ABSTRACT
Five deep shafts are under construction as part of the Central 1 (C1) section of High Speed (HS2) Phase One, a new high-speed railway linking London to the Midlands, UK. These shafts form an integral part of the 16km long twin bore Chiltern tunnel providing ventilation and access for emergency interventions. The depth of these shafts varies from 35m to 67m depending on the vertical alignment of the tunnel and topography. Four of the shafts provide intervention and ventilation facilities, have an internal diameter of 17.8m, are formed using diaphragm wall construction. The fifth shaft, required only for intervention, has an internal diameter of 10.8m and is formed using cast in-situ caisson sinking method. The ground conditions at the shaft sites consist of a weathered chalk sequence which are complicated by the presence of infilled dissolution features.
Along the C1 alignment the twin running tunnels are positioned at 25m centre to centre spacing. In the case of first four shafts the TBMs directly intersect the shaft walls forming the ventilation openings and service access routes into the tunnel. The fifth shaft has been located between the two running tunnels and service access to the tunnels is made via sprayed concrete lined adits. This paper describes some of the key design and construction challenges associated with these shafts.
