Geotechnical characterization of red shale and its indication for ground control in deep underground mining

Abstract

Geotechnical properties of red shale encountered in deep underground mining were characterized on both laboratory and field scale to reveal its unfavorably in geoenvironment. Its constituents, microstructure, strength properties and water-weakening properties were investigated. In situ stress environment and mining-induced fractured damage zone after excavation were studied to reveal the instability mechanism. The results show that red shale contains swelling and loose clayey minerals as interstitial filling material, producing low shear strength of microstructure and making it vulnerable to water. Macroscopically, a U-shaped curve of uniaxial compressive strength (UCS) exists with the increase of the angle between macro weakness plane and the horizon. However, its tensile strength reduced monotonically with this angle. While immersed in water for 72 h, its UCS reduced by 91.9% comparing to the natural state. Field sonic tests reveal that an asymmetrical geometrical profile of fractured damage zone of gateroad was identified due to geological bedding plane and detailed gateroad layout with regards to the direction of major principle stress. Therefore, red shale is a kind of engineering soft rock. For ground control in underground mining or similar applications, water inflow within several hours of excavation must strictly be prevented and energy adsorbing rock bolt is recommended, especially in large deformation part of gateroad.

摘要

软岩是一种对地质工程有危害的岩体。本文主要研究在深部开采时红页岩的实验室与实际应用 中的地质特性。通过成分分析,显微结构,强度特性等测试,尤其是水软化实验,揭示在原岩应力环 境中或开采扰动下特定巷道中红页岩的失稳机理。实验结果表明,红页岩在失稳时具有膨胀性,抗剪 强度变低,遇水易软化且失去粘土矿物作为充填介质的特性。随着软弱面与水平线的夹角不断增大, 在单轴抗压强度图像上可明显见到其曲线呈U 型,红页岩的失稳模式与这个角有很大的关联。强度实 验中红页岩的抗拉强度随这个角度的增大而单调减小,抗剪强度也比完整红页岩的低。当使用摩尔库 伦强度理论时,内聚力也会随这个角度的增大而减小。红页岩在浸水72 h 后,单轴抗压强度比原始状 态的减少91.9%。通过现场声发射实验,由于层理面的存在,当巷道沿主应力方向布置时会观察到一 个明显的不对称断裂面。根据研究与实验结果,红页岩为地质软岩,在深部开采地压控制或相同地质 情况下应严格控制水流量,在变形较大的巷道中使用预应力锚杆。