What decides whether a static load test "passes" or "fails"?
The pass criteria for a static load test are a set of numbers the design engineer writes into the project specification in advance, which are then compared with the values actually measured in a test to ASTM D1143/D1143M-20e1 and DPT 1251. Engineers look at three things together rather than drawing a conclusion from a single number.
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| What is checked | Typical criteria in Thai specifications | Fails when |
|---|---|---|
| 1. Gross settlement (Gross Settlement) at the maximum test load | At 200% of the design load, gross settlement must not exceed the value in the specification. Many specifications use about 25 mm, but some projects set a lower limit depending on pile size. | Gross settlement exceeds the specified value, even if the curve has not plunged |
| 2. Residual settlement (Residual / Net Settlement) after full unloading | Specifications usually limit permanent settlement after unloading to a few millimeters, or to a fraction of gross settlement. A sound pile should show substantial rebound. | The pile "sinks" permanently beyond the limit, indicating that the soil below the toe or around the shaft has been disturbed |
| 3. Curve shape and ultimate capacity (Curve Shape / Davisson) | The curve must not plunge steeply before the test load is reached, and the ultimate capacity by the Davisson criterion must be high enough that the allowable load (not more than 50% of ultimate capacity under the 2023 Ministerial Regulation on foundations (B.E. 2566)) covers the design load. | The curve plunges, or the Davisson line intersects the curve before 2 times the design load |
The 25 mm figure and the residual-settlement values in the table are only common examples. Each project sets different limits depending on pile size, building type and design engineer. SPN reports always state clearly which criteria the results were compared against. For how to read each value from the curve, see How to interpret static load test results, and for the origin of the 50% cap, see Allowable pile capacity under Thai law: the 40/50/40 rule
A common misunderstanding is that "failed" always means "the pile has broken". It does not. A pile that slightly exceeds the specified settlement but whose curve has not plunged may still carry load at a level lower than designed, whereas a pile whose curve plunges has truly failed. The remedies for these two cases are very different.
How many types of static load test failure are there? 3 patterns seen on site
Failures seen on site fall into 3 patterns: excessive settlement without plunging, a plunging curve, and abnormal jumps caused by the pile itself or the test system. They must be told apart before any decision is made, because each points to a different cause and remedy. The chart below compares the shape of the first two patterns with a pile that passes.
Type 1: Excessive settlement, but the curve has not plunged
The pile carries the full 200% load, but gross or residual settlement exceeds the specified value. The curve continues to bend smoothly, with no stage where settlement keeps going under constant load. In this case the pile has not failed by plunging. It is usually caused by a bearing layer weaker than assumed in design, or by soil that has not fully set up. The options a design engineer typically considers are therefore a re-evaluation of the allowable load, not abandoning the pile.
Type 2: Plunging curve — geotechnical failure
During one load increment the pile keeps settling without stopping, and the jack can barely hold the load. The curve drops almost vertically. This means the soil below the toe and around the pile can no longer carry the load (Geotechnical Failure). The load at the plunge point is the true ultimate capacity of that pile. A working pile that fails this way usually cannot continue to carry its original design load, because the surrounding soil has been permanently disturbed.
Type 3: Abnormal jumps — a problem with the pile itself or the test system
Settlement jumps suddenly at one load stage, there is a loud noise, the pile head cracks, or the readings from individual gauges do not agree. This may be a structural failure of the pile (Structural Failure), such as a separated spun-pile joint or concrete that did not reach strength, or a problem with the test system, such as an eccentric jack, a settling Kentledge support crib or a disturbed reference beam. Such results say nothing yet about the soil; the system and the pile must be checked first. For the components of the reaction system to check, see Kentledge vs reaction piles
What causes a pile to fail a static load test?
A pile fails a static load test because of 3 groups of causes: the soil, the pile itself and the test system. Knowing which group the problem belongs to is the most important step before deciding on a remedy.
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| Cause group | Common examples | How to confirm |
|---|---|---|
| Soil (Geotechnical) | Bearing layer deeper than the boreholes indicated · thin or discontinuous sand layer · sediment at the toe of bored piles (Soft Toe) · soil around the pile not yet fully set up (Soil Setup) · negative skin friction from fill | Compare the Boring Log with the actual pile length · drill an additional soil boring near the pile · review the drilling and concreting records |
| Pile itself (Structural) | Necking or a broken section in bored piles (Necking) · concrete below strength · incomplete welds in spun piles or micropiles · pile cracked during driving · pile shorter than designed | Seismic Integrity Test (DPT 1551-51) · compare the actual concrete volume placed with the theoretical volume · Cube/Cylinder Test results · Last 10 Blow records |
| Test system (Testing) | Eccentric jack or main beam · settling Kentledge support crib · reference beam exposed to sun or disturbed · Load Cell/gauges not calibrated · load held for less time than the standard requires | Equipment calibration certificates · check the line of loading and crib spacing · compare readings between gauges · review the load-holding time log |
In Bangkok's soft clay, "soil" causes are common, especially piles whose toes do not reach the first sand layer as designed. Read about the importance of soil layers in Bangkok soft clay vs provincial soils, and about piles that pass the test but still settle due to fill in Negative Skin Friction
The 2D drawing below is a section through a Kentledge static load test setup showing the points to check when the result fails, color-coded by cause group: soil, pile and test system. The next 3D view shows the additional checks around the pile, following the sequence of steps in the next section.
Steps after a failed static load test: what to do first?
After a failed result, follow the 7 steps below in order. The key principle is "collect data first, decide later", because site data cannot be recovered once the setup has been dismantled. This sequence is the approach SPN engineers use; the decision on remedial measures belongs to the project's design engineer.
- Do not dismantle the test setup yet — record every load stage and time in full, and photograph the pile head, jack, cribs and every gauge. If the load has not been fully released, unload in steps while measuring rebound, because the rebound value is key evidence of whether the failure is in the soil or in the pile.
- Check the test system before blaming the pile — review the calibration certificates for the Load Cell and gauges, the jack's line of action, the distance of the cribs from the test pile, and whether the reference beam was disturbed. If the system had a problem, the result cannot be used to judge the pile and the test must be repeated.
- Review the construction records for that pile — for driven piles, check the Blow Count and Last 10 Blow to see whether the driving refusal criterion was truly met; for bored piles, check the depth drilled, the concrete volume placed versus the theoretical volume, and the time between completing the hole and concreting. Then compare with the Boring Log of the nearest borehole. See how to read driving records in Blow Count and Last 10 Blow
- Check pile integrity with additional tests — perform a Seismic Integrity Test to DPT 1551-51 on the failed pile and adjacent piles to look for necking, cracks or a length different from the design. If the soil layers are in doubt, drill 1 additional soil boring near the pile, and PDA calibrated against the static result may be used to spot-check other piles in the same zone.
- Design engineer analyzes the cause and a three-party meeting is held — the owner, the design engineer and the contractor must see the same data set: the load-settlement curve, Seismic results, construction records and Boring Log, and then conclude whether the problem is limited to one pile or affects the whole zone.
- Choose a remedy — see the table below. The choice depends on the type of "failure", how far the result is from the criteria, and whether it is a test pile or a working pile.
- Verify by testing and record it in the report — every remedy must be closed out with a verification test as specified by the engineer, and the report must describe the whole sequence of events, not keep only the passing result. SPN reports are certified by a civil engineer holding a professional engineering license from the Council of Engineers Thailand (COE), for use by the design engineer and the relevant authorities in their assessment.
What are the remedies when a pile fails?
There are 5 commonly used remedies, ranging from re-testing, reducing the allowable load, adding piles and changing the pile length or type, to repairing or replacing the pile in question. The choice must come from the project's design engineer. This table gives owners and contractors an overview before the meeting.
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| Approach | Suitable when | Precautions |
|---|---|---|
| Re-test after a waiting period | Slight type 1 failure, the pile is sound (passes Seismic), and the soil is suspected not to have fully set up, or the test system had a problem | Needs a clear engineering reason, not "test until it passes". Re-testing a pile that has already plunged is usually meaningless. |
| Reduce the allowable load + add piles/revise the foundation | Settlement over the limit but still adequate capacity at a level below the design load; observe the 50% cap on ultimate capacity under the 2023 Ministerial Regulation on foundations (B.E. 2566) | All affected foundations must be recalculated, and other piles in the zone checked for the same behavior |
| Add adjacent piles + enlarge the pile cap | A working pile that has failed or lacks capacity, where the building column position must stay unchanged | The added piles must also be tested, and the pile cap must be designed to actually transfer load to the new piles |
| Change pile length/type for piles not yet installed | A test pile fails before production piling starts, or the problem is found to affect the whole zone | Must be based on soil investigation results, and a new test pile may be needed to verify the revised design. See the options in Which pile type should you choose? |
| Repair or replace the pile in question | Structural failure of the pile, such as a separated joint or concrete below strength | The construction-quality cause must be found, and other piles installed in the same period checked |
Do not re-test immediately without finding the cause, do not change the specification criteria after seeing the result, and do not use the result of a single pile to draw conclusions about the whole project, whether good or bad. Every decision must be signed off by the design engineer, who takes responsibility for it.
Test pile failure vs working pile failure: what is the difference?
A test pile is a pile installed for testing before production piling begins. A failed test pile is therefore "information", not "damage". Engineers use the result to adjust the pile length, size or type before installing piles across the whole project, which is the purpose of a test pile in the first place.
A working pile is a pile that will carry the actual building. A failure therefore means the foundation at that location must be remedied, and it raises the question of whether other piles installed in the same zone, by the same method, have the same problem. That is why many specifications require more piles to be tested when one fails. Read about the differences in depth in Test Pile vs Working Pile, and about how many piles must be tested in How many piles need to be tested?
Can you re-test, and when does a re-test make sense?
A re-test means performing another static load test on the same pile after a waiting period. It only makes sense if at least one condition is met: the first test system had a provable problem, or the pile is still sound and failed only slightly with Soil Setup suspected. In clay, setup can take several days to several weeks (read more about setup in What is a restrike test?). For waiting times based on concrete age, see At what concrete age can a pile be tested?
Conversely, once a pile's curve has plunged, the soil below the toe and around the pile has been permanently disturbed, and a re-test usually gives a lower capacity than before, so it is not a solution. Each re-test takes another 3–7 days on site; see the timeline in How many days does a static load test take?
How do you prevent a static load test failure from the start?
Most problems that lead to failure start before test day, from the soil investigation and pile installation through to preparing the test system. These 5 points greatly reduce the risk.
- Make the soil investigation deep enough and cover the site — boreholes must go deeper than the expected pile toe, and there must be enough of them for the project area. See the guidelines in How many boreholes, and how deep?
- Install test piles before production piling — so the test results come in time to adjust the design, rather than testing for paperwork after half the project has been piled.
- Control quality during pile installation — record the Blow Count, inspect welds, and control toe sediment and concrete volume for bored piles. See the checkpoints in What causes pile defects?
- Screen with Seismic Integrity Test before selecting piles for static testing — it is low-cost and can be done on almost every pile, so the static load test budget is not wasted on a pile that is already defective.
- Set up the test system correctly — equipment with calibration certificates, Kentledge cribs on firm ground, and an independent, shaded reference beam. See how to choose a testing company in Which pile testing company should you hire?
Summary
A failed static load test means the gross settlement, residual settlement or curve shape does not meet the criteria in the project specification. There are 3 failure patterns with different remedies: excessive settlement without failure, a plunging curve, and abnormal readings caused by the pile itself or the test system. The correct procedure is to collect complete data, check the test system, review the construction records and check integrity with a Seismic Integrity Test; the design engineer then chooses the remedy, from re-testing and reducing the allowable load to adding piles. What not to do is re-test without finding the cause, or use the result of one pile to draw conclusions about the whole project. If you need an engineer to help analyze a failed result or plan additional testing, contact SPN's pile testing team any business day.