Raft Foundation Design in Christchurch: Improvement and Seismic Performance

For a standard residential lot in Christchurch, a complete raft/mat foundation design package, including site investigation review, geotechnical modelling, structural design, and PS1 documentation, typically runs between NZ$1,980 and NZ$7,960. The spread depends on whether Improvement (such as stone columns or a densified gravel raft) is required, the complexity of the architectural layout, and the need for specific liquefaction mitigation calculations under the MBIE guidance. A straightforward single-storey build on TC2 land sits at the lower end, while a two-storey structure on TC3 with high groundwater and a complex floor plan moves toward the upper range.

A raft foundation is structurally designed to distribute the entire building load over the footprint, acting as a single rigid unit. Unlike a standard slab-on-grade, it incorporates deep perimeter and internal stiffening beams, heavy top and bottom reinforcement, and is analysed for differential settlement and seismic bearing capacity loss. In Christchurch, this design explicitly accounts for the cyclic softening of liquefiable layers and the potential for ground oscillation, which a standard NZS 3604 slab is not engineered to handle.

The choice depends on the depth and nature of the liquefiable layer. For shallow liquefaction within the top 3 to 4 metres, we often specify a densified gravel raft or vibrocompaction to create a non-liquefiable crust. For deeper layers of liquefiable sand, we may design stone columns at regular spacings beneath the raft to provide drainage and reinforcement. In areas with very soft organic silts, a surcharge preload period with wick drains can be specified before the raft construction begins, reducing long-term consolidation settlements to acceptable limits.

The structural design follows NZS 3404 for steel reinforcement detailing and NZS 4203 for seismic loading, while the geotechnical analysis adheres to the New Zealand Geotechnical Society (NZGS) guidelines for liquefaction assessment and the MBIE foundation design guidance for Canterbury. We also reference the relevant ASTM standards for soil testing, particularly ASTM D1586 for SPT data interpretation and ASTM D5778 for CPT correlations when calculating the post-liquefaction stiffness of the founding soils.