Wellpoint Dewatering – Northern Gateway

Project Overview

To enable construction of ~900 lin.m x 225mm Ø of Foul Sewer drainage, with the deepest excavation at 5.5m BEGL and rest groundwater level ~0.5m. At pipelaying rate of between 10m and 20m per day, in dry and stable ground conditions. BGL. A double-sided vacuum wellpoint system was installed.

Project Detail

• Loose, brown, silty, very gravelly, fine to coarse SAND. Gravel subangular is to subrounded, fine to coarse from GL to upto 15m BGL.
• Excavation Depth: upto 5.50m below surface
• Excavation support: Trench Box

Scope of Works

Modelling indicated that a minimum 48 hours of continuous pumping from a double sided wellpoint system located at a nominal 6m distance apart would generate a groundwater drawdown of 5m. Therefore a nominal 100m wellpoint pumping system was commissioned and ‘leap-frogged’ as drainage works occurred, that would enable 3 days (60m) pumping ahead and a 20m pumping behind any 20m working section. Wellpoints were installed by conventional water jetting methods. Where required wellpoints were installed from a reduced ground level to ensure they terminated at sufficient depth to achieve the required drawdown profile across the excavation area.