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PICNIC IN THE PARK

Vibro stone columns and precast modular block retaining walls are offering developers at Park Central in Birmingham considerable savings over the original piling scheme.

Park Central development in Birmingham is the first part of the Attwood Green Regeneration project, one of Europe's largest urban development sites.

This first phase is a joint venture between Crest Nicholson, Birmingham City Council, Optima and Mercian. Structural engineer is Curtins.

The site, between Five Ways Corner and New Street Station, was previously occupied by highrise local authority flats and light industrial units.

Work involves demolishing the run-down housing and replacing it with a mixed-use development set in landscaped parkland. Ultimately, Park Central will have 1,400 mixed-tenure homes and about 26,500m 2 of commercial space.

Ground conditions are typically made ground (which contains backfi led basements), overlying either sand or clay and then sandstone bedrock below about 4-5m.

The sloping site changes in height by 30-40m across the 1,500m long development. While there are no slope stability issues, a lot of retaining structures are needed to create level terraces and landscaping. The scheme was originally intended to make extensive use of piling for both load bearing and retaining.

Although the ground presents few problems geotechnically, value engineering has still delivered benefits says Simon Thompson, contracts manager with ground engineering contractor Keller, which has been installing foundations for many of the apartment blocks and houses.

Keller has adopted a range of techniques to better suit the different ground loadings, space requirements and variations in the soil profile.

For example, in areas where there is no fill, two-storey houses are being founded on traditional strip and pad foundations.

Where the fill is present, conventional strip footings can still be used for some of the low rise structures, but the fill is fi rst treated by vibro replacement.

Piling is still necessary for the apartment blocks, which are up to eight storeys high. These are founded on bored piles embedded into the sandstone.

Where the new blocks have undercroft car parking, Keller has installed a piled retaining wall, using 450mm diameter contiguous bored piles to form the basement area.

Further along the site a 300m long piled retaining wall was specifi ed to provide a 6m step in ground level to maximise the parking area below and provide fl at gardens for the houses on the upper level, which were also to be piled.

As an alternative, Keller proposed reducing the length of the piled wall and using it only where it ran next to existing buildings. As the line of the wall moved away from the site boundary, it was possible and cheaper to complete it in reinforced earth using Keller's Geolock system.

This consists of heavy precast concrete blocks that are dry laid with connecting geogrid reinforcement to form a gravity structure. It is available in various textures and colours - the blocks at Park Central will eventually be faced with a brickwork skin to match the walls on other parts of the development.

The two-storey structures up slope of the piled wall were founded partly over backfilled basement and new fill from the wall construction. Here Keller opted to use vibro stone columns as a more cost-effective approach to the proposed piling solution. The firm used one of its lightweight Minicat rigs to construct the columns, which had no problem operating in the limited space between the properties and the wall.

Built in England by Keller, the mini-rigs are fully instrumented and record construction details for each stone column and provide an immediate hard copy printout in the cab. The contractor believes this level of quality control is vital in ground improvement work.

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