To setup a cofferdam dewatering system, including boring, pump installation and outflow pipework and drainage tanks etc, typical rates are...

• Small 2 hole system, for an overrun tunnel, with a single drilling rig takes about 7 - 10 days to get up and running.
• A larger system, with 10 - 15 holes, for a station type cofferdam will take in the region of 3 weeks with 2 rigs. This can be speeded up with the use of more rigs to reduce the well boring times.

A typical rate of 2.5 days per pump installation can be used to calculate detailed pumping arrangements. The water level can be dropped in a "closed cofferdam" within a 6 to 8 hour period for a 25,000 m2 cofferdam with a 2 m water level reduction. For smaller closed areas it will take only a few hours at most to reduce the water level by 2.5 - 3 metres.

Recharge System - Average well depth of say 55 m deep, not into rock, 9 wells in 10 days or 1.11 days/no with an additional 4 - 5 days to connect pipework and recharge backup tanks etc.

Dewatering