Hobart grew fast after the 1960s, spreading from the Derwent estuary into the foothills of Mount Wellington. That expansion pushed construction onto variable geology — colluvial fans, alluvial terraces, and weathered dolerite slopes. In our experience, an exploratory test pit remains the most direct way to verify subsurface conditions before excavation. We dig pits down to 4.5 metres, log the profile in accordance with AS 1726, and document every layer. No guesswork. The visual record alone often saves redesign costs. Before committing to deep foundations, we recommend pairing the pit with a plate load test to confirm bearing capacity in the exposed stratum.
A well-logged test pit avoids 80% of foundation surprises. In Hobart, the change from colluvium to alluvium can happen within metres.
Methodology and scope
- Layer thickness and colour (Munsell)
- Soil texture and plasticity (field estimate)
- Moisture content and groundwater seepage
- Presence of fill, organics, or cobbles
Local considerations
In Hobart, many times we see builders assume uniform ground based on one borehole. That assumption fails when the test pit exposes a buried channel or a perched water table. The risk is real — differential settlement in a residential slab costs more than the entire geotechnical program. Another common issue is contamination from historic land use, especially in commercial zones near the port. An exploratory test pit flags these problems early. We also check for boulders in the dolerite talus; hitting one during pile installation can stop a job for days.
Applicable standards
AS 1726:2017 Geotechnical site investigations, AS 4678:2002 Earth-retaining structures, AS/NZS 1170.0:2002 Structural design actions
Associated technical services
Utility location and clearance
Before any digging, we mark buried services with GPR and dial-before-you-dig checks. This avoids gas, water, or power strikes in tight urban sites.
Hand-dug pits for confined access
When a machine cannot reach the location — backyards, steep slopes, or inside existing buildings — our crew excavates manually to 2.0 m depth with full shoring.
Continuous pit logging for linear works
For pipelines or retaining walls over 50 m, we excavate pits every 25–30 m and produce a continuous profile along the alignment. This picks up lateral variability fast.
Typical parameters
Frequently asked questions
How deep can an exploratory test pit go in Hobart?
Typically up to 4.5 metres with a excavator. Deeper pits require benching or shoring to meet safe-work regulations. In soft alluvium we stop earlier to avoid collapse.
What is the difference between a test pit and a borehole?
A test pit gives you a visual profile of the soil in place — you see layers, roots, and boulders directly. A borehole provides continuous SPT N-values but limited visual context. We often use both for critical structures.
Do I need a test pit for a small house extension in Hobart?
If the extension is within 2 m of an existing footing, yes. Many older Hobart homes sit on shallow strip footings that can be undermined. A pit reveals the actual foundation condition and soil type before you dig.
How much does an exploratory test pit cost in Hobart?
Prices range between AU$830 and AU$1,470 per pit depending on depth, access, and whether lab testing is included. Volume discounts apply for multiple pits on the same site.