Groundforce Shorco has delivered a bespoke support system for a major excavation in Dublin city centre.

A eight storey campus is being built by Contractor Bennett Construction in the city for the Royal College of Surgeons in Ireland (RCSI). Due to two of the building's eight storeys being below ground the site is currently occupied by a deep excavation more than 50 metres long and 30 metres wide.

The company has supplied modular hydraulic equipment to support a concrete retaining wall while Bennett Construction completes the excavation and installs the reinforced concrete basement slab.

Despite the excavation being mostly rectangular in shape, it contains irregularities that add complexity to the support engineering. Lateral forces, ranging from 215kNm to 300kN/m, are managed by Groundforce Shorco's MP150 and MP250 hydraulic props, connected through a waling beam utilizing the company's modular Mega Brace system. Where the shape of the excavation does not match the standard equipment bespoke beams and props have been designed and supplied to ensure that all of the concrete secant piles remain fully supported.

Five MP250 props support the middle of the excavation, spanning almost 30 metre. These props are fitted with 1,220 millimetre-diameter 'super-tube' (twice the diameter of standard extension tubes) to increase stiffness and prevent deflection.

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Each corner of the excavation is upheld by a mix of MP150 and MP250 props, ranging from just over 5 meters to more than 18 meters in length. These props act as angled knee-braces, supporting the Mega Brace beam at angles of approximately 45o. This arrangement imposes a significant shear force on the retaining wall.

Groundforce Shorco Major Projects Manager, Adam Fletcher, said:

"At certain locations there are very short lengths of retaining wall comprising only a few piles and these are unable to resist the shear from the knee braces.

"When the final pile layout was determined we decided to split the frame to suit two levels of propping and designed a bespoke corner brace to follow the pile line and limit the deflection where shear keys could not be used.

"Groundforce Shorco became involved when the design was at a very early stage and contributed to several revisions before the design was finalised, which also gave the opportunity to have a full picture of what was happening around the design. The bespoke solutions provided where the standard kit was not suitable are one of our strengths."

To help the waling beam resist shear forces at these spots, Groundforce Shorco created special steelwork that was welded to the Mega Brace beam on-site.

Chiara Morena, Groundforce Shorco Design Manager, said: "In standard procedures, the props are located against the retaining wall using gallows brackets. These not only ensure the correct positioning of the prop but also carry its unsupported weight and prevent any risk of slippage. On this project, only the hard secant piles could be used for supporting the beam via gallows brackets and that was an additional challenge. Once a partial built wall was provided, we had to verify the anchored connection of the gallows brackets for each beam extension around the excavation.

"However, due to the difference in level in the RCSI excavation, one of the MP250 props is installed at a 3o angle to the horizontal which introduces an upward force at the higher end of the prop as it is braced against the waling beam. To counteract this, we installed an additional gallows bracket upside down, in non-standard fashion, above the prop at this point."

Damien Watson, Project Manager with Bennett Construction, saidd: "Groundforce Shorco provided continuous assistance after completing the design and during the installation and removal phases. They even helped with elements that fell outside their responsibility as a supplier of temporary propping equipment."