Influence of joint anisotropy on the fracturing behavior of a sedimentary rock

节理面各向异性对沉积岩破裂特性的影响

Engineering Geology, Volume 228, 13 October 2017, Pages 224-237

Debanjan Guha Roy, T.N. Singh, J. Kodikara

Abstract:Fracture toughness (FT) and tensile strength (TS) indicate a rock's susceptibility to fracturing. Past studies showed that mechanical properties and FT of homogeneous rocks can be correlated well. In the present study, this capability has been extended to the jointed sedimentary rocks. Sandstone with a wide combination of analogue joints were tested in the laboratory, and the control of joint geometrical properties on the FT, TS, and development of fracture process zone (FPZ) were investigated. A FT prediction model was developed based on the TS of the jointed specimens. Multifractal scaling law (MFSL) was utilized to extend the laboratory results to the field scale. Further, the interaction of propagating crack with the already existing joints were investigated and post-peak behavior were studied. Results show that FT and TS decrease with decreasing joint spacing, but they are more sensitive at higher joint spacing. It is also possible to construct a linear prediction model between the FT and TS of rock. FPZ of jointed rock is less sensitive to joint orientation and more related to joint spacing. A negative correlation between the FPZ size and joint spacing is established in this study. Post-peak behavior of the jointed specimens indicate that crack-joint interaction increases the frictional resistance and dissipated fracture energy of the specimens. Further, comparison of mixed-mode fracture criteria with the experimental results show that Maximum Tangential Stress (MTS) criterion can successfully predict the mixed-mode fracture behavior of jointed sandstone.

A micro-mechanics based viscoplastic model for clayey rocks

基于微观力学的泥质岩粘塑性模型

Computers and Geotechnics, Volume 89, September 2017, Pages 92-102

F. Farhat, W.Q. Shen, J.F. Shao

Abstract: In this study, a micro-mechanics based viscoplastic model is proposed to describe time-dependent deformation for a class of clayey rocks. The heterogeneous rock is represented as a composite material containing a porous clay matrix and mineral inclusions at a mesoscopic scale. The clay matrix is composed of a solid phase and pores at the microscopic scale. The effective plastic yield criterion is determined from a nonlinear homogenization procedure (Shen et al., 2013). This criterion is extended and used as a viscoplastic loading function. Together with a suitable hardening law and a non-associated flow rule, the viscoplastic model is completed. A series of numerical assessments are presented to investigate the influence of porosity and mineral inclusions on the time-dependent deformation of clayey rocks. Comparisons between numerical results and experimental data are also performed and presented for different loading paths.

Temperature Induced Fracturing of Rock Salt Mass

温度变化引发的盐岩体破裂

Procedia Engineering, Volume 191, 2017, Pages 967-974

Kurt Staudtmeister, Dirk Zapf, Bastian Leuger, Marc Elend

Abstract: During the operation of gas storage caverns in rock salt mass the internal pressure changes during filling and withdrawal phases. Additionally temperature variations occur versus operation time. During withdrawal phases the temperature decreases which can lead to stress states in tensile regions at the cavern wall. Because the tensile strength of rock salt is relatively low compared to its compressive strength it is likely that tensile stresses lead to discrete fractures orthogonal to the direction of the tensile stresses. If fractures of this kind are created – whether vertical or horizontal – the gas will penetrate into the fracture at the relevant pressure and further extend the length of the fractures under certain circumstances. There are currently no theoretical approaches describing the manner in which the fractures might propagate into the not by temperature changes influenced rock salt mass during repeated cyclic pressure changes. This aspect is topic of prospective research. Salt caverns cannot be entered but only explored by sonar measurements, with which it is not possible to detect tensile fractures at the cavern wall. Within this paper examples from mining configurations will be shown where temperature changes lead to tensile fractures in the surrounding rock salt. These fractures have been well mapped while the temperature development is well documented. The paper deals with recalculations under consideration of different salt properties of the temperature distributions and the resulting stress state in the surrounding rock salt mass. The stress calculation results and the consequences for the dimensioning of natural gas caverns are going to be discussed and assessed.

Continuous-discontinuous cellular automaton method for cohesive crack growth in rock

岩石粘结裂纹扩展的连续-非连续元胞自动机法

Engineering Fracture Mechanics, In press, corrected proof, Available online 12 September 2017

Fei Yan, Xia-Ting Feng, Jia-He Lv, Peng-Zhi Pan, Shao-Jun Li

Abstract:A group of nonsingular enrichment functions is proposed for an exact representation of the cohesive stresses, which are consistent with a nonsingular asymptotic stress field around a crack tip. An improvement in the adaptive crack tip polar coordinate is proposed for treating the local polar coordinate error. Finally, a discontinuous cellular automation model was developed, including a cell model, cell state, and updating rules. Combined with these developments and the cohesive rock model, a continuous-discontinuous cellular automaton method (CDCA) for a cohesive crack analysis is proposed, through which the calculation is only limited to the cell locality without re-meshing, and therefore, no assembled global stiffness matrix is required. Numerical simulations reveal the accuracy and efficiency of the presented method.