A significant portion of Christchurch is built on reclaimed swamp and alluvial land - ground that does not perform well under direct load from a conventional concrete slab. This is why the rib raft (or waffle pod) foundation system became standard practice for new builds across the city in the post-earthquake rebuild period, and remains the dominant choice today.
What Is a Rib Raft Foundation?
A rib raft floor is a lightweight concrete foundation using polystyrene void-formers (pods) arranged in a grid, with concrete ribs running between them and a continuous top slab poured over the whole system. The result is a foundation that is significantly lighter than a full concrete slab - reducing the load on the ground below - while maintaining high structural strength. The polystyrene pods also provide good thermal insulation at the slab level.
Crucially, rib raft foundations sit largely on the ground rather than in it, which makes them more resistant to the differential settlement and lateral movement that caused so much damage to conventional foundations in the 2010-2011 earthquakes.
The Earthworks Process for a Rib Raft
Before the concrete crew arrives, the site needs to be prepared to the engineer's specification. This typically involves:
- Excavation to good bearing capacity. The existing ground is dug out to a depth determined by a geotechnical engineer - removing unsuitable material (soft silt, organic matter) and reaching the competent bearing layer below.
- Aggregate backfill, compacted in layers. The excavated zone is filled back with clean aggregate (typically AP65 or similar), compacted in controlled lifts to achieve the specified density.
- Geosynthetics if required. The engineer may specify geogrid (reinforcing mesh) and biddim cloth (silt barrier) at the base of the excavation and between aggregate lifts, depending on ground conditions.
- Final grade to rib raft formation level. The surface is laser-graded to precise tolerance before the concrete crew takes over.
TBT Earthworks provides full rib raft earthworks services in Christchurch and across Canterbury - from excavation through to final formation. We work closely with geotechnical engineers and concrete crews to keep new builds on programme. Call Trent on 027 858 0556 to discuss your project.
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