Mackay grew rapidly during the sugar boom of the late 19th century, with the city grid laid out over the alluvial flats of the Pioneer River. Those early roads were built directly on the natural floodplain — dark cracking clays and silty sands that compacted unevenly under dray traffic. Today, many of the original arterial routes still carry heavy loads from the port and the mining corridor to the west. An existing pavement evaluation in Mackay must account for this layered history: the original subgrade, decades of patch overlays, and the tropical moisture regime that keeps the water table within a metre of the surface in many suburbs. Our field team starts with a visual condition survey, then moves to coring and dynamic cone penetrometer testing to map layer thickness and in-situ strength before any overlay design begins.
The reactive clays of North Mackay swell during the wet season and crack when dry, demanding a pavement evaluation that captures both extremes.
Methodology and scope
Coastal humidity and summer rainfall exceeding 1,600 mm per year create a unique challenge for road pavements in Mackay. The reactive clays of North Mackay swell during the wet season and crack when dry, while the sandy terrace deposits near the airport drain rapidly but lose bearing capacity under saturation. For a reliable existing pavement evaluation in Mackay we combine Falling Weight Deflectometer testing with laboratory CBR on undisturbed samples. We also run Atterberg limits on the subgrade to classify its shrink-swell potential and use dynamic cone penetrometer profiles to locate weak layers below the pavement structure. When the pavement is thin or the subgrade very soft we deploy a plate load test at critical points to verify the design modulus directly. The results feed into a layer-by-layer structural number calculation that respects Austroads pavement design procedures.
Technical reference image — Mackay
Local considerations
Compare the sealed pavement on Shakespeare Street near the CBD with the haul road serving the sugar terminal at the port. The former sits on firm estuarine sands with a water table at 2.5 m; the latter is built on soft estuarine clays that have settled unevenly for decades. An existing pavement evaluation in Mackay that ignores this variability risks recommending a uniform overlay thickness — which will crack over the soft spots within a single wet season. The biggest risk we see locally is the hidden presence of old service trenches (water mains, sewer lines) that were backfilled with loose sand and capped with thin asphalt. These zones appear as localised depressions on the surface but can cause rapid fatigue failure in a new overlay if not identified by coring or ground-penetrating radar during the evaluation phase.
Peak force 50 kN; sensors at 0, 200, 300, 450, 600, 900, 1200, 1500 mm
Dynamic Cone Penetrometer (DCP)
DCP index (mm/blow); CBR correlation per Webster & Grau (1995)
Pavement coring
100 mm diameter; full depth to subgrade; photographed and logged in 50 mm lifts
Laboratory CBR (soaked)
AS 1289.6.1.1; 4-day soak; tested at 100% and 95% MDD
Layer thickness
Measured from core log ± 2 mm; verified by GPR where asphalt > 50 mm
Subgrade modulus (E_s)
Back-calculated from FWD deflection bowl; range 15 – 80 MPa typical for Mackay subgrades
Associated technical services
01
FWD Deflection Testing & Back-Calculation
Falling Weight Deflectometer survey at 20 m spacing along the project centreline. Each drop records nine deflection sensors. We back-calculate layer moduli using ELMOD software and compare them against typical values for Mackay subgrades. The output includes deflection bowl parameters and a structural number for each test point.
02
Pavement Coring & Laboratory Characterisation
Full-depth cores (100 mm diameter) extracted at representative locations. Each core is logged for layer type, thickness and condition. Asphalt cores are tested for density and binder content; granular bases for grading and particle breakdown; subgrade for soaked CBR, Atterberg limits and fines content.
Applicable standards
AS 1289.6.1.1 (CBR test method), Austroads Guide to Pavement Technology Part 2: Pavement Structural Design (AGPT02-17), AS 1289 (FWD deflection testing), AS 1289.6.3.2 (DCP test method)
Frequently asked questions
How deep do you core for an existing pavement evaluation in Mackay?
Core depth depends on the pavement structure, but we typically drill through the full asphalt or bitumen seal into the granular base and stop at the top of the subgrade — usually between 300 mm and 700 mm below the surface. If the subgrade is weak or the pavement thick we extend the core to confirm the base thickness and check for hidden soft layers.
What is the difference between FWD and DCP testing in pavement evaluation?
The Falling Weight Deflectometer measures the elastic deflection of the whole pavement under a controlled impulse load, giving the in-situ structural capacity and layer moduli. The Dynamic Cone Penetrometer measures penetration resistance per blow, from which we estimate CBR at discrete depth intervals. FWD covers the pavement system globally; DCP identifies weak zones within specific layers. Both are used together in a full evaluation.
Can you evaluate a pavement on reactive clay subgrade like the soils in North Mackay?
Yes, and it is essential to do so. Reactive clays in North Mackay can change volume by 30% between dry and wet seasons, which alters the pavement support. We evaluate the subgrade at its soaked condition (4-day soak CBR) and also measure the Atterberg limits to classify the shrink-swell potential. The design must account for the weakest seasonal state.
How much does an existing pavement evaluation cost in Mackay?
For a typical urban road section of 500–1000 m, the cost ranges between AU$1,750 and AU$5,230 depending on the number of core holes, FWD test points and laboratory tests required. A larger project with multiple lane-kilometres will reduce the per-metre rate. The exact scope is defined after a site walkover and a review of the pavement history.
