Coring damage extent of rock cores retrieved from high in-situ stress condition: A case study

Abstract

The distribution of damage in deep rock cores is critical for assessing its influence on rock mechanical characteristics. This paper presents a case study on estimating the coring damage extent induced by the stress redistribution through a combined method of numerical simulation and X-ray Computed Tomography (CT) scanning. Rock cores are taken at 1900 m depth from the test tunnel at Jin-ping Second Stage Hydropower Station (JPII) in China. Firstly, the stress path experienced by the rock core during coring has been examined by the numerical simulation, and the distribution of tensile zone in the cross-section of rock core is also discussed. Then two kinds of samples, collected from the same position under different stress levels, are scanned to obtain CT images, and a special CT value analysis strategy was adopted to evaluate coring damage intensities of these samples. The result indicates that the stress state can be regarded as the principal factor for the distribution of coring damage. During coring in the Test Tunnel of JPII under the quasi-hydrostatic stress condition, high tensile stresses (over 5 MPa) are observed at the exterior edge of rock cores, which may lead to the nucleation of microcracks around the core boundary, and the coring damage then propagates to the core centre. The CT scanning also illustrates that the coring damage zone may cover approximately 70~80% of the entire cross-section from the outside inside (stress level of 50 MPa), and that the central part of the core is less damaged or eventually undisturbed. Thus, intact rock samples can be expected to be obtained by overcoring the original rock cores. However, the applicability of the overcoring method seems to be largely dependent on the state of in-situ stress at the coring site. The whole core may be damaged if the lateral stress coefficient reaches a critical value (e.g. greater than 3.0), in which case a special coring equipment should be adopted to improve the stress state during coring.