31 Int J Petrochem Res. ISSN: 2638-1974 Volume 1 • Issue 1 • 1000107 International Journal of Petrochemistry and Research Research Article Open Access Applying Reactivation Tendency Analysis and Mohr-space to Evaluate Shear Strength Decrease and Anisotropy with Pre-existing Weakness(es) under Uniform Stress State Hengmao Tong 1 *, Hemin Koyi 2 and Hans Ramberg 2 1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China 2 Tectonic Laboratory, Department of Earth Sciences, Uppsala University, Uppsala, Sweden Article Info *Corresponding author: Hengmao Tong State Key Laboratory of Petroleum Resources and Prospecting China University of Petroleum Beijing 102249, China Tel: +86 10 89734607 Fax: +86 10 89734158 E-mail: tonghm@cup.edu.cn tong-hm@163.com Received: February 22, 2017 Accepted: April 7, 2017 Published: April 13, 2017 Citation: Tong H, Koyi H, Ramberg H. Applying Reactivation Tendency Analysis and Mohr- space to Evaluate Shear Strength Decrease and Anisotropy with Pre-existing Weakness(es) under Uniform Stress State. Int J Petrochem Res. 2017; 1(1): 31-39. doi: 10.18689/ijpr-1000107 Copyright: © 2017 The Author(s). This work is licensed under a Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Published by Madridge Publishers Abstract Understanding the mechanical controls on shear strength decrease due to preexisting weakness is a fundamental problem in tectonic studies. In this study, by applying reactivation tendency analysis theory, a theoretical framework and defined Shear-strength Coefficient (f d ) are developed for evaluating the shear-strength decrease and anisotropies due to the presence of preexisting weakness(es). The proposed study managed to overcome the restrictions of previous studies assumption that a pre-existing weakness plane contains the intermediate stress (σ 2 ) and vertical or horizontal orientations of principal stresses (Andersonian stress state). A new graphical technique (Mohr-space) was utilized to predict the shear-strength decrease and anisotropies caused by preexisting weakness(es). The Mohr-space technique made easier to visualize the state of stress and results of shear strength changes and able to build the quantitative and intuitive relationship between Shear-strength Coefficient (f d ) and weakness relative- orientation(θ’,φ›), weakness mechanical properties (C w and μ w ) and relative σ 2 ( 3 2 2 1 σ σ σ σ − − )in any uniform tri-axial stress state.. In this study, Shear-strength decrease and anisotropies of a rock sample are evaluated theoretically, and shear strength properties and deformation characteristics of a geological body with multiple pre-existing weaknesses are analyzed and predicted. Keywords: Shear strength; preexisting weakness; reactivation tendency; Mohr-space; sandbox experiment. Introduction It is well documented that pre-existing weaknesses (fracture planes or faults, layering, fabrics etc.) can lead to decrease of shear strength and strength anisotropies [1-3], and the potential strength anisotropy created by pre-existing weakness is considerable [4]. Ranalli (1990) [5] proposed a unified quantitative model to evaluate strength anisotropies by the pre-existing weakness in terms of the three tectonic faulting regimes when the weakness plane contains the intermediate stress (σ 2 ). However, to our knowledge strength anisotropies evaluation with pre-existing weakness is limited to two-dimensional cases (weakness plane containing σ 2 ) before slip-tendency theory was proposed [6]. However, strength anisotropies evaluation with slip-tendency [7] is confined to the qualitative analysis only. In addition, there is also some limitation in slip- ISSN: 2638-1974