Screw Pile Pull-Out Strength Test: Overview and Why It’s Essential

Screw piles are commonly used as foundations for solar power plants. However, before construction begins, it’s critical to assess the suitability of screw piles through a pull-out strength test. Failing to conduct this test can result in unnecessary expenses or structural failure.

This article explains the purpose, procedures, and increasing importance of pull-out tests for screw piles. It also covers why this test is becoming more common in both large-scale and residential solar projects.

What Is a Screw Pile Pull-Out Strength Test?

This test evaluates whether screw piles are appropriate for a specific site and helps determine the most efficient installation method. A screw pile is driven into the ground to a certain depth, then pulled upward using specialized machinery (e.g., applying 1,000 kg of force) to measure resistance.

• If the pile comes out easily, the soil lacks sufficient holding strength.
• If it holds firm, even a shorter screw pile may be sufficient, saving cost and labor.

🔧 Test Outcome Benefits:

• Reveals if soil reinforcement is necessary.
• Suggests optimal pile length.
• Informs appropriate installation method.

Why Is the Pull-Out Strength Test Important?

Screw pile foundations are cost-effective and quick to install. They don’t require curing time like concrete. However, they aren’t suitable for all ground types.

Examples of Risks:

• In hard soil, piles may bend or break without pre-drilling.
• In soft ground, insufficient resistance may lead to sinking or detachment.

Testing before the construction plan is finalized ensures:

• The selected pile type meets the strength requirement.
• Adjustments can be made early to avoid delays or accidents.

✅ Result: Safer construction and more efficient project execution.

Common Screw Pile Installation Methods

When the Ground is Hard

• Issue: Hard layers (e.g., bedrock) make direct driving difficult.
• Solution: Pre-drilling is essential.
  • Drilling methods include:
    • Air-blast rotary drilling.
    • Auger + vibration head drilling.

Pre-drilling improves accuracy and reduces pile damage.

When the Ground is Soft

• Issue: Easy to insert, but stability may be compromised.
• Solution: Use long piles or wide-blade screw piles (“large wing type”) that resist vertical and lateral forces better.
• In extremely soft soil, concrete foundations might be more practical.

What Does the Pull-Out Strength Test Include?

There are three main testing methods:

1. Pull-Out Test (Vertical and Horizontal Load Test)

• A screw pile is installed to depth and pulled vertically/horizontally.
• Equipment options:
  • Excavator with load gauge.
  • Hydraulic jack with tripod or reaction frames.

Typical loads tested: 200 kg to 2,000 kg depending on conditions.

Use Case:

To simulate wind uplift or lateral force from storms acting on solar panels.

2. SWS Test (Screw Weight Sounding Test)

• Also known as the Sweden Sounding Test.
• A weighted rod with a screw point is rotated into the ground.
• Measures N-values (soil resistance).

Test types:

• Manual
• Semi-automatic
• Fully automatic

🔍 Commonly used in Japan for residential sites. Offers cost-effective soil profiling up to ~10m depth.

3. Compression (Push-In) Test

• Measures ground support by applying downward force to a pile.
• Assesses risk of settlement under snow load or equipment weight.

Especially recommended in:

• Snowy regions (e.g., Hokkaido, Japan).
• Wind-prone areas where strong downward pressure is expected.

Why Strength Testing is Increasing

1. Regulation Changes

Japan implemented a “Pre-Use Self-Inspection System” in 2016 for systems above 500kW. Installers must now verify strength and report results before operation.

• Triggered by accidents like:
  • Panels flying off during typhoons.
  • Soil erosion and landslides during heavy rain.

Even low-voltage systems (<50kW) are increasingly tested due to rising quality standards.

2. Industry Trend: Safety Over Cost

Testing both pull-out and push-in strength helps ensure:

• No post-installation sinking.
• No equipment detachment due to wind or seismic events.
• Long-term project viability and stable returns.

Final Thoughts: Plan Your Foundation with Confidence

Screw pile strength tests are no longer optional—they’re essential for safe, efficient solar installations. Whether you’re working on a large-scale solar power plant or a small residential project, testing helps you:

• Prevent accidents.
• Avoid construction delays.
• Optimize costs.

👷 Need Help? Get in Touch with Firstsolar’s Screw Pile Experts

Let us assist you in choosing the right pile type, length, and installation method based on your site’s soil conditions.