Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (V_S1) and perpendicular (V_S2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.     

Influence of bedding structure on stress-induced elastic wave anisotropy in tight sandstones

To understand the evolution of stress-induced elastic wave anisotropy,three triaxial experiments were performed on sandstone specimens with bedding orientations parallel,perpendicular,and oblique to the maximum principal stress.P-wave velocities along 64 different directions on each specimen were monitored frequently to understand the anisotropy change at various stress levels by fitting Thomsen’s anisotropy equation.The results show that the elastic wave anisotropy is very sensitive to mechanical loading.Under hydrostatic loading,the magnitude of anisotropy is reduced in all three specimens.However,under deviatoric stress loading,the evolution of anisotropic characteristics(magnitude and orientation of the symmetry axis)is bedding orientation dependent.Anisotropy reversal occurs in specimens with bedding normal/oblique to the maximum principal stress.P-wave anisotropyε0 is linearly related to volumetric strain Sv and dilatancy,indicating that stress-induced redistribution of microcracks has a significant effect on P-wave velocity anisotropy.The closure of initial cracks and pores aligned in the bedding direction contributes to the decrease of the anisotropy.However,opening of new cracks,aligned in the maximum principal direction,accounts for the increase of the anisotropy.The experimental results provide some insights into the microstructural behavior under loading and provide an experimental basis for seismic data interpretation and parameter selection in engineering applications.     

对 Brandt 多孔隙颗粒介质弹性理论的修正及饱和土体的骨架波速

利用修正项来修正Ｂｒａｎｄｔ多孔隙颗粒介质弹性理论中的体积模量表达式，使其与Ｂｉｏｔ Ｇｅｅｒｔｓｍａ理论的结果一致起来。提出了计算骨架波速的新方法，探讨了一些物理参数对波速的影响。     

Estimation of elastic wave velocity through unsaturated soil slope as function of water content and shear deformation

The objective of this study was to estimate the elastic wave velocity of an unsaturated soil slope, and to verify its applicability. The elastic wave velocity in a silty sand was measured. The individual influence of the volumetric water content and the tilt angle on the normalized wave velocity through unsaturated soil were investigated through a series of varied slope model tests. The relationship function of the normalized wave velocity-volumetric water content-tilt angle was established. To verify the proposed estimation function, a series of fixed slope model tests was carried out. The relationship functions were used to estimate the behaviors of the wave velocity in rainfall-induced slope failure model tests. The applicability of the proposed relationship functions for the wave velocity behaviors was also presented. It was found that the estimation function is highly consistent with the measurements for the wave velocity behaviors through unsaturated soil slope in the presented test conditions. In addition, the effects of the rainfall duration/initial water content, density, slope angle and surface layer thickness on the decrease rate of the normalized wave velocity with the volumetric water content and the tilt angle within the test conditions in this study were seen to be small.     

线性回归法探讨马钢地区土层标贯锤击数与剪切波速值的关系

通过马钢地区各类土层标贯锤击数与剪切波速值的实测数据进行线性回归分析及拟合,得出了该地区以土层标贯锤击数计算剪切波速值的经验公式。     

苏州第四纪深厚地层剪切波速空间变化特征及其应用

土体剪切波速是工程场地地震效应评价的重要参数之一。基于苏州城区的大量剪切波速实测资料和工程地质特征,探讨了第四纪深厚沉积层剪切波速结构空间变化特征,给出了不同工程地质分区的土层剪切波速随深度变化的经验公式;基于不同深度范围内土层平均剪切波速间存在的显著相关性,建立了基于浅层剪切波速外推深层剪切波速的逐步外推法,验证结果表明:(1)苏州城区等效剪切波速ν_(s20)和ν_(s30)的空间展布具有显著的分区特征,覆盖层厚度d80m的Ⅲ类和Ⅳ类场地的等效剪切波速分界值ν_(s30)可以取为170 m/s;(2)平原区土层剪切波速随深度的变化呈幂律函数关系;丘陵区细化为3个小的区域后,各分区内土层剪切波速随深度的变化呈二次多项式关系;(3)逐步外推法具有很好的适用性,但当外推深度处的土层剪切波速存在剧烈变化时,剪切波速的逐步外推法失效;(4)基于剪切波速逐步外推法,可给出剪切波速不小于500 m/s和700 m/s的任意假想基岩面埋深H_(rock)及相应的微震动场地卓越周期T_g,发现T_g值和H_(rock)值的空间展布基本一致;选取剪切波速不小于700 m/s的土层顶面为地震基岩面时,各工程地质分区的T_g值和H_(rock)值空间展布的差异性显著增大。