Box culvert desilting, Hoi Fai Road to Olympian City

A long box culvert between Hoi Fai Road and Olympian City had silted up while still full of water. We compared three methods: a diver-operated sand pump feeding settlement tanks, a 0.3 m³ grab bucket, and jet-pump flushing. We costed each by the volume of solids removed per day.

Chosen method

• Settlement, percolation and storage tanks, a generator and diving equipment set up at the manhole.
• A diver, on surface-supplied air with a backup cylinder and constant voice communication, drags a 100 mm sand pump through the culvert.
• The pump stops when the tanks are full, while the water clears and drains off.
• Clean water is pumped away, and solids are moved to storage by grab and trucked off site.

Each team: a supervisor, a working diver, a standby diver, a tender and a communicator, working to the confined-space diving code.

Joint survey and wall cutting (2021)

In March 2021 divers went back into Cells 1 and 4 of the culvert and measured every open joint with callipers against numbered tags, so each gap could be recorded and repaired. In May 2021 we designed the cutting of a section of concrete wall inside the culvert.

From the drawings

Divers inspected cells 1 and 4 joint by joint, marking each joint as repaired, closed or uneven, and recording steel plates, expansive rubber and equalising holes. They worked on surface-supplied air with two 100 m umbilicals with communications, full-face masks, 12-litre bail-out cylinders and a hanging man-frame at the manhole. In May 2021 the work moved on to concrete cutting inside the culvert.