层状岩体滤波特性研究

Filter property of stratified rock mass

用振幅、相位透射、反射系数描述应力波在结构面的波形变化,充分考虑应力波在层间、层内多重透射、反射条件下建立层状岩体中透射、反射系数计算模型;分别计算的层状岩体透射系数及波形与已有研究结果吻合,表明该模型计算结果合理。讨论结构面刚度、岩层厚度及结构面数量对层状岩体滤波特性影响。结果表明,结构面刚度及岩层厚度是影响弹性层状岩体滤波特性的主要因素,层状岩体具有多尺度梳状滤波器特性,随岩层厚度与波长之比γ增加,振幅透、反射系数成两种尺度周期性变化,由层内多重透、反射形成大尺度周期γ=0.5,并存在一个带通、一个带阻,在带通内又存在多个小尺度周期,由层间多重透、反射形成;层状岩体具有多带通滤波特性,结构面刚度增加,岩体带通上、下限频率同时增加,且上限频率增加幅度较下限大,随频率增加,带通宽度近似成负指数减小;所有岩层厚度相同时带通中心频率约为岩层一阶固有频率的整数倍,各岩层厚度不相等时层状岩体仅在各岩层共同谐振频率处表现为带通。

The waveform change of stress wave propagating across the interfaces can be described by the transmission and reflection coefficients of its amplitude and phase. The calculation model for the transmission and reflection coefficients of stress wave propagating through stratified rock mass was established on the basis of considering fully the conditions of propagating through multiple interlayers as well as the intrastratal transmissions and reflections. The transmission coefficient and transmission waveform through stratified rock mass were calculated using the established model,and the results are of good agreement with the previous research results. The analysis results show that the model is reasonable. The effects of interface stiffness,thickness of stratum and interface number on the filter property of stratified rock mass were discussed. The study results indicate that interface stiffness and thickness of stratum are the main influential factors on filter property of elastic stratified rock mass. And the stratified rock mass has the multi-scale comb filtering properties. The amplitude transmission and reflection coefficients have two-scale periodic change with the increase of the ratio of stratum thickness to wavelength. The large-scale period, for intrastratal multiple transmissions and reflections of stress wave across stratified rock mass,is 0. 5. There exist a pass band and a stop band in a large-scale period. There are as well multiple small-scale periods in a pass band caused by multiple transmissions and reflections between interlayers. The stratified rock mass possesses the multiple band-pass filtering property. The upper limit frequency and lower limit frequency of pass band increase simultaneously with the increased of interface stiffness. The increased extent of upper limit frequency is more than that of lower limit frequency. The width of pass band decreases approximately in a negative exponential form with the increase of frequency. The center frequency of pass band is about an integral multiple of the first order inherent frequency of stratum when each stratal thickness is equal. But,the stratified rock mass has the band-pass property only at its resonance frequency when each stratum has unequal thickness.