Retaining walls on seismic sites

An earthquake does something to a retaining wall its static design never accounted for. The retained soil accelerates sideways against the back of the wall, and that push lands on top of the load the wall was already holding. Walls sound under normal conditions have gone over for exactly this reason.

How much extra push depends on the ground under the site, and that is the part changing. The seismic loadings standard the Building Code cites sorts a site into one of five subsoil classes, strong rock through to very soft soil, then works from assumed properties behind the label.

The newer document, SNZ TS 1170.5:2025, published last year and usable now as an Alternative Solution, uses seven classes based principally on a measured number. The number is the time-averaged shear-wave velocity through the top 30 metres of ground. Surface-wave testing can measure it without breaking ground, while CPT-based methods and other ground investigation information can also contribute to site classification.

Both documents are live, and a designer can work to either and be compliant. Loads generally rise under the newer one, because the 2022 national seismic hazard model put forecast shaking higher across most of the country.

The increase varies. Some sites barely move, and because assumed properties are conservative, a measured site can come back better than the assumption allowed.

So, a site investigation now has a second job. A hand auger and a lab test identify the soil. Neither tells you how fast a wave moves through it.

For retaining work, the reference is Module 6 of the earthquake geotechnical guidelines, published jointly by MBIE and the New Zealand Geotechnical Society.

It came out of the Canterbury rebuild and was generalised to cover routine retaining structures nationally, though the current version dates from 2021 and its hazard basis predates the 2022 model. It is guidance rather than a requirement, though that distinction wears thin in a dispute.

For reinforced concrete masonry retaining walls, continuity between the wall reinforcement and footing matters, as does complete and properly specified grout filling. Grout strength is the detail that goes missing.

NZS 4229 sets the minimum 28-day strength for grout fill at 17.5 MPa in exposure zone B, 20 MPa in zone C and 25 MPa in zone D, which covers sea spray. Its Appendix A carries details for some walls retaining soil, where its scope and limits apply.

Materials and workmanship sit under NZS 4210, design under NZS 4230. Ask early which loadings document the engineer intends to use. It changes the steel.