Modelling a mine-by test at the Mont Terri rock laboratory, Switzerland

An instrumented mine-by test was conducted at the Mont Terri rock laboratory, Switzerland in 1997–1998 to assess issues associated with tunnel excavation in a middle Jurassic claystone known as the Opalinus Clay. Excavation-induced stresses resulted in unusually large deformations and the development of an excavation disturbed zone (EDZ) around the tunnel where the observed dominant mode of yielding consisted of extensional fracturing. From field and laboratory observations, it has been observed that Opalinus Clay exhibits responses that often are not represented adequately by linear-elastic or elasto-plastic models. In particular, strong non-linear elastic behaviour at low stresses was observed in laboratory tests. This behaviour has been captured by a phenomenological-based model, known as the stress-dependent modulus (SDM) model. The concepts of the SDM model have been extended to a piece-wise pore pressure formulation that captures the hydromechanical rockmass response. These models were implemented into a finite difference method numerical code and used to simulate the short-term deformations and pore pressure response of the ED-B mine-by test. These simply calibrated models provided a reasonable fit to the field data, particularly in regions of unloading where rockmass deformations were not dominated by dilation.     

Experiments on thermo-hydro-mechanical behaviour of Opalinus Clay at Mont Terri rock laboratory,Switzerland

Repositories for deep geological disposal of radioactive waste rely on multi-barrier systems to isolate waste from the biosphere.A multi-barrier system typically comprises the natural geological barrier provided by the repository host rock e in our case the Opalinus Clay e and an engineered barrier system(EBS).The Swiss repository concept for spent fuel and vitrified high-level waste(HLW)consists of waste canisters,which are emplaced horizontally in the middle of an emplacement gallery and are separated from the gallery wall by granular backfill material(GBM).We describe here a selection of five in-situ experiments where characteristic hydro-mechanical(HM)and thermo-hydro-mechanical(THM)processes have been observed.The first example is a coupled HM and mine-by test where the evolution of the excavation damaged zone(EDZ)was monitored around a gallery in the Opalinus Clay(ED-B experiment).Measurements of pore-water pressures and convergences due to stress redistribution during excavation highlighted the HM behaviour.The same measurements were subsequently carried out in a heater test(HE-D)where we were able to characterise the Opalinus Clay in terms of its THM behaviour.These yielded detailed data to better understand the THM behaviours of the granular backfill and the natural host rock.For a presentation of the Swiss concept for HLW storage,we designed three demonstration experiments that were subsequently implemented in the Mont Terri rock laboratory:(1)the engineered barrier(EB)experiment,(2)the in-situ heater test on key-THM processes and parameters(HE-E)experiment,and(3)the full-scale emplacement(FE)experiment.The first demonstration experiment has been dismantled,but the last two ones are on-going.     

Safety-relevant hydrogeological properties of the claystone barrier of a Swiss radioactive waste repository: An evaluation using multiple lines of evidence

In Switzerland, the Opalinus Clay – a Jurassic (Aalenian) claystone formation – has been proposed as the first-priority host rock for a deep geological repository for both low- and intermediate-level and high-level radioactive wastes. An extensive site and host rock investigation programme has been carried out during the past 30 years in Northern Switzerland, comprising extensive 2D and 3D seismic surveys, a series of deep boreholes within and around potential geological siting regions, experiments in the international Mont Terri Rock Laboratory, compilations of data from Opalinus Clay in railway and motorway tunnels and comparisons with similar rocks.The hydrogeological properties of the Opalinus Clay that are relevant from the viewpoint of long-term safety are described and illustrated. The main conclusions are supported by multiple lines of evidence, demonstrating consistency of conclusions based on hydraulic properties, porewater chemistry, distribution of natural tracers across the Opalinus Clay as well as small- and large-scale diffusion models and the derived conceptual understanding of solute transport.     

Influence of tectonic shears on tunnel-induced fracturing

The Opalinus Clay is currently under investigation as a potential host rock for deep geological disposal of nuclear waste at the Mont Terri Rock Laboratory in Switzerland. Bedding in the Opalinus Clay at Mont Terri is ubiquitous and highly persistent leading to mechanical transverse isotropy. Adding to the complexity at the Rock Laboratory is the frequent occurrence of small-scale tectonic shears.This paper explores the influences of millimetre-thick tectonic shears and bedding on the development of induced fractures mapped in the EZ-B field experiment at the research facility. A series of numerical analyses were carried out by increasing the geological complexity of the host rock and comparing the redistributed stress field with geological maps of the induced fractures. The analyses show that if tectonic shears are not kinematically constrained, mobilisation of the shears can play a key role in the development of the induced fracture network and therefore, be a primary factor in the development of the excavation damaged zone. This illustrates that under certain conditions rock mass heterogeneity (in this case, resulting from the tectonic shears) may dominate over rock matrix anisotropy (in this case, resulting from bedding) and must be considered when predicting the induced fracture network of the excavation damaged zone.     

The excavation of a circular tunnel in a bedded argillaceous rock (Opalinus Clay): Short-term rock mass response and FDEM numerical analysis

The Opalinus Clay formation is currently being investigated as a potential host rock for the deep geological disposal of radioactive waste in Switzerland. Recently, a test tunnel was excavated at the Mont Terri underground rock laboratory (URL) as part of a long-term research project (“Full-scale Emplacement (FE) experiment”) aimed at studying the thermo-hydro-mechanical (THM) effects induced by the presence of an underground repository. The objective of this paper is twofold. Firstly, the results of the rock mass monitoring programme carried out during the construction of the 3 m diameter, 50 m long FE tunnel are presented, with particular focus on the short-term deformation response. The deformation measurements, including geodetic monitoring of tunnel wall displacements, radial extensometers and longitudinal inclinometers, indicate a strong directionality in the excavation response. Secondly, the deformational behaviour observed in the field is analyzed using a hybrid finite-discrete element (FDEM) analysis to obtain further insights into the formation of the excavation damaged zone (EDZ). The FDEM simulation using the Y-Geo code is calibrated based on the average short-term response observed in the field. Deformation and strength anisotropy are captured using a transversely isotropic, linear elastic constitutive law and cohesive elements with orientation-dependent strength parameters. Overall, a good agreement is obtained between convergences measured in the field and numerical results. The simulated EDZ formation process highlights the importance of bedding planes in controlling the failure mechanisms around the underground opening. Specifically, failure initiates due to shearing of bedding planes critically oriented with respect to the compressive circumferential stress induced around the tunnel. Slippage-induced rock mass deconfinement then promotes extensional fracturing in the direction perpendicular to the bedding orientation. The simulated fracture pattern is consistent with previous experimental evidence from the Mont Terri URL.     

Measurement of in-situ stress in weak rocks at Mont Terri Rock Laboratory, Switzerland

In-situ stress measurements in weak rocks, such as clay shales, that respond strongly to environmental changes are particularly difficult. An extensive in-situ stress measurement program has been conducted at the Mont Terri Rock Laboratory in Northwestern Switzerland. Hydraulic fracturing, Undercoring around a 600-mm-diameter borehole using CSIRO triaxial strain cells and the Borehole Slotter were used to establish the state of stress for the Opalinus Clay at this location. Conflicting orientations and magnitudes resulted from the measurement programs. Three dimensional elastic modelling was used in conjunction with tunnel and borehole observations to establish the most likely stress tensor. A stress measurement program using a borehole deformation gauge is currently underway to check the stress tensor resulting from the observational modelling.     

OPALINUS硬泥岩热反应的监控与模拟

目前，一些国家选择硬泥岩作为核废物地质深部处置的围岩。为深入理解硬泥岩在热-水-力（THM）耦合作用下的变化特征，在瑞士的Mont Terri地下岩石实验室对Opalinus硬泥岩进行了现场加热试验。该试验是利用-电加热器（直径=30cm，长度=5．4m）对硬泥岩进行了1a多的加热，然后降温0．5a。加热器表面的最高温度达100℃。在试验过程中，安装了80多个测试传感器对硬泥岩中的温度、孔隙水压、气体渗透以及变形进行了监控。通过观察发现围岩受热膨胀且产生明显的孔隙水压升高。距加热器约1m的区域，其孔隙水压从约1MPa增高到4MPa。最后对试验中观测的硬泥岩受热所产生的热-水-力全过程进行了模拟计算，并介绍了现场测量和数值计算的初步结果。     

A thermo-hydro-mechano-chemical formulation for modeling water transport around a ventilated tunnel in an argillaceous rock

Argillaceous rocks are being considered as potential host rocks for deep geological disposal. For the research work in DECOVALEX-2011, 5 participant research teams performed simulations of a laboratory drying test and a ventilation experiment for Mont Terri underground laboratory built in argillaceous rock formation. Our study starts with establishing a coupled thermo-hydro-mechano-chemical (THMC) processes model to simulate water transport in rock around the ventilated tunnel. Especially in this THMC formulation, a three-phase and two-constituent hydraulic model is introduced to simulate the processes which occur during tunnel ventilation, including desaturation/resaturation in the rock, phase change and air/rock interface, and to explore the Opalinus clay parameter set. It can be found that water content evolution is very sensitive to intrinsic permeability, relative permeability and capillary pressure in clay rock. Water loss from surrounding rock is sensitive to the change of permeability in clay which is induced by excavation damaged zone. Chemical solute transport in the rock near ventilation experiment tunnel is simulated based on the coupled THMC formulation. It can be estimated that chemical osmotic flow has little significance on water flow modeling. Comparisons between simulation results from 5 teams and experimental observations show good agreement. It increases the confidence in modeling and indicates that it is a good start for fully THMC understanding of the moisture transportation and mechanical behavior in argillaceous rock.     

Analysis of hydro-mechanical processes in a ventilated tunnel in an argillaceous rock on the basis of different modelling approaches

In this paper, a modelling benchmark exercise from the DECOVALEX-2011 project is presented. The bench-mark is based on the performance and results of a laboratory drying test and of the ventilation experiment (VE) carried out in the Mont Terri Underground Rock Laboratory (URL). Both tests involve Opalinus clay. The work aims at the identification, understanding and quantification of mechanisms taking place during the ventilation of a gallery in argillaceous host rocks on one hand and at investigating the capacity of different codes and individuals to reproduce these processes on the other hand. The 4-year in situ VE took place in a 1.3 m diameter unlined tunnel and included two resaturation-desaturation cycles. The test area was equipped with over one hundred sensors (including the global water mass balance of the system, relative humidity (RH), water content, liquid pressure, relative displacement and concentration of some chemical species) to monitor the rock behaviour during ventilation. The laboratory drying experiment, carried out before the VE, was designed to mimic the in situ conditions. The work was organized in a progressive manner in terms of complexity of the computations to be performed, geared towards the full hydro-mechano-chemical (HMC) understanding of the VE, the final objective. The main results from the modelling work reported herein are that the response of the host rock to ventilation in argillaceous rocks is mainly governed by hydraulic processes (advective Darcy flow and non-advective vapour diffusion) and that the hydro-mechanical (TM) back coupling is weak. A ventilation experiment may thus be regarded as a large scale-long time pump test and it is used to determine the hydraulic conductivity of the rock mass.     

Anisotropic modelling of Opalinus Clay behaviour:From triaxial tests to gallery excavation application

Deep repository in geological formations is the preferential solution considered in many countries to manage high-level nuclear wastes. In Switzerland, the Opalinus Clay is a candidate host rock. In this context, in situ and laboratory tests are conducted on Opalinus Clay to demonstrate the feasibility of deep disposal in this argillaceous formation. This paper presents a constitutive model able to fit the experimental data obtained from some triaxial tests conducted by Jahns(2013) on cores from borehole Schlattingen SLA-1. The elasto-plastic behaviour of Opalinus Clay is reproduced thanks to a DruckerPrager model, taking into account the anisotropy behaviour of this sedimentary rock. The objective is to employ a single set of parameters representative of the material. In a second version of the model, the stress-dependence of the elastic properties and damage are taken into account. Finally, the parameters calibrated with experimental tests are used to simulate the excavation of a gallery with a second gradient approach.     

Mechanical behavior of sandy facies of Opalinus Clay under different loadconditions

The mechanical behavior of sandy facies of Opalinus Clay at the Mont Terri underground rock laboratory(URL)in Switzerland was investigated with drained and undrained triaxial compression and extension,cyclic compression,and creep tests.Samples were taken from boreholes drilled parallel to bedding.Most of the samples were reconditioned to minimize sampling effects of desaturation and micro-cracking.The compression was accomplished by increasing axial stress at constant radial stress.The extension was carried out by increasing radial stress at constant axial stress.Moreover,extension was also achieved by simultaneously increasing radial stress and decreasing axial stress under constant mean stress.The test results showed elastoplastic stress-strain behavior with volumetric compaction until onset of dilatancy at high deviatoric stresses above 80%-90%of the peak failure strength.The strength is dependent upon load path and mean stress.The strength under triaxial compression is higher than that under extension.The respective strength increases with increasing mean stress.Desaturation enhances the stiffness and strength of the claystone.The deformation and strength of the elaystone are time-dependent.Under constant deviatoric stress,the claystone crept continuously with time,which can be characterized by a transient phase and a following stationary phase,and even a tertiary phase at high deviatoric stresses to rupture.     

Mont Terri试验场Mine-by巷道开挖近场响应模拟分析

 瑞士Mont Terri地下实验室于2008年构建Mine-by(MB) Niche巷道,并围绕其中的MB Niche 2进行开挖诱发Opalinus Clay黏土岩围岩变形及流固耦合特征的现场实测分析。以MB Niche 2巷道为背景工程,利用基于双应变胡克模型(TPHM)的数值模拟方法,对MB Niche 2巷道的开挖过程进行模拟分析,计算结果较为准确地反映开挖卸载诱发的近场围岩变形行为、损伤区及孔隙水压力演化特征。模拟结果显示,巷道拱顶径向位移可达甚至超过10 mm,两帮破坏深度可达1.4 m,孔隙压力的扰动区最远可出现在掌子面前方11 m左右,最大孔隙压力可抬升至3.3 MPa,模拟结果与现场实测值表现出较好的一致性。基于双应变胡克模型TPHM的计算结果较基于传统胡克模型SPHM的计算结果更趋于符合实际,这在一定程度上反映出TPHM较好地捕捉到岩石中孔隙、裂隙等的自然应变(真应变)的物理本质,强调合适的本构模型在卸荷开挖工程问题模拟分析中的重要性。     

核废料地质贮存介质黏土岩的三维各向异性热-水-力耦合数值模拟

为了解黏土岩在放射性废料长期贮存中的热-水-力耦合过程,结合MontTerri核废料贮存地下岩石试验工程中黏土岩各种物理量的各向异性特点,应用多孔介质力学耦合理论研究了该黏土岩在加热和冷却全过程中由于热荷载引起的耦合效应场。研究过程考虑温度升高引起的孔隙水黏滞性改变对渗透系数的影响。研究结果表明,岩体力学参数、水力学参数和热传导参数的各向异性特性是影响岩体的温度场、孔隙压力场和应力场分布的最主要因素。各向异性耦合模型与各向同性耦合模型的数值模拟对比研究结果表明：各向异性模型数值结果能更加客观地反映该地下岩石试验工程中黏土岩在受热状态下的热-水-力耦合效应;同时,也表明岩体在加热过程中一直处于受压状态,而在冷却过程中局部会出现拉应力,从而有可能导致拉裂缝的产生。     

Discontinuity networks in mudstones: a geological approach

Although usually masked by a veneer of weathered products, a discontinuity network has been observed in mudstones dated from the Lower Cambrian to the Miocene, principally in pits and quarries. Its origin is attributed to an early hydraulic fracturing of the initial muds with their drainage and progressive compaction. In contrast no natural discontinuities have been observed in the galleries of underground nuclear waste laboratories in Boom Clays at Mol, Belgium, and in Opalinus Clays at Mont Terri, Switzerland, or in deep reconnaissance borings. However, the observations were made immediately after the works. At depth, natural discontinuities are closed by the stress field. As there is no cement and with the temperatures of diagenesis being too low to allow any re-crystallization, they are sealed but not healed. They could reopen with the release of stress and/or desiccation.      

Back analysis of time-dependent behaviour of a test gallery in claystone

This paper presents a back analysis of the time-dependent behaviour of a monitored tunnel in Switzerland. It concerns a specific section of the Mont Terri reconnaissance gallery which was excavated in creeping rock. From a theoretical point of view, Lemaitre's viscoplastic model was implemented in the FLAC 2D finite-difference numerical code. Particular attention was paid to the stability of the numerical solution for such an explicit mode of resolution. Lemaitre model parameters were successfully fitted on creep tests and then on in situ convergence measurements after a complete numerical simulation. Based on the comparison of strain and stress measured in the lining with the calculated values, time-dependent behaviour can finally be extrapolated over a period of approximately 10 years. Scale effects between results of in situ and laboratory fittings are also discussed. The aim of these investigations is to justify the interest of Lemaitre's model for the analysis of time-dependent behaviour of underground structures over a long period of time.     

Investigation of damage-induced permeability of Opalinus clay

An excavation damaged zone (EDZ) around emplacement boreholes for radioactive waste represents a potential pathway for radionuclides due to its increased porosity and crack permeability. As clay is one of the potential host rocks for radioactive waste disposal, Opalinus clay samples from the underground rock laboratory at Mont Terri were investigated regarding their hydraulic properties – and related crack occurrence – after excavation and during stress-dependent crack closure. After determination of their hydraulic properties in untreated conditions, the samples were artificially cracked by tensile strength tests. The cracked samples were put into a triaxial pressure cell and the permeability and effective porosity were measured during stepwise increase and decrease of confining pressure. When the pressure was increased, a continuous decrease of permeability was found, which was similar for all test samples, and a mathematical expression was identified. When the pressure was decreased, no increase of permeability was observed until the samples were completely depressurized, leading to the assumption that during pressurization some kind of sealing process took place resulting in a permanent crack closure. In addition to the dependence on pressure, a time-dependent permeability reduction and thus crack closure at constant pressure was found, indicating a creep compaction behaviour of the clay. By knowing the initial permeability immediately after excavation of an emplacement borehole, the permeability reduction due to time-dependent stress variation can be calculated for use in long-term safety analyses.     

Study of the micro-mechanical behaviour of the Opalinus Clay: an example of co-operation across the ground engineering disciplines

The high degree of scientific cross-fertilisation possible between the three geo-engineering disciplines soil mechanics, rock mechanics and engineering geology, is demonstrated by means of a micro-mechanical model of the Opalinus Clay. After a brief review of Terzaghi’s effective stress principle and the importance of micro-mechanical models in general, a conceptual study of a micro-mechanical model of a claystone is presented in some detail. The model is based on the Particle Flow Code (PFC) developed by Itasca Corp. It introduces into the model the pertinent composition and structure of the Opalinus Claystone established in the local engineering geology of Switzerland and SW Germany. This includes elongated clay platelets, various layers of densified water around the platelets, free water in the pores and a specific texture of the platelets after consolidation. The model is numerically subjected to a series of loading stages. It is shown that the micro-mechanical model reproduces a number of features which have been known for a long time in soil and rock mechanics but which are often intractable in conventional generic models. The features include non-linear stress–strain curves with pre-failure damage and post-failure strain softening, a non-linear increase of the particle contacts with loading, distinct clustering of deformations, clustering of micro cracks leading to the development of shear bands and hysteresis in cyclic loading. It is concluded that micro-mechanical models are promising tools for further development of our understanding of the mechanical behaviour of geological materials. They offer an excellent opportunity for scientific co-operation between engineering geologists and soil and rock mechanics engineers.     

Three-dimensional FDEM numerical simulation of failure processes observed in Opalinus Clay laboratory samples

This study presents the first step of a research project that aims at using a three-dimensional(3D) hybrid finite-discrete element method(FDEM) to investigate the development of an excavation damaged zone(EDZ) around tunnels in a clay shale formation known as Opalinus Clay. The 3D FDEM was first calibrated against standard laboratory experiments, including Brazilian disc test and uniaxial compression test. The effect of increasing confining pressure on the mechanical response and fracture propagation of the rock was quantified under triaxial compression tests. Polyaxial(or true triaxial) simulations highlighted the effect of the intermediate principal stress(s2) on fracture directions in the model: as the intermediate principal stress increased, fractures tended to align in the direction parallel to the plane defined by the major and intermediate principal stresses. The peak strength was also shown to vary with changing σ2.     

三轴卸荷条件下大理岩扩容与能量特征分析

扩容现象是岩石变形破坏过程中的重要特征。基于MTS815 Flex Test GT岩石力学试验平台,采用室内三轴卸荷试验和塑性力学理论分析,揭示了大理岩在卸荷条件下的扩容特征及能量变化特征。结果表明,随着围压的增大,岩样的各特征应力随之增大,其扩容特征随之减弱;岩样的扩容参数——扩容指标以及剪胀角均具有围压效应,即扩容指标与围压呈良好的指数型分布,剪胀角与应力比呈线性分布;岩样的卸荷破坏过程中能量特征为初始时以可释放应变能为主到破坏时的耗散能为主,其间的转折点为初始损伤扩容点,同时卸荷条件下的特征能量值与围压具有良好的指数类型关系;在峰值点与残余点处,岩样的能量损伤值与剪胀角以及能量特征值与扩容指标均存在着较好的指数类型关系。     

Application of a local degradation model to the analysis of brittle fracture of laboratory scale rock specimens under triaxial conditions

Development of brittle fracture and the associated macroscopic behaviour of rock specimens in laboratory tests are simulated using a local degradation model for brittle fracture in heterogeneous rocks. We examine this subject because laboratory uniaxial and triaxial tests are widely used in the rock mechanics community to both characterise rock behaviour and to interpret fracture phenomena observed in natural rock, such as the Earth's crust, and developed around rock engineering structures. In addition, numerous historical efforts related to the detailed study of this subject have made available a great deal of information for use both as model input data and comparison results.A series of numerical experiments have been performed to investigate the influence of a number of parameters on rock fracture. In particular, rock fracture under various confining stresses has been explored. The results show that the degradation algorithm is capable of reproducing many characteristics associated with brittle fracture in heterogeneous rocks, including: the development of fracture from the elemental scale to the macroscopic scale; fracture pattern as a function of confining pressure; variation of fracture plane angle with respect to confining pressure; the complete stress–strain curve and corresponding strain energy dissipation characteristics; dependence of the stress–strain curve on confining pressure; and loading–unloading hysteresis loops.Independent investigations into the effect on rock fracture of (i) the degradation parameter embodied with the model and (ii) the Weibull shape parameter used to introduce heterogeneity distribution are described. The results indicate that the degradation parameter controls the degree of degradation relative to confining pressure. As this parameter increases, and the elemental degradation decreases, the number of failed sites generated prior to the formation of macroscopic fracture plane increases, and both the peak and ultimate strengths of the model increase.The Weibull parameter influences the formation of the final fracture plane. As this parameter increases, reducing the heterogeneity, the number of diffused failed sites and the angle of the eventual fracture plane to the major principal stress tends to decrease, and the brittleness of the resulting stress–strain curve increases. It is suggested that values in the range 2–4 are appropriate for this parameter in representing elemental strength distribution of rock materials.