Core discing characteristics and mitigation approach by a novel developed drill bit in deep rocks
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Core discing characteristics and mitigation approach by a novel developed drill bit in deep rocks ZHENG Min-zong(郑民总)1, 2, LI Shao-jun(李邵军)1, YAO Zou(姚邹)3, ZHANG Ao-dong(张奥东)3, XU Ding-ping(徐鼎平)1, ZHOU Ji-fang(周济芳)4 1. State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. Faculty of Engineering, China University of Geosciences, Wuhan 430074, China; 4. Yalong River Hydropower Development Co., Ltd., Chengdu 610051, China © Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract: Core discing often occurs in deep rocks under high-stress conditions and has been identified as an important characteristic for deep rock engineering. This paper presents the formation mechanism of core discing firstly. Then, the interaction between diamond drill bits and rock was analyzed based on numerical modeling. A novel drill bit with an inner conical crown for the mitigation of core discing was designed and verified by simulation experiments. The mitigation method was applied in the cavern B1 of CJPL-Ⅱ and satisfactory results had been achieved. The percentage of core discing had been obviously decreased from 67.8% when drilling with a rectangular crown drill bit, to 26.5% when an inner conical crown drill bit had been adopted. This paper gives full insight into core discing characteristics and provides a new method for core discing mitigation; it will potentially contribute to stress measurement in deep rock engineering. Key words: core discing; mitigation; drill bit crown; deep rocks; in-situ stress measurements Cite this article as: ZHENG Min-zong, LI Shao-jun, YAO Zou, ZHANG Ao-dong, XU Ding-ping, ZHOU Ji-fang. Core discing characteristics and mitigation approach by a novel developed drill bit in deep rocks [J]. Journal of Central South University, 2020, 27(10): 2822−2833. DOI: https://doi.org/10.1007/s11771-020-4512-x.
1 Introduction Core discing refers to an engineering geological phenomenon in which the core is broken into discs or fragments [1], as shown in Figure 1. It is a type of rock failure caused by geological actions and external disturbances during the process of core drilling under deep burial depth and high stress environmental conditions. Many researchers have attempted to analyze the characteristics and mechanism of the occurrences of core discing
through laboratory testing, site observations and numerical analysis [2−6]. The interest in this phenomenon lies in the estimation of the in-situ stress from the shape and frequency of the failures along the core axis [7]. It was assumed that core discing may involve tensile failures, a combination of shear and tensile failures, or only shear failures. LIM et al [8] studied the relationship between in-situ stress and core discing for Lac du Bonnet granite and believed that the magnitude and direction of the stress had certain influences on the core disc thi
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