A single poorly chosen coefficient can mean the difference between a slope that performs acceptably in a design earthquake and one that fails catastrophically, burying infrastructure beneath thousands of cubic metres of displaced ground. For engineers working across New Zealand’s seismically active terrain, the selection of the horizontal seismic coefficient, kh, in pseudo-static slope analysis is not a routine box-ticking exercise. It is one of the most consequential judgement calls in geotechnical practice, and getting it wrong in either direction carries real costs: over-conservative values inflate construction budgets unnecessarily, while under-conservative values leave communities exposed to landslide hazard during the next significant earthquake.
Our North Shore geotechnical engineers use these methods when assessing sloping and clifftop sections.
This article sets out how practitioners should approach kh selection under MBIE Module 6 and NZGS guidance, with practical examples drawn from typical New Zealand ground conditions.
Why Pseudo-Static Analysis Still Matters
Despite the availability of sophisticated dynamic numerical modelling, pseudo-static-slope-analysis remains the workhorse method for seismic slope stability assessment across New Zealand. It converts the complex, time-varying inertial forces generated by earthquake shaking into a simplified static horizontal force, applied as kh multiplied by the weight of the potential failure mass. The appeal is obvious: it is computationally efficient, well understood by reviewers and consent authorities, and produces a single factor of safety that can be benchmarked against accepted thresholds.
The limitation is equally obvious. Pseudo-static-slope-analysis does not capture the transient nature of seismic loading, nor does it predict displacement. A slope with a calculated factor of safety below 1.0 under pseudo-static loading does not necessarily fail outright; it may simply undergo some permanent deformation during strong shaking and then regain equilibrium once shaking ceases. This is precisely why MBIE Module 6 (Earthquake geotechnical engineering practice, Module 6: Permanent and transient slope deformation) positions pseudo-static analysis as a screening tool, to be followed by Newmark-type displacement analysis where the factor of safety falls below acceptable limits or where consequences of even modest displacement are significant.
Understanding this hierarchy is critical. Practitioners who treat the pseudo-static factor of safety as the final word, without considering displacement-based verification, risk either unnecessary conservatism or an incomplete assessment that fails to capture the true performance of the slope.
The MBIE Module 6 Framework for Selecting kh
MBIE Module 6 provides New Zealand-specific guidance for deriving kh, moving away from generic international defaults towards values calibrated to our seismic hazard environment under NZS 1170.5. The starting point is the site’s peak ground acceleration (PGA), derived from the NZS 1170.5 hazard factor Z, adjusted for site subsoil class, return period, and importance level of the structure or facility the slope supports.
A commonly referenced starting relationship is kh in the range of 0.5 to 0.7 times the site PGA (expressed as a fraction of g), with the specific fraction selected based on the acceptable level of displacement for the slope in question. For a slope above critical infrastructure, such as a water treatment plant or a State Highway corridor, a higher proportion of PGA is typically adopted, reflecting lower tolerance for displacement. For a slope with lower consequence of failure, such as beneath a lightly trafficked access track, a lower proportion may be justified, consistent with accepting some permanent deformation.
NZGS guidance reinforces this by linking kh selection explicitly to the importance level (IL) framework under the Building Act and NZS 1170.0. An IL4 facility, such as a hospital access road or emergency services route, demands a materially more conservative kh than an IL2 residential driveway cut. This is not an arbitrary distinction; it reflects the differing societal tolerance for post-earthquake functionality.
Dr. Kevin McManus, a prominent contributor to New Zealand’s earthquake geotechnical guidance, has noted that “the selection of seismic coefficients should never be divorced from an understanding of the consequences of slope displacement, nor from the return period appropriate to the facility being protected.” This principle underpins the entire Module 6 approach and should guide every kh determination a practitioner makes.
Worked Considerations: Regional Variation Across New Zealand
New Zealand’s seismic hazard varies enormously across the country, and kh selection must reflect this. A slope assessment in Wellington, sitting atop a Z factor of 0.4, demands a fundamentally different kh than an equivalent slope in Auckland, where Z is typically 0.13. Applying a Wellington-derived coefficient to an Auckland slope would produce needless over-design; applying Auckland assumptions to a Wellington cut would be dangerously non-conservative.
Consider a practical example: a 12-metre high cut slope in weathered greywacke colluvium supporting a residential subdivision access road in the Wellington region. With a Z factor of 0.4, a subsoil class D site, and an IL2 classification, the derived PGA for the appropriate return period might sit around 0.55g at ultimate limit state. Applying a kh of approximately 0.65 times this PGA yields a design coefficient in the order of 0.35, a figure materially higher than coefficients commonly applied in lower seismicity regions such as Northland or coastal Otago, where kh values of 0.10 to 0.15 may be appropriate for comparable slope geometries.
Canterbury presents its own complexities following the Canterbury earthquake sequence. Practitioners working in the Port Hills and surrounding areas must account for near-fault directivity effects and the well-documented amplification observed in loess-derived soils, both of which can justify kh values toward the upper end of the Module 6 recommended range, even where the underlying Z factor might suggest a more moderate coefficient.
Key Takeaways for Practice
- Anchor kh to site-specific PGA, not generic defaults. A blanket kh of 0.15, sometimes seen in older reports, is inappropriate across most of New Zealand’s higher seismicity zones and should be replaced with a value derived from the site’s NZS 1170.5 Z factor and subsoil class.
- Match kh to importance level and consequence. A slope beneath an IL4 lifeline route warrants a materially higher proportion of PGA than a low-consequence IL2 slope, even where both sit on identical geology.
- Treat pseudo-static-slope-analysis as a first-pass screen. Where the resulting factor of safety is below approximately 1.0 to 1.1, follow up with Newmark displacement analysis per Module 6 before concluding the slope is unacceptable.
- Account for regional and local amplification effects. Loess soils in Canterbury, liquefiable deposits along the Hutt Valley, and near-fault zones along the Alpine and Wellington faults all warrant upward adjustment beyond baseline Module 6 recommendations.
- Document the derivation transparently. Consent authorities and peer reviewers increasingly expect a clear audit trail showing how Z, subsoil class, return period, and importance level combined to produce the adopted kh, rather than a single unsupported figure in a calculation sheet.
Common Pitfalls Worth Avoiding
Several recurring errors undermine the credibility of pseudo-static-slope-analysis in practice. The most frequent is adopting a kh value from a previous project in a different region without re-deriving it for the current site’s Z factor and subsoil class. Soil parameters, groundwater conditions, and slope geometry rightly receive careful site-specific attention, yet the seismic coefficient is sometimes carried over by habit. This inconsistency is precisely what NZGS guidance seeks to eliminate.
A second common pitfall is ignoring the interaction between pore pressure response and seismic loading. In saturated or marginally saturated slopes, particularly those containing sensitive clays or loose sandy silts prone to cyclic softening, the appropriate response may not be a pseudo-static adjustment at all, but a fully coupled liquefaction or cyclic softening assessment. Module 6 is explicit that pseudo-static methods are unsuitable where materials are liquefaction-susceptible, and practitioners must screen for this before defaulting to a pseudo-static approach.
Call to Action
Selecting kh is not a matter of consulting a single table and moving on. It requires a defensible, project-specific derivation grounded in NZS 1170.5 hazard parameters, MBIE Module 6 methodology, and NZGS practice guidance, tested against the consequences the slope in question actually presents. At Chambers Consultants, our geotechnical team routinely works through this derivation for clients across New Zealand’s full range of seismic environments, from Auckland’s lower hazard setting to the demanding conditions of Wellington and Canterbury. If you are planning a development involving cut or fill slopes, retaining structures, or lifeline infrastructure in seismically active terrain, contact Chambers Consultants to ensure your pseudo-static-slope-analysis is grounded in current New Zealand guidance and stands up to regulatory and peer scrutiny.