The ring plain surrounding New Plymouth conceals a complex sequence of andesitic lahar deposits and volcanic ash layers that govern groundwater movement in ways borehole logs alone cannot predict. With annual rainfall exceeding 1,400 mm across Taranaki, perched water tables develop rapidly within paleosol horizons, creating drainage conditions that must be measured in situ rather than inferred from grain-size curves. Our team runs Lefranc and Lugeon permeability tests across the district, providing the hydraulic conductivity data that geotechnical models depend on when designing retention basins, cut-and-cover tunnels, or basement slabs below the static water level. For projects near the Waiwhakaiho River corridor, where coarse alluvium alternates with low-permeability silts, we often combine field permeability with in-situ permeability profiling to capture vertical variability that drives dewatering design.
Lugeon values measured in jointed lahar breccia near New Plymouth frequently exceed 15 Lugeon units, a condition that cannot be detected from core recovery alone.
Methodology and scope
Local considerations
The most expensive mistake we see repeatedly in New Plymouth is relying on laboratory permeability from Shelby tube samples and ignoring field-scale hydraulic conductivity before finalising drainage specifications. Volcanic soils contain root casts, fissures, and open framework gravel lenses that a 75 mm diameter specimen cannot represent. When contractors install a permanent dewatering system sized for lab-derived k values, the pumps either run continuously without lowering the water table or fail within months because the formation yields ten times more water than expected. The cost to retrofit an underdrain beneath an occupied building on Egmont Street can exceed the original dewatering budget threefold. A single day of Lugeon testing in the design phase replaces assumptions with measured transmissivity, and that data holds up when the council drainage engineer reviews the consent application.
Applicable standards
NZS 3404: 2014 (Concrete structures - durability and groundwater assessment), NZS 4203: 1992 (General structural design - groundwater loads), NZGS Soil and Rock Permeability Guidelines (2016), Houlsby (1976) Lugeon test interpretation methodology
Associated technical services
Lefranc constant-head test in overburden
Performed in cased boreholes through tephra and lahar silts, this method maintains a fixed water level while recording flow rate until steady state is reached. Ideal for shallow foundation drainage design in New Plymouth's residential subdivisions.
Lefranc falling-head test in low-permeability soils
Applied in paleosol horizons and weathered ash layers where conductivity is too low for constant-head measurement. We monitor head decay over time and compute k using the Hvorslev shape factor for the test cavity geometry.
Single-packer Lugeon test in fractured rock
A pneumatic packer isolates a 3 m to 5 m test interval in the borehole. Water is injected at up to five pressure stages, and the Lugeon value is calculated from the pressure-flow relationship. Standard for dam foundation assessments in the Taranaki ring plain.
Double-packer Lugeon test for isolated zones
Two packers isolate a discrete interval, eliminating leakage past the packer seat. Used when testing specific joint sets or when the rock mass is too fractured for single-packer reliability. Common in deep excavation projects near New Plymouth's CBD.
Typical parameters
Frequently asked questions
How much does a Lefranc or Lugeon permeability test cost in New Plymouth?
Field permeability testing in New Plymouth typically ranges from NZ$960 to NZ$1,970 per test interval, depending on borehole depth, access conditions, and whether a single or double packer setup is required. Mobilisation to sites across the Taranaki region is quoted separately. We provide a fixed-price proposal after reviewing the borehole logs and site access constraints.
When is a Lugeon test required instead of a Lefranc test?
A Lugeon test is specified when the zone of interest lies in rock that cannot be tested by Lefranc methods, typically below the soil-rock interface in New Plymouth's andesitic basement. Lugeon testing applies water under pressure through a packer-isolated interval and is mandatory for dam foundations, tunnel alignment investigations, and any structure where rock mass permeability controls the groundwater model.
How long does a field permeability test take on site?
A single Lefranc test in soil typically requires 45 to 90 minutes depending on the hydraulic conductivity of the formation. A complete five-stage Lugeon test in rock takes approximately two to three hours per interval, including packer inflation, pressure stabilisation at each step, and final depressurisation. We usually schedule one to three test intervals per drilling day.
Can you test permeability in existing monitoring wells?
Yes, provided the well has a known screen interval and adequate annular seal. We run falling-head slug tests in existing piezometers and monitoring wells across New Plymouth, which is a cost-effective alternative to drilling new boreholes when the well construction details meet NZGS guidelines for hydraulic testing.