SPN engineers testing pile integrity with the Seismic Integrity Test to locate defects in the pile shaft
Most pile defects lie below ground and cannot be seen by eye, so detection relies on measuring reflected waves or resistance during testing (Photo: SPN Soil Engineering)

What Is a Pile Defect?

A defective pile is one with a structural flaw that reduces its load capacity or cross-section integrity below the design. The flaw may take the form of a crack, a break, a reduced section or neck (necking), a cavity or void in the concrete, or segregation of the concrete.

A defective pile may stand normally at first, but under full load or over time it risks differential settlement, with cracks propagating up into the superstructure. Finding the defect before the pile cap is cast over the pile head is the point at which it can be fixed at the lowest cost.

What Causes Pile Defects? 6 Main Causes

Pile defects fall into 6 groups of causes. Causes 1, 3 and 5 mainly affect driven piles, cause 6 affects bored piles, and causes 2 and 4 can affect both. Because the construction methods differ, the risk points and test methods differ too (see how testing differs between bored and driven piles)

1. Overdriving or Excessive Repeated Driving

Driving with energy beyond the pile's strength, or continuing to drive after the pile has reached refusal, easily causes head damage. Transverse cracks mid-shaft (tension cracks) are caused by reflected tension waves, typically while the pile toe is in soft soil with low toe resistance. If the toe suddenly strikes a hard layer or a boulder, the high compressive stress can also damage the toe.

2. Substandard Concrete

Concrete with compressive strength below specification, incorrect mix proportions or insufficient curing makes the pile brittle and prone to cracking during driving. In bored piles, concrete with poor flowability segregates and leaves voids inside the shaft. Testing a pile before the concrete is old enough therefore gives results that do not reflect reality (see minimum concrete age before testing)

3. Incomplete Welded Splices

For piles spliced by welding steel end plates, a weld that is incomplete, eccentric or below standard turns the joint into a weak point that cracks or breaks easily under load. Joints also produce a clear reflection in Seismic Integrity Test results, so a "normal joint" must be distinguished from a "crack" (see inspecting welded pile joints)

4. Soft Soil Alternating with Hard Layers or Boulders

Abrupt changes in soil layers can deflect the pile or chip the toe when it hits boulders and buried debris. In bored piles, soft soil collapsing into the borehole before concreting causes necking. This group of risks drops sharply when a soil investigation report with adequate site coverage is available from the start

5. Improper Handling, Lifting and Stacking

Lifting a pile at the wrong points (not at the pick-up points), stacking it without timber bearers, or impact during transport causes cracks even before driving, which may then propagate under actual driving forces. This group of damage is often overlooked because it happens off site.

6. Defects from Casting Bored Piles in Place

Cast-in-place bored piles are prone to voids, necking and inclusions in the concrete if concreting under drilling fluid with a tremie pipe, casing withdrawal or the support slurry level is not adequately controlled.

The diagram below brings all 6 causes into one view. Read from left to right: cause → typical damage → suitable test method. The bottom band shows the sequence used in practice: screen with Seismic first, then confirm with a more detailed method.

Diagram matching the 6 causes of pile defects with the typical damage and suitable test methods A six-row table-style diagram following the article's six causes. Each row has three boxes joined by arrows: the left box is the cause with an icon, the middle box is a small pile segment showing the typical damage, and the right box holds test-method tags, where solid tags are the primary method and outlined tags are supplementary or confirmatory methods. Row one: overdriving or repeated driving causes head damage and transverse cracks mid-shaft; test with the Seismic Integrity Test, supplemented by visual inspection of the pile head and PDA during driving. Row two: substandard concrete makes the pile brittle or segregated with voids; test with Seismic, supplemented by CSL for large bored piles, and test only once the concrete is old enough. Row three: incomplete welded splices cause the joint to crack or break; test with Seismic, distinguishing a normal joint signal from a crack. Row four: soft soil alternating with hard layers or boulders causes deflection, a chipped toe, or necking in bored piles; test with Seismic, supplemented by PDA when capacity is in doubt, and reduce the risk with soil investigation. Row five: improper handling, lifting or stacking causes cracks before driving that then propagate during driving; mainly visual inspection before driving, then Seismic after driving. Row six: faulty casting of cast-in-place bored piles causes voids, necking and inclusions; test with CSL, which requires sonic tubes to be installed before concreting, supplemented by Seismic. The colors of the number circles indicate the pile type concerned: driven, bored, or both. The bottom band summarizes the sequence: quick screening of many piles with Seismic, confirmation with CSL, PDA or retesting, then an engineer assesses the severity, location and effect on load capacity. Conceptual illustration. 6 causes → typical damage → detection method Not to scale 6 main causes Typical damage Suitable method 1 Overdriving / redriving Driven past refusal · hit hard layer Head damage · cross crack Head Damage · Tension Crack Seismic Visual PDA Head: visible · shaft: test 2 Poor concrete Low strength · bad mix/cure Brittle · segregation voids Segregation · Void Seismic CSL big bored Test when concrete is aged 3 Incomplete weld Partial weld · eccentric Joint cracks or breaks Splice = weak point Seismic Tell normal joint vs crack 4 Soft/hard layers / boulders Abrupt soil change Deflected · chipped toe Bored: cave-in → necking Seismic PDA for capacity Cut risk: enough boring 5 Bad handling/lifting Wrong pick-up point · no bearers Cracked pre-driving Spreads when driven Visual pre-driving Seismic Off site, often missed 6 Faulty bored casting Poor tremie·casing·slurry Void · neck · inclusion Void · Necking · Inclusion CSL Seismic Sonic tubes before concreting Circle color: Driven Bored Both Solid Primary Outline Extra / confirm Conceptual 1 Screen Seismic on many piles 2 Confirm CSL · PDA · retest 3 Engineer assesses Severity · location · capacity
Diagram matching the 6 causes of pile defects with typical damage and suitable test methods — solid tags are primary methods, outlined tags are supplementary or confirmatory methods, and the number-circle color shows the pile type concerned (conceptual illustration; the actual method should be chosen by the responsible engineer)

Which Test Detects Each Type of Pile Damage?

There are 3 main groups of methods for detecting pile defects: the Seismic Integrity Test (low strain), Cross Hole Sonic Logging (CSL) and the Dynamic Load Test (PDA), chosen according to the type of damage and the information needed. The table below helps match each symptom to a suitable test method.

← Scroll to see the full table →

Symptom / Defect typeSuitable methodWhat it detects
Cracked, broken or necked pile; location by depth neededSeismic Integrity Test (Low Strain)Locates defects and estimates pile length from reflected waves
Voids or separation in large bored pilesCross Hole Sonic Logging (CSL)Checks concrete integrity between sonic tubes across the full section
Load capacity suspected below design criteriaDynamic Load Test (PDA)Measures load capacity and pile integrity during driving
Pile head damage visible by eyeVisual inspection + Seismic to confirmConfirms whether damage extends below ground
📌 Note

This table is a general guide. The method suited to an actual site should be chosen by the responsible engineer based on the pile type, size, depth and project-specific conditions.

Seismic Integrity Test — Whole-Pile Integrity Check, Fast and Economical

The Seismic Integrity Test (Low Strain Integrity Test) involves tapping the pile head with a hammer and measuring the reflected waves to locate defects such as cracks, necking or an abnormal pile toe. It is fast with a low cost per pile (approx. THB 100–150 per pile), so it is widely used for testing large numbers of piles. It is carried out to DPT standard 1551-51 of the Department of Public Works and Town & Country Planning.

Its limitation is that it suits piles whose length-to-diameter ratio is not too high, and it shows where a defect is but does not directly give load capacity. How to read signals and tell joints from cracks is covered in the Seismic Integrity Test result interpretation guide

Reflected-wave graph from a Seismic Integrity Test showing the signal of a pile with necking
Reflected-wave signal of a pile with necking — a reflection returning before the pile toe indicates where the cross-section is reduced (Image: SPN Soil Engineering)
Reflected-wave graph from a Seismic Integrity Test showing the signal of a pile with a transverse crack
Signal of a cracked pile — it differs from a normal splice joint signal in position and waveform, so it must be read by an experienced engineer (Image: SPN Soil Engineering)

Cross Hole Sonic Logging (CSL) — the Most Detailed Test for Large Bored Piles

CSL sends acoustic waves through water in sonic tubes cast into a bored pile and measures the travel time between tubes to assess concrete integrity across the full cross-section. It suits large bored piles where high confidence is required, as it offers greater resolution than the Seismic Integrity Test. Test method guidance refers to ACI PRC-228.2.

The trade-off is that sonic tubes must be planned and installed before concreting, so it cannot be applied retroactively to piles already cast. If this window is missed, the remaining options are the Seismic Integrity Test or other indirect methods. Read more about the test method in What Is Cross Hole Sonic Logging (CSL)? or see the scope of work on the pile testing service page

Dynamic Load Test (PDA) — Checks Both Capacity and Integrity

PDA (Dynamic Load Test) attaches force and acceleration sensors to the pile head during driving, then analyzes load capacity while assessing shaft condition from the Beta value (BTA). It is carried out to DPT standard 1252-51 and is suited to cases where the pile may not reach the design capacity, not merely where a crack is suspected.

Legally, PDA results can also be used in foundation calculations within the limits set by the foundation Ministerial Regulation B.E. 2566 (2023) (see legal criteria for allowable pile load), which the Seismic Integrity Test cannot do.

Approximate costs: Seismic Integrity Test THB 100–150 per pile and PDA from approx. THB 5,500 per pile, while CSL depends on pile size and quantity. Contact us for a free quotation within 24 hours. These prices exclude travel and equipment mobilization, which are charged extra according to site distance.

Each type of damage occurs at a different stage of piling work, so it is detected at a different time. The 3D view below places all 6 causes on a single site, and the right-hand panel follows the timeline of what to check at each stage and why testing should be completed before the pile cap is cast.

3D model of a single site combining where all 6 causes of pile defects occur, with a panel showing when and by which method each type of damage is usually detected Conceptual 3D view of a foundation construction site on a block of ground cut away to show a soft clay layer, a hard layer with boulders, and a sand layer. At back left is a stack of precast piles, and another pile is being lifted by a crane with slings tied near its end, away from the pick-up points, causing a crack mid-length, number five. In the center is a pile-driving rig with leader and drop hammer, splicing a pile with a welder working on the joint above ground, number three; in the soil section, the lower pile toe hits a boulder in the hard layer and deflects, number four. Next to it is a pile already driven but overdriven, with head damage and a transverse crack mid-shaft shown in section, number one. On the right, a concrete truck pours concrete via a chute into a tremie hopper, number two, substandard concrete, and a bored pile in a hole with upper steel casing; the section shows support slurry, concrete necking in the soft clay and soil mixed into the concrete, number six. The right-hand panel is arranged by time of detection. First, before driving, in storage and during lifting, cause five: visual inspection of every segment for cracks. Second, during driving and splicing, causes one and three: PDA to DPT standard 1252-51 during driving, plus checks of the pile head and weld. Third, after driving or casting and before the pile cap, causes one to four and six: Seismic Integrity Test on every pile to DPT standard 1551-51, and CSL for large bored piles to ACI PRC-228.2, which requires sonic tubes before concreting. Last, after the pile cap is cast over the pile heads, testing becomes much harder and only indirect methods such as Parallel Seismic remain. In summary, the cheapest point to fix defects is before the pile cap is cast. Not to scale. 3D model: 6 causes on one site + when found Not to scale 5 1 1 3 4 6 2 Crane lifting Rig + hammer Concrete truck Soft soil Hard + rock Sand Red-circle no. = cause in article Red dashed = typical defect spot 1 Overdriving / redriving 2 Poor concrete 3 Poor weld joint 4 Soft/hard / boulder 5 Bad handling·lift 6 Faulty bored cast Conceptual, not a real site When found · by which method The later, the harder to fix Pre-driving: storage / lifting 5 Visual crack check, every piece Driving / splicing 1 3 Check head · check weld PDA in driving · DPT 1252-51 After driving/pour, pre-cap 1 2 3 4 6 Seismic Integrity Test, all piles Per DPT standard 1551-51 CSL big bored · ACI PRC-228.2 Sonic tubes before concreting After cap cast over heads Much harder, indirect only e.g. Parallel Seismic Finish testing before cap Lowest-cost time to fix
3D model: where all 6 causes of pile defects occur on a single site (red-circle numbers match the cause headings) and when they are usually detected, from before driving to after the pile cap is cast — conceptual illustration, not a real site

What to Do When a Pile Defect Is Found

When test results show a defect, the first step is for the responsible engineer to assess its severity, location by depth and effect on load capacity. Possible responses range from retesting or supplementary testing with another method, to adding piles (additional piles), to revising the foundation layout, depending on the actual data for each pile.

If the pile cap has already been cast over the pile head, testing becomes much harder. The remaining option is an indirect method such as the Parallel Seismic Test, which requires drilling a borehole beside the pile to measure the waves.

SPN does not give specific remedial design advice through articles, as decisions require site data and certification by a licensed engineer. If a pile fails testing, we recommend reading Pile Failed the Seismic Test: What Next? and then consulting the project engineer to plan the next steps.

Summary

Pile defects can arise from 6 main causes: overdriving, substandard concrete, incomplete welded splices, abrupt soil changes or boulders, improper handling and lifting, and defects from casting bored piles in place. This damage cannot be seen from outside, so testing is the essential tool — the Seismic Integrity Test for broad screening to locate defects, CSL for large bored piles with pre-installed tubes, and PDA when load capacity is also needed. See the full scope of services at pile testing services

⚠️ Disclaimer

This article provides general information on the causes of pile defects and how to detect them only. It is not engineering advice for any specific project. Damage assessment and remedial measures must be determined by a licensed engineer with complete site data.