As Canadian cities evolve and densify, one engineering challenge resurfaces over and over: How can we confidently assess the length of existing piles and deep foundations when documentation is missing or incomplete? This question is particularly important during rehabilitation, retrofitting, or underground construction (e.g., tunneling) near existing infrastructure.

When the exact embedment of a pile is unknown, it introduces serious design and safety risks, especially in dense urban corridors. In such cases, non-destructive testing (NDT) provides a reliable and practical solution. In this blog post, we explore 3 methods for evaluating pile length—Parallel Seismic (PS), Length Induction Testing (LIT), and the Guided Waves method—and explain how these technologies are applied on real projects.

Why Knowing Pile Length Matters

Deep foundations like driven piles, H-piles, caissons, and helical piles are often used to transfer loads to stable soil layers or bedrock. But many existing foundations—especially those built decades ago—lack detailed records. This becomes a critical problem when:

• New tunnels or utilities are installed near old foundations

• Engineers want to reuse piles for new structures

• Bridge or building safety assessments are needed

The unknown length of piles can compromise load transfer assumptions and lead to underestimated risks of ground movement or structural failure. Therefore, assessing the depth and condition of these foundations is essential for both safety and sustainability. Learn more

3 Methods for Evaluating Pile Length

1. Parallel Seismic Method (PS)

Overview

The Parallel Seismic (PS) method is one of the most widely used NDT techniques to determine the depth of unknown foundations. This method works by generating mechanical waves at the top or side of a foundation and recording how long it takes for the waves to reach a hydrophone placed in a nearby water-filled borehole.

How It Works

• A borehole is drilled 0.9–1.5 m from the pile

• A controlled impact is delivered to the pile cap or side

• A hydrophone detects wave arrival times at various depths

• The change in wave velocity helps estimate the toe (bottom) of the pile

Applications

• Accurate for concrete and steel piles

• Works for bridge abutments, towers, and buried piles

• Ideal when access to pile head is limited, but borehole access is possible

Limitations

• Requires a vertical, water-filled borehole

• Assumes direct wave path and minimal noise interference

In practice, the PS method can estimate pile length with ±5% accuracy, making it one of the most dependable NDT tools.

2. Length Induction Test (LIT)

Overview

The Length Induction Test (LIT) is a highly specialized method used to evaluate the length of metallic piles, such as H-piles, steel sheet piles, and pipe piles. It operates on principles similar to a metal detector by identifying changes in magnetic fields caused by steel.

How It Works

• A parallel borehole is drilled past the expected pile toe

• A probe with an electromagnetic field is inserted

• Changes in induced voltage are recorded as the probe passes near steel

• A sharp voltage drop or rise indicates the end of the pile

Applications

• Ideal for steel piles and helical piers

• Used in transmission towers, bridges, and sheet pile walls

• Effective when pile material is ferromagnetic

Limitations

• Best performance when the probe is within 450 mm of the pile

• Not effective for concrete piles or composite materials

LIT is particularly valuable when visual access is impossible and minimal disruption to the structure is essential.

3. Guided Waves Method

Overview

The Guided Waves technique is a newer and highly innovative NDT method based on wave propagation and dispersion analysis. It’s ideal for complex urban projects where access is restricted.

How It Works

• A series of small accelerometers are attached to the pile

• Stress waves are introduced at the top or side of the pile

• The phase shift between sensors is analyzed to estimate pile length

• Sophisticated algorithms like Spectral Element Method (SEM) are used for interpretation

Applications

• Useful for concrete, timber, and even steel piles

• Works even when no pile head is exposed

• Can be used in active service structures

Limitations

• More complex data interpretation

• May be sensitive to signal-to-noise ratio, especially in steel

The method is increasingly being used in Canada for tunnel risk assessments and foundation reuse studies. Read more

Real-World Applications: Two Case Studies

Case 1: H-Pile Foundations in Edmonton

During a pipeline construction project, engineers needed to determine the length of H-piles supporting a transmission tower. No as-built records existed.

NDT Methods Used:

• Parallel Seismic (PS)

• Length Induction Testing (LIT)

• Guided Waves

Key Findings:

• PS and Guided Waves agreed on pile lengths (~19–20.5 m)

• LIT results were inconclusive due to inconsistent voltage changes

• Multi-method approach allowed validation and cross-checking

Case 2: Helical Piles Beneath a Pedestrian Bridge in Toronto

In a second project, a pedestrian bridge was located above a proposed trenchless tunnel. Engineers needed to verify whether the helical piles extended below the tunnel.

NDT Methods Used:

• Parallel Seismic (PS)

• Length Induction Testing (LIT)

Key Findings:

• PS method estimated pile toe at ~10.5 m

• LIT captured an anomalous voltage rise indicating the helix depth

• Results confirmed pile lengths aligned with original design assumptions

These case studies highlight the value of combining methods to increase accuracy and reduce uncertainty.

How to Choose the Right Method

The choice of method depends on several factors:

In some cases, a single method may be sufficient, but for high-stakes infrastructure work, a multi-method NDT strategy is best practice.

Benefits of Using NDT for Pile Length Evaluation

✅   No excavation required
  ✅   Reduced risk and disruption
  ✅   Cost-effective compared to intrusive methods
  ✅   Applicable to a wide variety of foundation types
  ✅   Critical for tunneling, bridge retrofits, and urban development

Conclusion

Understanding the depth and integrity of existing foundations is essential for safe and sustainable infrastructure development. The 3 methods for evaluating pile length—Parallel Seismic, Length Induction Testing, and Guided Waves—offer engineers powerful, non-invasive tools for tackling this challenge.

At FPrimeC Solutions Inc., we specialize in multi-method NDT programs tailored to real-world constraints. Whether you’re planning tunneling under a bridge, verifying foundations for reuse, or evaluating risks near buried infrastructure, we can help you identify the most suitable method—or combination of methods—for your project.