| During the excavation of underground projects in western Sichuan, macroscopic and mesoscopic jointed and cracked rock was widely distributed which led to the brittle split,spalling, rockburst and water burst disasters of surrounding rock. They all endangered the construction and operation safety. Therefore, supported by the National Natural Science Foundation of China (NO. 41272321), this thesis started with the statistical analysis of geometric characteristics of natural cracks network in hard and brittle rock, and the following studies in the field of the mechanical characteristics and fracture mechanism of craked and hard rock were carried out:(1) In order to study the formation mechanism of cracks in the rock and the fracture mechanism of cracked tunnel surrounding rock, a quantitative analysis and evaluation technique for complex crack network geometries was proposed and applied to the Niba mount rhyolite. Mechanical and geometric model of this rhyolite and crack network were built.(2) Based on the mesomechanics of composites and the interaction model of interaction direct derivative (IDD), and compared with the Dilute method and Giordano model which neglect of inclusion interaction and the interaction modified Taylor model,IDD had advantages in the analysis of cracked rock. Then IDD was used to discuss the influence of density, shape, dip angle and infilling of cracks on the effective elastic modulus of cracked rock. Combined with the analysis and evaluation technique of complex crack network geometries above mentioned, a new calculation method of effective elastic modulus for complex cracked rock was proposed and its reliability was verified by experimental results.(3) Three phase composite rock model consisted of "crack-infilling-rock" was proposed. Uniaxial compression tests were carried out on the specimens of composite rock model to study the compression-shear fracture mechanism. The concept of volume fracture energy Gf-V was proposed by introduced "fracture volume" to replace the "fracture area".Based on the results of three experimental groups, the bilinear model of front boundary effect was built, and then it was used to analyzed the influence mechanism of crack in heterogeneous infilling on the fracture characteristics of the composite rock.(4) "semicircular arch+straight wall+flat floor" arched tunnel model specimens were machined in the naturally cracked rhyolite dug from the Niba mount tunnel region.Biaxial compression tests were performed to measure the dynamic fracture process and the typical surrounding rock strain,which revealed the fracture mechanism of naturally cracked, hard and brittle surrounding rock. The results showed that the floor and roof of tunnel crack initiation and the tensile crack propagated to the deep, the slabbing failure was occurred in the sidewall and propagated to the deep surrounding rock to form a "V-shaped" damage zone, and the crush zone was formed in local arch springing.(5) Biaxial test was used to study the fracture mechanism of an arched tunnel whose floor and roof was nearby two big cracks. It showed that the big cracks shear slip initiation,there were some vertical cracks appeared and led to its fracture influenced by the nearby big crack, the induced cracks in the floor propagated to the sidewall and springing line,which resulted in the unsymmetrical fracture modes.(6) The crack initiation, propagation and the propagation of initial cracks of intact and cracked tunnel surrounding rock were simulated by discrete element particle flow code PFC2D. The results of numerical in the aspect of mesco-fracture mechanism and macro-propagation trend of cracked and hard surrounding rock showed agreement with the experimental results(7) Based on the hard and brittle rock-like material, the arched tunnel model with pre-slabbing sidewall was manufactured. Under biaxial compression, the surrounding rock was brittleness failure, the roof was integrally sunk, the pre-slabbing sidewall was buckling fracture and broken to the inner tunnel to form wedge-shaped fracture zone which was similar with that of rhyolite tunnel model. However, due to the slab cracks, the tangential stress of middle and bottom sidewall decreased and the fracture zone was limited in 0.4R. |
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