Every year, New Zealand structural and geotechnical engineers sign off on shallow foundation designs for houses, warehouses, and commercial buildings across everything from stiff Auckland clays to loose Christchurch alluvium and windblown Tauranga sands. Get the bearing-capacity-calculation wrong, and the consequences range from unsightly cracking to full-blown settlement failure — the kind that ends up in a Weathertight Homes Tribunal file or a professional indemnity claim. Understanding how to correctly apply New Zealand’s dual-check framework isn’t academic; it’s the difference between a foundation that performs for 50 years and one that becomes a liability within five.
Why NZ Practice Demands Two Separate Checks
Unlike some overseas codes that treat bearing capacity as a single pass/fail number, New Zealand practice under NZS 3604, the Building Code Verification Method B1/VM4, and NZGS guideline documents requires engineers to demonstrate adequacy against two distinct limit states: the Ultimate Limit State (ULS) and the Serviceability Limit State (SLS).
The ULS check confirms the foundation won’t experience a bearing capacity failure — a shear failure of the supporting soil — under factored ultimate loads. The SLS check confirms that under everyday working loads, settlement (total and differential) stays within tolerances that won’t damage the structure or its finishes. A foundation can pass the ULS check comfortably and still fail in practice because of excessive settlement, which is precisely why B1/VM4 insists both be demonstrated, not just one.
As the New Zealand Geotechnical Society’s Guideline for Foundations on Shallow Soils notes, “bearing capacity alone does not guarantee serviceability; settlement behaviour, particularly on variable or compressible soils common across New Zealand’s alluvial basins, must be independently assessed and reported.” This dual requirement reflects lessons learned from post-earthquake investigations in Christchurch, where liquefiable and soft soils produced settlement problems well before ultimate bearing failure became relevant.
Step One: The Ultimate Limit State Bearing Capacity Calculation
The starting point for any bearing-capacity-calculation in New Zealand is Terzaghi’s general bearing capacity equation, refined with Vesic or Meyerhof bearing capacity factors depending on the soil conditions and foundation geometry. The general form is:
qult = c’Nc + q’Nq + 0.5γBNγ
Where c’ is the effective cohesion, q’ is the effective overburden pressure at founding depth, γ is the soil unit weight, B is the foundation width, and Nc, Nq and Nγ are dimensionless bearing capacity factors dependent on the friction angle φ’.
Worked Example: Strip Footing on Auckland Residual Clay
Consider a 900 mm wide strip footing founded at 600 mm depth, supporting a two-storey timber-framed dwelling on typical Auckland residual clay. Site investigation returns c’ = 15 kPa, φ’ = 26°, and γ = 18 kN/m³. Using Vesic’s factors for φ’ = 26°: Nc = 22.25, Nq = 11.85, Nγ = 12.54.
Calculating:
- q’ = 18 kN/m³ × 0.6 m = 10.8 kPa
- Term 1: c’Nc = 15 × 22.25 = 333.75 kPa
- Term 2: q’Nq = 10.8 × 11.85 = 128.0 kPa
- Term 3: 0.5γBNγ = 0.5 × 18 × 0.9 × 12.54 = 101.6 kPa
This gives qult ≈ 563.4 kPa. This is the gross ultimate bearing capacity before any reduction factors are applied — and that’s exactly where NZ practice diverges from a simple textbook approach.
Applying Geotechnical Strength Reduction Factors
B1/VM4 and AS/NZS 1170.0 require that ultimate geotechnical resistance be reduced by a strength reduction factor, φg, before comparison against factored design actions. For bearing capacity of shallow foundations, NZGS guidance typically recommends φg values between 0.45 and 0.55 for standard site investigations with moderate uncertainty, rising to 0.6 where high-quality site-specific testing (such as CPT with correlated laboratory data) has been undertaken, and dropping as low as 0.35–0.40 for sites with limited investigation or higher variability.
For our Auckland example, adopting a moderate φg = 0.50 (reflecting a standard borehole and SPT-based investigation), the design geotechnical strength becomes:
qult,design = φg × qult = 0.50 × 563.4 = 281.7 kPa
This design value is then compared against the ULS bearing pressure demand, calculated from factored load combinations per AS/NZS 1170.0 (typically 1.35G or 1.2G + 1.5Q). If the two-storey dwelling imposes a factored bearing pressure of, say, 95 kPa on this footing, the ULS check passes comfortably with a utilisation ratio of roughly 0.34 — well within acceptable limits, and indicative of a foundation that could likely be narrowed if settlement governs instead.
Step Two: The Serviceability Limit State Check
Passing ULS is only half the job. Settlement must now be checked against the serviceability criteria in NZS 3604 and B1/VM4, which generally limit total settlement to 25 mm and differential settlement (angular distortion) to 1:500 for typical residential and light commercial structures, tightening further for structures sensitive to distortion such as those with masonry veneer or rigid cladding systems.
Using elastic settlement theory or one-dimensional consolidation methods (depending on whether the soil is granular or fine-grained), the same Auckland footing under sustained working load of 60 kPa might be estimated to settle 18 mm using typical residual clay compressibility parameters (mv = 0.00025 m²/kN across the influence depth). This sits within the 25 mm total settlement limit, and — provided adjacent footings are founded on comparable material at consistent depth — differential settlement is unlikely to breach the 1:500 criterion.
Critically, on variable sites such as those with fill, peat lenses, or transitional alluvial deposits common around Hamilton, Napier, and parts of Christchurch, settlement — not ultimate capacity — usually governs footing size. Engineers who size footings purely from the ULS bearing-capacity-calculation without running the parallel SLS settlement check routinely under-design for real-world performance.
Worked Example: When Settlement Governs
Take a 3.0 m square pad footing on loose-to-medium dense sand (Auckland volcanic-derived or Pukekohe-type soils) supporting a light industrial column load of 850 kN. A ULS bearing-capacity-calculation using Meyerhof’s method for φ’ = 32° might yield qult ≈ 720 kPa, and after applying φg = 0.45 (reflecting looser granular material and moderate investigation confidence), design capacity is 324 kPa — comfortably exceeding the factored demand of roughly 130 kPa.
Yet settlement analysis using Schmertmann’s method against CPT cone resistance data might predict 32 mm of immediate settlement — exceeding the 25 mm serviceability limit. In this scenario, the engineer must increase the footing width to reduce net applied pressure, or introduce ground improvement (dynamic compaction or a stone column array), even though the ULS check passed with margin to spare. This is a textbook illustration of why B1/VM4’s dual-check requirement exists.
Key Takeaways for Practising Engineers
- Never treat ULS as sufficient on its own. The Christchurch and Auckland case files are full of foundations that satisfied bearing capacity but under-performed on settlement.
- Match φg to investigation quality, not convenience. A φg of 0.55–0.60 requires robust CPT or laboratory-verified strength parameters — not an assumed φ’ from a desktop study.
- Settlement typically governs on loose granular or soft cohesive sites — Hamilton peats, Christchurch silts, and reclaimed land around Tauranga and Wellington all demand rigorous SLS checks.
- Differential settlement matters more than total settlement for most structural damage claims — always model founding conditions across the full footprint, not just at one borehole location.
- Document both checks explicitly in the geotechnical completion report; PS1 and PS4 certifications increasingly require traceable calculations for both limit states under current council scrutiny.
Get Your Foundation Design Right the First Time
Bearing capacity and settlement calculations are only as reliable as the site investigation and engineering judgement behind them. If you’re scoping a foundation design — whether a straightforward residential strip footing or a complex commercial pad on variable ground — Chambers Consultants’ geotechnical and structural engineers apply rigorous, code-compliant ULS and SLS assessments tailored to your site’s specific soil profile. Contact our team today to discuss your project and ensure your foundations are engineered to perform, not just to pass.