Main contractor Barhale used no fewer than 57 trench boxes with associated handrails to support the sides of the 1.5 m wide, 2.0 m deep trench in poor, made-up ground where the pipe passes through a petrochemical refinery at Harwich in Suffolk.
The contractor’s task was to install a new 1,000 mm diameter Polyethylene pipe to replace an existing concrete pipe which had failed and was leaking treated effluent into the surrounding ground. The client is Anglian Water’s capital projects joint venture, @One Alliance.
In addition to the trench boxes, a row of interlocking steel sheet piles braced with Groundforce’s 900 Series hydraulic frames were installed on one side of the trench to reduce flooding by the surrounding groundwater.
Throughout the project, the old concrete pipeline remained in operation while the new PE pipe was installed alongside. This meant that it was impossible to locate the special welding machine needed to join the PE pipe sections directly over the trench.
Instead, the machine was set up several metres away from one end of the trench and the pipe welded together and pulled into the excavation in a single 200m length. For this reason, the entire trench had to remain open throughout the operation.
Of the 57 trench boxes employed 55 were Groundforce’s Standard models. The remaining two were Premier Trench Boxes, used because they gave 500 mm more vertical clearance than the Standard boxes at the end of the trench where the pipeline entered the excavation.
Groundforce also supplied formwork and 900 series frames to allow Barhale to cast 4m x 4m concrete chambers at either end of the pipeline. These chambers were constructed over the existing outfall to facilitate the final connections to the new section of pipeline.
“This project was made difficult by the very poor silty ground and the fact that the pipeline runs through an oil refinery and is surrounded by the petrochemical company’s pipelines and other equipment” comments Barhale site agent Darren Shanks.
“All the time we were working in the trench we had the old concrete pipe in poor condition to contend with and we had to take great care not to further damage it” he adds.
