Geotechnical Survey for Solar: Scope, Cost, Pile Depth

The global geotechnical instrumentation and monitoring market is projected to reach $5.12 billion in 2026, growing at 9.9 percent a year, according to The Business Research Company (2026). That growth is not driven by skyscrapers alone. Solar developers now account for a large share of subsurface investigation work because ground-mount and utility-scale projects cannot afford foundation surprises.

Last updated: November 2026

Foundations are one of the few ground-mount costs that depend on what is underground. The geotechnical report is the single document that tells you whether piles will drive easily, hit rock, or need a different foundation. Leased area is also not usable area: setbacks, access roads, drainage and poor ground all shrink the space left for modules, and that is easier to learn before the PPA capacity is fixed.

This guide explains what a geotechnical survey is, when to order one, what it costs, and how to turn the report into a better foundation design.

In this guide, you will learn:

• What a geotechnical survey includes and how it differs from a topographic survey
• Which investigation methods apply to residential, commercial, and utility-scale solar
• How to read the four parameters that drive foundation cost
• How soil type maps to pile type, embedment depth, and budget
• Common mistakes that turn a small survey into a large change order
• When to commission the survey in the project timeline

What Is a Geotechnical Survey for Solar?

A geotechnical survey (also called a geotechnical investigation, or soil testing for a solar project) is a field and laboratory study of what is underground. It collects soil samples, measures bearing capacity, identifies rock layers, maps the water table, and tests for corrosive soil chemistry. The findings feed directly into foundation design.

For ground-mount and utility-scale solar, the survey answers three questions:

1. Can the soil support the racking loads? Bearing capacity and soil stiffness determine whether driven piles, helical screws, ballasted footings, or concrete piers are appropriate. See our ground-mount solar design guide for a full foundation workflow.
2. How deep must foundations go? Embedment depth depends on frost line, wind uplift, lateral loads, and the soil layers encountered.
3. What site preparation is needed? Cut-and-fill requirements, dewatering, soil replacement, and drainage all come from the geotech report.

A geotechnical survey does not tell you where to place panels. It tells you what is underground so the structural engineer can design a foundation that stays in the ground. PV design software integrates this data with structural load calculations to optimize pile lengths across a site. For the technical definition and related terms, see our geotechnical survey glossary entry.

Geotechnical Survey vs. Topographic Survey

These two surveys are often confused because both happen early in project development. They serve different purposes.

A topographic survey maps surface features. It records elevation contours, existing structures, roads, drainage paths, vegetation, and overhead lines. Designers use it to place arrays, plan access roads, and model shading.

A geotechnical survey maps subsurface conditions. It records soil layers, bearing capacity, rock depth, groundwater, and corrosion potential. Engineers use it to design foundations.

You need both. A site can look flat and simple on a topo map but hide shallow bedrock or soft clay below ground. The opposite is also true. A hilly site may have consistent dense soil that is easy to pile. Combine the topo survey with the geotechnical report before finalizing layout and foundation design.

Deliverables You Should Expect

A complete geotechnical report for solar typically includes:

• Site description and project overview
• Boring logs with SPT N-values or CPT profiles
• Laboratory test results for grain size, moisture, Atterberg limits, and corrosion potential
• Groundwater observations and seasonal fluctuations
• Soil classification by zone
• Foundation recommendations with pile type, embedment depth, and spacing
• Construction considerations such as dewatering, pre-drilling, or rock drilling

Why Geotechnical Data Matters for Solar Foundations

Foundation design is one of the few places where a small upfront investment prevents large downstream losses. Nuance Energy states that a geotechnical report for a solar project can cost $3,000 to $10,000, without giving a basis for the range. Compare that with the cost of redesigning foundations after piling has started.

The most common failure modes are predictable:

• Early pile refusal on shallow rock. Piles hit bedrock before reaching design depth. The contractor must mobilize rock drilling equipment, which is billed per day and adds schedule risk.
• Failed pull-out tests in soft layers. Soft clay or loose sand that was not detected requires longer or larger piles across the site.
• Groundwater during excavation. The water table measured in a dry season may rise during construction. Inverter pad excavations fill with water and require dewatering.
• Corrosion in aggressive soils. Low pH, low resistivity, or high chloride and sulfate levels shorten pile life. Uncoated or thin-walled steel may not last 25 years.

Projects that skip soil testing, or rely on a single borehole for a large site, carry the full risk of these surprises into construction. No public benchmark gives a reliable overrun percentage, so treat any single figure with caution. The geotech report is not a line item to cut. It is a small cost next to the foundation package it protects.

Investigation Methods Explained

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

Borehole Drilling with Standard Penetration Test (SPT)

A drill rig advances a borehole to a depth below the planned foundation tip. PVRack suggests at least 3 m (about 10 feet) below the design tip, or to refusal if that comes sooner. At regular intervals, a split-spoon sampler is driven into the soil. The blow count required to drive the sampler 12 inches is recorded as the SPT N-value.

SPT N-values indicate soil density and strength. In the PVRack guide, N under 4 is very soft, 10 to 30 is medium, and 30 to 50 is stiff or dense. Dense soil supports shorter driven piles, while very soft layers call for deeper embedment or a different foundation. SPT is the most common method on solar sites.

Cone Penetration Testing (CPT)

A truck-mounted hydraulic ram pushes an instrumented cone into the ground at a constant rate. The cone records 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 borings might miss. CPT is frequently used alongside SPT borings on large solar sites to fill in data between borehole locations.

Pull-Out Testing

A test pile is driven or screwed into the ground at the actual site. It is then loaded in tension and compression to measure real-world resistance. Pull-out testing validates the pile design derived from SPT or CPT data.

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. Each test location adds cost, but one failed test caught early is cheaper than a redesign across thousands of piles.

Desktop Study and Geologic Review

Before any drilling begins, geotechnical engineers review published geologic maps, USDA soil surveys, FEMA flood maps, historical aerial imagery, and any existing reports from nearby sites. The desktop study looks for red flags such as shallow bedrock, expansive clay, high water tables, fill material, or contamination.

It also determines how many boreholes are needed and where they should be placed for maximum coverage of site variability. A good desktop study prevents wasted drilling and ensures the field investigation targets the areas of highest uncertainty.

Geotechnical Survey Scope by Project Size

The scope of a geotechnical investigation for a solar project scales with acreage and site complexity. The table below summarizes the spacing and cost figures published in the sources linked in this post. Treat it as a planning guide, not a quote.

PVRack’s minimum rule is one boring per 5 acres on uniform sites and one per 2 to 3 acres on variable sites, and its FAQ tightens spacing for projects over 10 MWp. Add borings at inverter pads, substation foundations, access road alignments, and retention ponds. More geologically variable sites need closer spacing than uniform ones. Figures come from PVRack’s soil and geotechnical guide and Nuance Energy, both checked November 2026. Your engineer’s scope and local quotes govern.

Reading the Report: Key Parameters

Four parameters from the geotechnical report drive most foundation decisions on solar projects.

Soil Bearing Capacity

Bearing capacity is the load the soil can support without excessive settlement. It is expressed in kilopascals (kPa) or pounds per square foot (psf). Higher values mean shorter piles or smaller footings. Lower values mean deeper embedment, larger sections, or ground improvement.

PVRack lists typical allowable bearing capacity of 40 to 75 kPa for soft clay and 300 to 600 kPa for dense sand and gravel. The chart below shows the range. Your report gives the real values for your site, and the foundation engineer uses them to calculate pile length and section size.

Frost Depth

In cold climates, piles must extend below the local frost line. Frost heave can lift shallow foundations and misalign trackers. A pile embedded above the frost line will move seasonally and damage the array.

The required embedment is governed by two inputs. It must exceed the structural embedment depth and the local frost depth. PVRack’s rule is frost depth plus 200 mm, and it lists frost depth of 0.8 to 1.5 m in continental temperate climates and 1.5 to 2.5 m in sub-arctic ones. Local frost maps and code govern.

Water Table Depth

Groundwater affects excavation, dewatering, and corrosion. A high water table near inverter pads or underground conduit can add significant cost. Seasonal fluctuations matter. The water table measured in October may be 5 feet lower than in April.

If the report shows groundwater within a few feet of planned excavation depths, the project needs a dewatering plan. This is often overlooked in early budgeting.

Corrosion Potential

Soil resistivity, pH, chloride content, and sulfate content determine how aggressively the soil attacks steel piles. Thresholds differ between guides. PVRack, for example, treats:

• Resistivity below 1,000 ohm-cm as very aggressive and above 10,000 ohm-cm as non-aggressive.
• pH below 5.5 as acidic and accelerating steel corrosion.
• Chloride above 100 mg/kg and sulfate above 200 mg/kg as corrosion accelerators.

Use the criteria your engineer, galvanizer, or lender specifies.

In corrosive environments, hot-dip galvanizing may not be enough. Thicker pile walls, epoxy coatings, or stainless-steel hardware may be required. These decisions add cost but prevent failures at year 15 or 20.

Foundation Type Decision Matrix

The soil type encountered during a geotechnical survey directly determines the pile type, embedment depth, and foundation cost. The matrix below is a qualitative guide to that relationship for common solar site conditions. It carries no depths or costs because those come from the engineer’s design.

This matrix is a starting point. The final design must be stamped by a licensed structural or geotechnical engineer who reviews the actual report data. For a deeper look at residential foundations, see our guide to residential ground-mount solar foundations and permits.

Standards and Codes That Govern Solar Geotechnical Work

Geotechnical investigations for solar projects in the United States typically follow ASTM standards and ASCE loading codes. The most relevant references are:

• ASTM D1586: Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils
• ASTM D3441: Standard Test Method for Mechanical Cone Penetration Testing of Soils
• ASTM G57: Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
• ASCE 7-22: Minimum Design Loads and Associated Criteria for Buildings and Other Structures, including wind and seismic loads on solar arrays
• IBC and local amendments: Adopted building codes that reference geotechnical reports for foundation design

International projects may use Eurocode 7, IS codes in India, or local equivalents. The investigation methods are similar. The reporting format and safety factors differ by jurisdiction. Always confirm which standard the AHJ or lender requires before commissioning the report.

Common Mistakes and How to Avoid Them

Even experienced developers make the same errors. Here are the most costly ones.

Ordering the Survey Too Late

A common workflow is sign land, design layout, then drill geotech. This is backwards. A geotechnical report before lease signing can prevent a layout redesign after capacity is committed. Drill a grid of boreholes across the site before committing to capacity.

Using One Borehole for a Large Site

A single boring cannot represent 100 acres. Soil conditions change across swales, hilltops, cut-and-fill transitions, and former agricultural fields. Two extra borings cost little compared with redesigning foundations for half the site.

Ignoring Seasonal Groundwater

Reports often state groundwater depth at the time of drilling. If construction occurs in a wet season, the actual water table may be higher. Plan dewatering contingencies for inverter pads, trenches, and underground conduit runs.

Designing for Worst-Case Everywhere

A 100-acre solar farm rarely has uniform soil. Use borehole and CPT data to create a soil zone map. Assign pile specifications by zone rather than using a single worst-case design for the entire site. Pile steel is a large part of the foundation package, so zoning can save material.

Skipping Pull-Out Testing on Borderline Soils

When SPT or CPT data shows marginal conditions, pull-out testing is not optional. It is the only way to verify that the actual pile will resist wind uplift. A failed production pull-out test after thousands of piles are driven is a project crisis.

When to Commission the Survey

Timing is the most important decision. The survey should be commissioned after site control is secured but before detailed engineering begins. The ideal sequence is:

1. Site selection and desktop review. Use USDA soil maps, geologic maps, and FEMA flood data to screen the site.
2. Topographic survey. Map surface contours, drainage, and access constraints.
3. Geotechnical survey. Drill borings, run CPT, and perform pull-out testing concurrent with or immediately after the topographic survey.
4. Foundation design. Use the geotech report to specify pile type, depth, and spacing.
5. EPC pricing. Share the report with bidders so they price foundation work to actual conditions.
6. Construction. Verify production piles with spot pull-out tests and document refusal conditions.

Commissioning the survey before the EPC contract is the best way to avoid foundation cost overruns. Contractors who find soil conditions that differ from the bid assumptions will submit change orders.

Conclusion

A geotechnical survey is not a formality. It is the data foundation for the entire ground-mount solar project. The report determines foundation type, pile depth, site preparation, and construction risk.

Three actions will improve your next project:

• Commission the survey before signing the EPC contract. The cost is small next to the change orders it can prevent.
• Match scope to project size. Small sites may need a few borings. A utility-scale farm needs systematic spacing, such as PVRack’s one per 2 to 5 acres, plus targeted borings at critical structures.
• Use the report to create soil zones. Designing by zone instead of worst-case can save pile steel.

Treat geotechnical work as an early input, not a late check. For large projects, pair the geotech report with our solar farm design guide and test foundation assumptions in SurgePV’s generation and financial tool. When you are ready to lay out the array, book a SurgePV demo.

Frequently Asked Questions

What is a geotechnical survey for a solar project?

It is a subsurface investigation that collects soil samples, measures bearing capacity, maps rock layers and groundwater, and tests soil chemistry. The results determine foundation type, pile embedment depth, and site preparation needs for ground-mount or utility-scale solar.

How much does a geotechnical survey cost for solar?

Nuance Energy states that a solar geotechnical report can cost $3,000 to $10,000, and PVRack quotes $15,000 to $80,000 for a 10 to 50 MWp project. Cost rises with acreage, number of borings, and laboratory testing, so get a written quote for your site.

When should you commission a geotechnical survey for solar?

Commission the survey after site control is secured but before detailed engineering or EPC contract signing. Late surveys force redesigns and change orders. Early surveys let designers optimize foundations to actual soil conditions.

How many boreholes are needed for a solar farm?

PVRack’s guide suggests one boring per 5 acres on uniform sites and one per 2 to 3 acres on variable sites, plus extra borings at inverter pads, substations, and access roads. Your geotechnical engineer sets the final count from the desktop study.

What happens if you skip the geotechnical survey?

You design foundations blind, so problems show up during construction. Common ones are early pile refusal on shallow rock, failed pull-out tests in soft layers, and groundwater that forces dewatering. Each one means redesign, delay, or change orders.

What is the difference between SPT and CPT for solar foundations?

SPT uses borehole drilling with a split-spoon sampler to measure soil resistance at discrete intervals. CPT pushes an instrumented cone continuously into the ground to produce a full soil profile. Most large sites use both methods together.

Can you use a geotechnical survey for roof-mounted solar?

No, roof-mounted systems need a structural assessment of the roof framing and load capacity instead. Geotechnical surveys apply to ground-mount, carport, tracker, and utility-scale solar projects where foundations interact with soil.