The coastal sand deposits behind the Gold Coast beachfront are notorious for meta-stable structures. Dune sands and old beach ridges here can stand with a high void ratio, then collapse abruptly when wetted under load. This is not a slow settlement — it is a sudden volume loss that can crack a slab or tilt a footing overnight. Our collapsible soil evaluation follows AS 1726-2017 procedures to detect these hidden collapse potentials before concrete is poured. The test involves loading an undisturbed sample to a representative stress, then flooding it while measuring the vertical strain. If the strain exceeds 1%, the soil is classified as collapsible and requires treatment.
A soil that passes all standard compaction tests can still be collapsible if its dry density is low and its saturation degree is below 70%.
Methodology and scope
Local considerations
We use a fully automated oedometer frame with a load cell and LVDT to measure collapse strain. The rig sits inside a temperature-controlled lab, and each test runs for 48 hours to ensure full inundation equilibrium. The biggest risk in Gold Coast is that collapsing sands are often masked by a competent crust. A 300 mm thick surface crust can pass a plate load test, then the underlying loose sand collapses when the slab is loaded and the first rain saturates the ground. That is why we always pair the collapse test with a classification of soils to identify the fines content that controls the collapse mechanism.
Applicable standards
AS 1726-2017 Geotechnical site investigations, AS 1289.7.1 Standard test method for measurement of collapse potential of soils, AS 2870-2011 Residential slabs and footings (collapsible soil provisions)
Associated technical services
Double-Oedometer Collapse Test
Two identical undisturbed samples are loaded in separate oedometers — one kept at natural moisture, the other inundated at a fixed stress level. The collapse strain is the vertical strain difference between the two curves at the target stress. We report Cp values, collapse index, and a collapse classification (slight to severe).
Field Infiltration-Collapse Correlation
We combine a field infiltration test with laboratory collapse testing on adjacent block samples. This correlation predicts the collapse magnitude for a given rainfall event or irrigation regime. Useful for Gold Coast sites with fill imported from the hinterland, where sand sources vary widely.
Typical parameters
Frequently asked questions
What is the difference between collapsible soil and expansive soil?
Collapsible soil loses volume when wetted under load, while expansive soil swells when wetted. The two mechanisms are opposite. In Gold Coast, collapsible soils are typically clean sands or silty sands with low clay content, whereas expansive clays appear in the Neranleigh-Fernvale beds west of the M1.
How much does a collapsible soil evaluation cost in Gold Coast?
The cost ranges between AU$1.290 and AU$4.470 depending on the number of collapse tests, sample depth, and whether field infiltration correlation is included. A standard residential evaluation with two oedometer collapse tests and a field visit is around AU$1.800.
Can collapsible soils be treated after construction?
Post-construction treatment is difficult and expensive. The best approach is pre-construction mitigation: pre-wetting, deep dynamic compaction, or replacement with non-collapsible fill. If collapse is detected after a slab is cast, the options are limited to underpinning or slab jacking, which rarely restore full performance.
What collapse potential value is considered dangerous?
Per AS 1289.7.1, collapse potential (Cp) values above 2% are considered moderate to severe. For Gold Coast residential slabs on fill, we flag any Cp above 1.5% as requiring mitigation. Values above 5% usually require complete removal and replacement of the collapsible layer down to competent natural sand.