What are Seismic Integrity Test results read from?
The Seismic Integrity Test (or Low Strain Pile Integrity Test) is read from a plot of wave velocity (Velocity) versus time. The pile head is struck with a hand-held hammer, and a stress wave travels down the pile. When it meets a change in cross-section or concrete quality (Impedance Change), part of the wave reflects back up to be picked up by the sensor on the pile head.
The location of whatever the wave meets is calculated with a simple equation:
Depth = (wave speed × reflection return time) ÷ 2
The wave speed in typical concrete piles is about 3,500–4,000 m/s, depending on the concrete strength and age.
Hypothetical example: wave speed 4,000 m/s and a reflection returning at 0.010 s → depth = 4,000 × 0.010 ÷ 2 = 20 m
Testing and interpretation in Thailand mainly reference DPT 1551-51. The international method ASTM D5882-16 was withdrawn by ASTM in 2025, so if it is cited in documents, state the edition year and its status.
Seismic testing is fast and economical, so it is widely used to screen the integrity of large numbers of piles on a project (see the basics of the method in What Is a Seismic Integrity Test)
What does the graph of a normal pile look like?
The graph of a sound pile has two main wave peaks: the Impact Pulse at the pile head and the Toe Reflection at the design length, with a flat section in between.
- Impact Pulse — the first peak from the hammer blow, near zero (the pile head)
- Toe Reflection — the reflection from the pile toe, appearing at the position matching the design pile length (e.g. a 21 m pile should show a reflection at about 21 m)
Between these two points, the graph should be flat, with no distinct spurious peaks, meaning the pile cross-section is uniform along its length.
Which graphs indicate a pile defect?
A graph with a wave reflecting back before the pile toe indicates that the cross-section or concrete may change at that depth. The direction of the reflection relative to the Impact Pulse tells the type: the same direction means a reduced cross-section, the opposite direction means an enlarged cross-section. The table below summarizes 5 common patterns.
← Scroll to see the full table →
| Signal pattern on the graph | Possible meaning | Common in |
|---|---|---|
| Reflection in the same direction as the Impact Pulse before the pile toe | Reduced cross-section — necking (Necking), crack (Crack), void (Void), poor-quality concrete | Bored piles through soft soil / cracked driven piles |
| Reflection in the opposite direction to the Impact Pulse | Enlarged cross-section — bulging (Bulging) from concrete flowing out into soft soil layers | Wet-process bored piles |
| Rhythmic repeated reflections from a single location | A clearly separated crack or joint; most of the wave reflects back at that point | Multi-section driven piles / piles cracked by overdriving |
| No Toe Reflection at the design length | The wave is fully absorbed before reaching the toe (long pile/high friction), or a major defect blocks it | Long piles in stiff clay |
| Toe Reflection appears shallower than the design length | The actual pile may be shorter than designed, or fully cracked through at that point | Must be checked against construction records |
Real signal examples from tests on model piles in DPT 1551-51 Appendix D make each case easier to picture.
The same signal pattern can have several causes. For example, a mid-pile reflection may simply be a normal driven-pile joint rather than a crack. Conclusions must therefore be drawn by an engineer who reviews all supporting data: pile type, driving/concreting records and soil profile data.
Once you know the signal patterns, the figure below arranges graph reading into 3 questions: is the signal reliable, is the Toe Reflection seen at the design length, and is there a reflection before the toe and in which direction. Each answer leads to a result group from the table above, along with what to do next.
What makes graphs hard to read (not always a defect)?
Some factors create defect-like signals or weaken the Toe Reflection even though the pile may not be damaged. These factors are why signal quality is checked first (question 1 in the figure above).
- Uneven pile head or loose, spalling concrete — high noise; fix by grinding the pile head before testing (see Pile Head Preparation Before Testing)
- Very long pile relative to its cross-section — soil friction absorbs the wave until the Toe Reflection fades or disappears
- Abrupt changes in soil layers — sudden changes in friction can create defect-like reflections
- Testing when the concrete is too young — the wave speed is low and unstable, so calculated positions are inaccurate
What limitations of the seismic test should you know before concluding?
The Seismic Integrity Test is an integrity screening tool, not a load capacity test, and it has 3 key limitations.
- It cannot tell load capacity — it only shows the integrity of the pile shaft. To confirm load capacity, a Dynamic Load Test (PDA) or Static Load Test is required.
- Very small defects may go undetected — especially very deep defects or ones that are small relative to the pile cross-section
- Defects below the first defect cannot be seen — if there is a major crack in the upper part, most of the wave reflects back, and data below that point is unreliable
Abnormal signal found: what next?
Do not conclude straight away that the pile is defective. The first step is to confirm that the signal really comes from the pile, then investigate in depth only the piles that remain abnormal. Engineers usually proceed in this order:
- Retest the same pile at several positions on the pile head to confirm that the signal really comes from the pile and is not noise
- Compare with construction records such as the actual concrete volume placed, driving records and pile joint locations
- Test additional neighboring piles to see whether the problem is limited to one pile or affects the whole zone
- Investigate in depth with other methods such as concrete core drilling (Coring) or Cross Hole Sonic Logging (CSL) for large bored piles with pre-installed access tubes; where no Toe Reflection is seen and the length must be confirmed, a Parallel Seismic Test may be used
- Refer the results to the design engineer — deciding whether a pile is acceptable or needs remediation is the authority of the project's responsible engineer
The 3D view below shows an illustrative grouping of results into 4 levels: green is sound, orange needs comparison with construction data (bulging, joints), red is a suspected defect (necking, cracks), and gray is unreadable and must be retested.
If a pile fails the seismic test, read the detailed guidance in Pile Failed a Seismic Test: What to Do, and see an overview of choosing among all test methods in All Pile Testing Methods
Summary
Reading Seismic Integrity Test results means looking at where reflected waves come from and in which direction. A sound pile shows a clear Toe Reflection at the design length and a flat middle section, while a reflection before the toe is a signal that needs further investigation.
Do not conclude that a pile is defective from the graph alone, because the same signal pattern can have several causes. Correct interpretation requires an engineer to consider the pile type, construction records and soil profile data together, and the seismic test does not indicate pile load capacity.
SPN Soil Engineering provides complete pile testing services, including Seismic Integrity Test, Dynamic Load Test (PDA) and Static Load Test to the relevant DPT and ASTM standards, with more than 20 years of experience in all 77 provinces of Thailand, and reports certified by a civil engineer licensed by the Council of Engineers Thailand (COE).