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The Engineer's Guide to Specifying Helical Piles for Australian Solar Foundations

October 15, 2025
An engineer’s guide to specifying helical piles for solar foundations in Australia. Covers geotechnical analysis, AS 2159 design, and torque verification.

Helical piles represent an engineered deep foundation system that provides both compressive and tensile (uplift) resistance.

For solar farm foundation design, where large, lightweight structures are exposed to significant aerodynamic forces, this dual capability is indispensable.

Unlike a traditional concrete pier which relies primarily on mass and skin friction, a helical pile functions like an anchor, with the helical plates providing a defined bearing surface deep within a competent soil stratum.

Summary

  • This guide provides a technical framework for Australian engineers to design, specify, and verify helical piles for solar foundations.
  • It focuses on the critical interplay between geotechnical investigation, design compliance with Australian Standards (AS 2159), and on-site installation verification.
  • The content addresses key technical challenges including wind loading, corrosion, and variable soil conditions across Australia.
  • It also outlines the project benefits from an engineering and management perspective, including schedule acceleration and cost certainty.

The Starting Point: Geotechnical Investigation and Site Characterisation

A helical pile is only as effective as the ground it is installed in. Therefore, a comprehensive geotechnical investigation is the non-negotiable first step in any solar farm foundation design. A superficial soil log is insufficient; a detailed report is required to inform the engineering design.

Beyond a Soil Log: What Engineers Need from a Geotechnical Report

For a robust helical pile design, the geotechnical report must provide quantifiable data. This includes:

  • Soil Profile and Classification: Detailed logs identifying soil layers, their depths, and classification according to the Unified Soil Classification System (USCS).
  • In-Situ Testing Data: Standard Penetration Test (SPT) ‘N’ values or Cone Penetration Test (CPT) results (tip resistance qc and sleeve friction fs) are essential for calculating soil strength parameters.
  • Soil Strength Parameters: Undrained shear strength (Su) for cohesive soils and the angle of internal friction (φ) for cohesionless soils.
  • Corrosivity Analysis: Soil resistivity, pH levels, and the presence of chlorides and sulphates are critical for determining the required level of corrosion protection and ensuring the foundation’s specified design life.

Mapping Australian Soil Profiles: From Reactive Clays to Sandy Plains

Australia’s diverse geology presents unique challenges. A foundation solution that works in the sandy soils of coastal WA may be unsuitable for the reactive clays of Victoria. Helical piles offer the versatility to adapt, provided the design is informed by local conditions.


Table 1: Common Australian Soil Types and Helical Pile Design Considerations
Soil Type Common Location Primary Challenge Helical Pile Design Consideration
Reactive Clays Adelaide Plains (SA), Western Melbourne (VIC) Significant shrink-swell movement with moisture changes. Pile must be founded below the zone of seasonal moisture variation. A void former or slip sleeve may be used on the upper shaft.
Saline / Acid Sulfate Soils Coastal regions, Murray-Darling Basin Highly corrosive to steel. Requires enhanced corrosion protection, such as thicker hot-dip galvanization (>100 µm) or specialised coatings.
Loose Sandy Soils (Alluvium/Dunes) Perth (WA), Coastal QLD Low bearing capacity; requires deep embedment. Utilise larger diameter helices and/or multi-helix piles to maximise bearing area and achieve required uplift capacity.
Calcrete / Hardpan Layers Regional WA, SA Can cause premature refusal during installation. May require pre-drilling a pilot hole or using a pile with a hardened rock-cutting tip. Torque monitoring is critical.

The Core of the Design: Load Calculation and Pile Specification

With a clear understanding of the ground conditions, the engineer can proceed with the design, ensuring it is both safe and compliant with Australian law.

Adhering to Australian Standards: AS 2159 and AS/NZS 1170.2

The design of helical piles in Australia is governed primarily by two standards:

  1. AS/NZS 1170.2 – Structural design actions, Part 2: Wind actions: This standard is used to calculate the design wind loads on the solar array. For large solar farms, this is a complex calculation that considers site wind speed, terrain category, topography, and the aerodynamic shape of the panels to determine the critical upward and lateral forces.
  2. AS 2159 – Piling – Design and installation: This is the foundational code for all piling work. It outlines the methodology for determining the design geotechnical strength and structural strength of the piles. It mandates the application of safety factors and risk assessments to ensure the foundation performs safely over its entire life.

Sizing the Pile: Matching Helices and Shafts to Loads

The ultimate geotechnical capacity of a helical pile is calculated based on the bearing capacity of the individual helices. A simplified method involves summing the capacity of each helix, which is a function of the helix area and the soil’s shear strength. The design must satisfy that the design action (load) is less than or equal to the design geotechnical strength. Engineers select a combination of shaft diameter, wall thickness, and helix configuration (number, diameter, and spacing) to efficiently resist the calculated design loads while remaining structurally sound.

Ensuring Long-Term Performance: Corrosion Protection and Design Life

A key specification for utility-scale solar foundations is a design life of 50 years or more. To achieve this, steel piles must be protected from corrosion. The standard method is hot-dip galvanization in accordance with AS/NZS 4680, which requires a minimum average coating thickness of 85 µm for steel sections thicker than 6 mm. In more aggressive soil environments (identified in the geotechnical report), a thicker coating or alternative protective systems may be specified to guarantee longevity.


Graph 1: Estimated Steel Pile Design Life vs. Galvanization Thickness (AS/NZS 4680)
Design Life (Years) Soil Environment

25+ Years

40+ Years

50+ Years

Highly Corrosive
Moderately
Mildly Corrosive

*Note: Illustrative data for a standard 85µm galvanized coating. Thicker coatings or cathodic protection can extend design life significantly.*


Installation as Verification: The Power of Torque Correlation

One of the most compelling advantages of helical piles from an engineering perspective is the ability to verify the capacity of each pile as it is installed. This is achieved through a well-established method known as torque correlation.

What is Torque Correlation?

This is an empirical method that relates the amount of rotational energy (torque) required to install the pile to its ultimate geotechnical capacity. Extensive testing has shown a direct, near-linear relationship between installation torque and pile capacity in most soil types. The principle is simple: the harder it is to screw the pile in, the greater its capacity will be.

Real-Time Quality Assurance on Site

During helical pile installation Australia, the hydraulic drive head used to rotate the pile is fitted with a torque transducer. This device provides a continuous, real-time readout of the installation torque. The installer advances the pile until the torque reading consistently meets or exceeds the minimum value specified by the design engineer. This process effectively turns every pile installation into a load test, providing an unparalleled level of quality assurance across the entire project site.

From Theory to Practice: Applying the Torque Correlation Factor (Kt)

The relationship is expressed by the formula: 𝑄𝑢 = 𝐾𝑡×𝑇

Where:

  • Qu is the ultimate pile capacity.
  • Kt is the empirical torque correlation factor.
  • T is the final installation torque.

The Kt factor is dependent on the pile’s shaft geometry (e.g., round vs. square shaft) and the soil type.

While there are generally accepted ranges (e.g., Kt ≈ 30 m⁻¹ for 89mm round shafts), AS 2159 suggests that project-specific Kt factors should be confirmed with on-site load testing, especially for large and critical projects.

Addressing Technical and Logistical Challenges

While versatile, specifying helical piles requires an awareness of potential challenges and their engineered solutions.

Dealing with Difficult Ground: Rock Refusal and Obstructions

In ground with dense cobbles or shallow bedrock, piles can encounter refusal before reaching their design depth. Solutions include:

  • Pre-drilling: Using an auger or down-hole hammer to create a pilot hole through the obstruction.
  • Specialised Pile Tips: Employing piles with hardened points or dedicated “rock-cutter” lead sections.
  • Design Modification: If refusal is widespread, the engineer may revise the design to rely on end-bearing on the rock layer, provided it is verified as competent.

Lateral Stability for Sloped Sites and High Wind Zones

Standard vertical piles have limited lateral capacity. For sites with significant slopes or in high wind regions (such as cyclonic areas in WA and QLD), lateral stability is critical. This can be enhanced by:

  • Increasing Shaft Section: Using a larger diameter or thicker-walled pile shaft to increase bending stiffness.
  • Batter Piles: Installing some piles at an angle (typically 5° to 15°) to brace against lateral forces.
  • Concrete Collars: Pouring a small concrete collar around the top of the pile at ground level to increase lateral restraint.

The Remote Site Advantage: Logistics and Accessibility

For the vast, remote locations of many Australian utility-scale solar projects, logistics are a major project cost and risk. This is where helical piles offer a profound advantage. A single semi-trailer can transport hundreds of piles and all necessary installation equipment. This is in stark contrast to the logistical demands of concrete, which requires a continuous supply of aggregate, cement, and water, as well as a fleet of heavy concrete agitator trucks traversing often unsealed access roads.

On Specifying Helical Piles for Solar Foundations

For the Australian engineer tasked with delivering solar farm foundation projects, helical piles offer a technically superior solution. They provide a predictable, verifiable, and highly efficient system that aligns with the core engineering principles of safety, compliance, and performance. 

By leveraging detailed geotechnical data and adhering to Australian Standards, helical piles can be precisely designed to withstand the country’s unique environmental loads.

The ability to verify the capacity of every pile during installation provides a level of quality assurance that traditional foundations cannot match, ultimately de-risking the project and accelerating the delivery of renewable energy.

Frequently Asked Questions About Helical Piles for Solar Foundations

What Are the Design Considerations for Solar Panel Foundations?

The primary design considerations are geotechnical conditions, design life, and structural loads. Key loads include the permanent downward load (self-weight) and the transient loads from wind, which create significant uplift and lateral forces. The design must also account for corrosion resistance and compliance with AS 2159 and AS/NZS 1170.2.

How Do You Calculate the Load Capacity of a Helical Pile?

The ultimate capacity is calculated based on soil mechanics principles, using strength parameters from a geotechnical investigation. It is then verified on-site during installation by monitoring the installation torque and applying a proven torque correlation factor (Kt) to confirm the design capacity has been achieved.

What Is the Main Disadvantage of Helical Piles in Solar Projects?

The main disadvantage is their limitation in ground with shallow, hard bedrock or numerous large boulders, which can cause installation refusal. This limitation is typically identified in the geotechnical investigation, and it can be overcome with techniques like pre-drilling, though this can add time and cost.

How Does Soil Type Affect Solar Foundation Design?

Soil type is the most critical factor. Weak, compressible soils require piles with larger or more numerous helices installed to a greater depth to find a competent bearing layer. Corrosive soils demand enhanced galvanization or protective coatings. Reactive clays require the pile to be anchored below the zone of seasonal movement to prevent jacking.

What Are the Australian Standards for Piling?

The primary Australian Standard for all piling work, including helical piles, is AS 2159:2009 – Piling – Design and installation. This standard provides the framework for design, installation, and testing to ensure safety and performance.

Are Screw Piles the Same as Helical Piles?

Yes, in the Australian context, the terms “screw pile” and “helical pile” are used interchangeably to describe the same type of foundation system: a steel shaft with one or more welded helical plates that is screwed into the ground.

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