The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures All specimens were tested at their natural water content with loading approximately normal to the bedding Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation The unconfined compression strength of the tested specimens averaged 69 MPa The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress

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Experimental Study of the Brittle Behavior of Clay shale in Rapid Unconfined Compression

The numerical analysis of tunnels in rock masses, which exhibit a squeezing behavior, is considered, with reference to the use of two elastoviscoplastic constitutive laws implemented in finite-difference and finite-element codes. Because of the fact that in the most severe squeezing conditions time dependence is observed during excavation and may last for a long time, both the reinforcement of the tunnel face and the support of the tunnel section are to be considered at the design and construction stages. As a consequence, the need arises to analyze the tunnel excavation-construction sequence in detail and to account for time-dependent behavior. Numerical analyses have been carried with the Saint Martin La Porte access adit, along the Lyon-Turin Base Tunnel, taken as a case study. The interest stems from the performance monitoring data available for a significant tunnel length in the carboniferous formation, which exhibits a severely squeezing behavior. Also of interest is the adoption of a novel yielding support system, which revealed to be successful in coping with the difficult conditions encountered. Following a series of preliminary back analyses, carried out in axisymmetric conditions and meant to calibrate the rock mass parameters, attention has been focused on the support system. The actual excavation-construction sequence has been simulated in detail to investigate the tunnel response and the stress changes in the support. The numerical results are compared with the monitoring data, and the potential of the two different elastoviscoplastic constitutive models in describing the tunnel behavior is assessed in detail.

Time-Dependent Modeling of Tunnels in Squeezing Conditions

Thermal loading of rocks at high temperatures induces changes in their mechanical properties. In this study, a hard gabbro was tested in the laboratory. Specimens were slowly heated to a maximum temperature of 1,000°C. Subsequent to the thermal loading, specimens were subjected to uniaxial compression. A drastic decrease of both unconfined compressive strength and elastic moduli was observed. The thermal damage of the rock was also highlighted by measuring elastic wave velocities and by monitoring acoustic emissions during testing. The micromechanisms of rock degradation were investigated by analysis of thin sections after each stage of thermal loading. It was found that there is a critical temperature above which drastic changes in mechanical properties occur. Indeed, below a temperature of 600°C, microcracks start developing due to a difference in the thermal expansion coefficients of the crystals. At higher temperatures (above 600°C), oxidation of Fe2+ and Mg2+, as well as bursting of fluid inclusions, are the principal causes of damage. Such mechanical degradation may have dramatic consequences for many geoengineering structures.

Damage and Changes in Mechanical Properties of a Gabbro Thermally Loaded up to 1,000°C

Triaxial creep tests were performed on diabase specimens from the dam foundation of the Dagangshan hydropower station, and the typical characteristics of creep curves were analyzed. Based on the test results under different stress levels, a new nonlinear visco-elasto-plastic creep model with creep threshold and long-term strength was proposed by connecting an instantaneous elastic Hooke body, a visco-elasto-plastic Schiffman body, and a nonlinear visco-plastic body in series mode. By introducing the nonlinear visco-plastic component, this creep model can describe the typical creep behavior, which includes the primary creep stage, the secondary creep stage, and the tertiary creep stage. Three-dimensional creep equations under constant stress conditions were deduced. The yield approach index (YAI) was used as the criterion for the piecewise creep function to resolve the difficulty in determining the creep threshold value and the long-term strength. The expression of the visco-plastic component was derived in detail and the three-dimensional central difference form was given. An example was used to verify the credibility of the model. The creep parameters were identified, and the calculated curves were in good agreement with the experimental curves, indicating that the model is capable of replicating the physical processes.

Time-Dependent Behavior of Diabase and a Nonlinear Creep Model

It is important to note that creep is only one aspect of the time-dependent behavior of rocks. In Fig. 1, three cases are illustrated with respect to the complete stress–strain curve: creep, i.e., increasing strain when the stress is held constant; stress relaxation, i.e., decreasing stress when the strain is held constant; and a combination of both, when the rock unloads along a chosen unloading path. This ISRM suggested method deals only with the case of creep, which is particularly relevant for cases where the applied load or stress is kept constant. Creep tests have also been carried out on soft rocks such as tuff, shale, lignite, and sandstone, medium-hard rocks such as marble, limestone, and rock salt, and hard rocks such as granite and andesite (i.e., Akagi 1976; Akai et al. 1979, 1984; Ito and Akagi 2001; Berest et al. 2005; Doktan 1983; Passaris 1979; Serata et al. 1968; Wawersik 1983; Okubo et al. 1991, 1993; Masuda et al. 1987, 1988; Ishizuka et al. 1993; Lockner and Byerlee 1977; Boukharov et al. 1995; Fabre and Pellet 2006; Aydan et al. 1995; Chan 1997; Cristescu and Hunsche 1998; Hunsche 1992; Hunsche and Hampel 1999; Ito et al. 1999; Mottahed and Szeki 1982; Perzyna 1966; Slizowski and Lankof 2003; Yang et al. 1999). These experiments were mostly carried out under compressive loading conditions. There are few studies on rocks using creep tests under a tensile loading regime (Ito and Sasajima 1980, 1987; Ito et al. 2008; Aydan et al. 2011). In particular, shallow underground openings may be subjected to a sustained tensile stress regime, which requires the creep behavior of rocks under such conditions. Please send any written comments on this ISRM suggested method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey.

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ISRM Suggested Methods for Determining the Creep Characteristics of Rock

Creep and time-dependent damage in argillaceous rocks

In the study of time dependent behavior of rock, the main difficulty is to predict delayed failure, which is of the utmost importance in assessing the safety of underground structures, such as deep underground facilities designed for high-level radioactive waste disposal. In this context, the viscoplastic behavior associated with the rock damage must be taken into account. As the longitudinal and transversal wave velocities are related to the physical and mechanical characteristics of materials, ultrasonic measurements can give valuable information about the development of damage. In this study, P-wave velocity measurements were used to monitor damage evolution during uniaxial strain in controlled compression tests and long-term creep tests. These measurements were performed using sensors in a piezoelectric copolymer of polyvinyl-difluoride, which were placed on both ends of cylindrical rock specimens. Throughout the experiments, the dilating behavior of an argillite could be correlated with a decrease of...

Geraldine Fabre

Papers

Creep and time-dependent damage in argillaceous rocks

Damage Evaluation with P-Wave Velocity Measurements during Uniaxial Compression Tests on Argillaceous Rocks