Abstract: Severely affected by 921 ChiChi earthquake and devastating flood that followed due to Typhoon Mindulle and heavy rain, thick debris accumulation with flood, blocking the tailrace tunnel and other exports, and even flooding the underground powerhouse, causing serious damage to power generation facilities. In order to re-operation, the underground powerhouse and other facilities must be dredged and rebuilt. In view of flooding, the ventilation shaft extended upward 241 m to the slag field platform, that was filled by loose material, where must be adopted pipe roofing method and allcasing pile wall method to reach slope stability and to deal with backfill issue, then raise boring method to facilitate slag material down into the connection tunnel, accelerating the slag operation.

Key words: ventilation shaft, pipe roofing method, all-casing pile wall method, raise boring method