Maroochydore sits on the low coastal plain between the Maroochy River and the beach, and that is the ground the Queensland Government's soil guidance is describing when it says coastal areas lower than 5 metres AHD are likely to have acid sulfate soils present, with more buried beneath newer soils below 20 metres AHD. Undisturbed, those soils are harmless. Excavated for a basement, a pool or a services trench, or drained, the pyrite in them oxidises to sulfuric acid, which the same guidance says can attack concrete and steel, slowly destroying pipes, roads, bridges and building foundations. Many of them are unconsolidated estuarine muds and clays with low load-bearing capacity, so foundations and earthworks on them may settle or subside unevenly. The Sunshine Coast Planning Scheme 2014 carries an acid sulfate soils overlay code and a flood hazard overlay code for exactly this reason. Whether either applies to your lot is a question for your planner; what the ground actually is, and how deep the soft material runs, is what the boreholes answer.
That is why the residential, commercial and development work on this page starts from the same visit. Queensland's building regulator explains that engineers design footings to AS 2870 and that the design is matched to the site's classification, the measured expectation of how much the ground will move; its own table runs from Class A sand and rock with no expected movement, through S, M and H, to Class E clays that have moved 100 to 150 millimetres in South-East Queensland. The regulator also notes that sandy sites and rocky terrain are usually not prone to that shrink and swell, and much of Maroochydore is sand. But sand near the beach, estuarine mud along the river and clay further back are all within this one suburb, and reactivity is not the only question on soft ground: bearing capacity and sulphate attack are, which is why the same samples can go through sulphate soil and water analysis in our laboratory before anyone pours concrete.
The development end of town makes the point at scale. The Maroochydore City Centre Priority Development Area, 60 hectares of central Maroochydore including the former Horton Park Golf Club, was declared on 19 July 2013 to create a new central business district for the Sunshine Coast, with commercial, retail and high and medium density residential development, and Economic Development Queensland, not the council, assesses the development applications inside it. Larger commercial and multi-unit buildings on this ground are designed from deeper investigation than a house slab: geotechnical drilling and sampling at depth, laboratory analysis, and a report an engineer can size piles, rafts or ground improvement from. Wagner provides that investigation as well as the standard classification, and it is the same NATA-accredited laboratory analysing both.
Rain drives the seasonal cycle on any clay in the mix. The Bureau of Meteorology's Sunshine Coast Airport station, a few kilometres up the coast, averages 1,492.6 millimetres a year, with a February mean of 217.5 millimetres against 51.1 in September: the wet-summer, dry-spring swing that makes a reactive clay swell and shrink, and that keeps a low, poorly drained lot wet for months at a time.
Whatever the project, the method is the one Wagner publishes. We come to your site, drill two boreholes two metres deep near the proposed footing area, run two DCP or bearing-capacity checks beside each hole, and take the samples to Wamuran. For a house or a townhouse row you get an AS 2870 site classification in one of the six classes, A through to P, that your engineer and certifier design the footings to. For a commercial site or a development lot the investigation goes deeper and the report is written for the engineer sizing the structure. Where the ground is low and estuarine, sulphate soil and water analysis tells you before the pour whether the soil is aggressive to what you are burying in it.
Maroochydore is a suburb where the sand at the beach, the mud along the river and a new city centre on a former golf course all sit within a few streets of each other: the class, the bearing and the chemistry have to be measured on the site, never assumed from the address.
