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Time to settle down

GROUND IMPROVEMENT

A combination of ground improvement techniques is helping to fast-track a warehouse development at a former Midlands colliery site. Claire Symes reports.

 

The decline of traditional heavy industry in the UK means many redundant sites are today being regenerated for a wide variety of uses.

 

One such site is Kerseley Colliery near Coventry. The deep coal mine closed in the early 1990s and is now being redeveloped as a warehouse for distribution giant Pro-Logis.

 

While the mine shaft was capped and the site levelled soon after closure, the backfill was not engineered and is highly variable in depth and content. This, along with potential obstructions, meant that piled foundations would be expensive, so main contractor Winvic asked geotechnical contractor Roger Bullivant to provide an alternative ground improvement solution.

 

The material used to level the site was mostly clay, containing lenses of gravel and sand layers as well as other colliery and construction waste. Made ground at the colliery was underlain by sandstone from the Carboniferous Coal Measures but the depth to this solid material varied from 2m to 7m.

 

‘Clay fill materials can take up to 30 years to settle naturally but the fill material at Kerseley Colliery was placed just over 10 years ago, so was only partially consolidated, ’ says Bullivant ground improvement manager Jason Redgers.

 

‘Some form of ground improvement was essential to speed up the consolidation process and to reduce overall settlement and control the differential settlement of the new warehouse.’ Bullivant’s answer was to use two different ground improvement techniques to treat the building footprint, enabling the 30,000m 2warehouse to be built on conventional pad foundations.

 

‘We proposed dynamic consolidation using our NRG cone, which involves applying dynamic compaction to vibro stone columns, and two stages of lime cement stabilisation, ’ Redgers says. ‘Using two techniques may sound like more work but it is actually quicker and more cost effective than a traditional piled solution.’ The NRG system is a dynamic compaction technique that uses a cone-shaped tamper dropped from a leader rig rather than traditional flat-bottomed tamper dropped from a crane. Because the tamper is dropped in true free fall rather than being attached to a rope, Bullivant says the applied energy is increased, as is the speed and efficiency of ground improvement. An added benefit is reduced noise and vibration.

 

Bullivant has used its NRG Cone dynamic consolidation system to improve recently placed clay fill and in materials where stone columns cannot be used because of potential obstructions or boring problems. The technique can be used in almost any type of made ground and on materials where all processes of wet and dry, top and bottom feed vibro techniques can be used.

 

According to Redgers, use of dynamic compaction on its own would have improved the ground at depth but raised pore water pressures in the clay fill. ‘The increase in porewater pressure would reduce the effective stress and soften the ground until the porewater pressure dissipated.

 

‘By installing the vibro stone columns first and then applying the dynamic compaction, the top of the column ‘bursts’ to create a drainage pathway and a stiffened soil raft. This way the dynamic compaction can improve the ground, and the porewater can drain more rapidly through the granular columns.’ Before dynamic consolidation could get under way, Bullivant needed to create a firm platform for the rigs. The traditional solution is to import granular material but the firm opted to stabilise the top 300mm of soil by adding lime cement, just in advance of dynamic consolidation.

 

Soil stabilisation began in midJuly using Wirtgen equipment adapted for the contract. Work on installing the 5,000 vibro stone columns got under way soon after, using Bullivant’s in-house designed and built machinery.

 

The 7m long, 600mm diameter vibro columns were formed by a 600mm poker fitted to a crawler rig, with 45mm to 75mm aggregate. Column spacing varied depending on the loading and settlement requirements of floor slabs and foundation pads.

 

Aggregate was then poured into the hole in stages using a hopper mounted on the machine’s mast. This was compacted at the end of each stage using the poker, creating a dense column of stone with a greater diameter than the hole created by the poker.

 

‘At the peak we had three rigs at the site and were installing the columns at a rate of about 1,000 a day, ’ says Redgers.

 

Following on behind throughout the contract were two NRG dynamic compaction rigs.

 

Dynamic compaction was carried directly on the columns using a 10t weight over a 10m drop. This improved the ground bearing capacity to 150 kN/m 2below the pad foundations and 50kN/m 2below the floor slabs. The resulting imprints in the top of the columns were backfilled with stone.

 

Work has begun on the second stage of soil stabilisation to form a structural layer for the new warehouse, equivalent to a Department of Transport Type 1 stone blanket, removing the need to import granular fill for the subbase. Warehouse construction begins soon, and handover of the completed building is expected to take place late this year.

 

‘Use of soil stabilisation and dynamic consolidation also has considerable environmental benefits, ’ says Redgers.

 

‘Stabilisation work drastically reduces the amount of imported material, while the displacing nature of vibro stone columns reduces spoil production.’

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