We were called to a site in East Auckland last year where the contractor had already placed 300 mm of basecourse over a clay subgrade. Within weeks, rutting appeared. The problem was clear: nobody had run a CBR study for road design before placing the pavement layers. That job taught us something we now apply every day in Auckland — the California Bearing Ratio isn't just a number; it's the difference between a road that lasts fifteen years and one that fails in the first wet winter. We soak the samples for four days, measure penetration resistance, and compare it to a standard crushed rock value. That result tells us exactly what thickness of pavement we need. Before we start the CBR, we often complement the investigation with a calicatas exploratory to log the soil profile and take representative samples for the lab work. It is a straightforward process, but skipping it costs real money.
A CBR value below 3 in Auckland's volcanic clays means you need at least 450 mm of pavement to avoid structural failure within five years.
Methodology and scope
Our lab uses a motorised CBR press with a 50 kN load ring and a dial gauge that reads to 0.01 mm. The moulds are 152 mm in diameter, and we compact the sample at optimum moisture content following NZS 4404:2010. The procedure itself is methodical: we place the surcharge weight, seat the plunger, and apply the load at 1.27 mm per minute. We record load at 2.54 mm and 5.08 mm penetration. The reported CBR is the higher of the two values, capped at 100. For projects where the subgrade is particularly soft, we also run a densidad cono de arena in the field to check the in-situ density before we extract the block samples for the CBR study for road design. This combination of field and lab work gives the design team a complete picture. Every test is documented with a chain of custody, and the results are reviewed by the lead technician before release.
Technical reference image — Auckland
Local considerations
NZS 4404:2010 is the governing standard for pavement design in New Zealand, and it explicitly requires a CBR study for road design to determine subgrade strength. In Auckland, the risk is compounded by the variety of soils we encounter — from the soft alluvial clays in the South Auckland area to the stiff residual soils over Waitemata sandstone in the North Shore. If you skip the test or use a default value from a nearby project, you risk under-designing the pavement. The consequence is premature cracking, edge failure, or even full-depth rutting within the first two winters. We have seen it happen on subdivision roads where the developer assumed a CBR of 8 and the actual lab value came back at 3. That mistake adds NZ$80,000 in remedial work for a 500-metre road.
2.5 (soft clay) to 15 (Waitemata sandstone residual)
Associated technical services
01
CBR test in our laboratory
We receive block or bulk samples from your site in Auckland, compact them at the specified Proctor energy, soak for 96 hours, and run the penetration test. You get a certified report with swell data, moisture content, and dry density.
02
Field CBR with portable equipment
For projects where you need an answer the same day, we bring a portable CBR press to the site. This is common for cut-to-fill transitions on road alignments in Auckland where the subgrade conditions change every 50 metres.
03
CBR-based pavement design support
Our team takes your CBR results and cross-references them with NZS 4404 design charts to recommend pavement layer thicknesses. We do not design the road, but we give the structural engineer the numbers they need to apply their own methodology.
Applicable standards
NZS 4404:2010 — Land development and subdivision infrastructure, NZS 4402 — Standard Test Method for CBR of Laboratory-Compacted Soils, AASHTO T-193 — CBR of Compacted Soils, NZTA P/11 — Pavement design for state highways
Frequently asked questions
How long does a standard CBR study for road design take from sample delivery to final report?
The whole process takes five working days. Day one is sample preparation and compaction at OMC. Days two through five are the 96-hour soak period. After that, we run the penetration test and issue the report within 24 hours. If you need a quicker turnaround, we offer a fast-track service that uses a 48-hour soak with a correlation factor, but we recommend the full soak for final design.
What is the difference between CBR and California Bearing Ratio — are they the same thing?
Yes, they are the same. CBR stands for California Bearing Ratio, named after the California State Highways Department that developed the test in the 1930s. It measures the resistance of a compacted soil to penetration by a standard plunger relative to a crushed rock standard. In New Zealand, we follow NZS 4404, which aligns with NZS 4402. The term CBR study for road design is used interchangeably in the industry.
Can you run a CBR on a sample that contains gravel or roots?
It depends on the particle size. The CBR test is designed for material passing the 19 mm sieve. If your sample contains larger particles, we either scalp the oversize material and correct the result using a density correction factor, or we run the test in a 152 mm mould with a larger plunger. Roots or organic matter should be removed before compaction. For highly organic soils common in the Auckland peat areas, the CBR test may not be appropriate, and we would recommend a plate load test instead.
How much does a CBR study for road design cost in Auckland?
A standard CBR test in our laboratory costs between NZ$280 and NZ$460 per sample, depending on whether you require swell measurement, moisture content, and dry density. The price includes the 96-hour soak, the penetration test, and a certified report. For field CBR with portable equipment, the price is NZ$400 to NZ$600 per test point, which includes mobilisation within the Auckland region. Volume discounts apply for more than ten samples from the same project.
