A geotechnical site investigation is the systematic process of exploring subsurface conditions to provide information needed for foundation design, earthworks, slope stability, and pavement design. In Australia, site investigations follow AS 1726-2017.
What Is a Geotechnical Site Investigation?
A geotechnical site investigation determines the physical and mechanical properties of soil and rock at a site. It provides the data needed for:
- Foundation design (type, depth, bearing capacity)
- Earthworks specifications (fill quality, compaction)
- Slope stability assessment
- Pavement design
- Groundwater management
- Retaining wall design
- Contamination assessment
Investigation Phases
Phase 1: Desktop Study
Review existing information before any fieldwork:
- Geological maps and memoirs (Geoscience Australia, state surveys)
- Aerial photography and LiDAR
- Previous geotechnical reports for the site or adjacent sites
- Council records (known hazards: mine subsidence, landslide, flood)
- Site history — previous land use (fill, contamination, mining)
Phase 2: Site Walkover
A visual inspection of the site:
- Topography and slope conditions
- Existing structures and infrastructure
- Vegetation (tree location, species, root zones)
- Surface drainage and erosion
- Evidence of ground movement (cracks, settlement, slope distress)
- Access constraints for drilling equipment
Phase 3: Field Investigation
Subsurface exploration through boreholes, test pits, and in-situ testing:
| Method | Depth | Information Obtained |
|---|---|---|
| Boreholes | 3–30 m | Continuous soil profile, samples, SPT, groundwater |
| Test pits | 1–4 m | Visual soil inspection, bulk samples, rock assessment |
| CPT (Cone Penetration Test) | 5–40 m | Continuous profile, strength, pore pressure |
| DCP (Dynamic Cone Penetrometer) | 1–3 m | Upper soil strength, CBR correlation |
| Geophysical surveys | Variable | Stratigraphy, bedrock depth, voids |
Phase 4: Laboratory Testing
Samples are tested to determine engineering properties:
| Test Type | Parameters |
|---|---|
| Classification | Moisture content, Atterberg limits, particle size distribution |
| Strength | Triaxial (UU, CU, CD), direct shear, UCS |
| Compaction | Standard/Modified Proctor, field density |
| Permeability | Constant head, falling head |
| Chemical | pH, sulfates, chlorides, organic content |
| Reactivity | Shrink/swell index, linear shrinkage |
Phase 5: Analysis and Reporting
Interpretation of field and lab data produces:
- Soil/rock profile — layer descriptions and depths
- Geotechnical parameters — design values for strength, compressibility, reactivity
- Foundation recommendations — type, depth, bearing capacity
- Slope stability assessment — if relevant
- Groundwater conditions
- Excavatability assessment — rippability or rock excavation
Investigation Depth Requirements
Residential (AS 2870)
- Minimum 2 boreholes to 3 m depth
- Additional depths for highly reactive clays or P class sites
Commercial / Industrial
- Boreholes to depth of stress influence (typically 1.5–2× foundation width)
- At least one borehole to competent bearing stratum
Infrastructure (Roads, Bridges)
- Boreholes to 5–10 m below formation level
- Additional investigation at bridge pier locations
Sample Types
| Sample Type | Quality | Use |
|---|---|---|
| Disturbed (DS) | Suitable for classification, moisture, compaction | Bulk samples, auger cuttings, SPT samples |
| Undisturbed (UDS) | Preserves in-situ structure | Strength, consolidation, permeability testing |
| Core sample | Intact rock core | Rock quality, joint logging |
In-Situ Testing
| Test | Parameters | Application |
|---|---|---|
| SPT | N-value, strength correlations | All soil types |
| CPT | Cone resistance, friction ratio, pore pressure | Continuous profiling |
| DCP | Penetration rate, CBR correlation | Pavement subgrade, shallow fill |
| Shear vane | Undrained shear strength | Soft clays |
| Pressuremeter | Modulus, strength, limit pressure | Deep foundations |
| Plate load test | Bearing capacity, modulus | Shallow foundations |
Reporting
A standard geotechnical investigation report includes:
- Introduction — project description, scope of work
- Site conditions — geology, topography, access
- Field investigation — methods, borehole logs, photographs
- Laboratory testing — results tables and summaries
- Geotechnical model — interpreted soil profile and parameters
- Analysis — foundation, earthworks, slope stability as applicable
- Recommendations — design parameters, construction guidance
- Limitations — applicability of findings, need for additional work
Australian Standards
| Standard | Title |
|---|---|
| AS 1726-2017 | Geotechnical site investigations |
| AS 1289 Series | Soil testing methods |
| AS 2870-2011 | Residential slabs and footings |
| AS 2159-2009 | Piling — design and installation |
| AS 3798-2007 | Earthworks |
| AS 4326-2008 | Site preparation (Landslide-prone areas) |
Frequently Asked Questions
How many boreholes are needed?
For a typical residential block, 2 boreholes is the minimum. For larger sites, the number depends on the area and variability — typically 1 borehole per 500–1,000 m².
How long does a geotechnical investigation take?
Desktop study (1–2 days), fieldwork (1–2 days for standard residential), laboratory testing (3–7 days), analysis and reporting (2–5 days). Total: typically 7–14 business days.
Do I need a geotechnical investigation for a small shed or garage?
Check your council requirements. Some structures below a certain size may not require formal geotechnical input, but any structure that needs engineering certification will need site classification or investigation.