Underground Singapore 2011 2.
DESIGN CONSIDERATION
2.1. Site layout The majority of connection tunnels had to be constructed beneath the base slab of the existing cut and cover tunnels at the South East corner of Circle Line (CCL) Promenade station. In addition, 2m to 3m of the bottom part of the existing diaphragm wall together with the toe of two bored piles supporting the underground linkway of CCL Promenade station, had to be removed. Therefore, two NATM/SCL connection tunnels are proposed to fulfil this purpose. These two tunnels are stacked vertically and have similar circle shape and internal size. Figure 2, Figure 3 and Figure 4 show the layout plan and section of connection tunnels respectively. In this paper, we will focus on the upper connection tunnel.
Figure 2 –Layout Plan of Connection Tunnels to existing CCL Promenade Station
2.2. Ground Condition Subsurface condition along the connection tunnels was well established from adjacent boreholes. It revealed that the soil types from ground to tunnel invert comprise mainly the old alluvium, which was overlain by Fill and Kallang Formation. The soil to be encountered within tunnel face was described as hard Sandy Clay and hard Sandy Silt with SPT values more than 100.The depth to upper tunnel axis was approximately 28.7 m below ground level.
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