Helical piles vs. driven piles vs. ground screws for solar projects

Ground-mount solar lives and dies at the ground line. The structural steel above grade is visible in every rendering. The foundation system is visible in every overrun.

Driven piles, helical piles, and ground screws are not interchangeable skus. They are different installation mechanics, different soil matches, and different risk registers. Ballasted and concrete systems belong in the same conversation when rock or pullout makes deep foundations unattractive, but this article focuses on the three steel systems estimators are asked to “just pick.”

This comparison is conceptual. It is not a site-specific recommendation and does not replace geotechnical investigation or sealed professional design.

Installation

Driven piles are installed with impact or vibratory hammers. The section — often H-pile or pipe — is advanced to a target depth or to refusal. Installation is fast in compatible soils and noisy. Alignment and damage at the head are practical issues.

Helical piles are rotated into the ground. Capacity in many methods is related to installation torque and to plate geometry in the bearing strata. Installation is typically quieter and can be more equipment-specialized.

Ground screws are also rotated, with a screw geometry rather than a helical plate stack. They occupy a similar construction family to helicals but are not the same product: thread form, diameter, and manufacturer methods differ. Do not paste a helical torque correlation onto a screw without a method that supports it.

Equipment

Driven work wants a pile crew and a hammer matched to the section. Helical and screw work want torque motors with enough reaction and reach. Sites with tight access, slope, or low overhead can eliminate a system before capacity does.

Equipment availability is a commercial input. A theoretically cheaper screw that requires a machine your contractors do not have is not cheaper in the bid window.

Soil suitability

Driven piles like soils they can penetrate without early refusal and without collapsing a thin section. Dense gravels, cobbles, and shallow rock are hostile.

Helicals want strata in which plates can advance and bear. Soft clays, peat, and uncontrolled fill are difficult for everyone; helicals are not magic.

Screws are often discussed for weaker near-surface soils and for speed, but they still need a competent installation specification. Marketing soil ranges are not a borehole log.

Axial capacity

Compression and tension should be checked with the method that matches the system: driving formulas and static methods for driven piles; torque and plate bearing methods for helicals; manufacturer and independent methods for screws. Mixing methods across systems in one table without stating the method is how apples-to-oranges charts get into board decks.

Capacity is only as good as the soil interpretation and the factor of safety or resistance factor required by the governing code.

Uplift

Uplift is frequently governing on solar. Driven friction, helical plates, and screw threads mobilize uplift differently. Frost adfreeze can add demand that a “wind-only” spreadsheet never saw.

If the geotechnical report is silent on frost and you are in a frost region, you do not have an uplift design. You have a gap.

Lateral behavior

Array foundations see shear and moment from wind and, for trackers, from operational loads. Embedment, section stiffness, and head fixity matter. A short screw that is adequate in tension may be inadequate in deflection.

Lateral demand must come from the structural model. A typical “2 kN per pile” cell is not a model.

Refusal risk

Refusal is the signature risk of driven piles on heterogeneous sites. It is also a risk for screws and helicals that cannot advance. Pre-drilling, point reinforcement, relocation, or a system change are the usual responses — each with cost and schedule.

Unknown bedrock depth is not a small uncertainty. It is a fork in the foundation strategy.

Rock and bedrock

Shallow rock can eliminate driven piles and screws, push toward rock sockets or anchors, or move the project to ballast where the surface allows. Deep rock can be an end-bearing gift for driven piles and a depth tax for everything else.

Do not average rock depth across a site that has a ridge.

Corrosion

All three systems are steel in soil. Coating, sacrificial thickness, and stray current matter. Driving can damage coating; rotary installation has its own abrasion. Galvanizing specification belongs in both the engineering note and the steel cost build-up.

Constructability and schedule

Driven spreads can cover large acreage quickly in friendly soils. Rotary systems can be more selective and sometimes better on noise-sensitive or access-limited sites. Weather, water table, and testing (PDA, torque records, proof tests) change the calendar.

Schedule risk is part of foundation selection. A system that cannot be tested to the engineer’s satisfaction will not close.

Cost implications

Compare at least:

• steel mass and section
• coating
• installation hours and equipment
• testing
• expected change orders from refusal or redesign
• lead time

A low unit price with a high refusal probability is not a low expected cost.

How to use a comparison

Use a structured evaluation to ask better questions of the geotechnical engineer and the installer. Do not use it as a ranked shopping list.

Infrenta GeoLab can support that structured view as decision-support. Explore GeoLab, GeoLab documentation, and Foundation evaluation. The engineer of record still owns the choice.