The stress-strain-permeability behaviour of clay rock during damage and recompaction

Characterisation and understanding of the stressestrainepermeability behaviour of a clay host rock during damage and recompaction are essential for prediction of excavation damaged zone and for assessment of its impact on the repository safety. This important issue has been experimentally studied in triaxial compression tests on the Callovo-Oxfordian clay rock in this study. The samples were sequentially loaded by(1) hydrostatic precompaction to close up sampling-induced microcracks,(2)applying deviatoric stresses to determine damage and permeability changes, and(3) recompression along different loading paths to examine reversibility of the damage. The critical stress conditions at the onset of dilatancy, permeability percolation, failure strength, and residual strength are determined. An empirical model is established for fracturing-induced permeability by considering the effects of connectivity and conductivity of microcracks. The cubic law is validated for the variation of permeability of connected fractures with closure. The experiments and results are also presented and discussed.

Thermo-hydro-mechanical behavior of clay rock for deep geological disposal of high-level radioactive waste

In the context of deep geological disposal of radioactive waste in clay formations, the thermo-hydro- mechanical （THM） behavior of the indurated Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated in our laboratory under repository relevant conditions： （1） rock stress covering the range from the lithostatic state to redistributed levels after excavation; （2） variation of the humidity in the openings due to ventilation as well as hydraulic drained and undrained boundary conditions; （3） gas generation from corrosion of metallic components within repositories; and （4） thermal loading from high-level radioactive waste up to the designed maximum temperature of 90 ~C and even beyond to 150 ~C, Various important aspects concerning the long-term barrier functions of the clay host rocks have been studied： （1） fundamental concept for effective stress in the porous clay-water system; （2） stress- driven deformation and damage as well as resulting permeability changes; （3） moisture influences on mechanical properties; （4） self-sealing of fractures under mechanical load and swelling]slaking of clay minerals upon water uptake; （5） gas migration in fractured and resealed claystones; and （6） thermal impact on the hydro-mechanical behavior and properties, Major findings from the investigations are summarized in this paper,

Thermal effects on clay rocks for deep disposal of high-level radioactive waste

Thermal effects on the Callovo-Oxfordian and Opalinus clay rocks for hosting high-level radioactive waste were comprehensively investigated with laboratory and in situ experiments under repository relevant conditions:(1) stresses covering the range from the initial lithostatic state to redistributed levels after excavation,(2) hydraulic drained and undrained boundaries, and(3) heating from ambient temperature up to 90℃-120℃ and a subsequent cooling phase. The laboratory experiments were performed on normal-sized and large hollow cylindrical samples in various respects of thermal expansion and contraction, thermally-induced pore water pressure, temperature influences on deformation and strength, thermal impacts on swelling, fracture sealing and permeability. The laboratory results obtained from the samples are consistent with the in situ observations during heating experiments in the underground research laboratories at Bure and Mont-Terri. Even though the claystones showed significant responses to thermal loading, no negative effects on their favorable barrier properties were observed.

Examination of effective stress in clay rock

This paper examines the effective stress in indurated clay rock theoretically and experimentally.A stress concept is derived from the analysis of the microstructure and of the pore water in the highly-indurated Callovo-Oxfordian and Opalinus clay rocks,and subsequently validated by various experiments performed on these claystones.The concept suggests that the interparticle or effective stress in a dense clay ewater system is transferred through both the adsorbed interparticle pore water in narrow pores and the solidesolid contact between non-clay mineral grains.The experiments show that the adsorbed pore water in the claystones is capable of bearing deviatoric effective stresses up to the failure strength.The applied stresses are for the most part or even totally transferred by the bound pore water,i.e.the swelling pressure in the interparticle bound pore water is almost equivalent to the effective stress.This stress concept provides a reasonable view to the nature of the effective stress in argillaceous rock and forms the fundamental basis for studies of the hydro-mechanical properties and processes in clay formations.

Clay Mineralogical and Geochemical Studies of Detrital Rocks of the Upper Cretaceous Salt-bearing Strata,Simao Basin,SW China:Implication for Provenance and Source Weathering

1 Introduction Geological studies established on several sections in Lanping-Simao basin have shown that the salt-bearing strata of Mengyejing formation(Yunlong Fm.in Lanping basin)are constituted by an alternation of salt layers and interbedded facies.The latter consists mainly of mudstones,and mudstone-rich conglomerate.The mineralogy and geochemistry of salt-bearing beds and

Sealing of fractures in claystone

The sealing behavior of fractures in clay rocks for deep disposal of radioactive waste has been comprehensively investigated at the GRS laboratory. Various sealing experiments were performed on strongly cracked samples of different sizes from the Callovo-Oxfordian argillite and the Opalinus clay under rel- evant repository conditions. The fractured samples were compacted and flowed through with gas or synthetic pore-water under confining stresses up to 18 MPa and elevated temperatures from 20 ℃ to 90℃. Sealing of fractures was quantified by measurements of their closure and permeability. Under the applied thermo-hydro-mechanical （THM） conditions, significant fracture closure and permeability decrease to very low levels of 10^-19 to 10^-21 m^2 were observed within time periods of months to years. The properties of the resealed claystones are comparable with those of the intact rock mass. All test results suggest high sealing potentials of the studied claystones.

Deformation mechanism of roadways in deep soft rock at Hegang Xing’an Coal Mine

Engineering geomechanics characteristics of roadways in deep soft rock at Hegang Xing'an Coal Mine were studied and the nature of clay minerals of roadway surrounding rock was analyzed. This paper is to solve the technical problems of high stress and the difficulty in supporting the coal mine, and provide a rule for the support design. Results show that mechanical deformation mechanisms of deep soft rock roadway at Xing'an Coal Mine is of ⅠABⅡABCⅢABCD type, consisting of molecular water absorption (the ⅠAB -type), the tectonic stress type + gravity deformation type + hydraulic type (the ⅡABC -type), and the ⅢABCD -type with fault, weak intercalation and bedding formation. According to the compound mechanical deformation mechanisms, the corresponding mechanical control measures and conversion technologies were proposed, and these technologies have been successfully applied in roadway supporting practice in deep soft rock at Xing'an Coal Mine with good effect. Xing'an Coal Mine has the deepest burial depth in China, with its overburden ranging from Mesozoic Jurassic coal-forming to now. The results of the research can be used as guidance in the design of roadway support in soft rock.