| Project name: | Manchester Fort Retail Park |
| Location: | Manchester, UK |
| Keller company: | Keller Ground Engineering |
| Client: | HBG and Nuttall |
| Techniques used: | Vibro compaction, deep dynamic compaction, driven cast in place piling |
| Project duration: | 12 weeks carried out in Autumn 2003 |
Project summary: Keller developed an integrated geotechnical package to support a £25m retail park on a 14m-deep disused landfill in Manchester
Keller provided both piling and ground improvement at the Manchester Fort Retail Park in the Cheetham Hill district of Manchester, UK. Keller’s work for main contractor HBG, working in partnership with Nuttall, was part of an integrated geotechnical package needed in advance of the £25m redevelopment of what was a 1960s disused landfill.
The first problem is that the landfill is unusually deep, extending to 14m. Furthermore, the site was first used as a brick pit, and some parts of it still contain voids within buried kilns and tunnels, adding further to the challenges of implementing a successful ground improvement scheme.
The basic strategy of any ground improvement is to accelerate long-term settlement. At Cheetham Hill, Keller achieved this through the use of deep dynamic compaction over most of the site area, preceded by vibro stone columns in areas where the fill is deeper than 8m.
Dynamic compaction - in which a crane-suspended heavy weight is repeatedly dropped onto the ground - is the classic, low-cost option for treating large areas of loose fill. But it can only be relied upon to treat up to a depth of 6-8m, hence the need for the vibro stone columns, to treat the deeper fills.
Dynamic compaction was also used below the building footprints to densify the ground to provide additional support for the piling equipment.
The project consulting engineer, Scott Wilson, specified an overall ground improvement settlement criteria of 50mm in 25 years, although the actual design of the treatment process was contractor led.
The age of the fill provides the starting point for designing ground improvement, in that it gives you a good indication of how much of the fill's total settlement has already occurred through its self-weight plus biodegradation of the fill constituents - and therefore gives you an indication of how much more it is likely to settle.
Thoroughness during construction of the vibro stone columns is essential, in that their integrity depends on the amount of stone placed in the ground and the care with which the driver packs and repacks the stone. Keller, for the record, consistently uses more stone than its competitors, because this leads to more competent columns and better settlement control.
Controls on vibro work are also absolutely vital. The underlying principle is that you are modifying the poor quality ground, unlike piling which structurally by-passes the problem. To this end, all Keller's Vibrocat rigs are fully instrumented to give installation logs of every column, providing evidence of the degree of compaction achieved along the entire length of the column.
Keller also has three cast-in-place piling rigs on site, which are working on the foundations and floor slabs for the retail units.
Because it is a displacement method it creates no spoil, which in this age of increasing landfill costs and sustainable construction is highly desirable, particularly when the spoil is potentially putrescible domestic waste.
Furthermore, the method lends itself to the use of enlarged pile heads. Floor slab support piles are formed with integral 900mm diameter heads, which replace conventional piles caps and remove the need to trim down the piles.
