Constructions of underground excavations are of great interests to mining engineers. Geological features, geomechanical parameters of rock mass and stress conditions, have essential role in underground openings. Analysis of stresses and deformations due to construction of openings, are important in stability considerations. In this research, 2D and 3D continuum analyses of the underground openings excavated in extension phase of Masjed-E-Soleiman hydroelectric project, Khuzestan province, Iran, are compared. 2D and 3D models have been developed with assuming that the rock mass obeys Mohr-Coulomb criterion. The study revealed that for inhomogeneous rock mass with weak zones, 3-D elasto-plastic analysis exhibits more agreement with the field observations, however 2-D elasto-plastic analysis yields conservative results.
The use of underground excavations varies from straight-line tunnels to complex excavations in hydroelectric projects. For considering the stability of excavations, analysis of stresses and deformations is necessary. Construction of underground excavations in the rock mass causes a newdistribution of stress (induced stress). For analysis of stresses and deformations, Numerical modeling is one of the useful methods. Numerical analyses are divided into 2-D and 3-D analyses. For straight-line excavations 2-D analyses are used, with assumption of plain strain conditions. However, according to discontinuous nature of rock mass and presence of joints, beddings, faults and induced stresses, it seems that this assumption is not valid. On the other hand due to cyclic excavation and support installation, at least in the near of working face, this assumption is not valid. In complex geometries and geologies, two-dimensional (e.g. plain strain) method is inadequate. In these situations 3-D numerical analysis is necessary. 3-D numerical analyses are extremely difficult, time consuming and expensive, however, they will present better results (Vermeer et al. 2003). Duddeck (1991) noted that if the engineering design requires knowledge of the induced stresses and deformations of the tunnel structure, the geometrical changes at the working face and the sequences of excavation and support must be considered. Studies of Pan and Hudson (1988) and Kielbassa and Duddeck (1991) have shown that two-dimensional plane strain models, in comparison to three-dimensional equivalent of same models, are inadequate when stresses and tunnel convergence near the tunnel face are modeled. Dahawan et al. (2002) investigated 2-D and 3-D elastoplastic analyses for a set of four underground openings. This study revealed that for weak and inhomogeneous rock mass, deformations obtained with 3-D elasto-plastic analysis are more as compared to 2-D analysis. Ahmadi et al (2006) considered 2-D and Quasi-3-D for underground openings. Their study demonstrated that for inhomogeneous rock mass, deformations in the weak rocks, determined from 3-D analysis are more in comparison to Quasi-3-D analysis. Whereas, for strong rock mass deformations are lesser for 3-D analysis as compared to Quasi-3-D analysis that is in agreement with results of Dahawan's studies.
