Leighton Contractors (Asia) Ltd made an open vertical excavation, mostly in rock, of up to 40 m in depth prior to construction of the reinforced concrete underground railway station and concourse at North Point in Hong Kong. The site of the station is adjacent to a heavily trafficked main road, is surrounded by heavily populated high rise buildings, lies at the foot of steeply sloping ground rising to 300 m above sea level and is separated from the sea by about 100 m of reclaimed ground. At the site there was a thin cover of fill and weathered rock overlying fresh pinkish grey fine- to medium-grained widely jointed granite belonging to the Hong Kong Granite formation. Rockhead was found to dip to the north (towards the sea) with a zone of less severe but deeper seated weathering in the northwest corner of the excavation.
The stability of the excavation faces was dominated by steeply dipping discontinuities and in view of the serious consequences of failure on adjacent land and property it was decided to assume infinite persistence of major joints when calculating stability.
To ensure that stability was maintained during excavation, and because of the tight restrictions on working space, a system of temporary support utilizing rock anchors and bolts was adopted. The location, and capacity, of the support was limited by:
The presence of running tunnels adjacent to the main station wall. A separate contract for the tunnels was undertaken at the same time as the main excavation proceeded