Quick answer
A geotechnical survey investigates soil, rock and groundwater to inform project-specific foundation and earthworks engineering, with scope and limitations matched to the proposed structures and site.
Answer: A geotechnical survey investigates the ground conditions relevant to a solar project’s foundations and earthworks. It combines existing information, site observations, field testing and laboratory analysis to characterize soil, rock and groundwater. Its findings inform engineering decisions, but do not automatically establish pile depth or foundation approval. The investigation scope must match the site variability, proposed structures and applicable requirements.
What the investigation establishes
Ground-mounted PV requires foundations that work with the actual subsurface conditions and design loads. The investigation can identify fill, shallow rock, compressible or expansive soils, groundwater and chemical conditions relevant to buried materials. Its value is in a defensible ground model and design recommendations, including limitations.
A topographic survey describes surface geometry; a geotechnical investigation addresses the ground below it. Their coordinates and elevations need to be reconciled, but one does not replace the other. See topographic analysis.
USDA’s Web Soil Survey supports general planning and explicitly notes that certain engineering applications need onsite investigation. Published soil mapping is useful screening information, not a foundation design for a specific pile location.
Methods and their limits
| Method | What it can contribute | Important limitation |
|---|---|---|
| Desktop and reconnaissance review | Geology, historical use, terrain, existing reports and possible investigation targets | Maps and nearby investigations may not represent the proposed foundation locations |
| Borings and sampling | Soil or rock descriptions, samples and observations at selected locations and depths | Sampling extent and quality must be documented; a boring does not observe every part of the site |
| Standard penetration testing | Penetration resistance measured under the applicable test method | Interpretation depends on procedure, corrections, soil and engineering correlations; N-value is not directly a universal bearing capacity |
| Cone penetration testing | Depth profiles of cone resistance and sleeve friction; pore pressure where instrumented | Interpretation and feasibility depend on ground conditions and equipment |
| Laboratory analysis | Selected physical, strength, compressibility and chemical properties | Test selection and sample quality must fit the engineering question |
| Foundation load testing | Response of a defined test foundation under a specified loading procedure | Uplift, compression and lateral tests are distinct; one pull-out result does not prove every design limit |
FHWA’s geotechnical exploration reference discusses matching investigation to geology, design requirements and variability. It concerns pavement projects; its investigation-planning principles are useful context, not a solar-specific boring-spacing standard.
Scope the work around the project
Provide the geotechnical team with the current layout, proposed foundation concepts, load information, grading, inverter or transformer pads, roads and other relevant structures. Investigations should address representative conditions and material changes, with additional work where the responsible engineer identifies uncertainty.
There is no single borehole-per-acre rule that makes every solar site adequately investigated. Area alone does not describe geology, fill, groundwater, terrain or structural sensitivity. Small project capacity also does not prove that a desktop review is sufficient. Obtain a scope appropriate to the actual site and requirements.
Before fieldwork, establish access permission, utility-location procedures and other required authorizations. Competent personnel must plan and conduct the work under applicable site and safety requirements.
Translate findings into foundation decisions
Driven piles, helical foundations, drilled foundations and ballast have different design and installation constraints. Soil labels alone do not prescribe one required type or embedment depth. The engineering assessment may consider axial and lateral resistance, uplift, movement, installation feasibility, frost effects, groundwater, durability and the design loads.
Corrosion allowance is usually a material-loss or protection consideration; it is not a length to add to frost and structural embedment in a universal pile-depth equation. Frost, load resistance and durability require their appropriate engineering checks rather than a dimensionally inconsistent shortcut.
Groundwater observations record conditions at the time of investigation. They do not guarantee excavation will be dry later. Likewise, an assumed design life or a coating name does not establish the durability of a particular foundation in the measured environment.
Deliverables and construction feedback
A useful handoff includes investigation locations and elevations, dates and methods, logs, test results, interpreted ground conditions, recommendations, limitations and unresolved issues. Keep the report revision linked to the ground-mount system design.
Share the same controlled information with bidders and record how ground risk is allocated. Obtain itemized scope-based quotations rather than treating a generic survey cost or foundation-cost percentage as a project budget.
During construction, document unexpected ground, installation refusal and test results against the specified criteria. Escalate discrepancies to the responsible engineering team; do not shorten foundations or substitute a design solely because installation is difficult. Changed grading or layout may also require review of the investigation’s applicability.
For utility-scale solar, the investigation should remain connected to civil, structural and procurement decisions. General solar-layout software does not replace geotechnical interpretation or qualified foundation design.
Frequently asked questions
Is a soil map enough to design solar foundations?
Not automatically. It helps screening, but the responsible team must establish what onsite investigation and engineering are needed for the particular project.
Does an SPT N-value give pile depth directly?
No. Test interpretation, ground conditions, design loads, foundation geometry and applicable methods all matter. One penetration-resistance value is not a complete foundation design.
Is a pull-out test also a compression or lateral test?
No. These address different loading directions and require appropriate procedures and acceptance criteria. Record which test was actually performed.
When should the investigation be undertaken?
Coordinate it with site control and design development so relevant ground information is available before foundation decisions are released. Update the assessment when material site or design changes affect its scope.
Related Glossary Terms
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