Real-time computerized tomography (CT) experiments on limestone damage evolution during unloading
Introduction
Stress redistribution induced by excavation of underground engineering and slope engineering results in the unloading zone in parts of surrounding rock. The mechanical behaviors of rock under unloading are different from those of rock under loading [1], [2], [3], [4]. Study on the mechanical behaviors of rock under loading is mature. In macro-investigations, most of research has been basically limited to the influence of the loading rate on rock strength and rock fracture toughness. In the micro-mechanical approach, the nucleation, growth and coalescence of micro-cracks are studied and their influence on mechanical properties are reflected in certain ways [5], [6], [7], [8], [9]. Study on the mechanical behaviors of rock under unloading is lacunose. Load-path dependence of rock materials is one of the fundamental factors in slope engineering and underground engineering. Investigating the load-path dependence of crack initiation and propagation and coalescence in geological processes may provide valuable fundamental data for long-term stability of rock cavern, seismic mechanisms.
Research so far has been mainly concentrated on the strength of rock under loading [10], [11], [12].There has been little research on the internal damage evolution of rock under unloading. The constitutive relation of rock under unloading has not yet been properly established. Hence, effective modeling and behavior prediction are impossible.
Mechanical experiments are an efficient means of exploring the evolution law of internal damage during rock breakage. Normalized strain gauge measurements provide a simple means to describe the deformation characteristics of rock samples during the progressive accumulation of microfracturing damage. These measurements include the plastic strains associated with crack nucleation and growth and coalescence, as well as elastic strains and plastic strains associated with the deformation of rock materials. The direct correlation of these strains to the loss of cohesion, however, becomes somewhat more difficult. Acoustic emissions provide a direct measure of discrete damage events in brittle rock material such as pore collapse, crack initiation, propagation and grain boundary movements [13]. Moreover, Acoustic emission data was analyzed to develop a simple relationship between AE activity and the gradual loss of cohesion and the accumulation of damage. Total AE event counts up to each crack threshold were normalized with respect to the total number of events recorded at failure process so that comparisons could be made between individual tests [14]. The crack distribution on a rock surface in a certain stress state can be observed by optical or electronic microscopic [15]. Furthermore, the crack distribution on an internal cross-section under a certain stress state can be established by using the computerized tomography (CT) technique [16], [17]. CT images are obtained by scanning the damaged rock specimen at a single given stage in the past [16]. That procedure cannot obtain real-time CT images at different stress stages during the full process of rock damage evolution. However, the present method can obtain real-time results, which are required to study the real-time process of rock damage evolution under unloading.
This paper reports experimental research on the damage evolution properties of limestone under unloading by using the real-time CT testing technique. CT images and CT values for limestone cross-sections under different unloading levels were obtained. The mechanism of damage was analyzed and the new failure criterion of rock material was obtained under unloading.
Section snippets
Testing device and process
The rock specimen is a 50 ∗ 100 mm2 cylinder. The tests have been conducted at the CT laboratory, Cold and Arid Region Environmental and Engineering Research Institute, Chinese Academy of Sciences. The X-ray CT scanner, SIEMENS SOMATOM plus and triaxial loading system designed specially for the CT scanner were used (Fig. 1). The dimensions of the triaxial loading system are 240 m ∗ 1000 m, the maximum designed axial compression is 400 kN with a maximum confining pressure of 20 MPa and a maximum axial
The failure criterion of rock under unloading
Rock is a geotechnical material with high difference of tensile and compressive strengths. Its properties vary with rock type. The rock in underground engineering and slope engineering is subjected to unloading states. The main objectives of the study of strength theory (yield and failure criteria) are to understand the rock materials’ intrinsic response to loading and unloading, to identify the failure conditions in different stress states, and ultimately, to formulate necessary design
Definition of the CT value
According to Professor Hounsfield, one of the inventors of CT machine. The CT value Hrm is defined bywhere and are mass absorbing coefficients for rock matrix material and pore air, respectively. The unit name of the CT value is Hu (Hounsfield Unit), and 1000 is scaling factor of Hu. According to the definition. The CT value is −1000 Hu for air and 0 Hu for pure water. The CT value of a material essentially reflects its density, i.e. the higher the CT value of the
Conclusions
The CT technique is an effective method for studying the rock damage evolution process under unloading. From the CT experiments described here, the main conclusions are summarized as follows:
- (1)
The CT technique has been successfully used in unloading experiment of damage evolution process of limestone.
- (2)
The quantitative analysis of the internal rock damage evolution process is also possible via the CT value and a damage variable has been proposed, Eq. (53).
- (3)
The phenomena of the inhomogeneous
Acknowledgements
The author would like to express his sincere thank to Professor G.C. Sih for his kind help and remarks. This work was supported by the National Natural Science Foundation of China (Nos. 50778184, 50679097) and the supporting program of New-century Talents by the Ministry of Education (No. NCET-07-0911).
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