The construction of the emergency spillway tunnels of Imha Dam is being in progress on the granite region including fault fractured zone. Considering that this tunnel is being excavated as triplets tunnels, pillar width between each tunnel and face distance between each tunnel face have been examined. The influence of the fault fractured zone for the tunnel stability has been investigated by numerical modeling in 3D. Various geophysical investigations and rock engineering field tests have been carried out for these purposes. It was suitable that the second tunnel would be excavated in advance, maintaining the face distance between each tunnel face of minimum 25 m. The numerical analysis indicates that the roof displacement and the convergence of the second tunnel are insignificant, and the maximum bending compressive stress, the maximum shear stress of shotcrete and the maximum axial force of rockbolt are also insignificant. Therefore, it is estimated that the stability of the spillway tunnel ensured.
It is frequently flooded due to the unusual change of weather, and the design standard for the dam's flood water has been strengthened to the probable maximum flood (PMF). Since the probable maximum precipitation (PMP) calculated in the consideration of localized heavy rain in each dam's drainage area has increased than those days of design, it is necessary to secure the safety of existing dam during the inflow of the probable maximum flood. As similar, there is necessity for securing the safety of Imha Dam and its spillway to protect life and property of citizen. Thus, the construction of the emergency spillway tunnels has been propelled. However these spillway tunnels pass through granite region with developed fault fractured zone, and are constructed as the triplets tunnels, thus pillar width between each tunnel and face distance between each tunnel face have been examined, and roof displacement and convergence of tunnels have been analyzed through three dimensional numerical analysis of the curved area of tunnels influenced by the fault zone. Also the stability analysis of the spillway tunnels has been examined by analyzing the maximum bending compressive stress of shotcrete, the maximum shear stress of shotcrete and the maximum axial force of rockbolt,.
The region of Imha Dam is located at 16km above Banbyun stream which is main tributary of the upper stream of Nakdong river, and it is included at western part on the Taeback Mountains which stretches from the southern extremity to the southwest extremity. The Taeback Mountain range adjoined to this region consists of Baekam mountain, Mupo mountain to the east, and Go mountain, Guam mountain and etc to the west, and creates a watershed with this mountain system. Fig. 1 shows the geological feature of the surrounding Imha Dam. The Precambrian gneissic granite with the crystallized limestone is distributed around Ahgi mountain located on the central part of the dam area. On the upper part, the sedimentary rocks of Gyeongsang System.
