Groundforce digs deep for Belfast tunnelling project

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Groundforce has supplied specialist shoring equipment to support three deep excavations on a flood alleviation project in Northern Ireland.

The medium-duty hydraulically adjustable Maxi Brace bracing system was used by specialist contractor Active Tunnelling to support two launch pits and one reception pit during a crucial phase of NI Water’s Ravenhill Avenue Flood Alleviation project in Belfast. The main contractor is Geda Construction.

This £7m project in South Belfast is designed to reduce the risk of out-of-sewer flooding caused by surcharges of the existing Victorian brick-lined sewers during periods of heavy rainfall.

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NI Water’s contractor Geda Construction is replacing and upgrading 2km of sewer tunnel along Ravenhill Road, Ravenhill Avenue, and nearby streets.

As part of this project, Coventry-based Active Tunnelling was employed to bore a new 2m-diameter sewer tunnel below the busy Ravenhill Road to help reduce disruption during construction.

The tunnel extends 75m from the edge of Ormeau Park up Ravenhill Avenue; beyond this point, Geda Construction is employing more traditional open-cut methods to construct the remaining sewers and manholes.

The new tunnel follows a dog-leg in the street and therefore, since the tunnel boring machine cannot navigate around a sharp bend, it was bored in two drives from launch pits at either end towards a reception pit located at the bend in the middle.

Groundforce was employed early in the design process and was consulted throughout the entire 21-week scheme as the equipment was designed, installed and removed.

The two launch pits measure 5.5m x 3.5m, while the reception pit is just 5m x 3m – but all three are 6m deep and require substantial shoring to withstand the ground pressure.

The equipment supplied comprised three Maxi Brace frames for each excavation. These were installed to support the 7m-long KD6 steel sheet-piles driven by Active Tunnelling to create the cofferdams.

In each excavation, the bearing capacity of the three Maxi Brace frames increased with the depth of the excavation: 260mm-wide Maxi 4 leg braces (the lightest in the range) were installed at the top of the excavation; Maxi 5 leg frames (also 260mm wide but heavier in construction) were installed half-way down and Maxi 8R leg braces – 350mm-wide were installed at the bottom.

“We designed the pits so that the tunnelling machine could be lifted into the excavation without encountering any obstacles,” says Kristian McKinley, Groundforce area manager for Northern Ireland. “We also designed them so that once a 200mm concrete base slab had been cast, the lowest frame could be removed and six of the steel sheet piles could be lifted to allow the tunnelling machine to enter the ground under the road.”

The high lateral forces (up to 81.7kN/m) within the excavations, and the fact that Pit 2 was located right next to the Ravenhill Presbyterian Church, meant that careful monitoring was required to ensure minimal ground movement. Groundforce supplied the proprietary load-monitoring system to record loadings within the frames and additional vibration monitoring was also employed.

Rob Graham, contract manager with Active Tunnelling, commented: “This was a very delicate job, but we had excellent support from Groundforce throughout. Their service was very professional and the equipment was first class”. In addition to the Maxi Brace frames, Groundforce also supplied its Edge Safe and Davit Arm products to ensure safe working around the open excavations.

Although this is the first contract Groundforce has completed for Active Tunnelling, the two companies developed a strong working relationship during the project. They are currently in discussions regarding future contracts in Southern Ireland, including a forthcoming project in Roscommon.

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