Why Is the Weld the Weakest Point of a Spliced Pile?
The weld is the weakest point because it is the only part of the pile made on site, yet it must carry compression, tension and bending moment. Spun piles and micropiles are manufactured as prestressed concrete sections under factory quality control and then joined on site by welding the steel end plates around the circumference. Welding is done under the sun, in the rain, in tight pits and under time pressure, so joint quality varies more than any other component of the pile.
Mechanically, the joint does not carry compression alone. During driving, the stress wave traveling down the pile reflects back up as tensile stress, especially while the pile toe is still in soft soil. In addition, when the pile is out of alignment or the lateral soil is non-uniform, the joint must also carry bending moment. A weld that does not cover the full section is therefore likely to separate at the joint before the pile body is damaged.
What is more dangerous is that most defects do not show up immediately. The pile still drives, and still passes the load on test day, but small cracks or pores that let moisture reach the steel become the starting point of corrosion and failure years later. An incomplete welded joint is therefore one of the 6 main causes of pile defects
A micropile made of 1.50-meter sections driven to 21 meters has about 13 welds per pile. Every one of them is a chance for a defect and needs the same level of quality control — not just the first joint.
What Defects Occur in Pile Welds? 5 Common Types on Site
There are 5 weld defects commonly found in pile splicing: incomplete joint penetration/incomplete fusion, undercut, porosity, slag inclusion and cracks. Each has different causes and inspection methods. Some can be seen visually; others are hidden inside the weld metal and can only be found by non-destructive testing (NDT). For an overview of NDT methods for steelwork, see the article Steel Weld Inspection with NDT
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| Defect type | Appearance | Common causes | Inspection | Effect on the pile |
|---|---|---|---|---|
| Incomplete penetration / incomplete fusion Incomplete Joint Penetration / Incomplete Fusion |
Two conditions that often occur together — incomplete joint penetration means the weld metal does not reach the full thickness of the joint, the weld is concave below the plate surface, or some stretches have no weld metal at all; incomplete fusion means the weld metal has not fused with the base metal, leaving a gap along the groove face | Welding current too low, travel speed too fast, unclean contact surfaces, weld not completed around the full 360° | Visual inspection (VT) with a weld gauge to measure weld size, confirmed by ultrasonic testing (UT) when the specification requires | Actual load-bearing section smaller than designed; risk of separation during driving or under tension |
| Undercut Undercut |
A long groove along the weld toe, eating into the base metal | Welding current too high, wrong electrode angle, uneven travel speed | Visual inspection (VT) and groove depth measured with a weld gauge | Reduces the effective thickness of the plate and creates a stress concentration where fatigue cracks start |
| Porosity Porosity |
Small round pores scattered on the weld surface, or gas cavities inside the weld metal | Moisture in the electrodes or on the steel surface, rust, oil, strong wind blowing away the shielding gas | VT for surface pores, Dye Penetrant (PT) for small surface-breaking pores, UT for internal cavities | Reduces the load-bearing area and lets moisture reach the steel, accelerating long-term corrosion |
| Slag inclusion Slag Inclusion |
Dark lumps of slag trapped in the weld metal, usually in multi-pass welds and not visible from the surface | Slag not removed before the next pass, groove angle too narrow for the electrode to reach the root | Mainly ultrasonic testing (UT), since it is an internal defect that VT cannot see | Makes the weld brittle with hidden weak spots; may crack under impact during driving |
| Crack Crack |
Fracture lines in the weld or in the heat-affected zone (HAZ), usually clearly visible after the weld cools | Cooling too fast from rain or water in the pit, hydrogen in the weld, or eccentric splicing causing bending | VT with Dye Penetrant (PT) or Magnetic Particle (MT) | The most serious; must be ground out and rewelded as directed by the supervising engineer — driving must not continue |
If the upper and lower sections are not in line (misalignment) or the gap between the plates is not uniform around the circumference, the weld must carry additional bending as soon as driving starts, and it often ends in cracking. This problem is fixed at the fit-up stage before striking the arc, not by adding more weld metal. Acceptable tolerances should follow the project specification and each pile manufacturer's requirements.
How Are Pile Welds Inspected, and What Is Done at Each Stage?
Pile weld inspection is divided into 3 stages: before welding, during welding and after welding. Most defects can be prevented in the first 2 stages, while the post-weld stage confirms the result and records evidence. Walking over to look once the weld is finished is therefore not enough.
1. Before Welding — Fit-up Check
- Both end plate surfaces are clean, with no soil, oil or flaking rust
- The upper and lower sections are in line, checked with spirit levels on 2 perpendicular sides
- The gap between the plates (root gap) is uniform around the circumference
- Electrodes are of the specified type and size, stored dry, with no rust or moisture
- Welders are qualified as required by the project, and the welding machine can be set to the current in the welding procedure specification
2. During Welding — Process Control
- Weld the full 360° circumference without skipping hard-to-reach areas
- Weld in the specified number of passes and remove slag from every pass before the next
- Weld alternately on opposite sides to reduce distortion from heat building up on one side
- Do not pour water to speed up cooling, as it makes the weld metal hard, brittle and prone to cracking
- If it rains or water collects at the pile head, cover it and drive off the moisture first
3. After Welding — Inspect and Record
- Always remove slag and clean the weld surface before inspection
- Visual inspection (VT) of 100% of joints is always the first line of defense
- Spot-check with PT / MT / UT using the methods and rates set in the project specification
- Take photos labeled with the pile number, joint location and joint depth
- Apply anti-corrosion paint or mortar encasement as per drawings before driving the joint into the ground
The inspection methods and sampling rates are not fixed numbers that apply to every project. Follow the designer's specification and the pile manufacturer's requirements. If the drawings do not specify them, discuss with the supervising engineer before driving the first pile.
The 2D diagram below shows a side view during joint fit-up and a detail of the V-groove filled with weld passes, while the 3D view shows on-site pile splicing with spirit levels on two sides and an alternating welding sequence. Read them together with the 3 inspection stages above.
What Needs Special Care with 1.5-Meter Micropile Sections?
Micropiles in 1.50-meter sections need care on 2 counts: many times more welds than spun piles, and welding conditions in tight pits. Micropiles for house extensions or foundation strengthening usually come in short sections because they must be carried into tight spaces and installed with a small rig. Driven to 15 meters they have about 9 welds, and to 21 meters about 13, whereas spun piles in 6-meter sections have only 2–3 welds at the same depth.
Besides the number, site conditions are harder. Welders must work in tight pits where they cannot reach every side, the welding angle is inconsistent, and water often collects at the bottom of the pit, making the weld cool too fast. All this makes weld quality control more important for micropiles than for other pile types
Because joints occur every 1.50 meters, reflections from the many joints overlap until it is impossible to tell which are normal joints and which are real cracks. SPN therefore recommends PDA (Dynamic Load Test) together with on-site weld quality control, rather than relying on seismic test results alone.
For house extensions, the article House Extension with Micropiles: Do You Need a Pile Test? explains why testing is advisable, suitable methods and common micropile problems.
How Do Welds Affect Pile Test Results?
Welds affect each pile test method differently. The Seismic Integrity Test sees joints as reflected waves but cannot directly tell weld quality. PDA can detect joints that separate during driving, while a compression Static Load Test may show no sign of a weld defect at all.
Seismic Integrity Test (DPT 1551-51) sends a low-strain stress wave down the pile. When the wave passes a joint with a change in cross-section or density, part of it reflects back and appears as a step in the velocity trace. The analyst must know the joint depths in advance to tell whether a step is a normal joint or a real crack (see How to Read Seismic Integrity Test Results)
PDA / Dynamic Load Test (DPT 1252-51, ASTM D4945) measures force and velocity at the pile head under hammer impact. If a weld has separated, a reflection returns at a time corresponding to the joint depth, the force trace drops abnormally compared with the velocity trace, the BTA value indicating the degree of cross-section damage falls, and CAPWAP analysis shows a discontinuity in the pile at that location.
Static Load Test (ASTM D1143) applies mainly compressive load to the pile head. An incomplete weld whose plates still bear tightly against each other may therefore pass the test, even though its tension and bending capacity is lower than it should be. This is why pile testing cannot replace on-site weld quality control — both must be done together.
Record the depth of every joint during driving and send the record to the test team before test day. This makes reading seismic test traces much more accurate and reduces cases where normal joints are reported as defects.
What to Check When Inspecting Pile Welds on Site: an 8-Point Checklist
Use these 8 points to inspect every joint, from before striking the arc until before driving the next section. If any point fails, correct it as directed by the supervising engineer before continuing to drive.
- End plate surfaces clean, dry, with no flaking rust or oil
- Upper and lower sections in line, checked with spirit levels on 2 perpendicular sides
- Correct electrode type, stored dry, welding current set as specified
- Full 360° weld with the specified number of passes
- Slag removed from every pass and the surface cleaned before inspection
- Visual inspection (VT) of every joint — no cracks, porosity or undercut beyond the acceptance criteria
- Spot-check with PT / MT / UT at the rate set in the project specification
- Photograph and record the pile number, location and depth of every joint
This checklist is general guidance. If the project has specific requirements from the designer or pile manufacturer, always follow the project documents.
Summary
The weld is the only part of a spliced pile made on site, so it is where quality varies most and is hardest to control. The 5 common defects are incomplete joint penetration, undercut, porosity, slag inclusion and cracks; for slag inclusion and cracks, visual inspection alone is not enough.
The practical approach is to check fit-up before striking the arc, control the process during welding, then inspect visually 100% and spot-check with NDT per the specification. For micropiles with closely spaced joints, use PDA together with weld quality control rather than relying on seismic test results alone. If you are unsure which weld inspection or pile testing method your project should specify, consult SPN's engineering team directly.