When buildings or infrastructure are already in place, inspecting the foundation piles becomes more challenging, but no less important. Piles may shift, deteriorate, or contain defects that aren’t visible from the surface but could impact the structure’s performance over time.
Pile integrity testing offers a way to check the condition of these foundations without excavation or disruption. Even when access is limited, such as under slabs or footings, specialist methods can identify internal flaws and confirm structural reliability.
This guide covers three practical approaches for testing piles under existing structures: low-strain pile integrity test, Crosshole Sonic Logging (CSL), and parallel seismic pile test. Keep reading to learn how each one works and where it’s best applied.
Key Takeaways
- Pile Integrity Testing (PIT) is essential for assessing hidden risks under existing structures.
- The low-strain pile integrity test, also known as the transient dynamic response pile test, is quick, non-invasive, and ideal when only part of the pile is exposed.
- Crosshole Sonic Logging (CSL) offers detailed internal profiling but requires access tubes – best for bored or cast-in-place piles.
- Parallel seismic pile testing is effective when pile heads are completely inaccessible – ideal for verifying embedded length and continuity.
- The best method choice depends on the pile type, accessibility, structural load conditions, and the stage of construction.
- Working with specialists, like CTS, ensures proper method selection, data interpretation, and compliance with relevant standards.
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What Is Pile Integrity Testing?
Pile foundations are deep, column-like structural elements used to transfer building loads across stable ground layers below. They’re often used where shallow soils are weak or unstable, such as near rivers, bridges, industrial zones, or high-rise developments.
Pile Integrity Testing (PIT) refers to a suite of Non-Destructive Testing (NDT) techniques used to evaluate the condition and quality of these piles – whether they’re cast-in-place (bored or Continuous Flight Augered CFA), driven (precast concrete, steel, timber), or mini / micro piles for confined areas.
The main objectives of pile integrity testing are to:
- Detect flaws, such as cracks, voids, inclusions, or honeycombing
- Verify the pile’s continuity, straightness, and any changes in cross-section
- Estimate the length and consistency of the pile material over depth
PIT is commonly used to evaluate piles before structural upgrades or building extensions, during bridge maintenance programs, or to investigate undocumented foundations during asset audits. It’s also critical in forensic investigations following unexpected settlement or load shifts.
Why Pile Integrity Testing is Crucial for Existing Structures
Once piles are hidden beneath a completed structure, accessing them for inspection becomes more difficult, but also more essential.
1. Undetected pile issues can lead to major failures
Piles may deteriorate over time due to poor installation, material segregation, corrosion, or ground movement. If these issues remain unseen beneath a floor slab or column base, they can compromise the entire building’s load-bearing system.
2. Access limitations affect testing accuracy
In existing buildings, pile tops are often buried beneath concrete slabs, equipment, or structural loads. In these cases, stress wave reflections may be distorted or obstructed, requiring special configurations or alternative methods to maintain accuracy.
3. Precision is harder to achieve during retrofits or extensions
Before adding new loads (e.g., extra floors, mezzanines, or adjacent buildings), it's crucial to understand the condition of the existing pile foundation. Testing must deliver reliable data without disturbing the structure, requiring careful method selection and specialist expertise.
3 Methods for Testing Inaccessible Pile Foundations
When pile heads are covered by slabs or surrounded by structure, conventional testing methods aren’t always feasible. The three techniques below are specifically suited for these conditions, offering reliable ways to assess pile integrity with minimal access, no structural disruption, and high confidence in the results.
Method 1 – Low Strain Pile Integrity Test
The low-strain pile integrity test, also known as the transient dynamic response pile test, is a go-to solution when only a small portion of the pile is accessible. It sends stress waves through the pile to detect changes in cross-section, continuity, or potential defects.
How it works:
- A handheld hammer is used to tap the exposed pile top
- A sensor (accelerometer) records the stress wave response
- The reflected signals are analysed to detect changes in pile material or geometry
When to use it:
- When pile heads are partially accessible due to slabs, columns, or coverings
- Suitable for concrete piles with minimal exposed length
- Fast and cost-effective, making it useful for large-scale inspections
Example application:
In a warehouse retrofit where only a small pile section is visible through a floor cut-out, a low-strain pile integrity test confirmed shaft integrity without full excavation or downtime.
Method 2 – Crosshole Sonic Logging (CSL) Adaptation
Crosshole Sonic Logging (CSL) is a high-resolution ultrasonic method for identifying internal defects. While it’s typically used during construction, it can be adapted for existing structures if access tubes are present or can be installed.
How it works:
- Steel access tubes are embedded inside the pile (during construction) or cored nearby
- A transmitter and receiver probe are lowered into the tubes
- Ultrasonic waves are sent across the pile cross-section
- Differences in arrival time, signal strength, or attenuation indicate flaws, such as voids, segregation, or inclusions
When to use it:
- When internal flaws are suspected and tubes are available
- For large-diameter bored piles or cast-in-place piles
- In critical infrastructure projects like bridges or retaining walls
Example application:
In a bridge foundation inspection, crosshole sonic logging revealed an inclusion zone near mid-depth, prompting targeted remedial grouting.
Method 3 – Parallel Seismic Pile Testing
Parallel seismic pile testing is highly effective when pile heads are completely inaccessible, such as beneath thick floor slabs, machinery, or deep footings. This method helps determine pile length, continuity, and potential defects when surface access isn’t available.
How it works:
- A borehole is drilled adjacent to the pile (usually within 0.5-1.5 metres)
- A seismic source (e.g., a hammer or drop weight) generates waves at the surface
- A downhole geophone detects wave arrivals at multiple depths
- Changes in wave arrival times help estimate pile toe depth and integrity
When to use it:
- When pile heads are fully embedded or inaccessible
- For verifying unknown pile lengths during asset surveys or retrofits
- In confined or built-up environments where traditional PIT isn’t feasible
Example application:
In a city centre building extension, parallel seismic pile testing was used to verify the depth of existing driven piles beneath the slab without any disturbance to the operational floor.
Need Help with Pile Integrity Testing? Talk to CTS
The best method for testing piles under existing structures depends on the pile type, access conditions, structural loads, and whether the pile is new or existing. In some cases, combining methods gives the clearest picture. Choosing carefully helps ensure accurate results and efficient decision-making.
At CTS, we provide practical, non-invasive solutions tailored to complex site conditions. Whether you're verifying old foundations or installing new piles beneath an active structure, our team can help you choose the right method and get the data you need to move forward with confidence.
Explore our Pile Integrity Testing Services or contact us for site-specific advice and a quote.