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Discontinuous Deformation Analysis (DDA) was used to study the behavior of deep excavation and tunnel excavation in clean cobble-gravel deposits. Preliminary results obtained indicates that DDA is capable of simulating the behavior of discontinuum medium consisting of distinct particles. Due to the particle shape and the deposition process, natural clean gravel deposit would exhibit anisotropic characteristics. This anisotropic characteristics would be favorable for the stability of open cut but unfavorable for tunnel excavation.
In northern and central Taiwan, there exists a vast deposit of Quaternary cobble-gravel formation. Especially in Taichung metropolitan area, a layer of clean cobble-gravel deposit over 20m to 200m thick posed a major problem in the construction of building foundation and underground work. Past experiences from open excavation for building foundation and slope excavation show that it can maintain a stable condition with very little support work. This fact cannot be easily explained by the results of in-situ strength test. The main reason for this contradicting behavior may lie in the anisotropic characteristics exhibited. Therefore, whether the experiences gained from open excavation is applicable to tunnel excavation remains questionable. Numerical simulation on tunnel excavation in granular deposit has been performed by using elasto- plastic continuum model. This modeling method may be appropriate if the particle size is small enough when compared with the concerned dimensions of the problem. But due to large particle size and the discontinuous nature of deformation in the particle system, i.e., contact sliding and particle rotation, the continuum model may not be suitable. This approach is further hindered by the difficulty in obtaining the mechanical properties of the coarse grained medium in-situ or in the laboratory. The DDA method considers the deformation and rotation of individual particle and sliding between particle contact while maintaining the equilibrium of the particle system. Therefore, it is more suitable in dealing with systems consisted of distinct particles. In this study, different sizes of 2-D polygons of 12 vertices and 12 sides approximately in elliptic shape were used to simulate the cobble-gravel particles. The deposits of particles were formed by gravity with the long axis lying either primarily in horizontal or vertical direction which were termed horizontal deposit or vertical deposit hereafter in this paper. Direct shear tests were performed numerically by DDA to demonstrate the anisotropic characteristics in a global sense of the horizontal and vertical deposits. Open excavations on both deposits were also simulated by DDA to investigate the influence of anisotropic characteristic. In addition, tunnel excavated in horizontal deposit was studied by DDA method to observe the performance of gravel mass around the tunnel.
Several modifications were done based upon the DDA computing code (DC, DI, DG) of 1993 version developed by Shi(1993) for better simulating the processes of particle generation, sedimentation, consolidation and excavation. Computing program DISC developed by Ting et al. (1988) were also utilized to reduce the executing time of the processes of particle generation as well as initial sedimentation and consolidation.
