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Abstract Most of deep rock mechanics focus on rock samples from the same depth in the existing literatures. People hope to investigate the effects of different depths on mechanical behavior through applying different confining pressures on samples. In addition，the physico-mechanical behaviors of different depth rocks have been reported in many literatures. However，it is well known that both the confining pressure and rock microstructure will change with the increasing depth. In Mentougou district in Beijing，there is a special area，where from the surface to the depth of 1 100 meters，there is basically a same lithology，namely basalt. Scanning electron microscope(SEM) system with loading devices has been successfully employed to investigate the mesoscopic failure behavior of Mengtougou basalt with centric preset notch in different depths，offsetting single notch and double notches basalt. We hope to reveal mesoscopic mechanism of basalts with changing depths. Experimental results indicate that the crack will initiate from preset notch and develop into a continuous main crack for most of basalt samples in three-point bending tests. Both the failure load and the fracture energy for different depth basalts increase with the depth. The main reasons are that not only the microstructure of deep basalt is compact，but the porosity is smaller with the increasing depth. Both peak load and fracture energy of basalt with single preset notch are approximately same with those of double preset notches for the same offsetting distance. In addition，peak load and fracture energy will linearly increase with the increasing offsetting distance. The main reason is that though the failure of basalts with offsetting notch is affected by both of shear force and bending moment；and the bending moment gradually decreases with the increasing offsetting distance which induce the increase of peak load and fracture energy increase.
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Received: 30 October 2012
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2013, Vol. 32 
