The Irish word for Cork, the second city of the Republic, is ‘Corcaigh’ which means marsh. And ever since St Finbarr founded a monastery in the sixth century on a small island in the swampy estuary of the River Lee, builders in Cork have been battling with some pretty poor quality ground.
More than 1500 years on, and developers still face similar problems. When site investigation contractors moved on to the 4ha site of a new commercial park in Blackpool, a northern suburb of the city, the ground was so boggy that they couldn’t get the site investigation rigs into the middle of it, and could only sink boreholes around the perimeter.
With a small tributary to the River Lee running through the centre of the site – and cutting across three of the five proposed main building footprints – there was understandably concern over the ground conditions.
The site had been partly “reclaimed” about ten years ago when it was used as a repository for material left over from construction of the nearby Blackpool shopping centre. But with no engineering considerations to this infilling operation, the quality of the ground was highly variable and some form of ground improvement was needed to treat the up to 5.5m thick fill in advance of the new commercial park development.
Architect and landscape planner Brady Shipman Martin’s proposals included re-routing the river around the edge of the site; once this was done, the boggy central area of the site soon dried up. Ground conditions in the troubled central area were fortuitously found to be consistent with the boreholes around the edge.
Even so, main contractor PJ Hegarty called in Keller Ground Engineering’s Irish division, to look at both the ground improvement and reinforced soil embankments to stabilise the new river channel.
Hegarty will build three of the five main development buildings using ground slabs bearing directly on vibro-stone columns. On the two other structures, the site level is being raised by up to 2.5m using imported engineering grade fill, which removes the need for ground improvement in these areas. Structural wall loads are carried by CFA piles, which penetrate through the fill into the underlying river gravels.