Ground Beams
Southbridge, Alfred McAlpine Homes
Aarsleff Piling has responded to requirements from the house building industry and expanded its operations to provide clients with complete foundations packages of piles, caps and cast in-situ reinforced concrete ground beams. The advantageous and efficient concept enables house builders to combine, what has historically been, two separate work packages of piling and ground beams, into one sub-contract. This reduces administration, is more economical and provides better control at the interfaces of the two different, but complementary processes.
The foundations package provides significant practical advantages, especially on sites with poor ground or areas containing low levels of contamination. Piles are driven, using Aarsleff Piling’s own piling rigs, without any spoil coming to the surface. Spanning ground beams can then be cast directly on top of the ground surface, which is covered with a thin layer of lean mix concrete, to act as the soffit. The system is very flexible and easily adapted to changes in layout. There is no excavation, muck away or dumping costs and with Government promoting the reuse of brownfield sites for housing, Aarsleff Piling’s concept of providing piling, capping and ground beams packages could prove increasingly more popular.
House builder Alfred McAlpine Homes (Midlands), based in Birmingham, benefited from an Aarsleff Piling foundations package being installed for the initial £5M phase of a private residential housing scheme on a brownfield site at Southbridge on the southern outskirts of Northampton. McAlpine redeveloped the former Pioneer Aggregates site with eight blocks of three and four storey town houses and apartments. The first phase had 73 units and would be followed by a second phase. Aarsleff Piling installed the precast concrete foundation piles together with cast in-situ ground beams for the initial phase which was completed on schedule. The project value was approximately £225,000.
"Aarsleff Piling’s all in package of piles and ground beams is a simple, straightforward and a very competitive, commercial and good technical solution, which is ideally suited to the ground conditions on this project," said Alfred McAlpine Homes (Midlands) contracts manager Martin Burchnall. "This is the first time I’ve used in-situ ground beams and believe the system is far more adaptable and flexible for our requirements than alternative fixed length precast beams, which can also cause problems with the layout of internal drainage because toes on the beams prevents you getting drains tight up against the walls."
"The system is also very fast and clean with no muck away," adds site manager Gary Davis. "There are no haulage or off loading costs or onsite storage requirements, needed for precast beams, which also require more care and time setting out. When using precast beams the ground has to be levelled and compacted to prevent settlement. But with in-situ you simply put down a thin level blinding on top of the ground, place the reinforcement, put shutters up and pour the concrete."
Aarsleff Piling, used a Banut 500 self erecting hydraulic drop hammer piling rig, initially drove 417 of its 250mm square, continuously reinforced precast concrete piles to a predetermined set to accommodate working loads up to 450kN. The piles varying in length from 9m to 12m, were driven through an approximate 3m thick layer of fill and into overlaying and alternate bands of limestone and mudstone. After piling Aarsleff Piling followed on cropping and capping the piles, which were then strategically linked with approximately 1500 metres of 600mm by 600mm and 600mm by 500mm concrete structural load bearing beams, cast in-situ using the ground as the soffit.
"Aarsleff Piling have been very professional and done an excellent job. They will cut at least six weeks off their programme and have been very easy to work with. I would have no hesitation in using them again on similar projects," added Burchnall. "And with Government encouraging brownfield regeneration this kind of piling and cast in-situ ground beam foundations system is likely to be used more and more."
McAlpine followed on behind placing an initial layer of fill up to the top of the ground beams prior to starting to build the sub-structure brickwork on top of the ground beams to tie in with conventional beam and block floor and damp proof course. The entire site was covered with a second and final layer of fill and topsoil to a level of about 500mm above the top of the ground beams.
