the
Loading-Collapse curve in elasto-plastic models, this failure curve triggers the application of
a source term to account for wetting-induced collapse.
Coupling between the hydraulic and mechanical processes is achieved through the stress
dependency of the water retention behaviour, the inclusion of a new coupling factor for the hydraulic
contribution to the mechanical process, and the dependency of numerical convergence
criteria on net mean stress. An explicit iterative calculation approach is employed. As a result,
the hydraulic and mechanical moving meshes are decoupled to allow volumetric dependent
parameters to be updated within process iterations.
The model is calibrated and compared to experimental data from the SEALEX experiments
conducted by the Institut de Radioprotection et de S ˆ uret´e Nucl´eaire (IRSN) at the Tournemire
URL, France. The experimental programme comprises standardised laboratory tests, a 1/10th
scale mock-up of a hydraulic seal with a uniform technological void, and a full scale in situ
performance test with a non-uniform technological void due to its horizontal geometry.
Using a model with 5 hydraulic parameters, 8 mechanical parameters with an experimentally
defined failure curve, and one coupling parameter, the major trends of behaviour in all
the SEALEX experiments can be recreated, including axial stress build up, water uptake, and
final deformation. However, the elastic method employed leads to an over prediction of the
rebound on loss of axial confinement in the 1/10th scale mock-up test.
Simulations suggest that the non-symmetric technological void in the full scale performance
test could have lasting effects on the development of heterogeneity in the hydraulic
seal. The development of heterogeneity does not adversely affect the permeability with respect
to the design criteria, but may have significant consequences for the development of a
heterogeneous swelling pressure.||en
|dc.publisher||The University of Edinburgh||en
|dc.relation.hasversion||Bond, A., Thatcher, K., Chittenden, N., McDermott, C., and Fraser-Harris, A. (2014). RWM Coupled Processes Project: First Annual Report for RWM participation in DECOVALEX-2015 Tasks A and C1. AMEC report 18040-TR-002 v2.1.||en
|dc.relation.hasversion||Bond, A., Thatcher, K., Chittenden, N., McDermott, C. and Fraser-Harris, A. (2015). RWM Coupled Processes Project: Second Annual Report for RWM participation in DECOVALEX- 2015 Tasks A and C1. AMEC report 18040-TR-003 v3.0.||en
|dc.relation.hasversion||Bond, A., Thatcher, K., Chittenden, N., McDermott, C. and Fraser-Harris, A. (2015). RWM Coupled Processes Project: Third Annual Report for RWM participation in DECOVALEX-2015 Tasks A and C1. AMEC Foster Wheeler report 18040-TR-004 v2.0.||en
|dc.relation.hasversion||Bond, A., Thatcher, K., Chittenden, N., McDermott, C., Fraser-Harris, A and Wilson J. (2015). Final Report of the Coupled Processes Project: Outcomes from DECOVALEX-2015. AMEC Foster Wheeler report 18040-TR-005 v2.0.||en
|dc.rights||Attribution-NonCommercial-ShareAlike 4.0 International||en
|dc.subject||radioactive waste disposal||en
|dc.title||Development of a new non-linear elastic hydro-mechanical model for the simulation of compacted MX-80 bentonite: application to laboratory and in situ sealing experiments for geo-repository engineered barriers||en
|dc.type||Thesis or Dissertation||en
|dc.type.qualificationname||PhD Doctor of Philosophy||en