Hydromechanical Modelling Of The Sealing System For Geological Repository

The sealing system is a crucial component of the high-level radiativ waste geological repository, the issues on the hydrmechanical(HM) processes of a sealing system is the key aspect among the studying community. In this paper the evolution equations of effective porosity and permeability were built on the basis of effective porosity, with respect to descripting the change of porous structure,exhibiting in porous tunnel open or closure, and the permeability of porous media physically. The implicit integration expressions of Barcelona Basic Model(BBM) were also built by approach of return mapping integration for a time-step request and capacity of OpenGeoSys for descriping the expansive performance of bentonite/sand mixture. The numerical modelling on the hydration and compression processes of MX80 bentonite/sand mixture were conducted, and the comparative analysis of the numerical results versus experimental data show that, the evolution equations of effective porosity and permeability and implicit integration of BBM are feasible in numerical modelling. The numerical simulation of excavation in confining rock gave the approach to analyzing the effect of stress evolution on hydration properties. In order to simpfy the numerical model of confining rock in a sealing system, a simpfying approach was proposed by calibrating the intrinsic permeability. Finally, the hydromechanical modelling of a sealing system was implemented with parameters and model in previous modelling, the analysis of modelling results releasized the interaction of stress and hydration in the sealing processes.The studying aspects and obtained results are listed as follow:(1) In the light of internal porosity and effective porosity, an evolution equation for describing the effective porosity with a consideration of effective stress or net strain was developed on the basis of true strain and the damage model proposed by Xie[1-3]. Then the potential equations, Kozeny-Carman equation and a power function which are associated with effective porosity, were discussed with referenced experimental data to discuss the feaaiblity of permeability evolution equations.The comparative results of effective porosity denote that the effective porosity evolution has good ability in explaining the impact of deformation on effective porosity, while the equation has limit for anistropic porous media. The effective porous evaluation equation is convenient for engineering application, and it is reasonable for explaining the change of effective porosity due to deformation. The comparative results of permeability indicate that the power evaluation of permeability has a relative wide range of application, and it is reasonable for explaining the change of hydraulic performance due to porous structural change.(2) Establishing the implicit integration expressions of elasto-plastic model for unsaturated soil. The common elasto-plastic model for expansive clay were analyzed comparatively, and a famous model, BBM was selected for describing the elasto-plastic behavior of bentonite/sand mixture. Applying the return mapping and associate flow rule, the expressions of numerical integration of BBM were finally deduced as implicit formal.(3) Numerical modelling of the excavation and hydration processes in confining rock. Excavation of near-field confining rock, in fact, is a disturbing process in mechanical and hydraulic field. Therefore, the change in stress and hydraulic performance of near-field confining rock should took into consideration into hydromechanical modelling of a sealing system for geological repository. The excavation of tunnel and borehole in confining rock were numerical studied firstly for evaluating the impacted area of stress change. Then employing effective proposity and permeability evaluation equations into the modelling of hydration in excavated borehole. In order to save the compututational time and simplifying the model of a sealing system, a equavelent model was constructed by applying a equavelent intrinsic permeability.Results of modelling in tunnel excavation in confining rock show that, stress change around the area 2 m away from the tunnel is very limited. The effective porosity and permeability evaluation equations were applied in modelling the hydraulic performance of near-field around borehole. Numerical results show that the incrising strain increase the permeability of the area around the end of the borehole, while decrease the jointing area of the end and side surface of borehole. The water mass in modelling fit well with the data of experiment, which indicate that the equivalent model and permeability is feasible.(4) Numerical study on the hydromechanical processes of bentonite/sand mixture. Employing the implicit numerical integration based on return mapping algorithm, at the same time, effective porosity and powerful permeability evolution equations, the process of hydromechanical processes of bentonite/sand mixture were studied from the SEALEX experiment, in which the study on water absorption aims at the data of water retention curve and corresponding physical and numerical parameters such as initial porosity, density and suction, infiltration of water took place for indicating the feasibility of the effective porosity and permeability evolution equations. The modelling study on comparsion of bentonite/sand mixture aims at proofing the return mapping integration expressions. At last, by employing the parameters of infiltration and compression study of small-scale sealing system into the numerical study of hydromechanical laborabitory test, the hydromechanical processed of bentonite/sand mixture performed in the sealing system was numerical studied.Numerical simulation results of bentonite/sand mixture in infiltration process, compression process, and small-scale laboriatory sealing processed show that the effective porosity and permeability evolution equations reflect impact of microscopic structure change on hydromechanical processes. Barcelona basic model based on regression mapping implicit numerical integration algorithm has good ability in description the elasto-plastic behavior of bentonite/sand mixture. The contrast analysis of experimental and numerical results shows that employed controlling equations and parameters in the numerical simulation are reasonable.(5) The modelling of a in situ sealing system was conducted for studying its hydromechanical coupling processes. The numerical study of geological repository cannot only depend on the laboritory experiments, which exist certain scale effect and some other factors which cannot be considered form the practical engineering. Therefore, modelling on a sealing system of geological repository aims at the applicability of the theoretical model adopted in the sealing system, equalvent numerical model and numerical parameters. In considering the change of the hydraulic field in surrounding rock after excavation, using the calibrated permeability of surrounding rock and the numerical simulation of axisymmetric model gives a equalvent approach to simplifying the model and saving numerical time.Comparative analysis of the results of modelling and experimental results shows that the numerical results can explain the mechanism of hydromechanical coupling process in a sealing system. But some complex factors did not considered in the modelling work, such as the contacting process and potential crack between sample bulk, and the processes of water dehydration due to comparsion of bentonite/sand mixture, some difference exists between the numerical model and experiment. Even through, the numerical results relegalized that the technological void descrease the stress level of a sealing system, the compressive stress decrease the hydraulic ability of the central area of bentonite/sand mixture bulk in the sealing system because of structural change, the difference between the pore suction confining rock and bentonite/sand mixture has certain impact on hydromechanical processes as well.