A piled raft is generally applied in high-risk buildings where internal and external damage is both significant and progressive. Unlike ground improvement or raking micropiles, the system can be easily proven by design and/or testing. The primary advantage of this system over the traditional pile and beam is that the piled raft will naturally cover the whole footprint of the building, thereby providing support for the floor also. Where appropriate, a pile raft can be considered the best solution to foundation movement.
A piled raft is constructed of a reinforced concrete slab, cast in one pour, supported on micropiles, which carry the load from the external walls via needle beams. These needle beams are short reinforced concrete beams that project from the raft into the rising walls. The raft will also support the internal walls via smaller needles. The normal requirement for weldmesh is two layers of A393. The centres of the needles will depend on the pile loadings, raft design and arching ability of the walls.
This system can only be applied if enough rising walls are present to allow for the raft (c. 300 mm) and insulated concrete screed (c. 150 mm) to be installed. If insufficient rising walls are present, traditional pile and beam may be the only viable option.