Photovoltaic spiral piles: efficient adaptation and technical practice of photovoltaic support foundation

In photovoltaic project foundation engineering, spiral ground piles, with their advantages of "no excavation required, quick installation, and strong load-bearing capacity," have become a key alternative to traditional concrete foundations. They are particularly well-suited for photovoltaic applications in complex terrain and ecologically sensitive environments. As the "underground support" for photovoltaic racks, their performance directly impacts rack stability and the project's lifecycle cost. This article focuses on the core characteristics of spiral ground piles, detailing their performance indicators, material selection, scenario adaptation solutions, and key construction quality control points, providing a practical guide for photovoltaic project foundation design.

I. Definition and Core Value of Photovoltaic Spiral Ground Piles

Photovoltaic spiral ground piles are steel (or composite) piles with spiral blades. They are installed underground through a rotary press-fit method, eliminating the need for excavation and maintenance. They are primarily used to secure photovoltaic rack columns and transfer the loads of the racks and panels (deadweight, wind load, and snow load) to the stabilized underground soil. Its core value in photovoltaic projects lies in three key aspects:

1. Cost Reduction and Efficiency Improvement: Shortened Construction Time and Reduced Consumables

Traditional concrete foundations require excavation, reinforcement, pouring, and curing (typically 7-14 days), costing approximately 300-500 yuan per foundation. However, spiral piles require no curing, and installation takes only 5-15 minutes per pile, resulting in a 40%-60% reduction in costs compared to concrete foundations. For example, in a 100MW ground-mounted photovoltaic project, the use of spiral piles can reduce foundation construction time by over 30 days, saving over 2 million yuan in labor and consumables.

2. Eco-Friendliness: Reduced Land Damage

Spiral piles replace excavation with "rotational implantation," resulting in less than 5% damage to surface vegetation. After the project is completed, the piles can be removed and recycled, reducing land restoration costs by 80%. They are particularly suitable for ecologically sensitive areas (such as grassland and farmland photovoltaic projects). For example, in a pastoral-solar hybrid project in Inner Mongolia, the use of spiral ground piles reduced the grass recovery period from two years with concrete foundations to three months, balancing power generation and livestock production.

3. Strong Adaptability: Coping with Complex Topography and Geology

Spiral ground piles are adaptable to a variety of geologies, including sandy soils, loam, and weathered rock. There's no need to adjust the foundation type based on the geology (for example, a concrete foundation in sandy soil requires deeper burial depth, while spiral ground piles only require adjusting the blade parameters). Furthermore, in mountainous photovoltaic projects with slopes ≤25°, the pile inclination angle (≥75° with the ground) can be adjusted to suit the terrain, avoiding large-scale earthwork and leveling.

II. Core Performance Indicators and Design Key Points for PV Spiral Ground Piles
PV spiral ground piles must meet the requirements of "reliable load-bearing, corrosion resistance, durability, and easy installation." Their core performance indicators and design parameters are tailored to the load requirements and geological conditions of the photovoltaic project. (I) Core Performance Indicators

Bearing Capacity: Must meet both pullout and compressive strength requirements. According to GB/T 38946-2020, "Specifications for the Design of Photovoltaic Mounting Structures," the pullout resistance of PV spiral piles should be ≥15kN (applicable to ordinary ground-based PV systems), and ≥25kN in high-wind-load areas (such as coastal areas). Compressive strength should be ≥30kN to prevent the support columns from sinking. For example, a PV project in the Gobi Desert of Xinjiang was designed with a pullout resistance of 22kN for the spiral piles, successfully withstanding force 12 gusts (wind load of 1.8kN/m²).

Corrosion Resistance: Must be compatible with the project's environmental corrosion level. For example, farmland PV systems (soil pH 5.5-8.5) must meet C3 corrosion resistance, while coastal PV systems (in salt spray environments) must meet C5 corrosion resistance. The service life should be consistent with the PV system (≥25 years). Installation Efficiency: Single pile installation time is ≤15 minutes (including positioning, placement, and testing). Compatible with small hydraulic pile drivers (weighing ≤1.5 tons), this design facilitates equipment delivery to locations with difficult transportation, such as mountainous areas and deserts.
(II) Key Design Parameters
Pile Structural Parameters:
Pile Diameter: Designed based on load requirements. Diameters of Φ89mm and Φ114mm are commonly used for ordinary ground-mounted photovoltaic systems, while Φ140mm is used for high-load scenarios (such as dual-axis tracking mounts). Pile wall thickness should be 3-5mm, ensuring a "strength-weight balance" (e.g., a Φ114mm pile with a 4mm wall thickness and a tensile strength of ≥345MPa). Spiral Blades: The blade diameter is typically 2-3 times the pile diameter (e.g., a Φ114mm pile with a Φ250mm blade). The blades are 2-3, spaced 150-200mm apart. The blades are 5-8mm thick, and the edges are "bladed" (30° angle) to reduce implantation resistance.

Pile Length: The design is based on the thickness of the geological soil layer. In sandy soils, the pile length is typically 2.0-2.5m, and in clay soils, it can be shortened to 1.5-2.0m. Ensure that the blades are embedded in stable underground soil (e.g., 0.5m below the groundwater level to avoid buoyancy).
Connection Design:
The pile top and support column utilize a flange connection. The flange thickness is ≥10mm, with 4-6 bolt holes (accommodating M16-M20 bolts) to ensure uniform load transfer.
The flange and pile body must be welded on both sides, with a weld seam height of ≥8mm to prevent load fracture caused by weak weld points.

III. Material Selection for PV Spiral Ground Piles: Balancing Environmental Adaptability and Cost
The corrosion environments and load requirements of different PV scenarios vary significantly, requiring specific selection of spiral ground pile materials. Core materials fall into three categories:

1. Hot-Dip Galvanized Carbon Steel Spiral Ground Piles (Mainstream Choice)

Material Characteristics: Made of Q355B carbon steel with a hot-dip galvanized surface (zinc layer thickness ≥85μm), it offers a tensile strength of 345-460MPa and a pullout strength of 15-25kN. Its corrosion resistance meets Class C3-C4 environments (e.g., inland ground photovoltaic and farmland photovoltaic). Cost and Lifespan: The cost per pile is 80-150 yuan (Ø89mm-Ø140mm), with a service life of 20-25 years. These piles offer the highest cost-effectiveness and account for over 70% of the PV spiral ground pile market share.

Case Study: A 100MW ground-mounted PV project in Shandong Province employed Ø114mm hot-dip galvanized spiral ground piles (zinc coating thickness 100μm). Five years after installation, testing showed no significant corrosion on the piles, and the pull-out strength degradation rate was only 3%, meeting design requirements.

2. Zinc-aluminum-magnesium-coated steel spiral ground piles (suitable for highly corrosive environments)

Material Characteristics: The base material is Q460 high-strength steel, coated with a zinc-aluminum-magnesium coating (60% zinc, 30% aluminum, 10% magnesium, coating thickness ≥ 60μm). The piles offer five times the salt spray corrosion resistance of hot-dip galvanizing, a pull-out strength of 25-35kN, and are suitable for Class C4-C5 environments (such as coastal PV and industrial PV). Cost and Lifespan: The cost per pile is 120-200 yuan, with a service life of 25-30 years. While this cost is higher, it reduces future maintenance and replacement costs.

Case Study: A coastal photovoltaic power station in Fujian Province used 140mm Φ zinc-aluminum-magnesium spiral ground piles. After three years of use in a salt spray environment with a chloride ion concentration of 50mg/m³, the coating corrosion rate was only 0.002mm/year, far lower than the 0.01mm/year of hot-dip galvanized steel.

3. Stainless Steel Spiral Ground Piles (Extreme Corrosion Scenarios)

Material Characteristics: Made of 316L stainless steel containing 16% chromium, 10% nickel, and 2% molybdenum, it is resistant to acid, alkali, and salt spray corrosion, with a pull-out strength of 20-30kN, making it suitable for extreme environments (such as photovoltaic systems in saline-alkali land and high-temperature, high-humidity photovoltaic systems).

Cost and Lifespan: The cost per pile is 300-500 yuan, with a service life of over 30 years. These are only used in limited applications (such as saline-alkali projects in Hainan). Case Study: A photovoltaic project in saline-alkali land in Hainan Province employed 114mm 316L stainless steel spiral ground piles. The soil pH ranged from 8.5 to 9.0. After four years of use, the piles showed no pitting or crevice corrosion, and their pullout strength retention rate was 98%.

IV. Spiral Ground Pile Adaptation Solutions for Different PV Scenarios

The topography, geology, and environmental differences of PV sites necessitate customized spiral ground pile designs. The following are adaptation strategies for four typical scenarios:

1. Plain Ground PV: Efficient Standardization

Scenarios: Gentle terrain (slope ≤ 5°), loam and sandy soils, and moderate load requirements (wind loads of 1.2-1.8 kN/m²). Adaptation Solution:
Select Φ89mm-Φ114mm hot-dip galvanized spiral piles, with a length of 1.8-2.2m and two blades (Φ200mm-Φ250mm).
Using a "batch positioning + mechanical placement" method, each hydraulic pile driver (15kW) can install an average of 300-400 piles per day. A single megawatt requires approximately 800-1000 piles, with a foundation cost of approximately 80,000-120,000 yuan per megawatt.

Case Study: A 200MW plain photovoltaic project in Henan Province used Φ114mm hot-dip galvanized piles. Twenty pile drivers completed the foundation in 15 days, saving 3.5 million yuan compared to a concrete foundation and shortening the construction period by 20 days.
2. Mountain PV: Terrain Adaptation and Slip Resistance

Scenario Characteristics: Complex terrain (slope 5°-25°), primarily weathered rock and gravel soil, requiring resistance to lateral landslide forces, making access to the installation site difficult.

Adaptation Solution:

Pile bodies utilize Q460 steel spiral ground piles (Ø114mm-Ø140mm), with a length of 2.2-2.8m and three blades (Ø250mm-Ø300mm) to enhance pullout and slip resistance.

A small crawler pile driver (width ≤1.2m) is used to accommodate narrow mountain construction channels. The piles are installed at a 90°±3° angle to the ground to avoid uneven loading caused by tilting.

For areas with heavy gravel, the pile tips are treated with a wear-resistant alloy to reduce blade wear during installation. Case Study: A 50MW mountain photovoltaic project in Yunnan Province, with a slope of 15°-20°, employed 140mm 3-blade spiral piles. Each pile demonstrated a sliding resistance of ≥30kN. After installation, the project experienced two heavy rainstorms (daily rainfall of 150mm) without any pile displacement.

3. Desert Photovoltaic Project: Wind and Sand Fixation and Abrasion Resistance

Scenarios: The geology is characterized by fine sand and silt, resulting in high wind loads (2.0-2.5kN/m²) and severe wind and sand abrasion. Protecting exposed pile surfaces from erosion by wind and sand is crucial. Adaptation Solution:
Pile bodies are constructed of Φ114mm-Φ140mm zinc-aluminum-magnesium-coated steel, with a length of 2.5-3.0m (penetrating at least 1.5m into the stabilized sand layer). The blade spacing is reduced to 150mm to enhance adhesion with the sand.
Fiberglass protective sleeves (30cm high) are installed on the exposed top of the pile to prevent wind and sand from abrading the coating.
A combination of spiral piles and straw grids is used for sand fixation. The piles are spaced 3m x 3m apart, and the straw grids form a windbreak system to reduce the impact of sand dune movement on the piles.
Case Study: A 100MW desert photovoltaic project in Inner Mongolia used Φ140mm zinc-aluminum-magnesium spiral piles, 2.8m long. After installation, annual wind and sand abrasion was ≤0.005mm, and the pullout strength of a single pile remained at 28kN, meeting design requirements. 4. Farmland Photovoltaic (Agricultural Photovoltaic Complementary): Ecologically Reversible

Scenario Characteristics: Soil structure must be protected; the land can be returned to cultivation after the project is completed. The geology is primarily composed of a tillage layer (0.3-0.5m thick) with a clay layer beneath.

Suitable Solution:

Use Φ89mm hot-dip galvanized spiral piles, 1.5-1.8m long (to avoid penetrating the tillage layer), with two Φ200mm blades to minimize damage to the tillage layer.

Use "low-pressure implantation" (pile driver pressure ≤ 50kN) to avoid soil compaction that affects crop growth.

The piles are designed to be removable, with removable flanges. Hydraulic equipment is used to remove the piles after the project is completed, resulting in a soil recovery rate exceeding 95%. Case Study: A 50MW agri-photovoltaic hybrid project in Jiangsu Province uses Φ89mm removable spiral ground piles to plant crops such as wheat and rapeseed. The piles reduce soil compaction by ≤5%, and crop yields per mu are only 3% lower than those on traditional farmland, achieving a "win-win" for both power generation and planting.

V. Key Points for Construction and Quality Control of Spiral Ground Piles

The installation quality of spiral ground piles directly affects the stability of the photovoltaic mounting system and requires strict control in both the construction process and testing methods:

1. Construction Process (Standardized Steps)

Step 1: Geological Survey and Positioning: Conduct a pre-construction geological survey of the construction area to determine the pile length and blade parameters. Use an RTK locator (with an accuracy of ±5cm) to mark the pile locations, ensuring that the spacing is consistent with the design (typically 3m×3m-4m×4m).​
Step 2: Equipment Commissioning and Installation: Adjust the pile driver speed (20-30 r/min) and pressure (30-80 kN), adjusting parameters based on the geology (high speed and low pressure for sandy soils, low speed and high pressure for clayey soils). Align the pile with the positioning point and rotate it to the designed depth (the top of the pile should be 10-15 cm above the ground to facilitate connection with the support).

Step 3: Pile Alignment and Securing: After installation, use a spirit level to check the verticality of the pile (deviation ≤ 1°). If tilted, remove and reinstall. Tighten the flange bolts using a torque wrench (accuracy ±5 N·m), using the torque value set according to the bolt specifications (80-100 N·m for M16 bolts).​
2. Quality Inspection (Key Indicators)
Pulling Strength Test: One pile out of every 500 will be randomly inspected using a hydraulic puller. The pulling strength must meet at least 1.2 times the design value (e.g., if the design pulling strength is 20kN, the test value must be ≥ 24kN). If the test fails, the inspection rate will be increased.
Coating Integrity Test: The zinc/zinc-aluminum-magnesium layer thickness is tested using a coating thickness gauge (three points per pile). If the thickness is insufficient, recoat the coating. Coating adhesion is tested using the cross-hatch method (adhesion grade ≤ Grade 1) to prevent coating detachment.
Verticality and Depth Test: Verticality is tested using a laser plumb line. Any deviation exceeding 1° requires correction. The pile planting depth is measured using a tape measure. If it is less than the design value, deepen the pile.​
VI. Development Trends of PV Spiral Ground Piles

As the photovoltaic industry develops toward "efficiency, greenness, and intelligence," spiral ground piles are also showing three major trends:

1. Integrated Design: Collaborative Optimization with Mounting Systems

In the future, spiral ground piles will be integrated with the photovoltaic mounting columns. An "adjustable flange" (with an adjustment range of ±5cm) will be integrated into the pile top, eliminating the need for on-site cutting or shimming to accommodate mounting errors. Simultaneously, a "lightweight pile body" (made of thin-walled, high-strength steel) will be developed, reducing the weight of each pile by 20% compared to traditional piles, lowering transportation and installation costs.

2. Intelligent Monitoring: Full Lifecycle Management

A "stress sensor + wireless transmission module" (with an accuracy of ±0.1MPa) will be embedded within the pile body to monitor pile stress changes under wind and snow loads in real time. A back-end warning will be issued when stress exceeds a safety threshold (e.g., 150MPa). Combined with Beidou positioning, this technology can monitor pile displacement due to geological subsidence or windblown sand, enabling early intervention. 3. Environmentally Friendly Recycling: Circular Economy Applications
Develop "biodegradable coatings" (such as bio-based resin coatings) to avoid environmental pollution caused by discarded traditional metal coatings. At the same time, establish a ground pile recycling system, using specialized equipment to remove the ground piles after the project is completed.

After rust removal and recoating, the piles can be reused (with a recycling rate exceeding 80%), reducing resource waste.

VII. Conclusion
As highly efficient, eco-friendly, and adaptable foundation components, photovoltaic spiral ground piles have become a key component in reducing costs and increasing efficiency in photovoltaic projects. Their excavation-free design reduces ecological damage, their fast installation shortens construction time, and their adaptability accommodates complex scenarios. In the future, with the advancement of material technology and the integration of intelligent technologies, spiral ground piles will further enhance their load-bearing capacity and environmental performance, providing reliable foundational support for the implementation of photovoltaic projects in a wider range of terrains and environments, and contributing to the sustainable development of the global photovoltaic industry.