A complete guide to planning, executing and reporting a scope-driven geotechnical site investigation.
Geotechnical Site Investigation
A geotechnical site investigation develops the ground model and engineering parameters needed to manage foundation, earthworks, pavement, retaining, groundwater and construction risks. The investigation should be designed around project decisions and credible ground hazards, not a generic number of boreholes or a universal drilling depth.
Note: This page must be scoped and interpreted by suitably qualified professionals using the applicable project requirements and current standards.
Define the purpose and design questions
Before selecting investigation locations or methods, define:
- Proposed structures, levels, loads and construction sequence
- Foundation, excavation, earthworks, pavement and retaining concepts
- Required design parameters and serviceability questions
- Groundwater and dewatering risks
- Natural hazards and site constraints
- Existing structures, utilities and access limitations
- Decisions required at concept, detailed design and construction stages
The investigation should be proportionate to project complexity, ground variability and consequence of uncertainty.
Investigation stages
Desktop study
Review available geological, geomorphological, topographic, historical, contamination, groundwater and previous investigation information. Existing data should be checked for location, datum, quality, age and relevance to the proposed work.
Site walkover
Observe landform, drainage, slopes, fill, exposures, retaining structures, vegetation, distress, erosion, seepage, access and neighbouring development. Record limitations such as buried surfaces or inaccessible areas.
Preliminary ground model
Combine the desktop and walkover evidence into an initial model of expected units, boundaries, groundwater and hazards. Use the model to target uncertainty and select field methods.
Field investigation
Fieldwork may include boreholes, test pits, hand augers, cone penetration testing, standard penetration testing, dynamic cone penetration, geophysics, groundwater installations and in-situ strength or permeability testing.
Method selection should reflect the ground type, required data quality, access, safety and intended design use.
Sampling and laboratory testing
Samples should be suitable for the planned classification, strength, compressibility, compaction, CBR, permeability, chemical or environmental testing. Sample disturbance, recovery and preservation directly affect data quality.
Interpretation and reporting
Field and laboratory data should be reconciled into a ground model that identifies units, variability, groundwater and uncertainty. Parameters should be selected for defined design conditions, with their sources and limitations recorded.
Investigation locations and depths
There is no universal spacing or depth that is suitable for every project. Coverage should consider:
- Structure footprint, load concentration and founding options
- Depth of the zone materially influenced by construction and loading
- Weak, compressible or permeable layers below stronger material
- Excavation, retaining and slope failure mechanisms
- Earthworks cut and fill depths
- Pavement variability and drainage
- Geological boundaries, fill and buried channels
- Data needed to reduce the identified risks
The scope should remain adaptable. Unexpected conditions may require additional investigation during fieldwork or construction.
Groundwater investigation
A water level observed during drilling is not automatically the long-term groundwater level. Drilling fluids, delayed response, perched water and seasonal variation can affect observations. Where groundwater matters, install and monitor suitable standpipes or piezometers over an appropriate period.
Data quality and traceability
Each record should include coordinates, ground level, datum, method, equipment, dates, personnel, calibration where relevant and any deviation from the planned procedure. Samples, photographs and laboratory results should link directly to the corresponding location and depth.
Digital records should preserve original field observations and track later corrections or interpretations.
Safety, access and environmental controls
Field planning may require utility locating, permits, traffic control, drilling exclusion zones, contaminated-ground controls, working-at-height or near-water procedures, spoil management, noise controls and site restoration.
Safety constraints can affect data coverage and should be documented as investigation limitations.
Ground model and engineering outputs
A report may provide:
- Investigation scope and methods
- Site setting and interpreted ground model
- Factual field and laboratory results
- Groundwater observations
- Parameter selection and design recommendations
- Foundation, settlement, earthworks, pavement or excavation advice
- Construction risks, hold points and verification requirements
- Limitations, uncertainties and recommendations for further work
Recommendations should be tied to the proposed development and the information available at the time of reporting.
Construction verification
Investigation does not remove all ground uncertainty. Footing inspections, proof rolling, excavation mapping, pile records, groundwater monitoring and additional testing may be needed to compare actual conditions with the design model.
Related resources
- Geotechnical Investigation for Site Classification
- Marine Geotechnical Investigations
- Geotechnical Modelling and Analysis
- Critical Hydraulic Gradient and Hydraulic Heave