Locating a new building on unstable, volatile or reactive soils can really eat into a projects’ budget. This is a site penalty that makes the substructure often complex and expensive. It really should be considered very carefully when selecting a site, as it may be a factor that compromises the project … or, at worst, stops it from proceeding.

The issue needs to be recognised, considered and costed early in cost planning and reviewed during design.

The extra cost involved with this includes items like:

  • Additional excavation and soil removal;
  • Imported fill and crushed rock to replace the existing soil;
  • Blinding concrete below the footings to make the pour easier and foundations more stable;
  • The concrete slab may need to be thicker and with more reinforcement, and maybe lost formwork, so that it spans from footing to footing… and really it then operates as a suspended slab, rather than a slab-on-ground;
  • The edge beams and internal thickenings to the slab may need to be much larger, deeper and also span some distance between supports,
  • And, perhaps between mass or bored piers, screw piles or underpinning may also be required.

As well as the net extra cost of these items, the project program may also need to be extended to cope with this extra work.