Groundforce stops the earth moving on St. Stephen’s Green

29 Apr

Nine of Groundforce’s 250 tonne capacity MP250 hydraulic props have been used to support the basement excavation for Canada House, a new office development with MB McNamara Construction as main contractor, on the corner of Earlsfort Terrace and St. Stephen’s Green in Dublin.

Hegarty Demolition selected Groundforce equipment to provide support to secant piled walls prior to carrying out an 11.0 m deep basement dig. Hegarty Demolition also installed, adapted and removed the hydraulic props in accordance with Groundforce’s specifications.

The city centre location required that the groundworks design met tight deflection constraints due to the site’s close proximity to an adjacent multi-storey office building and neighbouring properties.

Because of the limited space available on site for the proposed development, the groundworks designer, Byrne Looby, was required to limit pile sizes wherever possible in order to provide the largest basement footprint achievable.

As a result the piles vary in diameter from 900mm to 1,500mm, depending on their location around the perimeter of the basement.

Groundforce supplied nine of its MP250 props to support the retaining wall at capping-beam level.

“Four of the MP250s are arranged as knee-braces, one across each corner of the excavation” explains Groundforce general manager Liam Brew. “Another five were used as cross-props spanning 25m across the excavation”.

The original design required six cross-props, but this was reduced to five and the spacing between the two centre props increased from 7m to 8m to allow space for the building’s concrete stair-core to rise through the centre of the excavation.

“This means that while three of the MP250 cross-props use the standard 600mm diameter tubes, the middle two are fitted with 1,200mm diameter ‘super’ tubes to compensate for the missing prop” explains Brew.

The 10m-long knee-braces are attached to the capping beam via custom-designed steel stud-plates, rather than the usual method of cast-in studs, say Liam:

“On this project, the capping beam is part of the permanent structure and is densely packed with steel reinforcement. So instead of casting the studs into the beam, as we normally do, we had to design special 1.5m-long steel plate fixings and bolt them to the beam”. 

Close liaison with the permanent works designer ensured that the fixing bolts do not interfere with the rebar inside the beam.

Using a proprietary system such as Groundforce’s MP250s was both quicker to deploy and more practical than the alternative, says Liam.

“The alternative was to install a bespoke welded steel frame. It was feasible, but couldn’t be guaranteed to meet the deflection criteria. We could hydraulically pre-load our props before the excavation started, and ensure that there was minimal settlement due to lateral loadings”, he says.

The propping system was installed in October 2014 and the last prop will be removed in March 2015