Raft Failure on Fill

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Building on filled ground needs the careful selection of fill and controlled placement to ensure an adequate foundation material. However, secondary structures such as domestic garages and boundary walls, do not always get the respect they deserve. Structures built on the edge of filled ground, like this one in Co. Cork, require extra attention due to the risk of slip.

Like any works, the engineer must provide a specification and the builder must build to that specification. This must include a methodology for the placing and compacting of the fill including the type and mass of compaction plant required, the layer thickness and the number of passes. Typical layers would be about 200mm and should not vary by more than 15% to minimise the risk of differential settlement. Topsoil, any unsuitable material and other geohazards must be removed before filling can commence. Homebond state that ‘filling and compaction should be carried out under the supervision of an engineer. The engineer appointed should be qualified by examination, be in private practise and possess professional indemnity insurance.’

It is important to note that it is differential settlement in a building rather than uniform settlement than results in foundation failure. Raft failure will occur in one of two ways: geotechnically, where it will tilt (as one unit) in one direction, or structurally, where the raft will break, and superstructure cracking will result.  The factors that effect this include the reinforcement of the raft and the geometry of the building. In this case, the raft has structurally failed, due to a combination of landslip and excessive settlement of the fill