Napier sits on the gravels and silts of the Heretaunga Plains. The 1931 earthquake lifted the coastal plain by over a metre, leaving a mix of marine sediments and alluvial deposits that still define local construction today. That history matters when you are designing a pavement. Saturated silts and fine sands dominate many sites, and their bearing capacity can drop fast after rain. A laboratory CBR test gives you a reliable soaked strength value, removing the guesswork from subgrade treatment. We run the test on remoulded samples compacted to your target density, and we pair it with a grain size analysis when the fines content looks borderline. For deeper investigation we often recommend in-situ permeability testing to understand how water moves through the layered soils common in Taradale and Ahuriri.
A soaked CBR of 3% versus 5% can change the pavement thickness by over 100 mm—and the cost of imported aggregate in Hawke's Bay is not trivial.
Methodology and scope
NZS 4404:2010 for land development and subdivision sets clear CBR thresholds for subgrade acceptance. Councils across Hawke's Bay enforce them strictly. Our soaked CBR procedure follows NZS 4402 Test 4.2, with a 96-hour soaking period that replicates worst-case field conditions. The lab compacts three specimens at different moisture contents—usually near optimum, dry of optimum, and wet of optimum—to build a density-strength curve. We use a calibrated loading press with a penetration rate of 1.27 mm per minute. The result is a CBR value at 2.5 mm and 5.0 mm penetration. If the 5.0 mm value is higher, we repeat the test. That is not optional. It is a check written into the standard. The entire sequence takes four to five days, and we report both the raw data and the corrected curve so your pavement engineer can review every data point.
Questions and answers
How much does a laboratory CBR test cost in Napier?
A single-point soaked CBR with compaction curve typically costs between NZ$240 and NZ$340, depending on the number of specimens and whether a particle size distribution is included. Three-point tests with full reporting fall at the upper end of that range.
What is the difference between field CBR and laboratory soaked CBR?
Field CBR uses a portable apparatus pushed directly into the subgrade at natural moisture content. It is quick but gives an unsoaked value. Laboratory soaked CBR remoulds the sample, compacts it to a target density, and soaks it for four days. The lab value is almost always lower and is the one specified in NZS 4404 for pavement thickness design.
How many samples do I need for a road project in Napier?
The number depends on the length of the alignment and the soil variability. For a subdivision road under 500 metres, we usually recommend at least three sample locations with one CBR test per location. On highly variable sites near old river channels, you may need more. We can advise after reviewing a preliminary soil log.
Can you test gravel aggregates with the CBR method?
Yes, but the standard CBR mould limits particle size to 19 mm. If your aggregate contains larger stone, we sieve out the oversize fraction and note the percentage in the report. The result still provides a valid soaked bearing value for sub-base and basecourse materials under NZS 4404.
