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  Construction Experiences from Cross Passage in Very Hard Rock and Blasting Works near Segmental Lining

  2.3km long twin bored tunnels were constructed in one of the contracts under DTL2 which is located in the western part of Singapore. After the construction of twin tunnels namely Expo Bound and Bukit Panjang Bound tunnels, 7 numbers of cross passages were constructed by mining. These cross passage will serve as a fire escape passage way between the Expo and Bukit Panjang Bound tunnels. The cross passages are also located in different types of ground condition such as in rock, in soil and in mixed ground. This paper will focus on the 3 numbers of cross passages which are located in rock. The length of these cross passages varied from 11.0m to 18.0m and these were excavated using mined excavation approach. As the twin bored tunnels were built before cross passage excavation and the cross passages are of small cross-sectional profile, the use of heavy machinery is limited. Thus, the excavation and blasting works would need to be carried out in confined space using small machinery. In this paper, we will discuss the excavation procedures, blasting technique and methodologies, and the measures taken to ensure minimal damage to the bored tunnel segment lining during blasting. In addition, we will discuss the blasting parameter such as maximum peak particle velocity in the Bukit Timah Granite and make a comparison with the measured readings. The effect of the cross passage excavation such as ground settlement, water ingress, surface vibrations, will also be addressed in this paper.

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