| Project name: | Blackpool Commercial Park |
| Location: | Cork, Eire |
| Keller company: | Keller Ground Engineering |
| Client: | PJ Hegarty |
| Techniques used: | Vibro compaction |
| Project duration: | 4 weeks carried out in Autumn 2003 |
Keller has used vibro stone columns to improve marshy ground at the site of a new commercial park in Cork, Republic of Ireland
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 10 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.
Keller’s four-week ground improvement contract included installation of 1280 columns, with an anticipated average depth of 4.3m. Site investigation drillers’ logs recorded significant chiselling time and localised high SPT counts, and given the possibility of hitting obstructions, Keller is pre-boring column positions using a 300mm diameter rotary auger.
Keller used its “bottom-feed” vibro-replacement process, which provides lateral support to the displaced soil at all times during installation.
Bottom feed is a more sophisticated process than the traditional top feed system and is normally used in preference to the latter where there is a risk of column collapse during construction. This can be as a result of either a high water table or simply where the material being treated is very loose.
The process involves vibrating the poker into the ground to full depth, and then forming the column in lifts, by discharging the stone through the poker’s base and then compacting it into the surrounding soil in layers. This pushes the stone sideways and downwards, increasing the density of both the stone in the column and the surrounding ground.
The operator monitors the electrical current in the poker, which rises as the ground stiffens, until a threshold value is achieved. More stone is added and the process repeated until eventually the dense stone column reaches ground level.
The company emphasises the need for thoroughness and control during construction of the columns. Column integrity depends on the amount of stone placed in the ground and the care with which the driver packs and repacks the stone. Keller maintains that it consistently uses more stone than its competitors, claiming that this leads to more competent columns and so better settlement control.
It believes too that new levels of construction control available through rig instrumentation and monitoring improve the reliability of stone columns to such an extent that they can now treat a much larger range of soil types. To this end Keller has now fully instrumented all of its purpose built vibro rigs, to give installation logs for every column and providing evidence of the degree of compaction achieved along the entire length of its columns.
At Cork, the stone columns are typically spaced at 2.3-2.8m and have a nominal diameter of 500mm. Columns are designed to carry superimposed floor loading of up to of 30KN/m2 – and Keller’s post installation testing schedule includes 15 plate loading tests to verify their performance.
The Blackpool Commercial Park is being built in two phases. The euro 40m first phase includes a cinema, retail area, central plaza, car parking and a leisure centre, which should give a much-needed boast to the area when it is completed in Autumn 2004.
