What does it mean when a pile "fails" a Seismic Test?

"Fail" means the pile's signal shows an anomalous reflection that needs further investigation; it is not a verdict that the pile is defective. The Seismic Integrity Test is a screening pile testing method. It does not give a black-and-white "pass/fail" result; instead it gives a signal integrity rating. For example, an anomalous reflection before the pile toe may indicate necking, a crack, a void or poor-quality concrete at that location.

The key point is that the same kind of signal can come from several causes, some of which are defects and some not:

  • Splices in multi-segment driven piles (normal, not a defect)
  • Soil layers where skin friction changes abruptly
  • A poorly prepared pile head, causing high signal noise
  • A genuine defect in the pile shaft

So seeing an anomalous signal once is the start of an investigation, not the conclusion.

How to read each signal pattern, such as the direction of reflections and the toe position, is explained in How to read Seismic Integrity Test results. This article picks up from the point where the report has already flagged a pile as anomalous.

The chart below summarizes all 4 steps from left to right. Steps 1–2 are quick and low-cost. If they show it is not a defect, follow the green bar, which skips the in-depth investigation. Step 3 is used only for piles that remain inconclusive.

The 4-step sequence after a report flags an anomalous Seismic Integrity Test signal in a pile, from retesting and comparing construction records to in-depth investigation by other methods and the option chosen by the design engineer (conceptual image) A conceptual flowchart of four cards from left to right. It starts with a red label saying the report found an anomalous signal in one pile. Above, a gradient arrow shows that the early steps are quick screening with low cost per pile, while the later steps take more time and budget. Card one, retest, shows a top view of the pile head with several tap points and three small signal traces with an anomalous reflection repeated at the same depth. The tasks are: grind the pile head smooth, retest at several positions, compare the traces from every position, and test neighboring piles in the zone. If the signal repeats only in this pile, go to step two; if every pile in the zone looks similar at the same depth, it is most likely an effect of the soil layers. Card two, compare construction records, shows a section of two soil layers with a spliced pile whose splice is at the soil layer boundary, and an arrow pointing to the signal at the same level. The data compared are the actual concrete volume placed, the Concreting Curve, the driving and splice records, the final Blow Count, and the soil investigation results. If still inconclusive, go to step three; if the signal sits exactly at a pile splice or a soil layer boundary, it can usually be concluded that it is not a defect. Card three, in-depth investigation by other methods, only for piles still inconclusive, shows five methods with icons: Coring to see the concrete and its compressive strength; PDA to answer whether the pile still carries its design load; excavation for shallow defects near the pile head; CSL for large bored piles with pre-installed tubes; and Parallel Seismic when there are no tubes or a structure sits on top. All results then go to the design engineer. Card four, the design engineer chooses one option, from mildest to most severe: accept for use; accept with reduced load on that pile; repair for a shallow defect; add piles with a foundation redesign; and abandon that pile and install a replacement, considering severity, pile location, load, and cost and time. A green bar below the cards shows the shortcut when step one or two concludes it is not a defect, skipping the in-depth investigation to the accept-for-use option once the engineer confirms. The bottom bar stresses recording every step in a report interpreted and certified by an engineer, retesting after repair, and that this image is conceptual, not project-specific advice. After an anomaly: 4 steps, retest → engineer decides Not to scale Report: 1 pile anomalous Early = screening, fast · low cost/pile Later = more time and budget 1 Retest Confirm it is the pile Tap many pts Repeats same depth Grind pile head smooth Retest at many points Compare all traces Test nearby piles Only this pile Go to step 2 → Zone piles similar Likely soil layering 2 Check site records Find signal source Sign Pile splice Layer edge Soft soil Stiff layer Data to compare Actual concrete volume Concreting Curve Driving log / splices Final Blow Count Soil boring results Inconclusive Go to step 3 → At splice/layer change Usually not a defect 3 In-depth checks Inconclusive piles only Coring Concrete + strength PDA Design load OK? Excavate Shallow defect near head CSL Large bored w/ tubes Parallel Seismic No tubes / covered Send all results to design engineer → Match method to need Concrete · strength · length 4 Design engineer Has decision authority 1 Accept for use No / minor defect 2 Accept + less load Nearby piles share 3 Repair Shallow: cut and recast 4 Add piles Redesign footing 5 Abandon, new pile Most severe case Consider Defect severity Pile position in layout Load · cost and time Shown not a defect in step 1 or 2 Skip in-depth checks Option 1: accept for use once engineer confirms Record all in report Engineer interprets/certifies Repaired → retest Confirm repair works Conceptual Not project-specific advice
Conceptual flowchart after a report finds an anomalous signal: ① retest ② compare construction records ③ in-depth investigation only for inconclusive piles ④ the design engineer chooses 1 of 5 options. The green bar is the shortcut when it is confirmed not to be a defect — conceptual image, not project-specific advice
SPN engineer retesting a Seismic Integrity Test at several positions on a pile head to confirm an anomalous signal
Retesting at several positions on the pile head to confirm that the anomalous signal really comes from the pile itself (Photo: SPN Soil Engineering)

Step 1: How do you retest to confirm the signal?

Retest to determine whether the signal really comes from the pile or from a poorly prepared pile head and the soil layers. This step takes little time and costs little, so it should always come first.

  • Grind the pile head smooth and clean, then retest at several positions on the pile head
  • Compare the traces from every position — if the anomalous signal repeats at the same depth every time, it comes from the pile itself
  • Test additional neighboring piles in the same zone to see whether the problem is specific to one pile or a pattern caused by the soil layers — if every pile in the zone gives a similar signal at the same depth, it is most likely an effect of the soil layers, not a defect

Step 2: Which construction records should you compare?

Use the 5 items of site data in the table below and compare them with the depth of the anomalous signal. These data help separate signals from pile splices or soil layers from genuine defects.

← Scroll to see the full table →

DataWhat it checks
Record of actual concrete volume placed (bored piles)Much more than theoretical → possible soil collapse / less than theoretical → risk of necking
Concreting Curve / concrete level vs depthIdentifies depth ranges with abnormal concrete, compared with the signal location
Driving record and splice locations (driven piles)Separates normal splice signals from defects
Final Blow CountDriven piles with abnormal blow counts may have cracked below ground
Soil boring resultsLocations of alternating soft/stiff layers that create defect-like reflections

If the anomalous signal location matches a pile splice or soil layer boundary exactly, the engineer can usually conclude it is not a defect without further checks. Having good soil investigation results therefore makes interpretation much more accurate.

Step 3: If still inconclusive, which method confirms it?

Choose the method according to the question you need answered, such as concrete quality, load capacity or pile length. This step is only for piles that have been retested and compared with records but whose signal still points to a likely genuine defect.

Concrete core sampling (Coring)

Drill into the pile shaft to the suspect level to see the actual concrete and test the compressive strength of the cores. It is the most direct way to confirm a concrete-quality defect, but it costs more and takes longer, so it is used only when truly necessary. The drilling procedure and how to interpret cores are explained in Concrete Core Test, and see the service at Non-destructive structural testing (NDE)

Concrete core sampling (Coring) from a pile shaft to investigate a defect in depth
Concrete Coring — drilling concrete cores from a pile shaft to confirm a defect at the suspect location (Photo: SPN Soil Engineering)

Dynamic Load Test (PDA)

If the question is "can this pile still carry its design load?", a PDA (Dynamic Load Test) gives a direct answer on load capacity. In some cases a pile shows a minor defect signal, but PDA confirms its capacity is sufficient, and the design engineer may accept it for use. PDA testing references DPT 1252-51 and ASTM D4945.

Excavation (for shallow defects)

If the signal points to a defect in the shallow section near the pile head, excavating to look directly may be faster and cheaper than other methods. If a crack is found, it can be repaired right away following the approach set by the engineer.

Cross Hole Sonic Logging (CSL) — for large bored piles

Cross Hole Sonic Logging (CSL) measures the integrity of the concrete across the whole cross-section of a bored pile by sending sound waves between steel tubes cast into the pile during construction. If the waves travel more slowly or weaken in certain sections, there may be a void, segregated concrete or soil inclusions. This method gives more detailed cross-section data than the Seismic Integrity Test but requires tubes installed in advance, so it suits large bored piles where CSL was specified from the start. The current reference guideline is ACI PRC-228.2; ASTM D6760-16, used previously, was withdrawn by ASTM in 2025.

Parallel Seismic Test — when no tubes were installed

If the pile has no CSL tubes, or is an existing pile with a structure on top, the Parallel Seismic Test (ASTM D8381/D8381M-21) is an option for assessing pile length and continuity. A borehole is drilled beside the pile and a sensor is lowered to record the signal generated at the pile head. The point where the signal changes abruptly indicates the pile toe or a break in the pile, which makes the method well suited to investigating existing piles.

📋 Seismic shows integrity · PDA/Coring confirms in depth

The Seismic Integrity Test is a fast screening tool with a low cost per pile, while Coring and PDA are confirmation tools that give more targeted data at a higher cost, so they are used only for piles that remain inconclusive after retesting and comparing records.

A Seismic Integrity Test costs about THB 100–150 per pile, and PDA about THB 5,500 per pile. These prices exclude travel and equipment mobilization, which are charged extra according to the site distance. For Coring, CSL and Parallel Seismic, the cost depends on the number of piles and site conditions. Contact us for a free quote within 24 hrs.

Step 4: Who decides, and how many remedial options are there?

The design engineer has the authority to decide. Once all the data are in, one of the following 5 options is usually chosen, listed from mildest to most severe.

  1. Accept for use — once it is confirmed the signal is not a defect, or a minor defect does not affect strength or durability
  2. Accept with reduced load on that pile — adjust the load distribution in the pile cap so neighboring piles carry more
  3. Repair — e.g. a shallow defect that can be exposed by excavation: break out the bad concrete and recast
  4. Add piles — drive/bore additional piles beside the problem pile and redesign the pile cap to redistribute load; usually used when the defect is severe or too deep to be worth repairing
  5. Abandon that pile and install a replacement — the most severe case

The choice depends on the severity of the defect, the pile's position in the foundation layout, the load it must carry, and the cost and time of each option. The owner or contractor should not decide on remedial work without going through the design engineer, as this affects legal liability and the structural warranty.

Every option should be documented: Seismic traces from every test, the construction records used for comparison, confirmation results, the approach set by the engineer, and retest results after repair. Check completeness with the pile test report checklist

The model below places the options on the same pile group at 3 locations: A confirm first, B repair a shallow defect, and C add piles with an enlarged pile cap. The panel on the right summarizes who does what.

Conceptual 3D model of a pile group under a pile cap with one pile reported as anomalous, comparing the options of confirmation, repair and adding piles, with a panel on who does what and which documents to keep A conceptual 3D view of a long soil block cut open to show its cross-section, with soft soil on top and a sand layer below. On the block are three pile caps not yet cast, shown as grey dashed lines, each with four piles, and the front-right pile is reported as anomalous, marked with a red circle at the pile head and a dashed red circle at the suspect location on the pile section. Location A, confirm before deciding, has a concrete coring rig on the pile head and a dashed line showing the core hole down to the suspect level at mid-length. Location B, repair for a shallow defect, has an excavation around the pile head, with the top of the pile shown as an orange dashed line meaning the bad concrete is broken out and recast. Location C, add piles for a deep defect, has two new piles shown as orange dashed lines beside the problem pile and an enlarged pile cap outline as an orange dashed line around the original. A note below states that accepting for use and accepting with reduced load on that pile need no physical repair, and that installing a replacement pile follows a principle similar to C. The panel on the right shows the roles of three parties from top to bottom: the test team retests, confirms and reports results based on actual data; the design engineer evaluates all results and decides the approach; the contractor carries out the approach set by the engineer without making its own fixes. Below is a list of documents to keep: Seismic traces from every test, construction records, confirmation results, the approach set by the engineer, and retest results after repair. This image is conceptual, not an actual remedial design. 3D model: anomalous pile in group → options engineer weighs Not to scale A Confirm first e.g. Coring to depth B Repair · shallow defect Dig, break out, recast C Add piles + enlarge cap Deep / severe defect Dashed red = Signal-indicated spot Soft soil Sand Pile reported anomalous Added / recast part Cap outline, not cast Enlarged cap outline Accept / accept with reduced load on that pile = no physical pile repair New replacement pile = similar to C · the design engineer chooses every option Who does what Test team Retest · confirm Report actual data Report + supporting data Design engineer Evaluate all results Decides approach Approach set Contractor Follow set approach No own fixes Documents to keep Seismic traces, every test Construction records used Confirmation (Coring etc.) Engineer's set approach Retest after repair Conceptual · not a remedial design
Conceptual 3D model: the pile reported as anomalous (red circle) in a pile group under a pile cap not yet cast, with options A confirm by Coring · B repair a shallow defect · C add piles and enlarge the pile cap. The test team reports, the design engineer decides, the contractor carries it out — conceptual image, not an actual remedial design

When a pile fails a Seismic Test, what should you not do?

There are 3 common mistakes:

  • Do not ignore an anomalous signal and cast the pile cap over it — checking before casting the pile cap is usually far cheaper than fixing it later, because once the structure is up, foundation remediation is difficult and expensive
  • Do not conclude a pile is defective from a single trace without retesting or looking at supporting data — this can lead to unnecessary demolition or added piles
  • Do not let non-engineers interpret results and make decisions — a report that can be relied on must be interpreted and certified by a civil engineer licensed by the Council of Engineers (COE)
📌 Note

This article provides general engineering knowledge. Interpretation, choice of confirmation methods and decisions on remedial approaches must follow the judgment of the licensed design engineer and the specific requirements of each project. It is not design or remedial advice for any particular pile.