Raft Slab Failure - Reactive Soil

  • Front of dwelling
  • Rear of dwelling
  • Cornice separation
  • Cornice separation
  • Cornice cracking
  • Test hole through dwelling slab
  • Cracks in concrete slab beam

In early 1991, a cotton farmer engaged a project manager / architect, structural engineer and geotechnical consultant [which was a large multi-discipline corporation] to advise on the design and construction of a very large house in the black soil country of Moree.  The site was close to the Mehi River, and was subject to infrequent flooding;  as such, it was necessary to construct a large earth mound to raise the floor level of the building above flood level.

The resulting design for the house was a very substantial concrete raft slab on a gravelly clay mound, with edge beams nearly 1 m deep;  the earth mound & house was then constructed under the supervision of the project manager and structural engineer, and completed in mid-1992.

In early 1993, the first signs of movement within the house structure were noted, with the movements and cracking of the walls of the house becoming so significant that SCE was engaged 1993 to:

  • provide advice on the cause of the damage;
  • assist in the subsequent litigation.

By early 1994, it became apparent that the raft footing system had failed, with the amount of edge heave of the slab, as compared to the centre part of the slab, exceeding 230 mm.  In effect, the house was seriously bent in the middle and suffered serious internal damage.

In the ensuing litigation, the liability for the damage ended up being borne by the project manager / architect and structural engineer.  Also, whilst it was clear to all concerned that the geotechnical consultant had grossly underestimated the shrink / swell movement of the soil, the geotechnical consultant escaped liability because after the house had been constructed, the geotechnical company that had provided the geotechnical advice had been wound up and taken over by a different overseas owned corporation.