Ground Screw vs. Concrete Foundation: The Structural Case for Helical Piles in Utility-Scale Solar

The foundation is the part of a solar ground mount project that nobody photographs, but everybody depends on. It anchors 25 years of energy production against wind uplift, frost heave, soil settlement, and corrosion. The decision you make before the first pile breaks ground will define your project's timeline, total installed cost, and end-of-life flexibility.

Two systems dominate utility-scale ground mount racking today: the concrete anchor or cast footing, and the ground screw — also called a helical pile or screw pile. Each has a distinct structural logic, a different cost profile, and a specific soil niche. Getting this decision wrong costs money. Getting it right can shave weeks off your construction programme.

The Two Foundation Types Defined

Concrete anchor / cast footing: Concrete foundations for solar ground mount come in two main forms. Drilled or augered piers are formed by boring a hole into the soil, inserting a steel sleeve or cage, and filling it with cast-in-place concrete. Precast concrete footings are manufactured offsite and set into position, typically for smaller arrays. Both methods rely on the mass and compressive strength of concrete to resist lateral loads, overturning moments, and vertical pull-out forces. Once cured, a concrete foundation is a permanent structure.

Ground screw / helical pile: A ground screw (or helical pile) is a steel shaft — typically 60mm to 114mm in diameter — with one or more helical plates welded along its length. It is installed by rotating it into the ground using a hydraulic driver attached to an excavator or dedicated screw pile rig. The helical plates cut into the soil rather than displacing it, engaging the undisturbed soil matrix around the helix. Installation generates real-time torque data, which engineers use to verify bearing capacity without the need for load testing in most cases.

Both systems connect to the same racking hardware above grade. The structural difference begins — and largely ends — below the surface.

Installation Speed: The 28-Day Problem

At utility scale, time is one of the most expensive line items in the project budget. Concrete foundations introduce an unavoidable delay: full structural curing takes 28 days under standard conditions. On a 10MW ground mount site requiring several hundred poured footings, the concrete pour schedule must be completed well ahead of racking installation. Wet weather, cold temperatures, or supply chain disruption with ready-mix can push this window further.

Ground screws eliminate curing time entirely. A two-person crew operating a hydraulic driver can install 80 to 120 screws per day depending on soil conditions and screw diameter. Racking installation can begin on the same day as the first screws go in, on the opposite end of the site. This concurrent workflow compresses the overall construction programme by two to four weeks on a mid-scale project — a meaningful schedule advantage in markets with tight grid connection deadlines or seasonal construction windows, common across Latin America and Central Asia.

Structural Performance by Soil Type

Pull-out capacity — the force required to extract the foundation vertically — is the critical design parameter for solar ground mount racking. Wind uplift on a row of panels creates significant overturning moments that translate into vertical tension at the upwind foundation.

Ground screws perform best in:

• Dense loam, compacted silty clay, and firm sandy clay — the helix engages well and torque correlates reliably with capacity
• Gravelly soils with moderate fines content — good interlocking resistance
• Hard ground with rocky substrate — specialist rock-socket or tri-flighted screws with hardened tips can penetrate surfaces that would crack a concrete form mold

Concrete performs best in:

• Very soft clay or saturated silt — where torque resistance is low and pull-out capacity via helix engagement is insufficient
• Expansive (high-plasticity) clay — where soil movement around a screw can compromise bearing over time
• High water table sites — where a large-diameter augered pier with deeper embedment provides reliable bearing below the zone of saturation

A site with SPT N-values consistently above 10 across the embedment zone is generally suitable for ground screws. When N-values fall below 5 in the top two meters, a geotechnical engineer should review the design before specifying screws.

In marginal soils, ground screws are not automatically disqualified. A larger-diameter shaft, deeper embedment, or a double-helix configuration can recover capacity. But the cost of that upgrade must be factored into the comparison with concrete.

Reversibility and Land Restoration

Concrete footings are permanent. Removing a drilled pier at end-of-life requires excavation, breaking, and disposal — adding significant decommissioning cost and leaving disturbed soil that may require remediation. For agricultural landowners in Latin America, Southeast Asia, or Eastern Europe who are leasing land for a 25-year solar term, this is a real concern. They want assurance that the land can return to productive use after the lease expires.

Ground screws are fully reversible. The same hydraulic driver used for installation can reverse direction to extract the screw, leaving a hole no larger than the shaft diameter. The site can be restored to agricultural condition within days. This reversibility is a core enabler of agrivoltaic projects — installations where solar generation and active farming coexist on the same land area. Solaracks' agrivoltaic ground mount systems are designed around helical pile foundations precisely because land restoration commitments are a standard requirement of agrivoltaic lease agreements in Europe, Brazil, and Southeast Asia.

Cost Analysis at 1MW Scale

Foundation costs typically represent 20% to 40% of total mounting system cost at utility scale. The breakdown differs significantly between the two systems:

At a 1MW ground mount site with approximately 300 foundation points, ground screws typically reduce total installed foundation cost by 15% to 30% compared to cast-in-place concrete when labor and schedule costs are included in the calculation. The material premium on the steel screws is generally recovered through lower labor hours and the elimination of concrete logistics on remote or difficult-access sites — a frequent condition in rural Brazil, Colombia, and Pakistan.

Corrosion at the Soil Line: The Detail Most Procurement Specs Miss

The soil-to-air interface is the most corrosive zone on a ground screw installation. This is where oxygen availability, moisture cycling, and electrochemical gradients converge. A ground screw with inadequate coating at this interface will experience preferential corrosion at the transition point — the exact zone that carries the highest bending moment.

Minimum protection requirements for ground screws in standard agricultural soil (corrosivity category C2/C3): heavy-gauge hot-dip galvanizing (HDG) to ISO 1461, minimum 85µm coating thickness. In soils with elevated chloride content, high organic acid levels, or industrial contamination (C4/C5 equivalent below grade), ZAM-coated (zinc-aluminum-magnesium alloy) steel or stainless steel upper shafts are the appropriate specification. Cathodic protection via sacrificial magnesium anodes can extend service life in aggressive underground environments and is worth specifying on projects where soil resistivity measurements indicate high corrosivity.

This topic was covered in depth in the Solaracks #SolarMaterialFacts corrosion series — including the Article 5 deep-dive on coastal and high-corrosivity environments, which addresses coating selection for both above- and below-grade solar mounting components. The same material logic applies underground.

Geotechnical Input Requirements: Why a Soil Report Is Not Optional

The single most common foundation design error on solar ground mount projects is specifying a foundation system before receiving a geotechnical investigation report. Both concrete and ground screws require soil data to be designed correctly. Without it, the engineer is guessing — and on a utility-scale project, a guess that requires redesign mid-construction is expensive.

Parameters that matter:

• SPT N-value (Standard Penetration Test): the primary proxy for soil density and bearing capacity. Collected at regular depth intervals. N-values below 5 signal soft, potentially problematic soil; N-values above 30 indicate dense material that drives well.
• Soil classification (USCS or AASTHO): determines whether the site contains expansive clay, organic material, or loose fill — each of which influences foundation type selection.
• Frost depth: in cold-climate markets (Central Asia, Poland, highland Chile, northern Mexico), foundation embedment must extend below the frost line to prevent frost heave. Ground screws that terminate in the active frost zone will cycle vertically with freeze-thaw, damaging the racking connection above.
• Soil resistivity and pH: essential for corrosion-protective coating specification on ground screws. A soil investigation without resistivity data is incomplete for this application.

Solaracks provides SAP2000 structural analysis and foundation design review as part of its project engineering service, and requires geotechnical data — at minimum, a borehole log with SPT N-values to 3m depth — before issuing a certified foundation design under JIS C 8955:2017 or ASCE 7-16. This protects both the EPC contractor and the asset owner.

Common Procurement Mistakes

1. Specifying ground screws without a soil survey. Ground screws are fast and cost-effective in the right soil, but they are not a universal solution. Procuring screws without knowing the N-values and soil classification of the site risks arriving on site with hardware that cannot achieve the required torque — and therefore cannot meet the design pull-out capacity. This is a programme-stopping event.

2. Ignoring frost heave in cold-climate projects. Projects in Kazakhstan, highland Mexico, the Andes, or Eastern Europe that specify ground screws must verify embedment depth extends below the local frost line. The ISO 14690 and national building codes for each market define this depth. A screw that terminates at 1.2m in a climate with a 1.5m frost depth will fail structurally within the first winter cycle.

3. Undersized screw diameter for the design wind load. Screw diameter and shaft wall thickness determine the bending stiffness of the foundation column. In high-wind zones — coastal Latin America, open plains in Central Asia — an undersized shaft will deflect under the design load, inducing stress concentrations at the soil-to-air interface. The structural design must specify shaft diameter, steel grade, and minimum wall thickness as part of the procurement document, not leave these to the lowest-cost supplier.

4. Treating concrete as the safe default. Concrete is not inherently safer than ground screws. A poorly mixed pour in a remote location with variable aggregate quality and no independent testing is structurally less reliable than a factory-manufactured HDG ground screw driven to verified installation torque. On EPC projects in markets with less rigorous construction supervision, a ground screw with real-time torque logging can actually provide better documented proof of bearing capacity than a concrete pour with no slump test records.

Matching Foundation to Site: The Decision Framework

The table below summarizes the primary decision logic:

How Solaracks Approaches Foundation Design

Solaracks' ground mount racking product range covers both foundation systems. Fixed-tilt and tracker-ready structures are available with ground screw interfaces or concrete sleeve anchor connections, allowing the same above-grade racking hardware to be specified for either foundation type. This means EPCs are not locked into a foundation decision at the racking procurement stage.

For projects that require engineering sign-off, Solaracks delivers quote and structural design within 24 hours of receiving site parameters and a geotechnical summary. Foundation design is produced in SAP2000 and certified to JIS C 8955:2017 or ASCE 7-16 depending on the project's local standard. Procurement teams in Brazil, Chile, Mexico, Colombia, the Philippines, Vietnam, and Pakistan regularly receive certified load calculations with the commercial quotation — at no additional charge.

If you are at the pre-tender stage and need to evaluate foundation options for a specific site, contact Solaracks with your geotechnical borehole data and wind speed parameters. The engineering team will provide a foundation recommendation and preliminary design in one business day.

This article is part of the Solaracks #BuiltForTheField series — a product deep-dive into the engineering decisions that determine whether a solar installation performs for 25 years or creates problems from year one.