Micro and macro evaluation of tensile characteristics of anisotropic rock mass after high temperatures treatment-a case study of Lingshi Gneiss

Abstract

The study of macroscopic and microscopic fracture behaviour and tensile strength evolution of anisotropic rock mass at high temperatures are of great significance to evaluation of stability of enhanced geothermal system (EGS). In this paper, Brazilian split test was adopted to study the action mechanism of different temperatures and bedding angles on the tensile strength of gneiss; the digital image correlation (DIC) was used for tracking the principal strain evolution throughout the whole split. Furthermore, the micro X-ray computed tomography and thin-section petrographic analysis were combined in studying the microscopic crack propagation trend and mineral component physicochemical changes of gneiss at different temperature stages. Results show that: (1) under the same bedding angle, the evolution of tensile strength of gneiss within 25–700 ℃ can be divided into three stages: at low temperature stage (25–200 ℃), the tensile strength of gneiss slowly increased, at medium-temperature stage (200–400 ℃), the tensile strength of gneiss slowly decreased, and at high temperature stage (400–700 ℃), the tensile strength of gneiss drops sharply. Under the same temperature, with increased bedding angle, the tensile strength of gneiss first decreased and then increased, with a minimum value when the bedding angle is 30° and a maximum value when it is 90°; (2) under the same temperature, with the increase of bedding angle, gneiss shows a single bedding plane tensile failure (0°), shear-type composite failure (15–30°), tensile-shear failure (45–75°), and base particle-bedding tensile failure (90°); (3) when the temperature is under 400℃, the initiation location is distributed at both ends of disc, whereas when the temperature is equal or above 400 ℃, it is distributed at the centre of disc. The results obtained are of great significance to safety evaluation of EGS and other high-temperature rock engineering.