Experimental Research And Its Engineering Application Of Anisotropic Mechanics Characteristic In Jointed Rock Mass Under Unloading Condition

In the process of long geologic effect, the rock mass was cut by structural planes with different shapes and directions into discontinuous rock mass with certain structure, which led to its complicated mechanical characteristic. Acoustic wave velocity test, Acoustic Emission Testing, conventional triaxial compression test and triaxial unloading test have been conducted on the jointed samples to discuss about their acoustic characteristic, loading and unloading mechanical characteristic. At the meanwhile,the size effect of jointed rock mass has been researched by numerical simulations and the equivalent mechanical parameters are obtained. What’s more, the loading and unloading yield criterion, unloading damaged equivalent mechanics model of the jointed rock mass has also been put forward, and the results has been applied in practice. There are several points on the research contents and conclusions as follows:(1) The acoustic wave velocity test of jointed samples shows that wave velocity of intact and jointed samples follows normal distribution. As the internal defect, measuring error and acquisition system accuracy, there is certain dispersion between the wave velocities. For jointed samples, wave can through the joint in different incident angle by changing the location of sensor. Compared with the intact samples, the wave attenuation is different with the incident angle, and the attenuation shows a linear increase with incident angle.(2) During the uniaxial compression test, the Acoustic Emission Testing on the jointed samples shows that the process of acoustic emission are composed of the initial compression area, the rise area, the peak area and the descending area. It has one-to-one correspondence with the curve of the stress strain. During the AE, the variations of AE amplitudes, hitting number and energy are the same. There are significant differences among them as the rock samples are with different incident angles. Therefore, the acoustic characteristic of jointed rock mass is anisotropic.(3) By the uniaxial compression test on jointed samples, the result indicates that the uniaxial compressive strength and deformation modulus of jointed samples both decrease, while the lateral and longitudinal displacements increase. With the changes of jointed dip, these parameters present a U-curve trend, of which the minimum value appears at dip60°The jointed samples have two failure modes, the axial splitting failure and the sliding along the joints failure.(4) By the conventional triaxial compression test on jointed samples, it shows that the variations of young modulus, deformation modulus, triaxial compressive strength and cohesion are the same to that present in the uniaxial compression test, which all show a U-curve trend. With the increasing of confining pressure, the ratios of above parameters’ extremum value decrease gradually, which indicates that the mechanics characteristic differences of jointed samples decrease. And the internal friction angle linearly increases as the jointed dip rises.(5) By the triaxial unloading test on jointed samples, the results shows that during the unloading process, there is big difference in the stress strain of jointed samples. The stress-strain curves of the samples in45°and60°are almost horizontal lines without failure characteristic, and the deformation modulus declines exponentially. In the unloading failure process, the axial stress and cohesion presents a U-curve distribution, while the45°and60°samples’axial stresses nearly do not decrease. This is closely related to failure mode. Different unloading rates (0.01MPa/s,0.02MPa/s,0.05MPa/s) have impact on deformation modulus, peak intensity, intensity parameter and failure confining pressure.(6) By the numerical test on equally spaced parallel jointed rock mass, it shows that the jointed rock mass with different dips all have size effect and the variations appear differently. When the rock mass strength is controlled by joint plane, the deformation modulus increases with model size in exponential relationship. When the rock mass strength is controlled by rock strength, the deformation modulus decreases with model size in logarithmic relationship to stabilize. Jointed rock mass with different sizes are also obvious anisotropic. The deformation modulus along the joint strike is less affected by the joints. The deformation modulus along joint inclination and elevation is greatly influenced by joints and presents a symmetric relation.(7) Based on the loading and unloading test and the analysis on mechanical effects of joint surface of the jointed rock mass, the loading and unloading yield criterion of single jointed and stratified rock mass was proposed. At the same time, the yield criterion is divided into2sections, one controlled by joint plane and the other by rock mass strength.(8) The deformation modulus and strength parameters of jointed rock mass decrease largely during the unloading process, which is defined as unloading damage. An equivalent mechanical model on unloading damage of jointed rock mass was put forward by setting up the one-to-one relationship between deformation modulus decreasing amplitude and damage variable in the unloading process. (9) On the basis of the equivalent mechanical model, it makes a numerical simulation on the interaction of high water level operating dam and slope in Xiaowan project. The result suggests that the variations of calculation value is the same with the monitoring one, and the two reaches a good agreement on values. It turns out that the mechanical model can well illustrate the deformation characteristics of jointed rock mass.