Effect of decompression and suction on macroscopic and microscopic behavior of a clay rock (original) (raw)
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Rock Mechanics and Rock Engineering, 2014
A series of clay shale specimens in equilibrium with various humidity conditions were used to establish the water retention characteristics, the influence of suction on ultrasonic p-wave velocity and rock mechanical properties such as Young's modulus, Poisson's ratio, onset of dilatancy, unconfined compressive strength and Brazilian tensile strength. Opalinus Clay, a clay shale considered as host rock for the disposal of nuclear waste in Switzerland was utilized. The results showed that the p-wave velocity normal to bedding (v p,n ) dropped sharply upon desaturation until suction approached the air-entry value. The sharp decrease was associated with desiccation cracks solely oriented parallel to bedding. For suction in excess of the air-entry value, v p,n was constant, indicating no further desiccation damage. The suction at the shrinkage limit and at the airentry point is similar in magnitude. The p-wave velocity parallel to bedding (v p,p ) remained constant in the entire range of suction investigated in this study. The constant v p,p with increasing suction might be associated with the disproportional decrease in the Poisson's ratio and Young's modulus and its opposing effect on p-wave velocity. An almost linear increase in unconfined compressive strength, Brazilian tensile strength, stress at the onset of dilatancy and Young's modulus with increasing suction was observed up to a suction of 56.6 MPa. For suction larger than 56.6 MPa, relatively constant strength and stiffness was observed. The increase is associated with the net contribution of suction to strength/stiffness, which decreases nonlinearly with decreasing volumetric water content. The rate of increase in tensile strength and unconfined compressive strength with increasing suction is different depending on the rock anisotropy. Compared to the strength values (Brazilian tensile and uniaxial compressive strength) obtained from specimens loaded parallel to bedding, the tensile strength parallel to bedding and the unconfined compressive strength obtained from specimens loaded normal to bedding are considerably more affected by increasing suction or decreasing water content. The reasons for the different rates in strength increase are considered to be related to local variations in suction (i.e., local suction) as a consequence of zones of contrasting pore-size distribution. These variations may influence the effect of suction on strength, especially when the load is applied parallel to bedding and crack growth occurs predominately along bedding layers with comparably low suction.
Highlighting some mechanisms of crack formation and propagation in clays on drying-path
The objective of this research was to analyse the formation and propagation of cracks related to desiccation in clay and to provide a better understanding of the behaviour of two clays, a kaolinite, and a mixture of kaolinite and montmorillonite. At the macroscopic level, the strain tensors of the two clays during drying, and the cracking phenomenon were studied using digital image correlation. The method is based on the determination of the local two-dimensional strains and displacements fields using the programs VIC-2D and VIC-3D. Different mechanisms of crack formation were identified: opening mode, sliding mode and tearing mode. At the end of desiccation, the cracks form a network similar to thermal fatigue or thermal shock networks. Bifurcation and coalescence of cracks can also be observed in soil. Another objective was to explore the effect of mineralogy on the behaviour of these clays. It is the reason why two different mixtures of kaolin and montmorillonite were chosen, with liquid limits ranging from 40% to 82%. The results were found to be similar in both cases except concerning the kinetics which is faster in kaolin.
Applied Clay Science
The aim of this paper is to compare and discuss the values of strains and crack apertures associated with desiccation cracks measured in Tournemire clay rock at different scales (micrometer to decimeter). Experimental investigations in the laboratory were conducted on one clay rock sample subjected to a desiccation process. Two faces with dimensions of 20 × 20 mm 2 (i.e., macroscopic scale) and 5.1 × 4.1 mm 2 (i.e., mesoscopic scale) were analyzed. The induced hydric strains and desiccation cracking were monitored by digital image correlation combined with a new algorithm (H-DIC). The results were compared with the data of Hedan et al. (2014) at the gallery scale (decimeter) and those of Wang et al. (2013) at the microscopic scale (micrometer). Our laboratory study yielded the following phenomenological results. First, the displacement fields revealed the presence of sub-horizontal cracks associated with the direction of bedding planes and sub-vertical cracks, as previously observed in a gallery front in Tournemire Station. Second, when the relative humidity (RH) decreased between 98% and 33%, the crack aperture kinematics at the macroscopic scale (centimeter) was divided into three steps: (i) a phase of opening and closure, (ii) a phase of only gradual closure, and (iii) a final phase in which the desiccation cracks closed. Only phases (ii) and (iii) were observed at the mesoscopic scale (millimeter), revealing that the kinematics of cracks depends on the scale observed. The comparison of the strains at the mesoscopic and the macroscopic scales also highlights that their values depend on the study scale: the presence of cracks at the mesoscopic scale leads to a large overestimation of the values of the strains calculated at the macroscopic scale. In contrast to the observations in the laboratory, the desiccation cracks detected in the gallery systematically open when RH decreases. This difference and the differences observed in the geometrical organization of crack networks are explained by the different boundary conditions prevailing in both cases (i.e., free swelling/shrinkage in laboratory versus constrained swelling/shrinkage in the gallery). The interpretation of the entire dataset emphasizes the need for a multi-scale approach to understand and model desiccation cracking mechanisms and the associated hydric strains in clay rocks.
Bulletin de la Société Géologique de France, 2000
The influence of mineralogy and moisture content on mechanical behaviour of a claystone rock is studied by the way of uniaxial and triaxial compression tests and microscopic observations. Some parameters characteristic of phenomena like plasticity and induced anisotropic damage are discussed as a function of these two factors. Rock behaviour becomes more brittle when calcite content grows or when clay or moisture content decreases. At the microlevel, plasticity is induced by slip of clay sheets and induced anisotropic damage appears by growth of oriented microcracks at the interface between grains and matrix.
SELFRAC: Experiments and conclusions on fracturing, self-healing and self-sealing processes in clays
Physics and Chemistry of the Earth, Parts A/B/C, 2007
In assessing the performance of a deep HLW repository, the evolution of the excavated damaged zone with time is a key issue. In the framework of SELFRAC fracturing, self-sealing and self-healing processes of Opalinus and Boom clay were studied in laboratory and in situ experiments. Definitions for the terms excavation damaged zone (EDZ), excavation disturbed zone (EdZ), sealing and healing are presented. It is shown that sealing and partial healing occur and the consequences of the results for performance assessment of HLW disposal in argillaceous rocks are discussed. The results of several in situ experiments and observations at the HADES underground research facility are detailed. The origin and extent of excavation induced fractures are discussed and sealing and (partial) healing of these fractures is demonstrated. In the description of the hydraulic features of the EdZ, the anisotropic pore pressure distribution around HADES and its evolution with time are discussed. Pore pressure is influenced several tens of metres into the host rock and its evolution is influenced by the anisotropic in situ stress state and the anisotropic hydraulic conductivity of Boom clay. Around the connecting gallery, an increase of hydraulic conductivity is measured up to about 6-8 m into the host rock, outside this influenced zone values between 4 • 10 À12 m/s and 6 • 10 À12 m/s were obtained. The highest value measured (close to the gallery) was of the order of magnitude of 10 À11 m/s. The observed increase is caused by lower effective stress levels close to the gallery rather than by excavation induced fractures. Self-boring pressuremeter tests show that total stress is influenced up to 6-8 m into the host rock and material parameters such as undrained shear strength and shear modulus are influenced up to 2-3 m into the host rock. In situ seismic transmission measurements showed that the closure of a borehole influences the seismic parameters of the surrounding host rock: a decrease in seismic velocity is measured and higher frequencies disappear from the transmitted signals. After closure of the borehole, sealing of the damaged zone around it occurs, this is observed by the recovery of seismic velocity and the reappearance of higher frequencies. Fracture sealing is also demonstrated by seismic and hydraulic measurements on a reinstalled fractured clay core.
2016
A clay rock sample from the Tournemire Underground Research Laboratory (Averyon, France) was subjected to a fast desiccation in the laboratory, from 98 to 33% relative humidity. At the millimetre scale, fracture locations were identified and desiccation strains and fracture apertures were calculated by digital image correlation on a surface of 5.5x4.1 mm2. After the desiccation, the microstructure of this surface was mapped under scanning electron microscopy by a large mosaic of back scattered electron images in high resolution. The aim of the study is a quantitative comparison between local strains and crack apertures to the local proportion of clay matrix and rigid inclusions of the sample, in order to understand better the role of microstructure in desiccation mechanisms in clay rocks. The results have shown that: a) the crack apertures are heterogeneous and seem to be higher at some interfaces between rigid inclusions and matrix, and b) the strains are heterogeneous and their in...
Acta Geologica Polonica, 2019
This paper presents the qualitative and quantitative characteristics of microstructures of Neogene clays from Warsaw, Poland. Scanning Electron Microscope (SEM) studies were used for the microstructural analysis of natural clays and clay pastes. Qualitative microstructural changes were observed: from a honeycomb microstructure for the initial clay paste to a turbulent microstructure for the dried paste. It was also noticed that water loss caused by the increase of the suction pressure had a significant impact on the microstructural transformations. Significant changes in the quantitative values of the pore space parameters were also observed. Increase of suction pressure and water loss caused a decrease in porosity and changes in the values of morphometric parameters, such as pore distribution; for example, a significant increase of the number of pores of 0−10 μm size and changes in the geometric parameters of the pore space were noticed with the increase of suction pressure. The po...
Mechanical properties of clays at high pressure
Journal of Geophysical Research, 1980
The mechanical properties of preconsolidated clays are studied at high pressures under undrained, triaxial conditions. At confining pressures equivalent to those at midcrust, different clays have the following common characteristics which differ significantly from those at low pressures: (1) the clays possess significant strength of several hundred bars (10 bars = 1 MPa); (2) the constitutive relations are marked by ductile yielding and strain hardening, followed by a broad peak strength and a gradual decrease in strength at greater deformation; and (3) the change in volume during shearing is small. On the other hand, the behavior of montmorillonite differs from that of the other clays (illite, chlorite, and kaolinite) in the following ways: (1) its peak strength at a given confining pressure is about half of the strengths of the other clays; (2) its peak strength occurs at a shortening of about 10%, while for the illite and chlorite the peak strength occurs at a shortening of 20-25% (for kaolinite, strain hardening continues even at 30% shortening); and (3) the fracture surfaces of some montmorillonite samples deformed at relatively low confining pressures show features resembling those in natural clayey fault gouge, whereas at higher confining pressures, montmorillonite and the other clays remain unfractured at shortening up to 30-40%.