Humber Crossing – Feeder 9 Replacement

The existing pipeline is 5km long and runs in a trench along the river bed between Paull, on the north bank of the Humber, and Goxhill on the south bank. Over time, tidal patterns have eroded the river bed, covering the pipeline, leading to parts of it at risk of becoming exposed. Recognising this issue, National Grid developed an engineering solution to protect the pipeline by covering potentially exposed areas with gravel-filled bags with concrete frond mattresses placed over the top.

These incorporate plastic fronds to mimic seaweed and encourage the settlement of sand and silt. This solution ensured the pipeline could continue to operate normally in the short-term and inspections have shown it has been effective and successful for now but not for long term.

A number of project-specific features had to be taken into consideration when carrying out the works. For example, the construction site was set up on top of very poor-quality substrate, which necessitated extensive soil stabilization measures involving the installation of a base course made of an up to 1m thick layer of gravel. The restriction of traffic periods for lorries driving to and from the construction site to 5h/day was another complicating factor. As the construction site is situated near the coast, heavy winds often occur, which may make it necessary to interrupt crane operations. This was already factored into the tunnel servicing plans to minimise downtimes. High tides may also cause flooding of the construction site, which means that all the installations must be flood-proof. The narrow inner diameter and total length of the tunnel also placed increased demands on the safety arrangements and tunnel logistics. In addition, a dispersion of large quantities of soil in the fluid cycle is to be expected when using a Slurry-TBM in chalk formations. This also had to be taken into account when planning the separation plant. And finally, the Health & Safety requirements for planning and execution are much more stringent in the UK than in Central Europe.

PORR's experiences gained in the course of the Emscher BA 40 sewage drain project, which was successfully carried out under similar conditions, had a positive impact throughout the entire construction site planning process.

The construction launch site for all tunnel construction works is located in Goxhill. The launch shaft for the tunnel excavation works is 60m long, 7m wide and has an 8m deep portal. An approximately 140m long ramp with a 4% gradient connects the shaft to the surface of the terrain.

The tunnel boring machine (TBM) is serviced for the excavation works by a wheeled service vehicle along the ramp. Wind-related interruptions to crane operations caused by heavy winds from the nearby coast can thus be entirely avoided. It also meant that the TBM, with a total length of 160m, could be completely assembled before excavations began.

The launch shaft was constructed inside a construction pit protected by reinforced bored pile and sheet pile walls embedded in the chalk layers. The water levels in the construction pit were lowered during the excavation works using a well point extraction system. Once the in-situ concrete floor had been installed and anchored to the retaining wall, the extraction system was turned off. The reception shaft in Paull will be constructed inside a circular construction pit, 12m deep, with a diameter of 15m, and protected by bored piles. Another well point extraction system will be in place here during excavation works.