NZS 1170.5:2004 sets the framework, but in Blenheim, site class can change within a hundred metres. The Wairau Plains gravels thin out toward the south, and pockets of alluvial silt and loose sand sit right under light industrial and residential subdivisions. We learned this the hard way on a project near Riverlands where three boreholes within a single lot gave Vs30 values from 180 m/s to 420 m/s. A desktop site class assignment would have missed that completely. Seismic microzonation here is not just a planning exercise—it’s the only way to avoid under-design in soft pockets and over-design on stiff gravel, both of which cost money. We combine MASW with microtremor arrays and targeted SPT boreholes to build a ground model that reflects what the 2016 Kaikōura shaking already showed us: Blenheim’s soils amplify differently, block by block.
In Blenheim, site class can flip from C to D in less than 80 metres; a single borehole doesn’t tell you that—a velocity map does.
