Raft foundation design in Hervey Bay must account for the region's unique coastal geology, where loose sands and high groundwater levels dominate. AS 1726-2017 provides the framework for site investigation, requiring SPT testing and plate load tests to establish allowable bearing pressures. Without a thorough understanding of the soil profile, a raft slab can experience excessive differential settlement, particularly on the sandy dune systems near the esplanade. We integrate the ensayo de placa de carga into our investigation suite to directly measure modulus of subgrade reaction, ensuring the raft is proportioned for actual ground conditions rather than assumed values.
In loose coastal sands, the subgrade reaction modulus often falls below 20 MPa/m — a raft must be stiff enough to bridge soft zones without cracking.
Service characteristics in Hervey Bay
Critical ground factors in Hervey Bay
A common mistake in Hervey Bay is designing a raft foundation based on standard residential tables without a site-specific investigation. Builders often assume a uniform sand layer, but buried clay lenses or pockets of loose fill can cause one corner of the slab to settle twice as much as the rest. This differential movement cracks tiles, jams doors, and stresses the plumbing. The solution is straightforward: a minimum of two boreholes to 6 metres depth, with SPT at 1.5 m intervals, plus a plate load test at founding level. Skipping these steps saves a few thousand dollars upfront but risks tens of thousands in remedial work later.
Our services
We deliver a complete raft foundation package tailored to Hervey Bay's coastal conditions, from field investigation through to final design report.
Site Investigation & SPT Testing
Boreholes to 10 m depth with standard penetration testing at 1.5 m intervals. Includes disturbed and undisturbed sampling for laboratory classification and strength testing.
Bearing Capacity & Settlement Analysis
Calculation of ultimate and allowable bearing capacity using Terzaghi's method modified for rectangular rafts. Settlement predictions using elastic theory and consolidation testing on clay layers.
Raft Structural Design & Detailing
Reinforced concrete raft design per AS 3600-2018, including punching shear checks, flexural reinforcement schedules, and construction joint detailing. We provide CAD-ready drawings for contractor use.
Frequently asked questions
What is the typical cost for a raft foundation design in Hervey Bay?
For a standard residential raft on a single block, the geotechnical investigation and design typically ranges between AU$1,650 and AU$6,680. This covers boreholes, SPT testing, laboratory tests, and a certified design report. Larger or more complex sites with deep clay layers or high water tables may fall at the upper end of this range.
How deep should boreholes be for a raft foundation in coastal sands?
We recommend boreholes to at least 6 metres depth, or 1.5 times the raft width, whichever is greater. In Hervey Bay's sand dune areas, deeper holes may be needed to rule out buried channels or loose fill. AS 1726-2017 specifies a minimum of two boreholes for a standard residential raft.
Can a raft foundation be designed without a plate load test?
Technically yes, but it's risky. The plate load test directly measures the modulus of subgrade reaction, which is the key parameter for raft stiffness design. Without it, designers must rely on correlations from SPT N-values, which can overestimate stiffness in loose sands. We always recommend at least one plate load test for rafts over 100 m².