River crossing featured in leading civil engineering journal

GAS TO THE WEST Sheet Piled Pits and Auger Boring

Modular shoring equipment from Groundforce recently helped the Murphy GMC joint venture install a new gas main under the Colebrooke River at Maguiresbridge in Co Fermanagh.

Murphy GMC are installing the new gas pipeline for SGN Natural Gas as part of the £250M Gas to the West energy infrastructure project to bring mains gas supplies to eight towns in the western part of Northern Ireland.

A section of the new gas main runs parallel to the A4 Belfast Road and in two locations where the highway passes over the Colebrooke River – the buried pipeline has to be installed underneath.

At Maguiresbridge, this was done using auger-boring techniques – basically a standard rig operating from a sheet-piled drive pit, which was dropped close to the riverbank.

Conventional auger boring certainly has its uses. The downside is its accuracy over a long drive length, meaning that it’s not the greatest system to install gravity mains or pipelines with tight tolerances. The upside is that compared to microtunnelling – especially slurry systems, it is quite an economical way to get a pipeline from A to B.

During auger boring, a cutting head with a slightly larger diameter than the casing pipe excavates the soil just in front of the casing. The helical rotating auger blades carry the soil back to the machine where it is removed either by hand or machine.

The muckaway along with the line and level is the downside – although the auger-led Decon RVS microtunnelling systems that McNicholas and Delta ran with during the 1990’s had both of those cracked.

The auger boring machine itself runs along a track which is aligned with the casing pipe where it has to be installed. After the machine finishes the assigned track, the machine is disconnected, and the auger assembly is brought back to the starting point where a new casing segment is welded to the previously installed casing.

The process is continued until the entire pipeline is wide by 6m-deep cofferdam using Larssen AR605 sheet piles.

To support the excavation, Groundforce designed a structure comprising three hydraulic frames. The middle and bottom frames both consisted of Mega Brace units, each propped at the mid-point with one 80T capacity HSK80 hydraulic prop.

These two frames were designed to be removed as soon as the concrete base slab had been cast and reached full strength.

The top frame, however, had to remain in place throughout, in order to support the opening at the top of the excavation.

“For the top frame we used our Super Mega Brace,” explained the shoring specialist’s Technical Sales Executive, Richard Dunn.

“We couldn’t have a prop across the top of the excavation because the client wouldn’t then have been able to lift the auger boring machine into the pit. That’s why we used our strongest bracing system.”

After Groundforce had designed the support structure and assembled the necessary components, it was then discovered that the ground conditions were not conversant with the borehole report.

“The test borehole hit a low spot in the limestone bedrock and so our calculations were out: the piles were meant to be driven to 8m but there was a refusal at 6.5m,” added Richard.

Groundforce’s design engineers were able to modify the design, using only the equipment already on-site, to provide a workable alternative at short notice.

“There wasn’t enough depth for the sheet piles to toe-in, but overall this didn’t affect the schedule.”

The first river crossing was completed during the early summer and the Murphy GMC joint venture are now completing the second river crossing about 6.5km further downstream.

Groundforce continues to lead the way when it comes to introducing innovations in excavation support equipment, with the launch of the MP750 and hydraulic nut bolt technology.

Designed by the company’s in-house engineering design team, the MP750 is the largest proprietary strut on the market and capable of resisting working loads of up to 750T.

The strut connects directly to the 1220mm dia. Super Tube extensions. The integral hydraulic ram combined with the twin-mechanical screw jacks provides simple installation and fail-safe security.

With a high resistance to buckling, long spans up to 50m are possible, without additional restraint. As with all their propping systems, the MP750 is quick to install and remove compared to structural steel, site-fabricated solutions.

This new addition will further benefit contractors working on projects that require heavy-duty ground propping.

The MP750 can be used with the Senceive load and data monitoring system, ideal for observational methodology. It also employs the latest hydraulic nut technology.

Acting directly behind the locking screws, the hydraulic nuts offer the ability to both pre-load the MP750 strut to a high level on installation as well as providing a safe and controlled means of releasing the strut under residual load. Without this technology, traditional mechanical systems can be problematic to remove under load.

The nuts require a dedicated high pressure hydraulic actuation system. Installation and removal operations are supported by the company’s experienced site assistance teams.