Riccarton infill case study — TC2 ground class and the foundation system redesign
Updated: Jul 28
A Riccarton infill foundation quote was priced against a TC1 ground class assumption. The site's actual classification under the post-earthquake ground class maps was TC2 — and the foundation system specified didn't meet the TC2 deemed-to-comply pathway. Caught at tender it saved $40-70k of foundation rework.
By Steve Parker · Trueworks · NZ construction estimation · 5 min
The foundation quote was priced against the soils report. The soils report referenced TC1. The post-earthquake ground class map for the suburb shows TC2 across the street and TC2 over the back fence. Catching the classification gap at tender — before the foundation footprint was committed — saved a foundation system redesign mid-build.
By Steve Parker · Trueworks · NZ construction estimation · 5 min
What you'll learn in this case study
The TC1 vs TC2 ground class gap on Christchurch infill sites in the eastern and southern suburbs
The NZS 3604 §3 and Ministry of Business, Innovation and Employment (MBIE) guidance that governs foundation system selection
The dollar and programme effect of catching ground class at tender versus catching it at foundation excavation
Quick answer: A Riccarton infill new build had a foundation quote priced against a TC1 ground class assumption with a standard slab-on-grade specification. The site's actual classification under MBIE post-earthquake ground class mapping was TC2, requiring a foundation system designed to accommodate the higher liquefaction and lateral spreading risk. NZS 3604 §3 sets the framework for foundation selection by ground type, with MBIE post-earthquake guidance modifying the requirements in the affected Christchurch suburbs. Catching the ground class at tender saved $40-70k of foundation rework and 4-6 weeks of programme.
The build
An infill new build on a Riccarton site, build value in the $900k-1.3M range. Single-storey contemporary home with attached garage, light timber frame, on what would have been a standard slab-on-grade foundation in pre-2011 Christchurch. Site was an established residential subdivision with neighbouring properties intact. The head contractor sent the foundation contractor's quote across at tender stage as part of a standard quote-check engagement.
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What we found in the quote
The foundation quote was straightforward. Excavation and preparation for a standard slab-on-grade per NZS 3604 §3, reinforcing to the engineer's standard residential schedule, concrete pour, finishing to the floor level. The quote referenced the geotechnical soils report supplied by the head contractor, which described the soil profile as "stiff alluvial gravels overlying dense silty sands, suitable for shallow foundation construction." The report's recommendation was a NZS 3604 §3 standard slab-on-grade with no specific liquefaction mitigation.
The problem was that the soils report appeared to be a pre-2011 document or a document that hadn't been updated to reflect the post-Canterbury-earthquake-sequence ground class mapping. MBIE's post-earthquake ground class maps for the Christchurch urban area classify land into four technical categories (TC1 through TC3, plus the unmapped CBD area) based on liquefaction risk under design earthquake loading. The classifications drive foundation system selection:
TC1: Liquefaction unlikely to cause significant ground damage; NZS 3604 §3 standard foundations acceptable
TC2: Minor to moderate liquefaction damage possible; foundation systems must be designed to MBIE TC2 guidance, including stiffened slabs, deeper footings, or specific engineering review
TC3: Significant liquefaction damage possible; foundation systems require specific engineering design and typically use deep piles or rafts
The Riccarton site sits on the boundary of TC1 and TC2 land. The MBIE map showed TC2 across the adjacent properties on the same street. The soils report's recommendation appeared to be inconsistent with the MBIE classification — a not-uncommon occurrence on infill sites where the geotechnical brief didn't ask for an MBIE classification check.
The foundation quote, priced against a standard slab-on-grade, was about $35-50k. The TC2-appropriate foundation system (typically a stiffened slab with edge thickenings, additional reinforcement, and possibly shallow rigid raft elements) was likely to cost $75-110k for the same footprint. The gap was $40-60k of foundation scope that wasn't visible in the quote.
The harder cost wasn't the foundation itself. The harder cost was the programme. A foundation system change at the geotechnical-report stage costs design fees and a re-quote. The same change at the excavation stage — when the spoil is already coming out and the formwork is being placed — costs a stop-work order, a re-design, a re-consent if the change is significant, and 4-6 weeks of programme delay before pour can resume.
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How the code resolves it
NZS 3604 §3 (Foundations) is the standard reference for residential foundation selection. The standard provides the framework for slab-on-grade, footing-and-floor, and pile foundation systems within defined ground type envelopes. The standard's ground classifications (good ground, fair ground, poor ground) overlap with but aren't identical to the post-earthquake MBIE classifications, and on Christchurch sites the MBIE classification is the controlling document.
MBIE's "Repairing and Rebuilding Houses Affected by the Canterbury Earthquakes" guidance (and its subsequent updates) sets the foundation requirements for the post-earthquake-classified land. For TC2 sites the guidance requires either:
A stiffened slab foundation with specified edge beam dimensions, slab thickness, reinforcement schedule, and contraction joint pattern
A driven or screw pile foundation to a competent layer
A specific engineering design verified for the site's expected ground deformation
The choice depends on the soils report, the building geometry, and cost. For a single-storey residential infill, the stiffened slab is typically the cheapest TC2-compliant option and is the most common choice.
The reconciliation at tender stage required two steps: (1) confirm the MBIE ground class for the specific site by checking the published map and requesting an updated soils report if necessary; (2) re-issue the foundation scope against the correct ground class with an appropriate foundation system selected.
What it would have cost if caught later
| Stage caught | Cost range | Why | |---|---|---| | At tender (paper review) | $2,000-4,500 | Updated soils report or written confirmation of MBIE ground class; foundation re-design to TC2-compliant stiffened slab; re-quote from foundation contractor | | At pre-start (post-award) | $8,000-15,000 | Building consent amendment for revised foundation; foundation contractor re-quote; possible delay of 2-4 weeks awaiting engineering review and consent | | At excavation (post-mobilisation) | $25,000-50,000 | Stop-work order; foundation system re-designed against actual ground conditions seen on excavation; possible re-excavation if original cut was too shallow for TC2 system; 4-6 weeks of programme delay; formwork and reinforcement re-quoted | | At pour (post-formwork) | $40,000-70,000 | Foundation rejected at pre-pour inspection; formwork stripped and re-set to TC2 specifications; some materials wasted; 5-8 weeks of programme delay; possible damages claim from following trades | | Post-completion (under building consent challenge) | $80,000-200,000 | CCC withheld pending foundation re-design; possible partial demolition to access foundation; structural re-engineering of as-built structure |
The Riccarton case caught the ground class at tender. The variation that wasn't booked was $40-70k. The fix was a $2-3k geotechnical update and a re-quote.
The clarification we recommended
The clarification packet to the head contractor named the ground class gap, cited NZS 3604 §3 (with MBIE post-earthquake guidance as the controlling reference for the Christchurch site), and recommended three actions: (1) request a written confirmation of the MBIE ground class for the specific site from a chartered geotechnical engineer; (2) if the classification is TC2 or TC3, request a foundation design appropriate to the classification and re-quote the foundation scope accordingly; (3) confirm the building consent application referenced the correct ground class and revise if necessary before consent issue.
The geotechnical engineer confirmed the site as TC2 within five working days. The foundation engineer designed a stiffened slab with 300 mm edge thickenings, an enhanced reinforcement schedule, and contraction joint spacing per MBIE TC2 guidance. The foundation contractor re-quoted at about $40k above the original quote — well below the variation cost it replaced. The consent application was lodged with the correct foundation system and the correct ground class. No variation surfaced post-award.
What other Christchurch infill builds should check
The MBIE post-earthquake ground class map is the controlling document for any Christchurch foundation selection — even on sites where neighbouring properties were undamaged
A soils report dated before 2011 (or one that doesn't reference the MBIE classification explicitly) should be updated before the foundation quote is locked in
TC2 foundation systems cost typically 30-60% more than TC1 standard slab-on-grade for the same footprint; the cost has to appear on the foundation quote, not on the variation log
NZS 3604 §3 ground classifications and MBIE post-earthquake classifications are not identical — the MBIE classification controls on Christchurch sites
The soils report should be commissioned with an explicit instruction to confirm the MBIE ground class and recommend a compliant foundation system, not just describe the soil profile
FAQ — foundations on Christchurch infill new builds
Q1: How do I check the MBIE ground class for a specific Christchurch site? MBIE publishes the post-earthquake ground class maps online and they're searchable by address. A chartered geotechnical engineer can also confirm the classification with a site-specific written assessment. For consent purposes the written assessment is the document typically required.
Q2: Can a soils report override the MBIE map classification? A site-specific geotechnical investigation can demonstrate that the site's actual ground conditions differ from the MBIE map classification — typically by showing the depth and strength of competent layers below the surface. The investigation must be commissioned with that explicit purpose and the result must be reviewed by the consenting authority. The MBIE map is the starting point, not the final word.
Q3: What's the difference in foundation cost between TC1 and TC2 for a typical residential build? On a typical 150-200 m² single-storey residential build, the TC1 standard slab-on-grade costs about $35-55k. The TC2 stiffened slab equivalent costs about $55-90k. The difference is 30-60% and reflects the additional concrete, reinforcement, and engineering design.
Q4: Does TC2 ever require pile foundations on a single-storey infill? Rarely — the stiffened slab is typically sufficient for single-storey residential on TC2 land. Piles are more often required for two-storey or heavier structures or for TC3 sites. Where soils conditions show a soft surface layer over a deeper competent layer, piles may be cheaper than a deep stiffened slab.
Q5: What happens if the foundation is built to TC1 spec on TC2 land and the discrepancy is discovered later? The building consent will typically be challenged at the CCC stage. The remediation depends on what's been built — typically a chartered engineer reviews the as-built foundation against the actual ground class and either certifies it as fit for purpose (rare without significant additional work) or requires retrospective strengthening. The cost is typically 3-5 times what the correct foundation would have cost at the design stage.
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