卸荷过程岩体中弹性波波速变化分析

Analysis of variation of elastic wave velocity in rock mass under unloading condition

伴随岩体卸荷过程,岩体内部应力状态要进行调整,同时岩体内部几何结构也要发生变化。由于卸荷过程应力状态变化很复杂,文中利用卸载过程中岩体内部几何结构的变化来反映卸载过程,通过岩体几何结构对弹性波传播的影响来分析卸载过程的波速。为考虑弹性波作用下局部裂纹的相互作用,采用双裂纹模型近似分析。在双裂纹体系内部采用"相互作用"分析法,以部分考虑波在岩体内的多次散射;在双裂纹体系之间采用线性叠加分析法,以考虑岩体中缺陷影响的局部化。通过节理裂隙的张开程度和节理裂隙的张开率描述卸载过程对双裂纹体系的影响,由此,对比分析了开挖卸载过程中四种不同的玄武岩与主频25 kHz和主频1 kHz对应的弹性波波速的变化。结果表明,随着卸载过程的推移,主频25 kHz声波和主频1 kHz地震波对应的相对波速逐渐减小,但声波波速的减小幅度要比地震波减小的幅度小,声波波速可以降低到原来的80%,而地震波波速可以降低到原来的50%。结论对于水利工程、隧道工程等建基面的验收和评价有很好的指导意义。

The stress state and the geometrical structures in rock mass are adjusted during unloading.Since it is difficult to describe the process of stress adjustment,the geometrical structures are used in the present paper to investigate the process of rock mass unloading,and the elastic wave velocity is studied by considering the interaction of geometrical structure and elastic wave.A double-crack structure is employed to make approximate analysis of the elastic wave velocity in jointed rock mass.In the model,the analysis method of interaction within the double-crack structure is used to partly consider multiple scattering between the joints of rock mass,and the linear superposition method between double-crack structures is adopted to analyze the localization effect for defects in rocks.The influence of unloading on the rock bridge is described by the opening displacement and the opening rate of joints,and variation of elastic wave velocity of four different kinds of rocks corresponding to two dominant frequencies,25 kHz and 1 kHz,is studied under excavation unloading condition.The results show that in the process of unloading,relative acoustic velocity corresponding to 25 kHz and seismic velocity corresponding to 1 kHz are gradually decreased;however,the decreasing degree of acoustic velocity is less than that of seismic velocity.For example,acoustic velocity of highly unloaded rock mass reduces to be 80% of that of the original rock mass,and seismic wave velocity reduces to be 50% of that of the original rock mass.These conclusions are of good guidance for the inspection and evaluation of water conservancy project and railway tunneling construction.