The Influence of Soil Type on Ground-mounted Solar Array Foundations

The Critical Role of Soil Properties in Ground-Mounted Solar Stability

Ground-mounted solar arrays are increasingly deployed across diverse terrains—from arid deserts to fertile agricultural land. The long-term performance and structural integrity of these installations hinge on a fundamental yet often underappreciated variable: the soil beneath them. Soil type governs load-bearing capacity, drainage behavior, and volumetric stability, all of which directly influence foundation design. A thorough understanding of geotechnical parameters is not merely a technical detail but a prerequisite for cost-effective, durable solar energy systems. This article expands on the relationship between soil characteristics and foundation selection, providing actionable insights for engineers, developers, and project managers.

Understanding Soil Classification and Key Geotechnical Parameters

Soils are classified using systems such as the Unified Soil Classification System (USCS) or the AASHTO system, which group soils by particle size, plasticity, and organic content. For solar array foundations, the most relevant categories include gravels (GW, GP), sands (SW, SP), silts (ML, MH), and clays (CL, CH). Each behaves differently under load and environmental stress.

Critical geotechnical parameters for foundation design include:

• Bearing capacity: The maximum pressure soil can support without shear failure or excessive settlement. Coarse-grained soils like gravel and sand typically have high bearing capacities (up to 300 kPa or more), while soft clays may offer only 50–100 kPa.
• Settlement potential: Immediate (elastic) and long-term (consolidation) settlement. Clays are prone to significant consolidation settlement over years, which can tilt solar arrays and misalign tracking systems.
• Shrink-swell potential: Clays expand when wet and shrink when dry, exerting upward forces on foundations. The plasticity index (PI) is a key indicator: high-PI clays (CH) can cause severe structural damage if not addressed.
• Frost susceptibility: Soils with fine particles (silts and some clays) can heave when frozen, lifting shallow foundations. Frost depth must be considered in cold climates.
• Corrosivity: Low soil resistivity (indicative of high moisture and dissolved salts) can accelerate corrosion of steel piles and grounding systems. Soil pH and resistivity testing are essential for foundation longevity.

These parameters are determined through field and laboratory testing during a geotechnical investigation, which is the foundation of sound foundation design. The American Society of Civil Engineers (ASCE) provides guidelines (e.g., ASCE 7) for minimum geotechnical requirements in solar farm projects.

Geotechnical Investigation: The First Step to a Stable Solar Array

A proper geotechnical investigation for a ground-mounted solar array follows a systematic process. The scope typically includes:

1. Desk study: Review existing geological maps, soil surveys (e.g., USDA Web Soil Survey), and nearby construction records to identify potential hazards.
2. Field exploration: Test pits or boreholes at intervals of 100–300 feet across the site, depending on soil variability. Standard penetration tests (SPT) and cone penetration tests (CPT) provide in-situ strength and density data.
3. Laboratory testing: Grain size distribution, Atterberg limits, moisture content, compaction characteristics (Proctor), and triaxial compression tests for shear strength.
4. Report and recommendations: A geotechnical report summarizes soil profiles, bearing capacities, and foundation type recommendations. It also addresses groundwater table depth, which can affect pile embedment and concrete durability.

Investing in a thorough investigation—typically costing $5,000–$20,000 for a 10 MW solar farm—can prevent millions in remediation costs if foundations fail. The National Renewable Energy Laboratory (NREL) emphasizes that "geotechnical issues are among the top five causes of solar project delays and cost overruns." (NREL, 2020)

Foundation Options Matched to Soil Type

Ground-mounted solar array foundations fall into two broad categories: driven/drilled foundations and surface-mounted (ballasted) foundations. The choice depends on soil strength, depth to competent strata, and site grading. Below we examine common foundation types and their suitability for different soil conditions.

Driven Piles (Steel H-piles or Pipe Piles)

Driven piles are the most common foundation for large-scale solar farms because of speed and cost efficiency. They are suitable for sandy, gravelly, and cohesive soils with adequate density or consistency. In loose sands or soft clays, longer piles are needed to mobilize skin friction and end bearing. Piles can be driven to depths of 5–30 feet, with capacities typically 30–100 kN per pile. Advantages: rapid installation, minimal excavation, easy removal during decommissioning. Limitations: hard driving in gravelly soils may damage pile tips; in high-plasticity clays, pile uplift due to frost heave or shrink-swell must be analyzed.

Helical Piles (Screw Anchors)

Helical piles are ideal for soils with low bearing capacity or high variability. They consist of a steel shaft with helical plates that are screwed into the ground, transferring load through bearing. They excel in clayey and silty soils where driven piles may experience excessive settlement. Helical piles can be installed with smaller equipment, making them suitable for remote or tight-access sites. However, they are more expensive per unit than driven piles and may not be economical for very large arrays. (Geotechnical.com)

Concrete Spread Footings (Pad and Strip Foundations)

Spread footings are used when soil conditions require a larger bearing area to limit settlement. They are common on softer clays or loose sands where pile solutions are uneconomical. A concrete pad (typically 2×2 ft to 4×4 ft per post) distributes loads over a wider area. Strip footings are continuous strips supporting rows of panels. These foundations involve significant excavation and concrete curing time, increasing installation time and cost. They also require careful drainage to prevent water accumulation and frost heave. For sites with expansive clays, a stiffened raft (slab with ribs) may be used to resist differential movement.

Ballasted Mounts (Surface-mounted Systems)

Ballasted mounts use concrete blocks or piers placed directly on the ground without deep foundations. They are limited to sites with high bearing capacity (e.g., competent gravel or dense sand) and no significant slope. They avoid underground disturbance, making them appealing for sites with environmental constraints or shallow utilities. However, they require a large footprint and careful weight calculations to resist overturning from wind loads. Ballasted systems are typically used for small to medium-sized arrays (<1 MW) on well-drained, stable soils.

Case Studies: Soil-Driven Foundation Selection

Clay Site in the Southeastern United States

A 50 MW solar farm in Georgia was proposed on land underlain by high-plasticity clay (CH) with a PI of 45. Initial pile test drives showed refusal at 8–10 feet in a desiccated crust, but below that the clay was soft. The geotechnical engineer recommended helical piles with a 10,000 lb (44.5 kN) design load per pile. Helical piles were chosen because they could be installed without damaging the crust and could reach deeper stiff clay layers. Without proper geotechnical input, driven H-piles would have resulted in excessive settlement and potential tilting of the trackers—a failure mode seen in early solar projects. (ASCE News, 2021)

Sandy Site in the Middle East

A 200 MW solar plant in Saudi Arabia was built on windblown sand (SP) with very low cohesion. Bearing capacity was only 80 kPa. Driven piles would have required excessive lengths (30+ ft) to generate adequate friction. Instead, the team used concrete spread footings with a geogrid reinforcement layer to improve load distribution. The footings were designed as shallow pads 3 ft deep, with a gravel blanket to prevent erosion from wind. This approach saved 35% in foundation costs compared to deep piles.

Cost Implications and Performance Considerations

Foundation costs typically represent 5–15% of total solar project costs, but a poor foundation choice can multiply O&M expenses. The following table summarizes relative cost and suitability:

Foundation Type	Relative Cost (per kW)	Best Soil Conditions	Vulnerabilities
Driven H-pile	Low	Sand, gravel, stiff clay	Hard driving in gravel; uplift in expansive clay
Helical pile	Medium	Soft clay, silt, variable soil	Higher unit cost; slower installation
Concrete spread footing	Medium to High	All soils (with design adjustments)	Requires excavation; longer curing; frost heave risk
Ballasted mount	Low (material) but high land use	Dense sand, gravel	Limited to stable, level ground

Long-term performance monitoring is essential. Settlement gauges and tilt sensors can detect early movement. In one 2019 study of 12 solar farms in the US, 25% showed measurable foundation movement within five years, all on clay soils without proper geotechnical treatment. (Solar Energy Journal, 2019)

Environmental and Long-Term Factors

Erosion and Stormwater Management

Soil erosion can undermine foundations, exposing pile caps or causing scouring. On slopes or in high-rainfall areas, ground-mounted arrays require erosion control measures such as grass cover, riprap, or subsurface drainage. The type of soil influences the erosion rate: silty soils are most erodible, while gravelly soils resist erosion but may allow water to flow underneath panels.

Frost Depth and Seasonal Variations

In cold regions, foundations must extend below the frost line to avoid frost heave. The frost depth varies by climate—from 0 feet in the tropics to 10+ feet in northern Canada. For example, on silty soils (ML), frost heave pressures can exceed 200 kPa, easily lifting shallow footings. Helical piles or driven piles are preferred in these conditions because they can be installed to depths greater than frost penetration. Insulation layers (e.g., extruded polystyrene) around footings can also mitigate heave.

Seismic Considerations

Soil type amplifies or dampens seismic waves. Soft soils (silt, loose sand) can amplify ground motion, increasing demand on foundations. Liquefaction is a risk in loose, saturated sands during earthquakes. In seismic regions, geotechnical reports must include site-specific response spectra. Deep foundations that reach competent strata are generally safer than shallow footings. The International Building Code (IBC) provides seismic design categories based on soil profiles (Site Class A through F).

Conclusion: Soil-Informed Design as a Competitive Advantage

The influence of soil type on ground-mounted solar array foundations extends far beyond a simple list of dos and don'ts. It is a multidisciplinary challenge that integrates geotechnical engineering, structural design, hydrology, and project economics. Developers who invest early in comprehensive soil investigations and match foundation solutions to the unique conditions of each site will achieve lower levelized cost of energy (LCOE) through reduced failures, fewer O&M interventions, and longer system life. As solar energy expands into more marginal land, mastering soil-foundation interaction is not optional—it is the bedrock of success.