AS 1726 sets the standard for geotechnical site investigation in Australia, and in Mackay that standard is critical. The city sits on deep alluvial deposits and residual soils from the surrounding Pioneer Valley, which means settlement behaviour can vary significantly across a single block. Without a proper settlement analysis, you risk differential movement that cracks slabs and misaligns structures. We combine borehole data, undisturbed sampling, and laboratory consolidation tests to model how the ground will behave under load. The local soil profile demands attention to pre-consolidation pressure and compressibility, especially near the Pioneer River floodplain. Before pouring concrete, a consolidation test gives the settlement rate and magnitude. A dilatometer reading adds lateral stress data for deeper fills.
A settlement prediction that ignores pre-consolidation pressure will overestimate total movement by up to 50 percent in overconsolidated Mackay clays.
Methodology and scope
Our team uses a hydraulic consolidation frame with electronic displacement transducers, capable of loading up to 50 kN. We run incremental load tests on undisturbed ring samples trimmed directly from thin-walled Shelby tubes. The equipment meets AS 1289.6.6.1 for one-dimensional consolidation. For each load stage we record time-deformation curves, then apply Casagrande's log-time method or Taylor's square-root method to determine the coefficient of consolidation (Cv). In Mackay's clay layers, we typically see Cv values between 0.8 and 3.5 m²/year. The lab also measures swelling pressure when reactive clays are present. All results feed into a settlement prediction that accounts for foundation geometry, load distribution, and drainage conditions. We cross-check field data with a CPT profile to confirm layer boundaries.
Technical reference image — Mackay
Local considerations
A common mistake builders in Mackay make is assuming uniform settlement across the whole slab. The reality is that soil layering changes within metres here. One corner of the site might sit on firm clay while the opposite corner hits loose sand over soft clay. That differential movement can crack a waffle slab or tilt a steel portal frame. We have seen projects where the owner skipped settlement analysis and ended up spending three times the original foundation cost on underpinning. The risk is real, especially on sloping blocks near the Eungella ranges or in low-lying areas subject to fluctuating water tables. A proper settlement analysis identifies those zones before you pour.
Covers one borehole per 200 m², two undisturbed samples, consolidation testing on the most compressible layer, and a settlement versus time graph. Suitable for single residential dwellings and small commercial slabs.
02
Advanced Differential Settlement Study
Includes three or more boreholes, multiple consolidation tests across all compressible layers, CPT correlation, and a 3D settlement contour map. Designed for multi-storey buildings, tilt-up panels, and bridges.
Applicable standards
AS 1726 – Geotechnical Site Investigations, AS 1289.6.6.1 – One-Dimensional Consolidation Test, AS 2870 – Residential Slabs and Footings (settlement criteria)
Frequently asked questions
How much does a settlement analysis cost in Mackay?
A standard analysis for a house block costs between AU$1,050 and AU$2,840 depending on borehole depth and number of samples. The advanced differential study ranges from AU$3,500 to AU$6,200. These are typical figures for the Mackay region.
What is the difference between total settlement and differential settlement?
Total settlement is how much the whole foundation moves downward. Differential settlement is the difference in movement between two points on the same foundation. Differential settlement causes structural damage like cracked walls and jammed doors. In Mackay, differential movement is often triggered by a change from clay to sand within the foundation footprint.
How long does a consolidation test take to complete?
A single consolidation test with six load increments takes around 14 days because each load stage must reach 100 percent primary consolidation before the next increment is applied. We run tests in parallel to deliver a full settlement analysis within 3 to 4 weeks from sample arrival.
Can settlement analysis be done on an existing building?
Yes. We drill inclined boreholes from the perimeter, extract undisturbed samples, and run consolidation tests to back-calculate the soil's stress history. This helps determine if observed cracking is due to ongoing settlement or other causes. The approach follows AS 2870 for existing structures.
