The northern segment of the Sanyi Tunnel for a length of 413 m with a diametr of 10.6 m is being driven through gravel layer. It is a new double-tracked railway tunnel of the mountain line near central Taiwan. The gravel portion could be divided into two sections: a 183 m section going underneath the existing freeway with an overburden of 8~20 m, and a second section of 230 m is driven southward through the Miaoli hilly district. Due to the high ground water level above the tunnel, a pretreatment of grouting and forepiling is recommended before excavation.This paper describes the tunnelling methods adopted for the 230 m section, where the progress was only 5.7 m per month at the beginning, and had collapsed four times. Gradually, the progress rate was improved to 26 m a month after the grouting and tunnelling techniques were applied. The results were compared with that of the other section underneath the freeway, where the grout of microfine cement and the minipipe roofing were used.
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