Geotechnical Investigation in Australia
A field-practical guide to Australian geotechnical investigations: how scopes are planned, how drilling and sampling
are typically executed, how in-situ tests (SPT/CPTu/DMT/DCP) are selected, how NDD and utility controls are handled,
how logging is delivered in AGS format, and how safety and QA are managed on site.
Common Australian project types
- Residential & commercial developments
- Roads/rail, ports, renewables
- Basements, retaining, pavements
Key drivers
- Expansive/reactive clays (many regions)
- Fill variability (urban sites)
- Groundwater & dewatering constraints
Typical outputs
- Factual logs + test results
- Interpretive parameters & recommendations
- AGS digital deliverables (when required)
Overview
Australian investigations are usually staged: desk study → targeted fieldwork → lab testing → factual reporting →
interpretive design guidance. The scope is often driven by local ground risk (reactive soils, uncontrolled fill, groundwater,
soft alluvium, or weathered rock), plus practical constraints such as utilities, traffic, and access.
Australian reality: if you ignore services, you don’t “save time” — you schedule a service strike.
Australia-specific context
Ground conditions that commonly matter
- Reactive clays: seasonal moisture changes drive heave/shrink and footing movement.
- Uncontrolled fill: thickness and variability often dominate risk on urban sites.
- Alluvium/estuarine deposits: soft compressible layers, high groundwater, potential acid sulfate soils.
- Weathered rock profiles: variable “rockhead”, rippability, and permeable horizons.
How clients commonly specify work
- Residential: focus on site classification (reactivity), bearing/settlement, earthworks, drainage.
- Road/rail: tighter QA, more in-situ coverage (CPT/DCP), hold points, pavement subgrade focus.
- Major infra: structured deliverables (GDR/GBR style), digital AGS data, more probabilistic thinking.
Typical standards & guidance used in Australia
| Area |
Common Australian references (typical) |
Notes |
| Site investigations |
AS 1726 |
Framework for investigation planning, logging, reporting and QA expectations. |
| Soil testing |
AS 1289 series |
Classification + compaction + moisture/density + strength tests (method selection is key). |
| Residential footing context |
AS 2870 (site classification) |
Often drives investigation scope for house sites in reactive soil regions. |
| Road/rail projects |
State authority specs (e.g., Vic / NSW / QLD / WA) |
Commonly define frequencies, hold points, reporting format, NATA testing requirements. |
| Work health & safety |
WHS legislation + SWMS/JSA + traffic management |
Practically non-negotiable: services, pits, plant, road corridors, contaminated land. |
| Digital data |
AGS data format (client-required) |
Used to deliver logs/tests in a machine-readable way for models and auditing. |
Different clients and authorities will call up different document sets; the best practice is to list the governing
documents explicitly in your report header.
Planning & approvals
Before fieldwork
- Desk study: geology maps, local experience, previous reports (nearby helps a lot).
- Access: rig size, overhead clearances, pavement protection, noise/time constraints.
- Permits: council/authority approvals, rail corridor access, environmental constraints.
- Traffic management: if near roads/footpaths, treat it like a project deliverable.
Investigation design logic
- Pick locations to resolve the uncertainties that drive design (not “nice round numbers”).
- Combine methods: boreholes for sampling + CPT for continuous profiling + pits for near-surface/fill/services.
- Plan contingencies: refusal, water inflow, collapse, contaminated material, restricted access.
Utilities (DBYD) & Non-Destructive Digging (NDD)
On Australian urban sites, utility control is a primary risk. “Dial Before You Dig” plans are a starting point; NDD
(hydro-vac / vacuum excavation) is commonly used to positively locate services before intrusive works.
Typical workflow
- DBYD enquiry + review responses and limitations.
- Mark out assumed service corridors and exclusion zones.
- Use NDD to expose services at critical points (depth + offset confirmation).
- Set bore/CPT positions only after positive location where risk is non-trivial.
NDD field records
- Chainage/offset, depth, service type/material, diameter, condition.
- Photos with scale and depth reference.
- Reinstatement notes (backfill, compaction, pavement repair).
“We didn’t hit anything” is not a control measure. Positive location is the control measure.
Drilling & boreholes
Boreholes provide material for classification and lab testing, plus groundwater observations. Methods commonly used in Australia
include auger drilling (solid/hollow stem), rotary mud/polymer for unstable ground, rotary air for rock/weak rock transitions,
and coring where rock quality parameters (e.g., RQD) matter.
What to document every time
- Drilling method and changes with depth (and why).
- Returns (cuttings/core), losses, washouts, and any bore instability.
- Obstructions/refusal and how it was handled.
- Water strikes and time-stamped groundwater readings.
Fluids, lubrication & practical controls
- Bentonite/polymer fluids: stabilise bore walls, reduce collapse, control inflow.
- Rod/casing lubrication: improves penetration stability and reduces friction/heat.
- Control flushing: excessive wash can remove fines and destroy sample representativeness.
If your log says “sand” but you drilled with aggressive wash in silty clay… the ground model may be fiction.
Sampling practice
Disturbed sampling (classification)
- Bulk bags/jars for PSD, Atterbergs, moisture, compaction preparation.
- SPT split-spoon samples (useful but typically disturbed).
- Auger cuttings (lowest quality; still useful for trend if logged carefully).
Undisturbed sampling (strength/stiffness)
- Thin-walled tube samples for cohesive soils.
- Piston samples for very soft/sensitive clays where disturbance must be minimised.
- Careful handling/transport is part of the “test method”, not an afterthought.
Undisturbed sampling: tube sizes & piston samplers
Common tube diameters (typical Australian field practice)
- 75 mm thin-walled tube: very common where sample quality matters.
- 63 mm tube: common alternative.
- 50 mm and 38 mm: used where access/ground limits exist (or legacy rigs).
If you need high-quality stiffness/consolidation: prefer a larger, better-quality tube sample over “more” low-quality samples.
Piston sampling
Piston samplers help protect very soft/sensitive clay samples by controlling suction and limiting remoulding during insertion.
They are commonly selected when conventional push sampling risks smearing and loss of structure.
- Improves recovery and reduces sample disturbance.
- Better suited for oedometer and triaxial testing in soft clays.
- Needs disciplined sealing, orientation marking (“TOP”), and vibration/temperature control in transport.
Sealing, transport and chain-of-custody
- Seal ends immediately (caps + tape; wax/paraffin where required) to prevent moisture loss.
- Label: hole ID, depth interval, sample ID, date/time, method (tube/piston), “TOP”.
- Store upright when possible; avoid heat and vibration; deliver to lab promptly.
- Record recovery, sample condition, and any drilling effects (wash, smear, fall-in).
In-situ tests (Australia): SPT, CPT(u), DMT, DCP
| Test |
Common Australian use |
Strengths |
Watch-outs |
| SPT |
Borehole-based profiling (esp. sands) + legacy specs |
Simple, widely understood, works with sampling program |
Energy/procedure sensitivity; coarse gravels can give misleading blow counts |
| CPT / CPTu |
High-resolution profiling; soft ground, pavements, major projects |
Continuous data; CPTu adds pore pressure response |
Refusal in gravels/cobbles; needs good calibration and interpretation |
| DMT |
Settlement/stiffness-focused projects; some road/rail contexts |
Useful stiffness indices and stress state insight |
Technique/interpretation dependent; not as common as CPT |
| DCP |
Pavements/subgrades, earthworks QA, quick spatial coverage |
Fast and cheap; good for variability mapping |
Correlations vary; document equipment and refusal criteria |
A very Australian pattern: test pits + DCP for pavements/subgrade variability, plus a few deeper
boreholes and/or CPTs to anchor the ground model.
Test pits & trenches
Test pits are heavily used in Australia for near-surface variability (fill, pavements, services, relic footings),
bulk sampling, and visual inspection of fabric/structure that boreholes can miss.
Great for
- Fill thickness/variability and identifying uncontrolled materials.
- Pavement layer thickness and subgrade condition.
- Utility corridors and service-rich sites (paired with NDD).
Limitations
- Depth and groundwater constraints.
- Safety controls dominate method choice (batter/shoring/exclusion zones).
- Needs careful reinstatement, especially in public areas.
Logging & AGS deliverables (Australian projects)
Many Australian clients (especially government and large infrastructure) request AGS-format deliverables so data can be imported
into digital ground models and QA systems. At a minimum, your reporting should clearly separate factual and
interpretive content.
What typically goes into AGS-style outputs
- Borehole/test pit location metadata (coords, datum, method, dates).
- Strata descriptions with depth boundaries and descriptor codes.
- Sample registers (type, diameter, recovery, preservation notes).
- In-situ tables: SPT blows, CPT traces (often separate data files), DCP results.
- Groundwater observations with time stamps and method (strike vs stabilised).
Australian reporting habits that help
- State the coordinate system and vertical datum clearly (clients will ask).
- Include a “scope limitations” note (access, utilities, refusal, water).
- Summarise regional geology and expected hazards (reactive soils, ASS, contamination).
- Use consistent descriptors and a repeatable legend for strata.
Common “factual vs interpretive” split
- Factual: logs, photos, sample register, in-situ and lab results, groundwater readings.
- Interpretive: ground model, design parameters, derived correlations, recommendations, risks.
QA, transport & lab testing
Typical QA controls
- Chain-of-custody and sample tracking to lab.
- Time-stamped groundwater readings and method notes.
- Calibration records (CPT cone, DMT blade) where required.
- Photologging of cores/samples with depth markers.
Lab testing (common Australian patterns)
- Classification: PSD + Atterbergs + moisture/density.
- Strength: UU/CIU/CID triaxial, UCS (weak rock), direct shear (interfaces as needed).
- Compressibility: oedometer (especially where settlement matters).
- Earthworks: compaction and CBR-type programs (spec dependent).
Many clients specify NATA-accredited labs and particular test methods—treat the spec as part of your design input.
WHS & field safety (Australia)
Non-negotiables
- SWMS/JSA prestart brief, site induction, PPE.
- Traffic management where near roads/footpaths.
- Exclusion zones around plant; clear comms with drill crew.
- Barricading open holes and pits; secure covers where required.
- Utility strike controls: DBYD + NDD + spotter + permit-to-dig as applicable.
Test pit safety
- Do not enter unsupported excavations.
- Use battering or shoring; keep spoil/plant away from edges.
- Plan for groundwater inflow and collapse potential.
- Stop work if conditions deviate from assumptions (soft zones, running sand, unknown services).
A “quick pit” can still kill someone. Treat excavations seriously every time.
Field checklist (Australia-style)
- ✅ DBYD completed; responses reviewed; limitations noted.
- ✅ NDD completed where risk exists; services positively located and recorded.
- ✅ SWMS/JSA and induction done; traffic management implemented (if needed).
- ✅ Hole IDs + coordinates + datum confirmed; photos taken with markers.
- ✅ Groundwater readings time-stamped and method recorded (strike vs stabilised).
- ✅ Tube/piston samples sealed immediately; “TOP” marked; transport controlled.
- ✅ SPT/CPT/DMT/DCP procedures documented; refusal and obstructions logged properly.
- ✅ Site reinstatement done; hazards removed; as-built locations captured.
