ABSTRACT: This paper presents a case study on the optimum design of excavation and support system of a shallow tunnel which is faced with the unexpected bad geological condition during excavation, The detailed geological investigation shows that the rock mass in the region of concerns is heavily weathered and fractured with RMR value less than 20. Considering this, the concept of the new design is focused on the reinforcement of the ground preceding the excavation and two design patterns are suggested. One is L W -grouting & fore-poling with pilot tunnelling and the other is the steel pipe reinforced grouting. Numerical analysis by FLAC shows that these two patterns maintain the roof ground stable during the whole excavation process while the original pattern causes tensile failure around the tunnel and the severe floor heaving. This process of the design assessment can be a good guide in the construction of the tunnels in heavily fractured rock mass.

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