The Guu-Guann Dam, a concrete arch dam 82.5 meters in height, was damaged by the earthquake of Sept. 1999. The scope of repair work included sealing the leakage and repair shear cracks. The work included the use of borehole sampling and percussion echo detection of crack distribution, polyurethane resin injection grouting to fill oblique cracks; also grouting of horizontal leakage and verticalblock seams to effect water stoppage. The main difficulties and challenges facing this project were: improvement of resin injection grouting technology of very largestructures, the danger of frequent rock falls and aftershocks;and overhead operational safety.After completion of the grouting, the excellent effectiveness of all work was confirmed by drilling..This projects total contract amount was about12 million NT dollars.