Coffs Harbour sits on the Mid North Coast with an annual rainfall exceeding 1,600 mm, a figure that shapes the local geotechnical landscape. The Flat Dilatometer Test (DMT) is particularly valuable here because it measures in situ lateral stress and soil stiffness without the disturbance common to other soundings. In a region where colluvium and residual soils dominate the foothills, the DMT provides a direct read of modulus and horizontal stress index—parameters that drive foundation design for the growing number of low-rise residential and commercial developments. The test follows AS 1289.6.5.2 and the Australian standard AS 1726, ensuring the results are accepted by local certifiers. For sites where deeper profiling is needed, we often pair DMT with the ensayo SPT to cross-validate strength and deformation characteristics across different soil layers.
The DMT's ability to capture in situ lateral stress and modulus makes it the preferred tool for settlement analysis in Coffs Harbour's colluvial soils.
Method and coverage
A typical scenario in Coffs Harbour involves a 3-storey apartment block on a sloping block near the harbour. The DMT blade is pushed at a constant rate of 2 cm/s, and every 20 cm a membrane is inflated to record the lift-off and lift-off corrected pressures. These readings yield the material index (ID), horizontal stress index (KD), and dilatometer modulus (ED) with high repeatability. The test also supports liquefaction screening using the Youd-Idriss 2001 framework, which is relevant given the seismic zone factor of 0.08 for the region. A practical workflow includes:
Pre-drilling or cone penetration to the target depth
Blade insertion with a 20-tonne hydraulic push rig
Real-time pressure reading and correction for membrane stiffness
When the soils are very soft or interbedded with sand lenses, we integrate the tomografía sísmica to map shear-wave velocity profiles that complement the DMT's stiffness data. This combination gives engineers a reliable basis for calculating immediate settlement on Coffs Harbour's clay-rich profiles.
Technical reference image — Coffs Harbour
Regional considerations
A common oversight in Coffs Harbour is relying solely on SPT N-values from sandy layers to estimate settlement for shallow footings on the coastal clay crust. The SPT does not capture the in situ lateral stress or the constrained modulus with enough resolution for soft soils. This leads to overestimating bearing capacity and underestimating immediate settlement, especially after heavy rain cycles. The DMT fills that gap by providing a direct measurement of ED and KD, which feed into the settlement equations recommended by AS 4678 for earth-retaining structures. Skipping this test on sites with known soft clay horizons near the Boambee Creek floodplain has led to differential settlement exceeding 40 mm in past developments.
DMT for Settlement Analysis on Soft Coffs Harbour Soils
For sites with high moisture content and low preconsolidation pressure, we run DMT soundings at 1.0 m intervals to 15 m depth. The ED and KD profiles are used to compute immediate settlement under strip footings and raft slabs. Results are plotted against the Schmertmann method and reported with a certified test summary.
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DMT with Seismic Dilatometer (SDMT) for Dynamic Response
When the project requires shear-wave velocity profiles for earthquake design or vibration analysis, we add a seismic module to the DMT blade. The SDMT records both the dilatometer readings and the shear-wave travel time between two geophones, giving VS profiles that feed into site class determination per AS 1170.4. This is especially useful for critical infrastructure near the Coffs Harbour airport expansion.
Standards that apply
AS 1289.6.5.2 – Standard Test Method for Performing the Flat Plate Dilatometer, AS 1726 – Geotechnical Site Investigations (2017), AS 4678 – Earth-Retaining Structures (2002)
Top questions
What parameters does the Flat Dilatometer Test provide in Coffs Harbour?
The DMT gives the material index (ID), horizontal stress index (KD), and dilatometer modulus (ED). From these, engineers derive the constrained modulus (M), coefficient of lateral earth pressure at rest (K0), and overconsolidation ratio (OCR). In Coffs Harbour's colluvial soils, the KD is particularly useful for identifying overconsolidated crusts that influence shallow foundation design.
How does the DMT compare to the SPT for settlement prediction?
The SPT measures resistance to dynamic driving, while the DMT directly measures in situ lateral stress and stiffness. For settlement prediction in soft to medium clays, the DMT's constrained modulus is far more reliable than correlations from N-values. In Coffs Harbour, where clay layers can have high plasticity, the DMT reduces uncertainty in immediate and consolidation settlement estimates by up to 40% compared with SPT-based methods.
What is the typical cost range for a DMT investigation in Coffs Harbour?
A standard DMT sounding to 15 m depth with reporting typically ranges between AU$1,150 and AU$1,710. This includes mobilisation, blade insertion, data reduction, and a certified test report. Costs can vary depending on site access, number of soundings, and whether seismic modules are added. For a precise quote tied to your specific project geometry, contact our team directly.
