“Transverse Action” – A model benchmark exercise for numerical analysis of the Callovo-Oxfordian claystone hydromechanical response to excavation operations

The Callovo-Oxfordian claystone (COx) is considered as a potential geological formation to host an industrial radioactive waste repository in France. A detailed understanding of the thermo-hydro-mechanical (THM) behavior of the COx is a key issue for design of different repository structures and the safety calculations of the project. More particularly, numerical modeling of induced fracture networks around drifts excavated at the main level of the Andra’s Meuse/Haut-Marne Underground Research Laboratory (M/HM URL) and short and long term behavior of the COx around these drifts are of great interest. Several constitutive models have been developed/used in the framework of the R&D and simulation programs of Andra. A model benchmark exercise has been launched since 2012 to provide an overall view of the developed models regarding the in situ observations. In this view, two series of test cases, respectively at material point scale and at drift excavation scale are defined. Different kinds of constitutive models based on the elasto-visco-plasticity concept, continuum damage mechanics, the rigid block spring method and two-scale computational homogenized model (CHM) are used within this exercise. The obtained results show that accounting for material anisotropy and strain localization treatment techniques can improve the obtained results when elasto-visco-plastic models are used. Damage mechanics based approaches and methods accounting for discontinuities through discrete elements provide also interesting insights especially when fracturing processes must be modeled. However, more efforts are necessary to improve the robustness of these kinds of approaches in the complex context of COx response to excavation works.     

A micromechanical-based elasto-viscoplastic model for the Callovo-Oxfordian argillite: Algorithms,validations, and applications

For a potential geological barrier of high-level radioactive waste repositories in France, the long-term mechanical behavior of the Callovo-Oxfordian (COx) argillite is the most concern for engineers. In this paper, a micromechanical-based elasto-viscoplastic model is proposed, and its numerical realization is our main object. The COx argillite is considered as a three-phase composite consists of porous clay, quartz, and calcite. By assigning appropriate constitutive laws to those constituents, the macroscopic elasto-viscoplastic behavior of the COx argillite is determined with an extended Hill's incremental approach. The numerical aspects includes (a) a new formulation is proposed for the plastic multiplier when adopting the overstress (Perzyna) model to define the viscoplastic strain. Meanwhile, a new formulation is also proposed to solve it within the framework of an implicit returning mapping scheme. (b) The corresponding consistent tangent moduli are strictly derived by extending the method proposed for solving plastic problems; (c) the efficiency of the proposed integration algorithms for the local constitutive equations and the homogenization procedure are validated, receptively, by a built-in porous plasticity model of a commercial finite element (FE) program ABAQUS and by FE computations of a two-phase unit cell; and (d) the proposed micromechanical model is finally applied to simulate experiment data in short-term triaxial compression tests and long-term triaxial creep tests. And the numerical results show that it is able to reflect the variation of the mechanical behavior with respect to the varied mineralogical compositions.     

Numerical modelling of in situ behaviour of the Callovo–Oxfordian argillite subjected to the thermal loading

To enhance the understanding of thermal impact on the in situ behaviour of the Callovo–Oxfordian argillite, this paper presents an interpretation of an in situ heating experiment carried out in the Meuse/Haute Marne Underground Research Laboratory (M/HM URL). The argillite was heated successively by two constant heating powers: 277 W and 925 W. When subjected to thermal loading, the argillite exhibits an important volume change and a strong pore pressure response that significantly affect its hydraulic and mechanical behaviour. Numerical analysis has been performed by using a coupled theoretical formulation that incorporates a constitutive model especially developed for this material. Based on Biot's theory, this model includes the influence of interstitial pressure on the mechanical behaviour. The simulation obtained reproduces satisfactorily the results of the in situ experiment and the main observed patterns of behaviour. The interpretation and discussion of numerical results provide additional data that can help us to understand the thermo-hydromechanical behaviour concepts of saturated argillite formation.     

An insight into grain interaction in bentonite/claystone mixtures

Acta Geotechnica - This paper investigates the swelling mechanism of Callovo-Oxfordian (COx) claystone grain in its mixtures with bentonite which are potential candidate for engineered barriers in...     

Study of the Anisotropic Properties of Argillite Under Moisture and Mechanical Loads

Due to various factors, such as sedimentation, layered morphology of clay minerals, in situ stress, etc., argillite rocks often exhibit anisotropic behavior. In order to study the anisotropic properties of the Callovo-Oxfordian (COx) argillite of the Meuse–Haute-Marne site in France considered as a possible host rock for high-level radioactive nuclear waste repository, a series of tests including uniaxial compression and dehydration and hydration at different constant applied stress levels are carried out. In this study, a specific setup combining moisture and mechanical loading with optical observation is used and it allows to continuously capture surface images from which the full-field strains are determined by using Digital Image Correlation techniques. The results show evidence of the mechanical and hydric anisotropy of the material. The anisotropy parameters are identified, assuming the studied argillite as transversely isotropic. The shrinkage and swelling depend on the applied stress and the angle with respect to the vertical direction of the mechanical load and the stratification plane, and this dependence is quantified. The non-linearity and the hysteresis observed during dehydration and hydration cycles are discussed.     

Analysis of Long-Term Anisotropic Convergence in Drifts Excavated in Callovo-Oxfordian Claystone

The main purpose of this paper is to analyze the convergence measurements in drifts of the Underground Research Laboratory (URL) of the French National Radioactive Waste Management Agency (Andra), excavated in Callovo-Oxfordian claystone. These measurements show an anisotropic closure, which depends on the drift orientations with respect to the horizontal in situ stresses. This anisotropic character of the deformation is taken into account by assuming that the drifts section evolves following an elliptical shape. The characteristics of the deformed elliptical section are evaluated following the methodology proposed by Vu et al. (Rock Mech Rock Eng 46:231–246, 2013). Then, using the semi-empirical law proposed by Sulem et al. (Int J Rock Mech Min Sci Geomech Abstr 24:145–154, 1987), the convergence evolution is fitted independently for each axis of the ellipse. This method allows to distinguish two effects: the face advance and the time-dependent behavior of the ground. The results for the two drift orientations (along the major horizontal stress and perpendicular to it) show very close values for the parameters describing the time-dependent properties of the ground, the distance of influence of the face, and the extent of the decompressed zone around the drift. Finally, the model is validated by keeping these parameters as constants and by simulating the convergence data on a new drift. It is shown that with a period of about 40 days of convergence monitoring, the model can provide valuable insights for predictions of the convergence evolution in the long term.     

热−水−力耦合条件下黏土岩蠕变特性研究

黏土岩作为放射性核废料处置的备选介质,长期处于热-水-力耦合的复杂条件下。为研究围岩的长期稳定性,开展了一系列不同围压和偏应力下的黏土岩加温-降温排水蠕变试验。得出以下结论：温度的升高会提升黏土岩的蠕变速率,延长衰减蠕变阶段的时间,但是降温过程中试样主要以冷缩变形为主,并未见明显的蠕变变形;围压的降低以及偏应力的增加会提高黏土岩的蠕变速率,并且这种影响会随温度的升高明显加剧。基于试验结果,在Perzyna过应力理论的基础上引入蠕变硬化、蠕变损伤以及热损伤,建立了黏土岩热-水-力耦合蠕变模型。通过ABAQUS软件及其子程序对该模型进行数值实现,模拟结果与试验结果的对比初步表明该模型可以很好地描述热-水-力耦合条件下黏土岩的蠕变特性。     

Characters of Nummulites multiplicutiospiralis n. sp. from theQarara Formation,Maghagha, Nile Valley,Egypt

Nummulites multiplicutiospiralis n.sp.is described and illustrated for both generations from a claystone of Lutetian age in the Middle Eocene Qarara Formation, Maghagha,Nile valley,Egypt.Previously considered to be Nummulites gizehensis,we describe this form as a new species based on diameter of the test and protoconch sizes,septal shape and granulations.The primary feature of this species is spiral multiplication,seen in spiral section.The species is also characterized morphologically by lenticular to flat tests with rounded periphery,wrinkled to broadly meandering septal filaments and few granules on the periphery in microspheric forms.The granules are spirally arranged on the septal filaments in megalospheric forms.Chambers are longer than high and the spire is irregular.The new taxon belongs to the Nummulites partschi Group,Subgroup N.gizehensis.Reflexive coiling in the spiral may develop through ontogeny as in N.rollandi,the species which was considered to be an index species in the southern Tethys (Boukhary et al.,1982 and Boukhary et al.2010).The characters of the new species,as well as its retrogenesis and reflexive coiling,should be further studied in general in Nummulites spp.,to understand the importance of such characters phylogenetically.     

Arching within hydraulic fill stopes

Arching is a well known phenomenon, which effects stress developments which were investigated and compared using analytical and numerical solutions. Marston’s (1930) solution was extended to a generalised 3-dimensional rectangular stope and later modified for square and circular stopes for comparison with FLAC results. Aubertin et al. (2003) & Li et al. (2003) models were improved significantly by placing the backfill within narrow stopes as lifts or layers in numerical modelling where the normal stress variation with depth were found to be more realistic. The FLAC results were compared with analytical solutions which were developed by previous researchers and modified by the authors to evaluate the arching effects in backfilled placed in narrow and circular stopes. It appeared from the investigation herein that δ = 0.67 ϕ and K = K _o condition gives a very close match with the numerical model solutions obtained from FLAC. Many laboratory tests were conducted to find out friction angles for four Australian mines, which were between 30 and 49 degrees.     

Gas flow in anisotropic claystone: modelling triaxial experiments

Selected gas pulse tests on initially saturated claystone samples under isotropic confinement pressure are simulated using a 3D thermo‐hydro‐mechanical code. The constitutive model considers the hydro‐mechanical anisotropy of argillaceous rocks. A cross‐anisotropic linear elastic law is adopted for the mechanical behaviour. Elements for a proper modelling of gas flow along preferential paths include an embedded fracture permeability model. Rock permeability and its retention curve depend on strains through a fracture aperture. The hydraulic and mechanical behaviours have a common anisotropic structure. Small‐scale heterogeneity is considered to enhance the initiation of flow through preferential paths, following the direction of the bedding planes. The numerical simulations were performed considering two different bedding orientations, parallel and normal to the imposed flow in the test. Simulations are in agreement with recorded upstream and downstream pressures in the tests. The evolution of fluid pressures, degree of saturation, element permeability and stress paths are presented for each case analysed. This information provides a good insight into the mechanisms of gas transport. Different flow patterns are obtained depending on bedding orientation, and the results provide an explanation for the results obtained in the tests. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of mechanical behavior and strain localization of methane hydrate bearing sediments with different saturations by a new DEM model

This paper presents a numerical investigation into mechanical behavior and strain localization in methane hydrate (MH) bearing sediments using the distinct element method (DEM). Based on the results of a series of laboratory tests on the bonded granules idealized by two glued aluminum rods and the available experimental data of methane hydrate samples, a pressure and temperature dependent bond contact model was proposed and implemented into a two-dimensional (2D) DEM code. This 2D DEM code was then used to numerically carry out a series of biaxial compression tests on the MH samples with different methane hydrate saturations, whose results were then compared with the experimental data obtained by Masui et al. [9]. In addition, stress, strain, void ratio and velocity fields, the distributions of bond breakage and averaged pure rotation rate (APR) as well as the evolution of strain localization were examined to investigate the relationships between micromechanical variables and macromechanical responses in the DEM MH samples. The numerical results show that: (1) the shear strength increases as methane hydrate saturation S_MH increases, which is in good agreement with the experimental observation; (2) the strain localization in all the DEM MH samples develops with onset of inhomogeneity of void ratio, velocity, strain, APR, and distortion of stress fields and contact force chains; and (3) the methane hydrate saturation affects the type of strain localization, with one shear band developed in the case of 40.9% and 67.8% methane saturation samples, and two shear bands formed for 50.1% methane saturation sample.     

基于塑性损伤的黏土岩力学特性研究

一系列三轴压缩试验表明,黏土岩的力学特性与其应力状态相关。主要表现为：围压越大,黏土岩试样应力峰值越大,且峰值出现得越晚。这是因为高围压试样内部微裂纹闭合趋势更加明显,从而对于微裂纹的发展具有一定的抑制作用。但在软化过程中应力的跌落程度受围压影响不大,这是因为软化过程中,不同围压作用下试样内部的微裂纹均逐渐贯通,并发展成同类型的剪切面,对试样力学性能的削弱大致相同。在试验研究基础上,以微裂纹作为损伤基元,引入受应力状态影响的损伤演化方程（因微裂纹的发展受应力变化影响）,建立新的本构模型,通过Abaqus及子程序对新模型进行数值实现。模拟结果与试验结果对比表明,该模型可以有效地描述黏土岩的力学特性。研究成果为黏土岩相关地下工程施工提供了更可靠的理论指导。     

Impact of constitutive models on the numerical analysis of underground constructions

The constitutive model frequently used in numerical calculations of tunnel excavation is linear-elastic perfectly plastic with a Mohr–Coulomb (MC) failure criterion. Generally, this leads to shallower and wider surface settlement troughs than those observed experimentally. It is therefore necessary to use adapted constitutive models for the design of underground works. In this paper, three constitutive models are implemented in a two-dimensional simulation of an underground excavation in plane strain: a linear-elastic perfectly plastic model (the MC model), an elastoplastic model with isotropic hardening [the hardening soil (HS) model, Schanz et al., Beyond 2000 in computational geotechnics, Balkema, Rotterdam, pp. 281–290, 1999] and an extension of this model which implies an evolution of the stiffness modulus in the small-strain range according to the strain level (the HS model with small-strain stiffness “HS-Small”, Benz, Small-strain stiffness of soils and its numerical consequences. Ph.D. thesis, Universitat Stuttgart, 189 pp., 2007). The study is based on the results of drained triaxial compression tests representing an overconsolidated clay (Gasparre, Advanced laboratory characterisation of London clay. Ph.D. thesis, Imperial College London, 598 pp., 2005); and is then applied to a shallow tunnel. The impact of the constitutive model is highlighted as well as the limits of the simplest constitutive model.     

Characteristics and Hydrocarbon Accumulation Periods of the Fluid Inclusions in the Sandstone Reservoirs of Chang 9 Oil Groups in Yanchang Formation,Huaqing Area

Huaqing area is an important area for oil and gas exploration and development in the Ordos Basin, within which, the Chang 9 oil group of Triassic Yanchang Formation is one of the main intervals for exploration and development (Zhang et al., 2011). Fluid inclusions record abundant oil and gas accumulation information and important oil and gas migration and accumulation history information(Parnell et al., 1998; Parnell, 2010; Jiang et al., 2018). The occurrence, distribution and intersection of the fluid inclusions in diagenetic minerals were observed under microscope. The accurate formation periods of inclusions are key to determine hydrocarbon accumulation periods (Burrus et al., 1985; Zhou et al., 2000; Zhao, 2002). In addition, the fluorescence color and brightness of organic inclusions observed by fluorescence microscopy could be used to roughly identify the oil and gas evolution stages (Zhang et al., 2010). Therefore, the occurrence, distribution and fluorescence characteristics of oil and gas inclusions were important evidence for judging the hydrocarbon accumulation periods. The fluid inclusions in sandstone reservoirs of Chang 9 oil group in Huaqing Area were observed in detail, and the observation results revealed that fluid inclusions were widespread in Chang 9 sandstone and exhibited various types. Through comprehensive analyses, it was considered that the fluid inclusions in Chang 9 sandstone reservoirs indicated three hydrocarbon accumulation periods.     

Expansive bentonite-sand mixtures in cyclic controlled-suction drying and wetting

Expansive clay buffers in radioactive waste disposal designs experience cyclic drying and wetting paths during different stages of their design life. Clayey soils subjected to these processes develop swelling and shrinkage deformations, which give rise to the accumulation of compression or expansion strains during suction cycles. Experimental studies were undertaken using oedometer tests on an artificially prepared bentonite-sand mixture (80% bentonite by dry mass). In order to study these processes and to identify the most important features controlling soil behaviour, several wetting-drying cycles with suctions ranging between 130 and 4 MPa were applied using vapour equilibrium technique and covering a wide range of overconsolidation ratios (OCR). The tested samples showed cumulative shrinkage strains along the successive cycles, which became more significant at increasing vertical net stresses (low OCR values). However, no accumulation of expansion strains was detected at elevated OCR values. Test results were interpreted and predicted within the context of an elastoplastic model proposed by Alonso et al., 1999, [Alonso, E.E., Vaunat, J., Gens, A., (1999). Modelling the mechanical behaviour of expansive clays. Engineering Geology, 54, 173-183.] which takes into account the accumulation of strains. A good correspondence between measured soil response and model predictions was observed. The paper also presents the methodology to derive the constitutive parameters.     

Diamond in Oceanic Peridotites and Chromitites: Evidence for Deep Recycled Mantle in the Global Ophiolite Record

Diamonds have been discovered in mantle peridotites and chromitites of six ophiolitic massifs along the 1300 km‐long Yarlung‐Zangbo suture (Bai et al., 1993; Yang et al., 2014; Xu et al., 2015), and in the Dongqiao and Dingqing mantle peridotites of the Bangong‐Nujiang suture in the eastern Tethyan zone (Robinson et al., 2004; Xiong et al., 2018). Recently, in‐situ diamond, coesite and other UHP mineral have also been reported in the Nidar ophiolite of the western Yarlung‐Zangbo suture (Das et al., 2015, 2017). The above‐mentioned diamond‐bearing ophiolites represent remnants of the eastern Mesozoic Tethyan oceanic lithosphere. New publications show that diamonds also occur in chromitites in the Pozanti‐Karsanti ophiolite of Turkey, and in the Mirdita ophiolite of Albania in the western Tethyan zone (Lian et al., 2017; Xiong et al., 2017; Wu et al., 2018). Similar diamonds and associated minerals have also reported from Paleozoic ophiolitic chromitites of Central Asian Orogenic Belt of China and the Ray‐Iz ophiolite in the Polar Urals, Russia (Yang et al., 2015a, b; Tian et al., 2015; Huang et al, 2015). Importantly, in‐situ diamonds have been recovered in chromitites of both the Luobusa ophiolite in Tbet and the Ray‐Iz ophiolite in Russia (Yang et al., 2014, 2015a). The extensive occurrences of such ultra‐high pressure (UHP) minerals in many ophiolites suggest formation by similar geological events in different oceans and orogenic belts of different ages. Compared to diamonds from kimberlites and UHP metamorphic belts, micro‐diamonds from ophiolites present a new occurrence of diamond that requires significantly different physical and chemical conditions of formation in Earth's mantle. The forms of chromite and qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150‐380 km or even deeper in the mantle (Yang et al., 2007; Dobrthinetskaya et al., 2009). The very light C isotope composition (δ13C ‐18 to ‐28‰) of these ophiolitic diamonds and their Mn‐bearing mineral inclusions, as well as coesite and clinopyroxene lamallae in chromite grains all indicate recycling of ancient continental or oceanic crustal materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction (Yang et al., 2014, 2015a; Robinson et al., 2015; Moe et al., 2018). These new observations and new data strongly suggest that micro‐diamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. The in‐situ occurrence of micro‐diamonds has been well‐demonstrated by different groups of international researchers, along with other UHP minerals in podiform chromitites and ophiolitic peridotites clearly indicate their deep mantle origin and effectively address questions of possible contamination during sample processing and analytical work. The widespread occurrence of ophiolite‐hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in‐situ oceanic mantle. The fundamental scientific question to address here is how and where these micro‐diamonds and UHP minerals first crystallized, how they were incorporated into ophiolitic chromitites and peridotites and how they were preserved during transport to the surface. Thus, diamonds and UHP minerals in ophiolites have raised new scientific problems and opened a new window for geologists to study recycling from crust to deep mantle and back to the surface.