Bunbury began as a port town in the 1830s and grew into the largest city in Western Australia's South West region, with its urban spread pushing into the sandy coastal plain and around the Leschenault Estuary. The subsurface here is a mix of dune sands, estuarine clays, and occasional limestone bands — a combination that demands careful consideration when choosing a foundation system. For multi-storey residential or commercial projects, a raft or mat foundation often becomes the logical solution because it spreads loads across a wider footprint and reduces differential settlement. Before locking in the design, we always run a thorough investigation that includes asentamiento diferencial analysis to confirm the soil variability won't compromise performance. Our team has worked on dozens of Bunbury sites, from Glen Iris to Withers, and the pattern is consistent: the raft needs to be stiff enough to bridge weak pockets but not over-designed for the competent sand layers that occur at depth.
A stiffened raft with deeper edge beams bridged the soft clay zone and kept differential settlement under 15 mm for the Bunbury CBD project.
Methodology and scope
Local considerations
AS 2870-2011 classifies Bunbury sites predominantly as Class M or Class H due to moderate to high reactivity of the clay profiles in the eastern suburbs. That classification directly influences the raft design — edge beam depths and reinforcement spacing change significantly between a Class M and a Class H site. We've seen cases where a builder assumed a standard 300 mm slab only to find the clay had moved 50 mm after a dry summer, causing cracking in the masonry above. The risk picture also includes the possibility of loose sand layers near the Leschenault Inlet that can settle under dynamic loads, and we routinely specify a pre-construction site classification test to lock in the design assumptions before concrete is poured.
Applicable standards
AS 2870-2011 Residential slabs and footings, AS 3600-2018 Concrete structures, AS 1726-2017 Geotechnical site investigations, AS/NZS 1170.0-2002 Structural design actions
Associated technical services
Site Classification & Bearing Assessment
NATA-accredited laboratory testing of soil reactivity and bearing capacity, including Atterberg limits, shrinkage index, and plate load tests, all referenced to AS 2870 site classes.
Stiffened Raft & Edge Beam Detailing
We calculate required raft thickness, edge beam depths, and torsional reinforcement for Class M and Class H sites, with settlement predictions under working and ultimate loads.
Hydrostatic Uplift & Groundwater Control
Assessment of groundwater levels (including seasonal variation) and design of permanent or temporary dewatering to prevent flotation of the raft slab during construction and service life.
Typical parameters
Frequently asked questions
What is the difference between a raft foundation and a slab-on-ground in Bunbury?
A raft foundation is a thickened concrete slab designed to spread structural loads across the entire footprint, typically with deeper edge beams and stiffening ribs. A slab-on-ground is a thinner, unreinforced or lightly reinforced slab that relies on the soil for support. In Bunbury's reactive clay areas, AS 2870 usually requires a stiffened raft to control differential movement, whereas a slab-on-ground would risk cracking in the superstructure.
How much does a raft/mat foundation design typically cost for a Bunbury residential project?
For a standard residential raft design including site classification and geotechnical assessment, the range is AU$1.580 – AU$6.920. The final cost depends on the site class, the number of boreholes required, and whether plate load tests or settlement analyses are needed. We provide a fixed-price quote after reviewing the site location and proposed building dimensions.
How long does the design process take from soil investigation to final drawings?
Typically 10 to 15 working days for a single residential raft, including two to three days on site for drilling and sampling, five to seven days for laboratory testing, and two to three days for structural calculations and drafting. For larger commercial rafts with deeper excavations or complex groundwater control, the timeline extends to three to four weeks.