A seven-storey residential project on Pall Mall required a diaphragm wall to retain a deep excavation just metres from a heritage-listed building. We were called in to run the field investigation and lab testing that would feed the design parameters. The challenge in Bendigo is the mixed geology: you can drill through recent alluvial clays and hit Ordovician bedrock within the same borehole. For that reason, we always begin with a series of boreholes and SPTs, then correlate the results with a resistivity survey to map the soil-rock interface before any wall is sized. The design team then uses those profiles to set panel dimensions and reinforcement.
In Bendigo's mixed geology, a diaphragm wall design is only as good as the soil profile that supports it.
Methodology and scope
The local climate in Bendigo is semi-arid, which means the surface soils are often desiccated and cracked. That desiccation affects the active wedge behind a diaphragm wall, so we measure suction profiles on undisturbed samples in the lab. We also run triaxial tests under unsaturated conditions to capture the true strength. On a recent job near Lake Weeroona, the water table sat 2.5 m below the surface, requiring a seepage analysis before the wall geometry could be locked in. To refine the hydrogeological model we combined our borehole data with a GPR survey that traced old creek channels under the site. The combination of geophysics and direct sampling is what makes the design reliable in Bendigo's complex subsurface.
Technical reference image — Bendigo
Local considerations
Bendigo sits in a region of moderate seismicity (AS 1170.4 peak ground acceleration 0.08 g), so diaphragm walls here must also handle cyclic loading. The old alluvial channels that cut through the area can create abrupt stiffness changes across a single panel, leading to differential bending moments if not captured in the model. We have seen designs that assumed uniform soil conditions fail to account for a 3-metre lens of soft clay interbedded with stiff gravels. Our laboratory tests identify those lenses early, so the structural engineer can adjust reinforcement zones or specify longer panels that bridge the weak layer.
Continuous core and standard penetration tests at 1.5 m intervals to define soil layers and rock quality designation (RQD) for the wall embedment zone.
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Triaxial and direct shear testing
Consolidated-undrained triaxial tests on undisturbed samples, plus direct shear on remoulded specimens, to provide effective stress parameters for AS 4678 design.
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Permeability and groundwater monitoring
In-situ falling head tests and laboratory permeameter tests on the clay and sand layers, combined with standpipe piezometer readings over two wet seasons.
Applicable standards
AS 4678-2002: Earth-retaining structures, AS 1726-2017: Geotechnical site investigations, AS/NZS 1170.4:2007 (R2022): Structural design actions – earthquake, AS 3600-2018: Concrete structures (for wall panel design)
Frequently asked questions
What is the typical depth range for diaphragm walls in Bendigo?
In Bendigo, diaphragm walls are commonly designed for depths between 12 and 25 metres. The depth depends on the height of the retained excavation and the depth to competent bedrock or stiff clay. For basements with two to four levels, 15 m is a typical target.
How much does a diaphragm wall design investigation cost in Bendigo?
For a standard investigation including three boreholes, SPTs, triaxial testing, and a permeability assessment, the range is AU$2.960 to AU$10.950. The final cost depends on the number of boreholes, the depth of each hole, and whether geophysics (resistivity or GPR) is added.
Can diaphragm walls be built in Bendigo's stiff clay without temporary support?
Bendigo's stiff clays have sufficient stand-up time for short panels, but bentonite or polymer slurry is still required to maintain trench stability during excavation. The slurry density must be adjusted based on the groundwater level and the clay's plasticity index. Our lab tests determine the optimal slurry mix for the site.
