New Wear Crossing Bridge, Central Pier Temporary Works Cofferdam

Cofferdams

AGL were the designers of a temporary cofferdam structure at Sunderland, UK which facilitated the construction of the central bridge pylon of the New Wear Crossing Bridge. AGL carried out the design for Roughan O’Donovan/Farrans.

The key features of the cofferdam design carried out by AGL are outlined below:

• The ground conditions in the river bed comprised 13m of very soft alluvium, over a thin layer glacial till/gravels and then very weak to medium strong mudstone. Water depths of up to 7m in the river were considered. The very soft ground provided little lateral resistance for the cofferdam so provided a particular design challenge.
• The temporary cofferdam was designed as a fully-interlocked combi-wall cofferdam with sealant/welding applied to clutches to minimise the inflow of water into the excavation. The combi wall comprised Arcelor Mittal HZ-M king pile system.
• The design considered flood conditions which introduced high flow forces and water pressures as well as barge forces.
• The cofferdam was required to support a temporary working platform that allowed construction of the permanent bored piles for the bridge pylon. The cofferdam was design for the vertical and lateral loading of the piling rig and steel deck.
• The stability of the cofferdam relied on a system of dewatering/relief wells within the cofferdam to prevent an uplift failure occurring by lowering water pressures during excavation.
• The design was carried out using the PLAXIS 2D finite element program to allow soil-structure interaction and the interaction between the cofferdam walls to be considered.

Drivability analysis were carried out by wave equation programs GRLWEAP and PDPWAVE to check that sufficient embedment into to the underlying mudstone could achieved by the combi-wall for lateral stability and to select the appropriate driving hammer. Driving records for the piles were found to be consistent with the drivability analysis.