a b s t r a c tThis study presents crack initiation, propagation and coalescence at or near pre-existing open cracks or flaws in a specimen under uniaxial compression. The flaw geometry in the specimen was a combination of a horizontal flaw and an inclined flaw underneath. This flaw geometry is different from those reported in the previous studies, where a pair of parallel flaws was used. Three materials were used, PMMA (Poly Methyl MethAcrylate), Diastone (types of molded gypsum), and Hwangdeung granite. Crack initiation and propagation showed similar and different patterns depending on the material. In PMMA, tensile cracks initiated at the flaw tips and propagated to the tip of the other flaw in the bridge area. The cracks then coalesced at a point of the inclined flaw, which is affected by the flaw inclination angle. For Diastone and Hwangdeung granite, tensile cracks were observed followed by the initiation of shear cracks. Coalescence occurred mainly through the tensile cracks or tensile and shear cracks. Crack coalescence was classified according to the crack coalescence types of parallel flaws for overlapping flaw geometry in the past works. In addition, crack initiation and coalescence stresses in the double-flawed specimens were analyzed and compared with those in the single-flawed specimen. Numerical simulations using PFC 2D (Particle Flow Code in two dimensions) based on the DEM (Discrete Element Method) were carried out and showed a good agreement with the experimental results in the coalescence characteristics in Hwangdeung granite. These experimental and numerical results are expected to improve the understanding of the characteristics of cracking and crack coalescence and can be used to analyze the stability of rock and rock structures, such as the excavated underground openings or slopes, tunneling construction, where pre-existing cracks or fractures play a crucial role in the overall integrity of such structures.

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Scaled model tests were performed to investigate the effects of (1) faults, (2) weak planes, and (3) grouting on the stability of a tunnel. Numerical analyses were carried out and the results compared with those of the scaled model tests. When every parameter is properly scaled down in accordance with the scaled-down geometry, it is possible to represent the geological conditions of the operation site with the scaled model.Four different types of model were made to investigate the effect of a fault on the stability of a tunnel, and five different model types to examine the influence of weak planes and grouting effects. A mixture of sand, plaster and water was used to reproduce the scaled-down physical properties of rock and weak planes. The scale factor of the model was checked with field conditions. A hydraulic biaxial compression testing machine was used. Numerical analysis using FLAC 2D for the actual scaled model test was carried out as shown in Fig. 1. It was observed that the amount of deformation and the size of the plastic region increased as the distance from a fault to the tunnel decreased. When a fault was located across the upper part of the tunnel, the displacement of the crown was greater than when the fault was located to the left of the tunnel.The numerical analysis also showed that the tunnel was found to be stable when the cellular arch method (CAM) reinforcement was installed around the crown of the tunnel. However, better stability was obtained when the region above the crown of the tunnel was systematically grouted. Concerning the existence of a weak plane, the amount of displacement at the side wall of the tunnel was quite large, since the shear deformation occurred along the weak plane. Numerical analysis presented quantitative values comparable to the results of the scaled model tests. ARTICLE IN PRESSFig. 1. The failure zone and plastic region around the tunnel in the scaled model test, (a) pressure=3.0 kg/cm 2 , (b) plastic region by FLAC 2D analysis.