House extensions such as a rear kitchen, a carport or an extra side room often use micropiles because they can be driven in tight spaces with little vibration. But many homes skip pile testing and then run into settlement or cracking after the extension is finished. This article explains in plain terms for homeowners whether to test, which method suits micropiles, and whether it is worth it.
What Are Micropiles and Why Are They Popular for House Extensions?
A micropile is a small pile with a cross-section of about 20–30 cm, driven in short segments that are welded together as they go down until they reach a firm bearing layer. In this article, the term means the small precast piles driven in short segments that are commonly used for house extensions, such as I-section or hollow round sections.
Their main advantage is the small piling rig, which can get into narrow spaces beside a house, and vibration is lower than with ordinary driven piles. This makes them well suited to extensions built right against the existing house or a neighbor's house.
Note that micropiles are driven in segments of about 1.50 m each (1.00–2.00 m in some systems) and joined by welding steel plates at the segment ends. A 15 m pile is therefore made of about 10 segments, with about 9 welded joints along its length. This is why the choice of test method for micropiles differs from that for bored piles or long-segment driven piles.
Why Shouldn't House Extensions Skip Pile Testing?
Because problems with extension piles cannot be seen from the ground surface and usually show up as cracks after construction is finished. Micropile testing confirms 2 things: (1) the pile is intact, not broken, necked or separated at a joint, and (2) the pile reaches the firm layer and can carry the design load. The reasons not to skip it are:
- Bangkok's soft clay settles a lot The Bangkok Soft Clay layer means piles that are short or do not reach the firm layer can easily settle.
- Extensions often settle apart from the existing house If the extension piles have a problem, the extension settles differently from the original house, causing separation and cracks at the junction.
- Fixing it later costs far more Testing is charged per pile and usually only a sample of piles is tested, whereas repairing an extension that has already settled, such as underpinning the foundation or demolishing and rebuilding, usually costs many times more than the testing.
How many piles to test, which method to use, and the load acceptance criteria are at the discretion of the engineer responsible for the project. This article provides general information and is not a project-specific design.
What Problems Can Occur After Driving Micropiles?
There are 4 common micropile problems in house extensions: not reaching the firm layer, separated joints, broken or necked piles, and off-center piles.
- Pile does not reach the firm layer Driving stops before reaching the bearing layer, so the load capacity is insufficient.
- Joints between segments come loose or separate due to incomplete welding.
- Pile breaks, necks or cracks from driving through a hard layer or hitting debris underground.
- Off-center or misaligned so the load transfer differs from the design.
These problems usually show up as “the extension is settling” or “cracks where the old house meets the extension”. By the time the symptoms appear, they can rarely be fixed simply.
The section below brings these problems together in one image. The gray marks on the micropiles are the welds at every segment, the red dashed circles numbered 1–3 are the risk points, and the box on the right shows which method checks each one.
Which Test Method Is Best for Micropiles?
The short answer is the Dynamic Load Test (PDA) together with weld inspection and driving records for every pile. The Seismic Integrity Test (Low Strain), commonly used on bored piles and long-segment driven concrete piles, is not recommended for micropiles, because welds every 1.50 m make the reflected-wave graph impossible to read conclusively. Details follow in the next section.
← Scroll to see the full table →
| Test method | What it checks | Best for | Reference price |
|---|---|---|---|
| Dynamic Load Test (PDA) | Load capacity and pile damage (BTA value) | Recommended for micropiles Confirms the pile actually carries the load | ~THB 5,500/pile |
| Seismic Integrity Test (Low Strain) | Integrity of the pile shaft; detects defects such as breaks/necking | Bored piles and long-segment driven piles — not recommended for micropiles | ~THB 100–150/pile |
Prices in the table are reference prices per pile and exclude travel and equipment mobilization, which are charged extra according to site distance.
PDA and site quality control answer different questions. Site QC tells you whether each pile has its full length and well-made joints, while PDA tells you whether representative piles can carry the design load. They should therefore be used together.
Why Is the Seismic Integrity Test Not Recommended for Micropiles?
The Seismic Integrity Test, or Low Strain Integrity Test, is performed to DPT standard 1551-51 (with ASTM D5882-16 as the reference method, which ASTM withdrew in 2025). A small hammer taps the pile head, sending a stress wave down the pile shaft. When the wave passes a point where the pile's “impedance” changes, such as a neck, a break or the pile toe, part of the wave reflects back up to the pile head. The engineer reads the position and shape of the reflected waves to judge whether the pile is intact.
The problem is that a micropile is not a single continuous pile, but one built from short segments of about 1.50 m welded together, which conflicts with several basic assumptions of the method.
- Every weld is a reflection point The steel plate at each joint has a different impedance from the concrete, so every weld sends back its own set of reflections. A 15 m pile has about 9 welds, so the graph is full of reflection peaks from the joints, mixed with multiple reflections bouncing between joints, until it is impossible to tell which peak is a real defect.
- The method's resolution is too coarse for 1.50 m spacing The hammer produces a pulse with an impact duration of about 1 millisecond, while the wave travels through concrete at about 3,500–4,000 m/s, so the transmitted signal is several meters long. The method therefore struggles to separate anomalies less than about 1–2 m apart, which is close to the weld spacing of micropiles.
- The pile is too slender for the method to work well Micropiles have a 20–30 cm cross-section but are 12–21 m long, a length-to-section ratio (L/D) of about 40–100, while Low Strain usually gives clear results only on much less slender piles (a common guideline is around L/D 20–30). The wave is heavily damped by soil friction, and the toe reflection is often not seen, so the actual length cannot be confirmed.
- The cross-section is not uniform along the length Many micropile systems are prestressed concrete combined with steel (such as I-sections or hollow round sections) with thicker steel at the segment ends, so the reference wave speed and impedance are not constant. Locating damage from wave travel time is therefore inaccurate.
The overall result is an “inconclusive” report, with the risk both of reporting damage in a sound pile (from weld reflections) and of missing real damage hidden in the noise. You pay for the test but get no conclusion you can base a decision on, so it is not worthwhile for short-segment micropiles.
Why Is the Dynamic Load Test (PDA) Suitable for Micropiles?
The Dynamic Load Test, or PDA Test (DPT 1252-51, ASTM D4945), measures pile load capacity by dropping a hammer on the pile head and reading the signals from strain and acceleration sensors attached to the side of the pile. This method suits micropiles better because:
- It answers the question homeowners really want answered: “Can the pile carry the design load?”, not just whether the pile shaft is intact.
- The drop hammer delivers far more energy than a hand hammer, so the wave reaches the pile toe even in a slender pile.
- It provides a pile damage indicator (BTA, or Beta) from the same analysis. If a weld has separated so that it cannot transfer force, this shows up both in the signals and in a drop in the calculated load capacity.
For sampling criteria, see the article How many piles need to be tested. For choosing the method and number of piles, consult an SPN engineer through our pile testing services to suit actual site conditions.
So How Are the Welds and Pile Length Checked?
The answer is quality control on site during driving, for every pile. Without the Seismic Test, the driving records are the main evidence for the welds and pile length. They include:
- Visual inspection of every weld before the next segment is driven. Welds must be full, free of porosity or burn marks, and welders must be qualified as specified.
- Recording the number of segments and cumulative length to confirm the pile reaches the design depth.
- Recording the blow count or final penetration (Final Set), which indicates that the pile toe has reached the firm layer.
- Additional non-destructive weld testing when the work is critical and the engineer specifies it, such as non-destructive testing with Dye Penetrant or ultrasonics.
The advice above is a general rule for the short-segment micropiles used in house extensions. For small-diameter piles cast or installed as a single continuous length, Low Strain can still be used as normal. The final decision rests with the engineer responsible for the project, who sees the actual site conditions.
The 3D view below sets out the work sequence on a narrow plot beside the house: during driving, every joint is welded and inspected and the number of segments recorded; then a PDA test is carried out on representative piles before the foundations are cast. Read it alongside the box on the right, which lists the steps from consulting the design engineer to issuing the report.
Is Micropile Testing Worth It for a House Extension?
Yes, compared with the damage that could occur. A PDA Test costs about THB 5,500/pile, and usually only some piles are sampled as the engineer specifies. Repairing a settling house, demolishing the extension structure or underpinning the foundation usually costs many times more than the testing and disrupts daily life during the repairs.
The most cost-effective time is after the piles are driven and before the foundations are cast, because if a pile has a problem, it can still be fixed or supplementary piles driven before anything is built on top.
If your house is already settling before the extension, also read House settlement and extensions: why soil investigation comes first.
In addition, the test report is certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number. It can be used in the extension permit application to the relevant authorities and serves as a traceable record of the test results.
The prices above exclude travel and equipment mobilization, which are charged extra according to site distance. For a net price for your site, contact us for a free quotation within 24 hours.
Summary: How Should an Extension on Micropiles Be Tested?
House extensions on micropiles should have their piles tested, especially in Bangkok's soft clay areas, and the right method should be chosen. The Dynamic Load Test (PDA) suits micropiles because it confirms the actual load capacity and also gives a pile damage indicator. The Seismic Integrity Test is not recommended, because micropiles are driven in segments of about 1.50 m and the closely spaced welds cause joint reflections that mask the signals of real damage. Alongside testing, welds must be inspected and driving records kept on site for every pile. Testing charged per pile reduces the risk of settlement and cracking, which cost many times more to fix later.