Vibrocompaction Design for Whanganui Basin Soils

The most common misstep we see on Whanganui sites is treating a uniform sand profile as homogeneous—then wondering why compaction radii fail QA. The city sits on a deep basin of Quaternary alluvium, where the Whanganui River has laid down interbedded sands, silts, and occasional pumice lenses over millennia. Without a design that maps these lateral and vertical gradations, even a well-executed vibrocompaction grid can leave untreated pockets. Our CPT testing program feeds directly into the design phase, giving us continuous soil behaviour type profiles instead of discrete SPT blows, so we can calibrate spacing, energy, and depth for the actual stratigraphy. A design built on interpolation between boreholes alone misses the very lenses that cause differential settlement later.

Good vibrocompaction design in Whanganui is about predicting where the river buried a silt lens—and having a plan for it before the vibrator hits refusal.

Process and scope

In our experience, the Whanganui sand units respond differently depending on fines content—once silt exceeds about 15 percent, the vibrator's radius of influence drops sharply. We factor this into the triangular grid geometry, often tightening spacing near the riverbank where Holocene overbank deposits are younger and looser. A design review must also account for the water table, which sits high across much of the city and affects the backfill column behaviour during compaction. Where CPT tip resistance stays below 5 MPa for more than a metre, we evaluate whether stone columns offer a more reliable densification path than pure vibrocompaction, especially under structural loads exceeding 200 kPa. The design deliverable includes a detailed penetration and withdrawal log sequence, amperage targets per stage, and a verification testing plan tied to NZGS Module 5 guidelines.

Local geotechnical context

Whanganui's expansion east of State Highway 3 has pushed development onto terraces where the geological model shifts from fluvial sands to older, weathered marine sediments with higher silt content. The risk here is designing a vibrocompaction scheme that works beautifully in the sand but underperforms where the soil transitions into a silty sand or sandy silt at depth. We've reviewed projects where post-treatment CPTs showed good improvement in the upper 8 metres but negligible densification below a subtle silt marker bed—the design hadn't specified a staged energy ramp to breach that layer. A second risk is under-estimating ground vibration propagation toward heritage buildings along Victoria Avenue; our design always includes a vibration monitoring specification with peak particle velocity limits set in consultation with the council's building consent team.

Technical parameters

Parameter	Typical value
Typical design grid	Triangular, 2.0 to 3.5 m spacing
Target relative density (Dr)	70–85% depending on seismic category
Depth range treated	6 to 25 m below platform level
Vibrator power range	130–180 kW electric or hydraulic
Backfill consumption indicator	0.3 to 0.8 m³ per linear metre
Post-treatment verification	CPT before/after pairs, minimum 1 per 400 m²
Seismic performance criterion	Liquefaction factor of safety ≥1.2 per NZGS Module 1

Associated technical services

Vibrocompaction trial programme design

We define trial zone location, vibrator settings matrix, and acceptance criteria before full production. The trial validates grid spacing and energy input against the CPT-derived target relative density, saving costly rework on large Whanganui footprints.

Performance specification and QA documentation

We produce a NZS-compliant design report with construction drawings, instrumented monitoring requirements, and a stage-by-stage verification protocol. The package is structured for peer review and building consent submission.

Quick answers

What does vibrocompaction design in Whanganui typically cost?

Design fees for a vibrocompaction project in Whanganui generally range from NZ$2,770 to NZ$8,810. The final figure depends on the treated area, number of CPT verification locations, and whether a trial programme is included. We provide a fixed-price proposal after reviewing the geotechnical investigation data.

How do you determine the grid spacing for Whanganui sands?

We derive the initial triangular grid spacing from the Priestley method, calibrated against CPT tip resistance and friction ratio data collected on site. The spacing is then refined through a trial zone programme where we measure post-compaction CPT improvement and adjust for fines content variability across the Whanganui basin.

Can vibrocompaction mitigate liquefaction risk in Whanganui?

Yes. The NZGS Module 1 framework requires a factor of safety against liquefaction of at least 1.2 for typical structures. Our designs target a post-treatment relative density that achieves this threshold under the 500-year return period seismic event defined in NZS 1170.5, verified by before-and-after CPT pairs.

What verification testing do you specify?

We specify a minimum of one CPT pair per 400 m² of treated area, with a requirement that the post-treatment cone resistance meets or exceeds the design target profile. We also include zone load tests or plate load tests where the site will support shallow footings, to confirm deformation modulus improvement.