What Is Geotechnical Survey? Definition & Guide

Key Takeaways

A geotechnical survey for a solar farm characterizes subsurface soil, rock, and groundwater conditions to determine the correct foundation type and pile embedment depth for ground-mount racking
Common investigation methods include borehole drilling with Standard Penetration Testing (SPT), Cone Penetration Testing (CPT), pull-out testing, and desktop geologic review
Soil bearing capacity, frost depth, water table elevation, and corrosion potential are the four parameters that most directly affect foundation cost and structural performance
Skipping or under-scoping a geotechnical survey on a ground-mount solar project routinely leads to 20-40% foundation cost overruns during construction
Survey results feed directly into pile design, determining whether driven steel piles, helical screws, ballasted footings, or concrete piers are the most cost-effective foundation
NREL and DOE guidelines recommend a minimum of one borehole per 2-5 acres on utility-scale solar sites, with additional borings at inverter pads, substation locations, and access roads

What Is a Geotechnical Survey for Solar Projects?

A geotechnical survey (also called a geotechnical investigation or soil testing for a solar project) is a field and laboratory investigation of the subsurface conditions at a proposed solar farm site. The survey collects soil samples, measures bearing capacity, identifies rock layers, maps the water table, and tests for corrosive soil chemistry. These findings determine what type of foundation system will work, how deep piles must be driven, and what site preparation is needed before racking installation begins.

For ground-mount and utility-scale solar projects, the geotechnical survey is one of the earliest and most consequential steps in project development. Foundation costs typically represent 8-15% of total installed cost on a ground-mount solar site survey, and the geotech report is the single document that determines whether those costs land at the low end or the high end of that range.

A geotechnical survey does not tell you where to place panels. It tells you what is underground — so that the structural engineer can design a foundation that stays in the ground for 25-35 years without settling, corroding, or pulling out under wind load.

Types of Geotechnical Investigation

Four investigation methods cover the range of soil testing a solar project typically requires. Most utility-scale sites use a combination of at least two methods.

Field Testing

Borehole / SPT Testing

A drill rig advances a borehole to depths of 10-50 feet, collecting continuous soil samples. At regular intervals (typically every 5 feet), a Standard Penetration Test (SPT) measures resistance by counting the blows required to drive a split-spoon sampler 12 inches into the soil. SPT blow counts (N-values) directly indicate soil density and bearing capacity. This is the most common method for geotechnical surveys on solar farm sites and produces the data that structural engineers rely on for pile design.

Field Testing

Cone Penetration Testing (CPT)

A truck-mounted hydraulic ram pushes an instrumented cone into the ground at a constant rate, recording tip resistance, sleeve friction, and pore water pressure continuously with depth. CPT is faster than borehole drilling and produces a continuous soil profile rather than data at discrete intervals. It excels at detecting thin soft layers or lenses that boreholes might miss. CPT is frequently used alongside SPT borings on large solar sites to fill in data between borehole locations.

Site-Specific

Pull-Out Testing

A test pile is driven or screwed into the ground at the actual site, then loaded in tension (uplift) and compression to measure real-world resistance. Pull-out testing validates the pile design derived from SPT or CPT data and is especially valuable when soil conditions are variable or borderline. Many EPC contractors and racking manufacturers require pull-out test results before finalizing pile specifications. Tests are performed at multiple locations to capture site variability.

Pre-Field

Desktop Study / Geologic Review

Before any drilling begins, geotechnical engineers review published geologic maps, USDA soil surveys, FEMA flood maps, historical aerial imagery, and any existing geotechnical reports from nearby sites. The desktop study identifies potential red flags — shallow bedrock, expansive clay, high water tables, fill material, or contamination — and informs the field investigation plan. It also determines how many boreholes are needed and where they should be placed for maximum coverage of site variability.

Soil Types and Foundation Impact

The soil type encountered during a geotechnical survey for a solar farm directly determines the pile type, embedment depth, and foundation cost. This table summarizes the relationship for common soil conditions found on ground-mount solar project sites.

Soil Type	Bearing Capacity (psf)	Pile Type Required	Embedment Depth	Foundation Cost Impact
Dense sand / gravel	3,000 – 5,000	Driven steel W or C piles	6 – 8 ft	Low — standard pile driving, minimal refusal issues
Stiff clay	2,000 – 4,000	Driven steel piles or helical screws	8 – 10 ft	Low to moderate — may need pre-drilling in very stiff zones
Soft clay / silt	500 – 1,500	Helical piles or concrete piers	10 – 15 ft	Moderate to high — deeper embedment, larger pile sections needed
Loose sand	1,000 – 2,000	Helical piles or rammed aggregate piers	8 – 12 ft	Moderate — longer piles, possible ground improvement
Expansive clay	1,500 – 3,000 (variable)	Drilled concrete piers below active zone	12 – 20 ft	High — must penetrate below swell/shrink depth
Shallow bedrock (under 4 ft)	10,000+ (rock)	Rock anchors or surface-mount ballast	2 – 4 ft into rock	High — rock drilling or alternative foundation design required
Organic / peat soils	Under 500	Concrete caissons or soil replacement	15 – 25 ft to competent layer	Very high — poor bearing material, extensive ground improvement
Fill material	Variable / unreliable	Depends on fill depth and composition	Must penetrate through fill to native soil	Variable — requires detailed characterization of fill thickness

Pile Depth Calculation

The required pile embedment depth on a solar project is a function of multiple geotechnical and structural parameters. While the exact calculation varies by foundation type and engineering methodology, the core relationship follows this form:

Required Pile Depth (Simplified)

D_required = D_frost + D_structural + D_corrosion

Where D_frost = local frost penetration depth (pile must extend below frost line to avoid heave), D_structural = additional embedment required to resist lateral wind loads and vertical uplift based on soil bearing capacity (derived from SPT N-values or CPT data), and D_corrosion = sacrificial steel thickness allowance for corrosive soil environments (typically 1-3 mm added to pile wall thickness rather than depth, but aggressive corrosion may require deeper embedment into less corrosive strata).

The structural embedment component is where the geotechnical survey data has the most direct impact. In dense sand with SPT N-values above 30, a pile may only need 6 feet of structural embedment to resist design wind loads. In soft clay with N-values below 5, that same pile may require 12-15 feet of embedment to develop adequate lateral and axial resistance.

Solar design software that integrates geotechnical data with structural load calculations can optimize pile lengths across a site, accounting for the spatial variability of soil conditions rather than designing every pile for worst-case conditions.

The Real Cost of Skipping a Geotechnical Survey

Projects that skip soil testing for solar project sites or rely on a single borehole for a 50+ acre site routinely encounter 20-40% foundation cost overruns during construction. The most common failure modes: piles that refuse early on unexpected shallow rock (requiring rock drilling equipment mobilization at $15,000-$25,000 per day), piles that fail pull-out testing because soft layers were not detected (requiring redesign to longer or larger piles), and water table encountered during excavation for inverter pads or underground conduit (requiring dewatering). A thorough geotechnical survey on a utility-scale solar site typically costs $15,000-$50,000 — a fraction of the $200,000-$500,000 in change orders that inadequate soil data can trigger.

Geotechnical Survey Scope by Project Size

The scope of a geotechnical investigation for a solar project scales with acreage and site complexity. These guidelines reflect industry practice and NREL recommendations.

Residential ground-mount (under 5 kW): A desktop review of USDA soil maps and local frost depth data is often sufficient. If the site is on a slope, in a flood zone, or has known fill material, one borehole to 15 feet is recommended.

Commercial ground-mount (50 kW - 1 MW): Minimum two boreholes to 20-25 feet, plus one CPT sounding per acre. Include laboratory testing for corrosion potential (pH, resistivity, chloride, sulfate) and grain size analysis.

Utility-scale solar farm (5 MW+): One borehole per 2-5 acres, supplemented by CPT soundings between boreholes. Additional borings at inverter pad locations, substation foundations, access road alignments, and retention pond areas. Pull-out testing at a minimum of 3-5 representative locations. Full laboratory testing program including consolidation testing if soft soils are present.

Practical Guidance

Geotechnical survey data flows through the entire ground-mount solar project lifecycle — from site selection through construction and long-term performance.

Import geotech data into your design model early. Soil bearing capacity and frost depth directly affect pile length, which affects material cost, which affects the financial model. Waiting until after design completion to incorporate geotech data often means redesigning the foundation. Use solar design software that accepts geotechnical parameters as design inputs.
Map soil variability across the site. A 100-acre solar farm rarely has uniform soil conditions. Use the borehole and CPT data to create a soil zone map, then assign pile specifications by zone rather than using a single pile design for the entire site. This can reduce pile steel costs by 10-20%.
Coordinate with the topographic survey team. Geotech borings should be located based on both geologic variability and site topography. Hilltops, swales, drainage paths, and cut/fill transition zones are high-priority boring locations because soil conditions change most at these features.
Check corrosion potential before specifying pile coatings. Soil resistivity below 2,000 ohm-cm, pH below 5.5, or elevated chloride/sulfate levels indicate corrosive conditions. In these environments, hot-dip galvanized piles may not last 25 years, and epoxy-coated or thicker-walled piles may be needed.

Read the geotech report before mobilizing pile driving equipment. The report’s recommended pile installation method (impact driving, vibratory driving, or pre-drilling) determines which equipment to bring to site. Showing up with an impact hammer on a site with shallow bedrock wastes time and money.
Conduct production pull-out testing early in construction. Even with a thorough geotechnical survey, real-world pile performance should be verified. Install and test piles in the first row before committing to full production. If results differ from predictions, adjust pile lengths or types before thousands of piles are in the ground.
Document refusal conditions. When piles hit refusal (rock or very dense soil) before reaching the specified depth, stop and record the location and depth. The structural engineer will evaluate whether the shortened pile still meets design requirements or whether a rock socket or alternative foundation is needed.
Watch for water during excavation. If the geotech report identifies a water table at 8 feet and you are excavating a 6-foot inverter pad foundation, you should be fine. But water tables fluctuate seasonally. If construction occurs during a wet season, groundwater may be higher than measured during the survey. Plan for dewatering contingencies.

Budget for the geotech survey in early-stage project economics. A utility-scale geotechnical survey for a solar farm costs $15,000-$50,000 depending on acreage and number of borings. This is a pre-construction development cost that must be in the pro forma from day one. Do not defer it to save money — deferred geotech work creates far larger budget risk later.
Use geotech results for site selection due diligence. If soil testing reveals 15 feet of organic fill over soft clay, foundation costs may make the site uneconomic compared to an alternative parcel with dense sand at 3 feet. Geotech data is a go/no-go decision input, not just a design input.
Share geotech reports with EPC bidders. EPC pricing accuracy depends on foundation risk. Providing the geotechnical report to all bidders results in tighter, more competitive bids because contractors are not padding their numbers to cover unknown soil conditions.
Confirm landowner permission and utility clearance before drilling. Borehole drilling requires heavy equipment access. Confirm that the land lease allows geotechnical investigation, and call 811 (or local equivalent) for underground utility locates before any subsurface work begins.

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Sources & References

Frequently Asked Questions

What does a geotechnical survey for a solar farm include?

A geotechnical survey for a solar farm typically includes a desktop review of published soil and geologic maps, field investigation with borehole drilling and SPT testing at multiple locations across the site, laboratory testing of soil samples for grain size, moisture content, corrosion potential, and bearing capacity, and a final engineering report with foundation recommendations. Larger sites also include CPT soundings between boreholes and pull-out testing of prototype piles. The report specifies recommended pile types, embedment depths, and any ground improvement or site preparation needed before construction begins.

How much does soil testing for a solar project cost?

Soil testing costs for a solar project vary by site size, number of borings, and laboratory testing requirements. For a commercial ground-mount system (1-5 acres), expect $5,000-$15,000. For a utility-scale solar farm (50-500 acres), a full geotechnical investigation typically costs $15,000-$50,000, including mobilization, drilling, CPT soundings, laboratory analysis, and the engineering report. Pull-out testing adds $2,000-$5,000 per test location. These costs represent less than 0.5% of total project cost but directly influence foundation design, which accounts for 8-15% of installed cost.

When should a ground mount solar site survey be conducted?

A ground mount solar site survey should be conducted during the early development phase, after site control is secured but before detailed engineering design begins. The ideal timing is concurrent with or immediately after the topographic survey, so that both surface and subsurface data are available when the design team starts layout and foundation engineering. Conducting the geotechnical survey too late — after design is complete or during construction — means foundation specifications are based on assumptions rather than data, which frequently results in costly redesigns or construction change orders.