What Is a Seismic Test (Low Strain Pile Integrity Test)?

A seismic test, or low strain pile integrity test (also called a Pile Integrity Test — PIT, or Sonic Echo), is a non-destructive test of pile integrity. The pile head is tapped with a hand-held hammer and the reflected wave is measured. A low-strain stress wave travels down the pile and is reflected back when it meets a change in cross-section or impedance, such as a defect or the pile toe. An accelerometer mounted on the pile head records this signal for analysis.

In Thailand, the Department of Public Works and Town & Country Planning standard DPT 1551-51 (standard for pile integrity testing) is the main criterion. The international test method most often cited is ASTM D5882-16, which ASTM withdrew in 2025. If ASTM is cited in documents, the edition year and status should therefore be stated clearly.

The seismic test is a very popular quality control (QC) method for bored piles and cast-in-place piles, and can also be used on precast piles, because it is fast, economical and can test large numbers of piles without pre-installed tubes as required by cross-hole sonic logging.

Seismic test (low strain pile integrity test) checking pile integrity non-destructively, by the SPN engineering team
Seismic (low strain) testing — tapping the pile head with a hand-held hammer and recording the reflected wave with an accelerometer (Photo: SPN Soil Engineering)

Why Check Pile Integrity?

Because pile defects occur underground and cannot be seen from outside. Cast-in-place bored piles can be unintentionally defective during construction—for example, borehole wall collapse causing necking, concrete segregation forming voids, or soil mixed into the concrete. Precast piles may crack during handling or driving.

If left undetected, a pile may not carry its full load and may settle later. Integrity testing is therefore an important screening step before the superstructure is built.

How Does a Seismic Test Work?

The seismic test is based on wave reflection. When the pile head is tapped, a stress wave travels down through the concrete at wave speed (c) and is reflected back up whenever it encounters a change in impedance.

There are 2 ways to analyze the signal: the time domain (Sonic Echo / Pulse Echo Method), which looks at the position and direction of reflection peaks, and the frequency domain (Transient Response Method). The direction of a reflection also indicates the type of change: necking (impedance decrease) gives a reflection in the same direction as the impact wave, while bulging (impedance increase) gives a reflection in the opposite direction.

SPN engineer mounting an accelerometer on a bored pile head and tapping it with a hand-held hammer to generate a low-strain stress wave during a seismic integrity test on site
An accelerometer is mounted on the pile head, which is then tapped with a hand-held hammer to send a stress wave down and back (Photo: SPN Soil Engineering)

How Is Pile Length Calculated from the Reflected Wave?

Pile length, or depth to a reflector, is calculated from L = c·Δt/2, where Δt is the time for the wave to travel down and be reflected back to the pile head. Three formulas are used for interpretation:

Pile length / depth to reflector:   L = c · Δt / 2 Δt = round-trip travel time of the wave (divided by 2 for the down-and-back path) — a preliminary estimate that depends on the assumed wave speed c
Wave speed in concrete:   c ≈ 3,500 – 4,000 m/s Depends on concrete quality and age; use a value appropriate to the project
Pile impedance:   Z = ρ · c · A ρ = density, A = cross-sectional area — a change in Z is what causes a reflection

Hypothetical example: with c = 4,000 m/s and a toe reflection peak returning at Δt = 0.010 s (10 ms), the estimated length is 4,000 × 0.010 ÷ 2 = 20 m. With c = 3,500 m/s, it would be 17.5 m. The assumed c therefore directly affects the length.

Panel A of the figure below shows what happens in the pile when it is tapped, and panel B shows the signal recorded by the sensor. Note that the peak from the necking returns before the peak from the pile toe (2L/c).

Principle of the seismic integrity test: the pile head is tapped, a compression wave travels down and reflects back from necking and the pile toe, with a conceptual example signal plot Two-panel diagram. Panel A is a section of a bored pile in soft clay, stiff clay and sand layers. The pile head is trimmed to sound concrete and protrudes slightly above ground. A hand-held hammer taps the pile head with a low-strain impact; an accelerometer on the pile head is connected by cable to a signal recorder. An orange compression wave travels down the pile at speed c. At a neck at depth z, where the cross-section and impedance decrease, part of the wave is reflected back first, and the rest continues to reflect at the pile toe at depth L. Panel B is a conceptual plot of pile-head velocity versus time, with the impact peak at time zero, the reflection peak from the neck at 2z/c and the reflection peak from the pile toe at 2L/c. Below the time axis is a depth axis equal to c times t divided by 2. A formula box shows L equals c times Δt divided by 2, wave speed in concrete of about 3,500 to 4,000 m/s, and Z equals ρ c A. A reflection-direction box explains that a reflection in the same direction as the impact means an impedance decrease, such as necking, and the opposite direction means an impedance increase, such as bulging. The bottom band summarizes that this method answers questions of integrity and preliminary length, but does not measure load capacity, which requires PDA or a static load test. Seismic test: tap → wave travels down → reflects Not to scale A · On site: tap pile head, measure wave Section of bored pile in soil PIT recorder Hand hammer Low-strain impact Accelerometer z L Neck (Necking) Z drops · reflects first Wave travels down at speed c Pile toe Reflects at time 2L/c Soft clay Stiff clay Sand B · Velocity signal at pile head Conceptual example · Not measured data Velocity (v) → 0 2z/c 2L/c Time (t) → 0 z L Depth = c·t/2 → Impact Neck reflection Toe reflection Δt = round-trip time Length formula L = c · Δt / 2 c ≈ 3,500–4,000 m/s Z = ρ · c · A (impedance) Preliminary estimate Depends on assumed c Reflection direction Same as impact Z drops, e.g. necking Opposite to impact Z rises, e.g. bulging ✓ Answers: integrity Necks, cracks, voids, soil ✓ Answers: approx. length Toe signal must be visible ✕ Does not measure capacity Needs PDA or static
Principle of the seismic (low strain) test: a compression wave from a hand-held hammer travels down the pile, partly reflects at the neck (time 2z/c) and reflects again at the pile toe (time 2L/c). The time at which each peak returns to the pile head is used to calculate depth with L = c·Δt/2 — the plot is a conceptual example, not actual measurements, and this method does not measure load capacity.

What Can a Seismic Test Detect?

The seismic test detects significant changes in cross-section or concrete quality and can be used for a preliminary estimate of pile length, as follows:

  • Necking — reduced cross-section, reduced impedance
  • Bulging — increased cross-section, increased impedance
  • Crack / break (Discontinuity) — causes the wave to reflect back before reaching the toe
  • Void / honeycomb — concrete that is not dense and homogeneous
  • Soil inclusion in concrete — from borehole wall collapse during casting
  • Preliminary pile length estimate — comparing the estimated length with the design (an approximation that depends on the assumed wave speed and requires a visible toe reflection)

To see what each type of signal looks like on a plot, read How to Read Seismic Integrity Test Results and Reflectograms. And if a test shows an abnormal signal, it does not always mean the pile is unusable—see the checks and remedies engineers actually use in What to Do When a Pile Fails the Seismic Test

⚠️ Caution in interpreting pile length

• The pile length shown in the report is only the length derived from the reflected wave speed, or as advised from the field. The actual length should be judged from field information.

• For tested piles, driving or boring records should be reviewed before the piles are put into service—for example, pile casting and temporary steel casing extraction records, together with soil conditions, site exposure and pile eccentricity—with the field engineer or designer making the assessment before use.

• If the pile length needs to be known, a Parallel Seismic Test is recommended

What Are the Steps of a Seismic Test?

The test has 6 main steps and takes only a few minutes per pile once the pile head has been prepared.

  1. Prepare the pile head — cut/grind to a sound, smooth and clean concrete surface
  2. Mount the accelerometer on the pile head with a coupling material
  3. Tap the pile head with a hand-held hammer to generate a low-strain stress wave
  4. Record the reflected wave signal several times and average to reduce noise
  5. Analyze the signal to determine length and defect locations
  6. Classify the integrity level and prepare a report certified by a licensed civil engineer (COE)

A pile head ready for testing must be cut or chipped down to sound concrete with a smooth, clean surface, and the concrete must be old enough per the project specifications. See details in Pile Head Preparation Before Testing and Concrete Age Required for Pile Testing

The 3D view below simulates an actual site: green pile heads have been tested, gray ones are awaiting testing, and a pile with a rough head must be chipped first. The right-hand panel summarizes what must be prepared and the sequence of work on test day.

3D model of a seismic integrity test site with many pile heads, with a pile head preparation checklist and the test-day sequence Conceptual 3D view of a foundation area with trimmed bored pile heads arranged in a grid, on a soil block cut to show soft clay, stiff clay and sand layers. The front row of piles shows their shafts extending down to the sand layer. Green pile heads have been tested; gray pile heads have been trimmed and are awaiting testing. A technician crouches beside one front-row pile, tapping the pile head fitted with an accelerometer using a hand-held hammer, with a signal cable to a recorder on the ground. The shaft of this pile in the soil section shows a compression wave traveling down and a reflection from the pile toe traveling back up. In the right corner of the layout is a pile with a rough head, exposed rebar and concrete debris, meaning it is not ready and must be chipped first. The right-hand panel lists pre-test preparations: cut or chip the pile head to sound concrete, a smooth and clean surface, concrete old enough per the project specifications, and design length data with boring or driving records; followed by the test-day sequence: mount the sensor with coupling material, tap several times and average the signals, analyze length and anomaly locations, classify integrity level and prepare an engineer-certified report, with a note that each pile takes only a few minutes, so dozens of piles can be tested per day. 3D model: seismic test site, many piles per day Not to scale Tested (green ring) A few min per pile Testing Hammer + sensor + recorder Trimmed, awaiting test Not ready Rough head, chip first Wave travels down and reflects from the toe Soft clay Stiff clay Sand Pile head prep + sequence Before test day Cut/chip to sound concrete No loose concrete Smooth, clean surface No soil/mortar debris on head Adequate concrete age Per project specs Prepare pile data Design length + boring/driving log Test day 1 Mount sensor with couplant Firmly on concrete surface 2 Tap several times then average signals to cut noise 3 Analyze signals Length + anomaly locations 4 Classify integrity level Engineer-certified report A few min per pile so dozens of piles per day Conceptual · not an actual pile layout
3D model of a seismic integrity test site: each pile head is tapped with a hand-held hammer and the signal recorded with an accelerometer, a few minutes per pile, so dozens of piles can be tested per day — conceptual view, not an actual pile layout
SPN engineer performing a low strain pile integrity test on a bored pile head, with the signal recording and analysis unit (PIT) placed beside the pile
Testing on site — the reflected wave signal is recorded several times with the PIT unit for averaging and analysis (Photo: SPN Soil Engineering)

How Much Does a Seismic Test Cost, and What Are Its Pros and Limitations?

Seismic testing starts at about THB 100–150 per pile, depending on the number of piles and site conditions. This price excludes travel and equipment mobilization, which are charged according to the site distance. Compare prices of each pile testing method in Pile Testing Prices 2026, or contact us for a free quotation within 24 hours.

✅ Advantages

Fast (a few minutes per pile), low cost, non-destructive, no pre-installed tubes and lightweight equipment, so large numbers of piles can be tested in a short time.

⚠️ Limitations

Suitable for piles whose length-to-diameter ratio (L/D) is not too large (about 20–30). Very long piles or piles with high skin friction damp the wave so that no toe signal is visible. This method only detects significant defects and is not a load capacity test. Results must be interpreted by an experienced engineer.

Which Standards Apply to the Seismic Test, and How Does It Differ from Other Methods?

In Thailand, the seismic test mainly references DPT 1551-51, and it is only one in a set of pile tests, each answering a different question:

← Scroll to see the full table →

Test method / standardWhat it measures and key features
Seismic / Low Strain (DPT 1551-51; ASTM D5882-16 withdrawn in 2025)Integrity and preliminary pile length — fast, low cost, non-destructive, no tubes required, many piles can be tested, but limited by L/D
Cross-hole Sonic Logging (ACI PRC-228.2; ASTM D6760-16 withdrawn in 2025)Detailed internal integrity of large piles; test tubes must be installed in advance during casting
Dynamic Load Test — PDA (DPT 1252-51 + ASTM D4945)Pile load capacity using a high-strain impact; faster than static
Static Load Test (DPT 1251 + ASTM D1143)Load capacity measured directly with real load; the main reference method for determining load capacity

Typically, the seismic test is used to check the integrity of large numbers of piles, and selected piles are then tested for load capacity by dynamic (PDA) or static load test. For more on reading PDA results, see PDA Test Result Interpretation, or see the scope of all SPN pile testing methods on our pile testing services page.

Examples of pile integrity testing by the seismic (low strain) method by the SPN Soil Engineering engineering team, on both cast-in-place bored piles and precast piles, in many types of real projects across Thailand.

SPN engineer performing a seismic test on a bored pile head in the middle of a construction site with rebar and a crane
Testing bored piles in the middle of a large structural project
Engineer tapping an exposed bored pile head in an excavated area to test integrity by the low strain method
Testing pile heads in an excavated area
Integrity testing of trimmed piles in a foundation area, with an excavator working in the background
Foundation work: pile heads trimmed for testing
SPN engineering team testing the integrity of a round bored pile head by the seismic (low strain) method
Testing a round bored pile head
Engineer performing a seismic test on a large pile cap in foundation work
Testing on a pile cap
Integrity testing of square precast piles laid out in a yard before use
Testing precast piles before use
Aerial view of integrity testing of multiple bored piles in a foundation pit by the SPN team
Aerial view of testing multiple piles in a foundation pit
📌 Note

This article is provided as engineering knowledge. The choice of test method, number of piles to be tested and interpretation of results should follow the project specifications and the judgment of a licensed engineer.