Relationships between cracking, strains and proportions of clay matrix and rigid inclusions in Tournemire clay rock (original) (raw)
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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.
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.
Salient factors controlling desiccation cracking of clay in laboratory experiments
Géotechnique, 2013
This paper elucidates some of the controlling factors governing soil desiccation. The desiccation tests were conducted on three materials-clay, potato starch and milled quartz sand-all three featuring similar fracture energy. Two controlling factors were identified in desiccation cracking, regardless of the material. The first is the tensile stress and strain energy development within the material when the material is restrained against shrinkage. The distribution of the tensile stress will depend on the boundary conditions and material stiffness, and will dictate where cracks are likely to originate. The second factor is that the exact positions of crack initiations will be controlled by the flaws and/or pores within the material. For materials such as clay, with very fine particles, the cracking mechanism is governed by flaws, since the desaturation of fine pores would require very high suction stress, and this requirement leads to sequential cracking and orthogonal crack patterns. If the material has particles giving relatively large and uniform pore sizes with high moisture diffusivity leading to high shrinkage energy prior to cracking, then the fracture energy balance indicates that cracking can occur in near hexagonal patterns with 1208 crack initiations, which occur predominantly simultaneously. However, even for materials with lower moisture diffusivity, such as for clay, high desiccation rates can give rise to an 'effective layer' over which high suctions and strain energy develop, leading to almost simultaneous dense cracking.
International Workshop on Geomechanics and Energy, 2013
Digital image correlation was used for the first time in an underground gallery to monitor the argillaceous rock deformations during an annual climatic cycle. This experimental in situ investigation was carried out on a study area of 34.4×27.5 cm², located on the East96 gallery front at the Tournemire experimental station, during which the relative humidity (RH) and temperature (T) were continuously measured for more than one year and fluctuate naturally.Our results demonstrate the ability of the non-invasive DIC method to monitor clay-rock strains and the opening and closure of desiccation cracks. Moreover, our study provides the following results. First, the hydric strains were anisotropic; the strains perpendicular to the desiccation cracks were almost homogeneous and much larger than those parallel to the same cracks. Second, the changes in crack apertures calculated from the displacement fields and the strain fields were clearly correlated and concomitant with changes in RH and T. Third, contrary to direct measurements acquired at the Mont-Terri site, the crack apertures of the desiccation cracks were reversible after one year of data acquisition. Moreover, although the main desiccation cracks were sub-horizontal and associated with the direction of bedding planes, our work demonstrated the existence of sub-vertical cracks.
Filling the Gaps – from Microscopic Pore Structures to Transport Properties in Shales
The application of ion-beam milling techniques to clays allows investigation of the porosity at nm resolution using scanning electron microscopy (SEM). Imaging of pores by SEM of surfaces prepared by broad ion beam (BIB) gives both qualitative and quantitative insights into the porosity and mineral fabrics in 2D representative cross-sections. The combination of cryogenic techniques with ion-beam milling preparation (BIB and FIB, focused ion beam) allows the study of pore fluids in preserved clay-rich samples. Characterization of the pore network is achieved, first, using X-ray computed tomography to provide insights into the largest pore bodies only, which are generally not connected at the resolution achieved. Secondly, access to 3D pore connectivity is achieved by FIB-SEM tomography and the results are compared with 2D porosity analysis (BIB-SEM) and correlated with bulk porosity measurements (e.g. mercury injection porosimetry, MIP). Effective pore connectivity was investigated with an analog of MIP based on Wood's metal (WM), which is solid at room temperature and allows microstructural investigation of WM-filled pores with BIB-SEM after injection. Combination of these microstructural investigations at scales of ,1 mm with conventional stressstrain data, and strain localization characterized by strain-fields measurement (DICdigital image correlation) on the same sample offers a unique opportunity to answer the fundamental questions: (1) when, (2) where, and (3) how the sample was deformed in the laboratory. All the methods above were combined to study the microstructures in naturally and experimentally deformed argillites. Preliminary results are promising and leading toward better understanding of the deformation behavior displayed by argillites in the transition between rocks and soils.
Crack Initiation and Crack Propagation in Heterogeneous Sulfate-Rich Clay Rocks
Rock Mechanics and Rock Engineering, 2013
ABSTRACT Brittle fracture processes were hypothesized by several researches to cause a damage zone around an underground excavation in sulfate-rich clay rock when the stress exceeds the crack initiation threshold, and may promote swelling by crystal growth in newly formed fractures. In this study, laboratory experiments such as unconfined and confined compression tests with acoustic emission monitoring, and microstructural and mineralogical analyses are used to explain brittle fracture processes in sulfate-rich clay rock from the Gipskeuper formation in Switzerland. This rock type typically shows a heterogeneous rock fabric consisting of distinct clayey layers and stiff heterogeneities such as anhydrite layers, veins or nodules. The study showed that at low deviatoric stress, the failure behavior is dominated by the strength of the clayey matrix where microcracks are initiated. With increasing deviatoric stress or strain, growing microcracks eventually are arrested at anhydrite veins, and cracks develop either aligned with the interface between clayey layers and anhydrite veins, or penetrate anhydrite veins. These cracks often link micro-fractured regions in the specimen. This study also suggest that fracture localization in sulfate-rich clay rocks, which typically show a heterogeneous rock fabric, does not take place in the pre-peak range and renders unstable crack propagation less likely. Sulfate-rich clay rocks typically contain anhydrite veins at various scales. At the scale of a tunnel, anhydrite layers or veins may arrest growing fractures and prevent the disintegration of the rock mass. The rock mass may be damaged when the threshold stress for microcrack initiation is exceeded, thus promoting swelling by crystal growth in extension fractures, but the self-supporting capacity of the rock mass may be maintained rendering the possibility for rapidly propagating instability less likely.
Topology of desiccation crack patterns in clay and invariance of crack interface area with thickness
The European Physical Journal E, 2015
We study the crack patterns developed on desiccating films of an aqueous colloidal suspension of bentonite on a glass substrate. Varying the thickness of the layer h gives the following new and interesting results: (i)We identify a critical thickness h c , above which isolated cracks join each other to form a fully connected network. A topological analysis of the crack network shows that the Euler number falls to a minimum at h c. (ii) We find further, that the total vertical surface area of the clay A v , which has opened up due to cracking, is a constant independent of the layer thickness for h ≥ h c. (iii) The total area of the glass substrate A s , exposed by the hierarchical sequence of cracks is also a constant for h ≥ h c. These results are shown to be consistent with a simple energy conservation argument, neglecting dissipative losses. (iv) Finally we show that if the crack pattern is viewed at successively finer resolution, the total cumulative area of cracks visible at a certain resolution, scales with the layer thickness. A suspension of Laponite in methanol is found to exhibit similar salient features (i)-(iv), though in this case the crack initiation process for very thin layers is quite different.
(2009) Influence of cracking in the desiccation processes of clay soils
It is well known that clayey soils undergoing desiccation tend to shrink and eventually crack. Analysis of the behaviour and influence of cracks in these types of soils is very important in several engineering fields such as mine tailing dams, long-term radioactive waste storage, impervious core of earth dams, and in any situation where clay is used as a barrier to fluid flow. Loss of humidity and cracking changes the permeability of such barriers that may no longer work properly and pose potentially high risks to property and lives. This paper presents an analysis of cracking during drying of soils using a computer code de-veloped within the framework of the finite element and finite differences methods. A study of the influence of crack initiation and propagation in the desiccation process is also undertaken, with a comparative analysis of the phenomenon both with and without crack generation that allows some preliminary conclusions about the desiccation problem. The computer code has been implemented within the MatLab environment. The formulation is based on the principles of the unsaturated soil mechanics and the mechanics of a continuum medium. The partial differential equations that govern the problem are solved using the finite element (Galerkin) method in space and the finite differences method, using the Crank-Nicholson scheme, in time. Further developments of the code will include fracture mechanics principles to simulate crack propagation.
Expansive Clay Cracking Behavior through Digital Image Correlation
E3S Web of Conferences, 2020
Expansive soils may present cracks arising from the drying process and their evolution can cause irreparable damages to engineering projects. Investigating this phenomenon is vital to understanding its geomechanics. The objective of this article is to present numerical modelling of the formation and propagation of cracks in expansive soil. A desiccation experiment was therefore carried out using an expansive silty clay from Paulista, in northeastern Brazil. The drying process was monitored by measuring the temperature and relative humidity of the air, as well as by capturing images with a camera. The digital images were correlated using the Ncorr numerical tool in MATLAB. As a result, this study made it possible to conclude that the soil cracking dynamics presented a non-orthogonal pattern during the dryness test, while the image treatment made it possible to observe the tendency of cracks to appear and propagate on the soil surface, allowing for the detection of crack growth and pr...