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The stratum under the Taiwanese area has been impacted by tectonic force induced by earthquake. The discontinuities exist more and more in rock mass in the stratum. In the underground excavation, the fracturing pattern in the underground structure is mainly influenced by the rock masses formed by discontinuities. The stability of the whole rock masses could be lost if the key block in the rock masses is unstable. The instability of the key block will cause the initial movement of the rock masses and sequentially displace the whole rock masses. Therefore, the stability analysis of the key block is the major task of the underground excavation. The previous successful research of applying the Key Block Theory to the block stability analysis of Mintan underground electrical power plant is continued in this research in order to investigate the influence of the variable intersection angles between tunnels to the stability of underground excavated chambers in rock masses. The research results demonstrate that the choice of direction for the intersection dimension of an underground chamber would greatly influence the type, numbers, safety factor, mode of sliding, strength of sliding and volume of potential key blocks. In the Mintan electrical power station, the analytical result of the stability analysis of rock masses formed by the underground excavated chamber and jointed rock masses using Key Block Theory in this research obtains the optimum range of the angles between the direction of long side of the chamber and the rock discontinuity. The range could be between 15 to 75 degrees for the optimum layout of the underground chamber.
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