Hobart sits on a complex mix of Jurassic dolerite bedrock overlain by Quaternary alluvial deposits, particularly in the Derwent River valley. The water table sits relatively high near the waterfront, often between 2 and 5 metres deep, which directly affects the geotechnical design of deep excavations. In our experience, the stiffness contrast between the weathered dolerite top and the soft estuarine clays underneath demands careful consideration. Before finalizing a shoring system, we typically run a georradar survey to map buried boulders and old river channels, then cross-check with permeability tests in the field to estimate inflow rates into the excavation box.
Weathered dolerite top and soft estuarine clays under Hobart demand a tailored design for every deep excavation project.
Methodology and scope
- basal heave stability in the soft clay layer (factor of safety > 1.5)
- groundwater control using a combination of sump pumping and a low-permeability grout curtain
- deflection limits for adjacent heritage buildings (max 15 mm lateral movement)
Local considerations
Hobart's population of roughly 250,000 means many developments occur on infill sites with unknown buried obstructions. The 2013 Meehan Range earthquake (M4.5) reminded local engineers that seismic loads can govern retaining wall design in deep excavations, especially when the soil profile includes loose sand lenses susceptible to liquefaction. Ignoring the lateral earth pressure increase due to seismic shaking — as prescribed by AS 4678 — can lead to wall failure or excessive inward movement. We always include a site-specific seismic hazard assessment that factors in the local dolerite geology and the alluvial basin effects.
Applicable standards
AS 4678-2002 Earth-retaining structures, AS 1726-2017 Geotechnical site investigations, AS/NZS 1170.4-2002 Structural design actions – Earthquake actions (incl. Amdt 1)
Associated technical services
Shoring and Retaining Wall Design
From soldier pile and lagging to secant pile walls, we design support systems that keep excavations safe and dry. Our team calculates active and passive pressures using effective stress analysis, considers groundwater seepage, and verifies global stability for each stage of the dig.
Groundwater Control and Dewatering Design
We design dewatering schemes that lower the water table safely without causing harmful settlement to neighbouring structures. Systems include deep wells, ejector systems, and cut-off walls, all calibrated to Hobart's variable hydraulic conductivity.
Typical parameters
Frequently asked questions
What geotechnical parameters are most critical for deep excavation design in Hobart?
The stiffness and strength of weathered dolerite, the undrained shear strength of estuarine clays, and the height of the water table are the three most critical parameters. We also assess the coefficient of earth pressure at rest (K0) and the liquefaction potential of loose sand layers near the Derwent River.
How much does a geotechnical design for a deep excavation in Hobart typically cost?
The cost ranges between AU$3,650 and AU$14,460 depending on excavation depth, number of support levels, groundwater complexity, and whether a seismic analysis is required. A typical basement excavation for a commercial building falls in the middle of that range.
Do I need a shoring design if the excavation is only 3 metres deep?
Yes, in Hobart you generally do. Shallow excavations in soft clay or loose sand can collapse without support. Even a 3-metre cut in weathered dolerite may need rock anchoring if blasting or mechanical ripping is used. AS 4678 requires a design for any excavation deeper than 1.5 m that affects adjacent property.
How long does the design process take for a deep excavation project?
A typical shoring and groundwater design takes four to six weeks from the moment we receive the site investigation data. Complex projects with multiple support levels or heritage protection requirements may take eight to ten weeks. We always coordinate with the structural engineer to avoid delays.