- Stormwater drainage tunnel has 34 intakes and 32 dropshafts.
- Main tunnel diameter from 6.25m to 7.25m.
- Main tunnel length approximately 10.5km.
Arup is the consultant to the Hong Kong Drainage Services Department in carrying out the design, pre-qualification, tender and construction supervision for the Hong Kong West Drainage Tunnel (HKWDT).
The objective is to relieve flooding risk in the low-lying urban area of north-west Hong Kong Island. The main components of HKWDT are a 4.5km section with a diameter of over 6m and a 6km section with a diameter of over 7m.
Stormwater collection and drainage
The tunnel will intercept surface stormwater runoff from an upper catchment area at the mid-level area of Hong Kong Island and discharge up to 240m3/s at the final outlet portal to the west.
Stormwater will pass through a drainage system of horizontal shafts, 32 dropshafts up to 195m deep, an eastern inlet portal structure and over 30 other intake structures.
Tunnel boring and excavation
The main tunnels will be constructed by two tunnel boring machines (TBM). The horizontal adits will be constructed using the drill and blast method, carried out concurrently with the TBM excavation.
Most of the dropshafts will be constructed using raise boring. Disturbance at ground level is minimised by drilling a small shaft from above that is then expanded to the required dimensions from below.