高应力取芯卸荷损伤及其对岩石强度的影响

CORING-INDUCED UNLOADING DAMAGE TO ROCK SAMPLES UNDER HIGH STRESS CONDITION AND ITS IMPACT ON ROCK STRENGTH

合理准确地获得深部岩石的强度对岩爆等动力地质灾害的预报和防治具有重要意义。针对高应力条件下钻孔取样过程造成的应力卸载对岩样的损伤问题,通过现场取样、数值模拟和室内试验等方法,比较不同应力水平下钻孔取样的损伤范围及程度。数值模拟和现场所取岩芯的饼化现象证实,所设计的应力解除取样方案切实有效,可有效降低取样区域的应力水平(约30%),可在同一地点取得不同应力水平的岩样;CT扫描结果表明,在锦屏二级水电站1 900 m深条件下常规岩石取样导致的卸荷损伤范围可能超过岩样横截面的50%。低应力取样方案所获得的大直径岩样通过套钻取得中心标准样的平均强度要比常规取样条件提高5%～15%,平均约8%,表明卸荷损伤对岩样强度的影响不容忽视;另外,前者单轴压缩时加载初期的声发射事件数明显少于后者,这表明即便是经过套钻,也不能完全消除取样损伤的影响,与CT扫描的结果相符。

The reasonable and accurate evaluation of deep rock strength has great importance on forecasting and controlling rock bursts and other dynamic geological disasters during excavation.Aiming at the problems of rock sample damage induced by coring-induced unloading,the damage zones and extent of rock samples under different stress levels are studied through numerical simulation and field sampling and indoor experiments.The results of numerical simulation and the corediscing phenomenon both confirm that the stress relieving sampling scheme suggested in this paper is effective,which can reduce stress of the sampling area by about 30%;and make it possible to core rock sample under different stress levels in same place.According to the result of the compaterized tomography（CT） scanning of the rock samples obtained from tunnels of Jinping II hydropower station with a buried depth of 1 900 m,unloading-induced damage extent may exceed 50% of the whole section of rock samples.The average uniaxial strengths of the rock samples by means of low stress sampling and over-coring increase by approximately 5% to 15% than conventional sampling under high stress condition and over-coring,with an average of about 8%;so the influence of coring-induced unloading damage on rock strength can？t be ignored.And the number of acoustic emission events of the former at early loading stage is significantly less than the latter,which indicates that the coring-induced unloading damage can？t be eliminated completely even by over-coring;and this result is in accordance with that of CT scanning.