A combination of deep dynamic and high energy impact compaction and displacement piling has been used to prepare a landfill site for redevelopment in south London.
Urban landfills often have a number of costly technical and environmental issues to resolve before any building work can start.
Many contain a high percentage of organic matter in various stages of decay, often generating gases and producing dangerous leachates. Some of the tips have been filled far above the surrounding ground and require extensive levelling to make them accessible for development.
One such site is that chosen by distribution and services firm ProLogis for a new 22,250m 2distribution centre in Beddington Lane, Croydon.
Previously a landfill, it is in an area where much of the land has been used for tipping in the past.
However, its position in south London made it a prime location for redevelopment.
Tipping stopped about 10 years ago, leaving a plateau 7-8m above ground level, too high for the proposed warehouse development.
Reducing site levels meant potentially having to remove large quantities of contaminated soil off site at enormous cost. The challenge facing contractor McNicholas in its bid for the design and construct contract was to find a more economic alternative.
'The cost of this element alone could have potentially been well in excess of a million pounds and would have been the deciding factor on whether the client would proceed with this particular development or not, ' says McNicholas engineering design manager Adam Tyrie.
McNicholas invited geotechnical contractor Keller Ground Engineering to look at both the foundation requirements and at insitu methods of reducing site levels.
Keller proposed driven cast insitu displacement piles to support the frame and floor slabs and a slurry cut-off wall to control leachates, along with a gravel trench for leachate collection and gas ventilation around the site perimeter.
To reduce site levels without having to move spoil offsite, Keller proposed dynamic deep compaction (DDC), a system of high energy tamping which involves dropping large weights from controlled heights to compact the soil. From the site investigation, it was anticipated that an overall reduction of at least 250mm was possible.
However, with fill of such variable density, it was almost impossible to predict what reduction could be practically achieved. 'We needed something more, ' says Tyrie. 'With the costs involved, every extra 100mm of level reduction was critical to the project's overall viability.'
The client agreed to Keller carrying out an site trial to determine the order of volume changes that could be achieved.The trial demonstrated that DDC could reduce site levels by up to 900mm, depending upon fill densities, providing considerable savings on disposal costs.
'DDC proved an ideal method of reducing volume, ' says Keller engineering manager Barry Slocombe. 'Plus we were able to offer the treatment to improve the settlement and CBR performance beneath the roads and hard standing areas.'
The environmental impact of the method on neighbouring properties had to be considered.
Roads lie along two sides of the site and industrial units on the other two. A small brook runs along the north and easterly boundaries, with the refuse bank rising steeply up from it. A row of Victorian cottages sits on the opposite side of one of the roads.'DDC involves dropping a heavy weight from heights of up to 15m, ' says Slocombe.'A safe clearance of at least 40m has to be maintained between the treatment area and the surrounding properties.'
To ensure approval for the system, the project team arranged a meeting with residents to discuss any concerns. Keller proposed a range of measures including reduced energies and exclusion zones around the existing properties and the perimeter slurry wall. The firm also offered to carry out vibration monitoring to measure the effects of the ground compaction works.
Keller started work on site in August 2001 with construction of the 960m long and 600mm wide slurry cut-off wall around the site perimeter. A 5m working platform was prepared at the base of the embankment to enable the 7m deep slurry wall to be excavated.
A central mixing plant was set up to mix the cement/bentonite slurry and then pump it to support the trench during excavation. The slurry was a self-hardening mix designed to produce an insitu wall with a permeability of 10 m/s. Once complete, and with leachate flow now controlled, Keller was able to start the dynamic compaction operation.
Using two dynamic compaction rigs Keller completed compaction of the 20,000m 2area in five weeks, producing on average 750mm of settlement.
Cottage residents felt vibration levels were unacceptable, despite measurements indicating that they were well within acceptable potential damage levels.
Keller decided to change the system for the area closest to the cottages. To reduce vibrations but still provide the compaction required, Keller used its high energy impact compaction (HEIC) technique.
The system, new to the UK and offered by Landpac in collaboration with Keller, is a form of dynamic compaction. It uses a three- or five-sided dual roller module pulled by a tractor unit. Slocombe explains:
'As the drums roll, so the sides rise and drop, providing a rapid impact system that can induce compaction to depths of about 3m.'
HEIC energy inputs range from 10-28KJ/m 2with the drum weight ranging from 8-12t and the drop height from 150mm to 30mm. Keller says the system provides an efficient, low profile alternative to dynamic compaction that can be used within 10m of structures.
Using the five-sided roller, Keller completed treatment of this part of the site without causing undue disturbance to the neighbouring properties.
The HEIC roller system is fitted with a ground response measuring system so that a full picture of the underlying soils and their state of compaction may be observed to identify any soft zones that require additional treatment or removal.
Once site levels had been reduced, the piling platforms were prepared. The development comprises three units each up to 11,750m 2, supported on 2,300 piles. The design took into account the potential of negative loading imposed by the long term settlement of the fill. Ground conditions were 8m of refuse overlying a thin layer of terrace gravel underlain by firm to stiff London Clay.
The driven cast insitu displacement pile solution chosen satisfied the Environment Agency's concerns about potential vertical migration of leachates. Keller says the system provided a number of other benefits, the most important being displacement, again reducing the amount of spoil generated.
Another advantage was the system's heavy driving capability, particularly useful in tackling unknowns with the refuse. The 380mm diameter piles were driven to lengths of up to 17m for working loads of 600kN. To reduce punching shear, the slab piles were built with 900mm diameter enlarged heads. The smooth tops to the conical heads enabled continuous gas membranes to be laid directly over them before casting the floor slabs on top.