Firm sandstone and loose backfill are poor companions when a building needs to span both. David Hayward visits Newcastle to see how the potential for differential settlement was minimised.
Recycling of demolition waste during vibrodisplacement strengthening of an infilled quarry site near Newcastle saved time and money on groundworks for a new retail park.
Shop units for the development at Byker will be built spanning sandstone at ground level and, just metres away, deep uncompacted fill.
Main contractor Barr Construction considered several options for minimising differential settlement. The building could be extensively piled or the poor ground strengthened by dynamic compaction or vibro-displacement stone columns.
Dynamic compaction would have caused too much vibration for an adjacent metro line, while 15m deep CFA piling would have been expensive and also risked vibration problems. The mixed fill was so weak, and settlement tolerances so exacting, that the only other option seemed to be using more than 3,000 close centred stone columns over an area of only 10,000m 2- and this would have been equally costly.
Geotechnical designer Bachy Soletanche, called in for advice by Barr and Bullen Consultants, proposed halving the number of stone columns needed by first surcharging the fill area.
When the firm also suggested using demolition waste on the site both for the surcharge and the stone columns - saving the project about £70,000 by not having to import conventional column stone - the £120,000 contract was all but won by Bachy's ground treatment partner company Vibro Foundations.
'They came up with by far the best solution from both the design and commercial viewpoints, ' says Barr project manager Steve Russell. 'Initially I was unsure about the surcharge idea, but the whole scheme has worked out excellently with all our settlement criteria met.'
Part of the planned £12M retail park is located over a Victorian sandstone quarry worked until the 1920s.The 10m deep quarry was backfilled with a mixture of shale, ash, clinker and soft clay. It was later capped with a thin concrete slab as the base for a tram depot.
When developer Thornfield Properties arrived late last year to create the retail park, trams had been replaced by bus garages. Their subsequent demolition left a 50,000t pile of concrete and brick waste.
The main 150m long shop unit is to be built on the edge of the infilled area. About 20% of the two-storey building is just behind the near vertical quarry face, with hard sandstone outcropping at ground level, and the remaining 80% over the variable uncompacted fill.
'Without piles or ground treatment there was a real fear of significant differential settlement, ' says Vibro Foundations project engineer Jeff Lewis.'We had to guarantee no more than 1 in 500 differential settlement across the quarry face area.'
Ground consolidation using stone columns emerged as the most practical solution but, on its own, would have demanded a dense network to have any chance of achieving the settlement tolerances.
Vibro Foundations asked for the fill to be surcharged for three weeks so a more economic, less dense stone column design could be adopted.
The nearby pile of demolition waste produced an ideal material to form the 3m high, 28,000t surcharge built up in 500mm compacted layers and exerting a 50kPa pressure on the fill.
Settlement was monitored continuously, with readings covering a wide range consistent with the fill's variability and peaking at about 120mm.This allowed Bachy to design a network of 1,673 stone columns.
Varying load requirements in the building demanded two sizes of stone columns. Beneath concrete pad footings for its 70 steel columns, 700mm diameter stone columns had to be, on average, 10m deep and placed at 1.2m centres.
Under the building's concrete ground slab, loading is lighter, with the 500mm diameter stone columns formed 6m deep on a grid of up to 3m.
This specification enabled Vibro Foundations to show off both of its vibro-displacement techniques with top and bottom feed (see box).
The 400 longer bottom-fed columns were formed with conventional single size 40mm diameter limestone that had to be brought in.
But for the 1,273 shorter columns, the top feed method allowed a wider range of stone to be used. Normally this would be 75mm to 40mm limestone, but the design engineers were again eyeing up the pile of demolition waste.
After Barr had crushed the waste to the same size range, it proved just as suitable for the stone columns. Recycling 7,000t of the rubble not only saved the cost of importing stone, but provided the equally valuable environmental benefit of avoiding about 800 lorry journeys to or from this urban site.
Follow-on load tests, where the treated ground was subjected to 90t loads and 11t plate tests, confirmed the success of the ground treatment. Settlements of a maximum of 8mm were all within the range allowed, with improved ground strengths meeting the required 150kN/m 2applied loading from the building's steel columns and 40kN/m 2from the slab area.
Vibro Foundations brought in three vibro-displacement rigs to install more than 1,600 compacted stone columns.
Two of the rigs operated the top-feed method for columns beneath the main shop unit's ground slab, while the third used the bottom-feed technique for longer columns strengthening ground below the structure's steel columns.
The top-feed method involves a 7m long poker suspended from the arm of a Komatsu PC240 excavator. Powered by a 250kVA electric generator on the back of the excavator, the 400mm diameter poker, with its slightly wider vibrating head, forms a 6m deep vertical hole through the fill.
At Byker the normal 75mm to 40mm graded limestone forming each column was replaced by demolition waste. To introduce this crushed concrete and brick rubble, the poker is withdrawn totally from its hole.
From ground level a bucket shovel then feeds a 0.5t charge of material which is compacted by the reintroduced poker. This process is repeated with more 1m deep charges until the column is full.
No spoil is brought to the surface, instead the fill is pushed sideways to strengthen surrounding ground. To help displacement, air is forced down a central tube in the poker and released through side jets in its base.
To form the longer columns, designed to reach sandstone bedrock beneath the fill, a heavier, wider poker is mounted on a Liebherr crane.
After the poker has formed an average 10m deep hole, a square 1.5t capacity stone hopper fixed to its top drops single size 40mm imported limestone down a 100mm wide tube attached to the poker's side, to the bottom of the column - hence the technique's name.
Stone is compacted by marginally raising and lowering the poker. As it is not withdrawn until the column is complete, the poker protects the longer hole from collapse. This is particularly important at Byker as groundwater is about 6m deep.