低应变率加载速度影响脆性岩体锚固效果的试验研究

外载荷加载速度是影响锚杆支护硐室稳定的重要因素,锚固体加载速度效应的试验研究较少。锚固硐室围岩会更接近单轴受力状态,围岩压力主要为低应变率加载,结合脆性围岩锚杆支护破坏特点,对布设两根相似锚杆的砂岩进行了从0.001～0.100 mm/s等5种低应变率加载速度工况下的单轴压缩试验。试验表明低应变率加载速度对加锚试样和无锚试样力学性质及破裂特征的影响是有差异的。加锚砂岩弹性模量随加载速度增加有轻微提升,所有工况下,锚固砂岩整体轴向变形量仍与无锚砂岩的轴向变形量相近;无锚试样的单轴抗压强度随加载速度增大呈递增趋势,但加锚砂岩强度随加载速度增大出现相对劣化,对加载速度的敏感性相对降低,锚杆加固增强作用减弱;各加载速度下无锚试样均最终表现为拉剪破坏,初始可见表面裂纹均为轴向张拉裂纹;加锚试样随加载速度增加会使最终破裂形式由张拉破坏向拉剪破坏过渡,初始表面裂纹由轴向张拉裂纹转变为剪切裂纹。进一步,从能量理论与加锚岩体声发射特征、锚杆与岩体相互作用等方面探讨了低应变率加载速度增大导致加锚砂岩强度劣化的机制,声发射信号表明高加载速度条件下加锚试样受载初期就会产生较大损伤,耗散能量增加,同时,高加载速度亦使岩体与锚杆间的界面载荷传递不能发挥作用。研究结果表明,锚杆支护下的冲击地压巷道应防范锚固体的扰动失效问题,推广"锚支卸"联合防冲支护措施。

Experimental study on anchoring effect of brittle rock mass influenced by loading rates at low strain rate

The loading rate of external load can strongly influence the stability of surrounding rocks of roadways supported by rock bolts. Currently,the anchored rocks under different loading rates are seldom studied by experiments.Surrounding rocks of anchored roadways are mostly under uniaxial loading condition,moreover,the surrounding rock pressure is mainly loaded at low strain rate. Combined with those failure characteristics of brittle surrounding rocks of roadways supported by rock bolts,the uniaxial compression experiments on sandstone with two bolt-like iron bars areperformed under five kinds of loading rates at low strain rate ranging from 0. 001 mm/ s to 0. 100 mm/ s. The experimental results show that loading rates at low strain rate variously influence the mechanical properties and fracture characteristics of anchored and unanchored sandstone samples. The elastic modulus of anchored sandstone samples isslightly improved with the increase of loading rates at a low strain rate. Under all the loading conditions,the axial compressive deformations of anchored sandstone samples are mostly same as those of sandstone samples without bolts. Uniaxial compressive strengths of samples without bolt can increase with the increase of loading rates,however,high loading rates can result in the bad strength characteristics of anchored sandstone samples compared to those unanchored samples,its strength sensitivity to loading rate is relatively reduced which indicates that the reinforcement effect of bolts is weakened by the loading rate. The final fracture mode of all unanchored samples is the tensile-shear failure which always occurs with an initial tensile surface crack. Increasing the loading rate can make the final fracture mode of anchored samples change from tensile failure to tensile-shear failure unlike that of unanchored samples,also turns its initial tensile surface crack into the shear crack. In addition,the strength deterioration of anchored sandstone influenced by loading rates at low strain rate is discussed based on energy theory,the acoustic emission characteristics of anchored samples and the interaction theory of rock bolt and rock mass. Acoustic emission signals show that the anchored specimen has large damage much early after loaded at high loading rate and its energy dissipation increases,furthermore,the high loading rate also makes the interface load transfer between the rock mass and the bolt ineffective.These results show that the load dynamics failure of rock-bolt system should be prevented especially for anchored roadways in danger of coal bump and comprehensive rock bolts support methods need to be promoted necessarily with the relieving pressure technology.