Cork city is underlain by alluvial deposits with highly permeable sands and gravels. This, coupled with a tidal water table, meant significant excavation challenges when Irish Rail and SIAC Construction extended Kent Railway Station

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Every deep excavation in Cork city presents a challenge. The area is underlain by a series of alluvial deposits with highly permeable sands and gravels. This, coupled with a tidal water table, gives rise to significant dewatering and excavation support challenges.

These were exactly the challenges facing Irish Rail and main contractor SIAC Construction on its contract to extend Kent Station in the city.

img_20160408_144136The contract involves construction of a new entrance building to improve access and provide better links to the city centre. The entrance building has a basement concourse linked, via a new subway, to the underpass leading to the existing station building.

To allow the construction of the reinforced concrete basement and subway, SIAC and others installed a 25m long x 9m wide secant piled perimeter wall and a 4m deep mattress of permeation grout, 7m below ground level. This effectively sealed the excavation from penetrating ground water and provided a dry working area for the basement floor.

Given the poor bearing capacity of the surrounding soil, some 22 ground anchors were proposed on the original temporary works proposal. SIAC decided to review the propping options with Groundforce and proposed some value engineering to the client.

The alternative solution was to prop the capping beam internally using five hydraulic props and remove the need for tie-back anchors.

img_20160415_1630301“The props provided a significant time saving and resulted in a much lower risk of water ingress where the tie back anchors would have punctured the secant piles,” commented SIAC project manager, Adrian Farry.

He added: “With the props, there was much less finishing works than would have been required for the head of the tie back anchor. Installed on site in a day using a 60t crane and the 21t excavator, the props were a much simpler installation to execute. Once some of the reinforced concrete structure had reached sufficient strength, removal was as little as half a day.”

Ground anchors were installed on the underpass section of works, which was completed at an earlier stage of the works, and this provided the perfect case study to compare the two methods of temporary support.

“In many situations, hydraulic props can offer significant benefits over tie back anchors for the temporary propping of basements,” comments Groundforce general manager, Joseph Lenihan.

For this 14-week Groundforce project, there were significant benefits when compared against the anchors including the advantage of speed of installation, reduced dewatering and the option of live load monitoring – all contributing towards the successful construction of the basement reinforced concrete works.

http://www.engineersjournal.ie/wp-content/uploads/2016/10/20160415_141243-1024x576.jpghttp://www.engineersjournal.ie/wp-content/uploads/2016/10/20160415_141243-300x300.jpgDavid O'RiordanSponsoredconstruction,Cork,Iarnrod Eireann,SIAC Construction,transport
Every deep excavation in Cork city presents a challenge. The area is underlain by a series of alluvial deposits with highly permeable sands and gravels. This, coupled with a tidal water table, gives rise to significant dewatering and excavation support challenges. These were exactly the challenges facing Irish Rail and...