Stanley Pier reconstruction

Rebuilding a pier over open water means working below the tide line. We planned the whole job around a set of reusable waterproof steel formworks that are floated into position, sunk around each pair of piles to form a watertight mould for the concrete.

Sequenced in five stages

• Day 30: underwater welding of I-beam ties between piles, removal of nine platform piles, a crane platform, and the first formworks at A7/F7 and A6/F6 with five cross beams. A7/F7 concreted.
• Day 60: five more platform piles removed and the platform partly rebuilt, formworks moved to A5/F5 and A4/F4, four precast beams installed, A6–A5 concreted.
• Day 90: formworks at A3/F3, four more precast beams, A4/F4 concreted.
• Day 120: the A4/F4 formwork modified for A2/F2, formworks at A1–A2, precast stair beams.
• Day 190: the last precast units at A1–A3, final concrete pours, formworks and pile ties removed, and the upper-deck working platform built.

Formwork designed to float

Each formwork unit was drawn with its own weight and buoyancy so it could be towed and placed without heavy lifting: the A3/F3 unit weighs 4.8 t with 28 t of buoyancy, A7/F7 7.5 t with 58.5 t, and the larger A5–A4 unit 14 t with 127 t. A temporary working platform was added in December 2005.

From the drawings

The pile caps and beams were cast below water inside watertight steel formwork boxes (PB1–PB7, A1–A7 and F1–F7), built from 5–6 mm plate on channel frames with marine-glue seals. A full set weighs about 14 tonnes against 127 tonnes of buoyancy; the smaller set 7.5 tonnes against 58.5 tonnes. A temporary working platform carried the work, and the programme ran in stages over about 190 days: precast beams in, formwork fitted and stripped, concrete poured, pile ties welded underwater, platform piles removed. The same formwork was later adapted for Wong Shek Pier (2007).