What Is a Dynamic Load Test (PDA Test)?

Dynamic Load Test, or PDA Test (named after the Pile Driving Analyzer instrument), is a test of pile load capacity by high-strain dynamic testing in accordance with ASTM D4945 and the Department of Public Works and Town & Country Planning standard DPT 1252-51 (Standard for Dynamic Load Testing of Piles). A drop hammer strikes the pile head to generate a stress wave, and gauges measure force and velocity at the pile head for analysis based on stress wave theory.

Besides load capacity, a single test also provides pile integrity, driving stress and transferred energy, which is why it is widely used for acceptance testing of large numbers of piles, both driven and bored.

SPN engineers performing a Dynamic Load Test (PDA) at a pile head on site, warehouse and office building project
Dynamic Load Test (PDA) on a real site by the SPN engineering team — a hammer strikes the pile head while the force and velocity of the stress wave are measured (Photo: SPN Soil Engineering)

How Does a PDA Test Work? The High-Strain Principle

PDA uses a high-energy impact to send a stress wave down the pile, then reads how much resistance the soil provides from the difference between the force and velocity signals at the pile head.

When the hammer strikes the pile head, a stress wave travels down the pile. Strain transducers and accelerometers mounted near the pile head convert the signals into “force” and “velocity” over time. In the early part of the record, before reflections arrive, force and velocity are proportional. As the soil along the shaft and at the toe resists the wave, the two signals separate, and the engineer uses this difference to evaluate load capacity. For a step-by-step guide to reading force versus Z·v plots, see PDA Test Result Interpretation

Drop hammer lifted by a crane above the pile head to create a high-strain impact in a PDA test
A crane lifts the drop hammer and releases it onto the pile head, creating a high-strain stress wave that mobilizes the soil's resistance (Photo: SPN Soil Engineering)

Test Setup (Side Elevation and 3D Model)

The setup consists of a crane lifting a steel hammer that drops along a hammer guide onto the pile head through a pile cushion. Before testing, a pit is excavated around the pile head with a clearance of about 0.8 m from the pile surface and a depth of about 1.5B–2.0B (B = pile cross-section size), subject to each project's specifications, so that the gauges can be mounted at least 1.5B below the pile head as required by ASTM D4945. Strain transducers and accelerometers are mounted in pairs on opposite sides of the pile, and the cables are connected to the PDA unit.

Side elevation of the Dynamic Load Test (PDA) setup — a crawler crane lifts a steel hammer along a hammer guide, a pit around the pile head 0.8 m wide and 1.5B–2.0B deep, and the positions of the strain transducers and accelerometers
Side elevation of the Dynamic Load Test setup — steel hammer, hammer guide, pit around the pile head (0.8 m clearance, 1.5B–2.0B deep) and gauge positions (Photo: SPN Soil Engineering)
3D model of a Dynamic Load Test (PDA) showing a crawler crane, a steel hammer in its guide, the test pile in a cutaway pit, and a close-up of the strain transducers and accelerometers cabled to the PDA unit
3D model of the PDA test setup — a soil cutaway showing the test pile below the pit, with a close-up of the strain transducer / accelerometer positions at the pile head (Photo: SPN Soil Engineering)

What Formulas Does PDA Use to Calculate Capacity?

PDA relies on three main formulas from stress wave theory: force from measured strain, pile impedance, and the CASE Method formula, which uses signals at two instants 2L/c apart to estimate total resistance:

Force from strain:   F = E · A · ε E = elastic modulus of the pile, A = cross-sectional area, ε = measured strain
Pile impedance:   Z = E · A / c c = wave speed in the pile — in the early part of the signal, force relates to velocity by F = Z·v
Total resistance (CASE Method):   Rtotal = ½ [ F(t1) + F(t2) ] + (Z/2) [ v(t1) − v(t2) ] t2 = t1 + 2L/c — static resistance = total resistance − soil damping (using damping factor Jc)

What Is the Difference Between the CASE Method and CAPWAP?

The CASE Method gives an immediate estimate on site, while CAPWAP is a detailed office analysis that confirms the static capacity. PDA results are therefore analyzed at two levels:

  • CASE Method — a quick closed-form calculation in the field using a soil damping factor (Jc), giving an approximate load capacity immediately on site
  • CAPWAP (Signal Matching) — an in-depth office analysis that matches the measured signals to a model to obtain a reliable static capacity, together with the distribution of shaft friction, end bearing and a simulated load–settlement curve (read more in What Is CAPWAP Analysis)
Pile Driving Analyzer (PDA) displaying force and velocity signals with the load capacity estimated by the CASE Method in the field
The PDA unit records and displays force/velocity signals in the field and estimates capacity with the CASE Method before the in-depth CAPWAP analysis (Photo: SPN Soil Engineering)

What Does a PDA Test Measure?

A single PDA test provides four types of data at once:

  • Load capacity — both shaft friction and end bearing
  • Pile integrity (BTA) — detects cracks or reductions in cross-section
  • Driving stress — maximum compressive and tensile stresses, to prevent pile damage during driving
  • Transferred energy — evaluates the efficiency of the pile driver/hammer

What Are the Steps of a Dynamic Load Test?

PDA work has four stages: site preparation, field testing, CAPWAP analysis and the certified report. The first two take place on site; the last two are done in the office.

The diagram below summarizes the whole PDA process in one view. Read the cards from left to right: cards ①–② are site work and cards ③–④ are office work. Then check the bottom panels for what PDA results can answer and which questions need other methods.

Overview of the Dynamic Load Test (PDA) in four stages, from site preparation, field testing and CAPWAP analysis to the engineer-certified report, with a summary of what PDA does and does not answer Conceptual infographic with four cards arranged left to right and connected by arrows. Cards one and two sit under a bracket labeled “On site”; cards three and four sit under a bracket labeled “In the office”. Card one, site preparation before test day: a section showing the pit around the pile head with about 0.8 m clearance from the pile surface and a depth of 1.5B to 2.0B; preparation items are pile age ready (bored piles wait for concrete age, driven piles rest 1 to 14 days before restrike), pile head flat and square with holes drilled for gauge anchors, and crane, hammer, hammer guide and pile cushion ready. Card two, field testing, about 5 to 10 points per day: the image shows a steel hammer in its guide above the pile head, with gauges mounted not less than 1.5B below the pile head and cabled to the PDA unit; two strain transducers and two accelerometers are mounted on opposite sides; the hammer is dropped with enough energy to mobilize the soil resistance; the PDA records force and velocity and gives an immediate estimate by the CASE Method. Card three, CAPWAP analysis: an illustrative plot of the measured signal as a solid line overlaid with the modeled signal as a dashed line; results are a static capacity more reliable than the CASE value, the split between shaft friction and end bearing, and a simulated load–settlement curve. Card four, certified report within a few working days: a document image with an engineer's signature and seal, signed by a licensed civil engineer (COE), stating the standards ASTM D4945 and DPT 1252-51 for PDA testing, and used for pile acceptance with the designer and relevant authorities. Below each card is a tag pointing to a follow-up article: pile head preparation, PDA result interpretation, CAPWAP and PDA timeline. The bottom-left panel lists what PDA answers from a single test: load capacity including shaft friction and end bearing, the pile BTA value, maximum compressive and tensile driving stresses, and energy transferred from the hammer. The bottom-right panel lists what PDA does not answer: capacity under actual static loading, which must be calibrated with a Static Load Test on important projects; static capacity from the CASE value alone, which requires CAPWAP analysis; full capacity when the hammer energy is insufficient; and long-term capacity from end-of-initial-drive results in clay, which requires a restrike test. B is the pile cross-section size; actual values follow the project specifications. Dynamic Load Test (PDA): 4 stages in one view Not to scale On site In the office 1 Site preparation Before test day 0.8 m Depth 1.5B–2.0B Pile age ready Bored: wait concrete age Driven: rest 1–14 days Head flat, square Drill gauge holes Crane + hammer + guide Pile cushion More: pile head prep 2 Field testing Approx. 5–10 points/day ≥1.5B PDA Gauges 2 + 2 Strain + Accelerometer Opposite sides Drop the hammer Energy to mobilize soil CASE Method on site PDA records F and v Instant estimate More: PDA results 3 CAPWAP analysis Refines the CASE value Meas. Model Signal matching Measured ↔ model (Signal Matching) Static capacity Reliable value Shaft / toe split + simulated load–settlement More: CAPWAP 4 Certified report In a few working days PE States standards used ASTM D4945 DPT 1252-51 (PDA) Signed by licensed CE Certifies test results For pile acceptance To designer & authorities concerned More: PDA timeline What PDA answers 4 data types from one test Load capacity Shaft friction + end bearing Pile integrity BTA value Driving stress Max. compression / tension Transferred energy Hammer / rig efficiency What PDA does not answer and what to add Capacity under real static load → calibrate with Static Static capacity from CASE alone → use CAPWAP Full capacity if hammer weak → use a bigger hammer Long-term capacity from EOID (clay) → Restrike B = pile section size · Plot in card ③ is illustrative only · Actual values follow project specifications
Dynamic Load Test (PDA) overview in 4 stages: ① site preparation (pit with ~0.8 m clearance, 1.5B–2.0B deep) ② field testing at about 5–10 points per day with immediate CASE values ③ CAPWAP analysis in the office ④ report certified by a licensed civil engineer (COE) within a few working days — the bottom panels summarize what PDA does and does not answer. The plot in card ③ is illustrative, not measured data (Image: SPN Soil Engineering)

The detailed steps, in the actual order of work, are:

  1. Trim the pile head flat and square, drill holes for gauge anchors, excavate a pit around the pile head, and set up the crane and drop hammer
  2. Mount 2 strain transducers and 2 accelerometers on opposite sides of the pile, at least 1.5B below the pile head
  3. Drop the hammer onto the pile head (high strain) with enough energy to mobilize the soil resistance
  4. The PDA unit records force and velocity over time for each blow
  5. Make a preliminary evaluation in the field with the CASE Method
  6. Perform CAPWAP analysis in the office and prepare a report certified by a licensed civil engineer (COE)

Before testing, check whether the pile concrete is old enough to test, complete the pile head preparation before testing, and plan the number of piles to sample for testing to suit the size of the project

SPN technician mounting strain transducers and accelerometers on the side of the pile head to prepare for a Dynamic Load Test (PDA)
Drilling anchors and mounting the gauges (strain transducer and accelerometer) on the side near the pile head before high-strain testing (Photo: SPN Soil Engineering)
📋 EOID and Restrike

Testing during initial driving is called End of Initial Drive (EOID). However, pile capacity in clay usually increases with time (soil setup), so a repeat test after the soil has rested for 1–14 days (restrike) is commonly performed to reflect the long-term capacity.

How Long Does a PDA Test Take, and How Much Does It Cost?

Field testing takes little time per pile. A typical planning figure is about 1 day per 5–10 points when the piles are in the same area (subject to each project's specifications). CAPWAP analysis and the certified report take another few working days. For the timeline from booking to receiving the report, see How Long Does a PDA Test Take

PDA testing starts at about THB 5,500 per pile, depending on pile size, number of piles and site conditions. This price excludes travel and equipment mobilization, which are charged extra according to the distance to the site.

What Are the Advantages and Limitations of PDA Testing?

The main advantages of PDA are speed and economy; the main limitation is that it is an indirect evaluation, so the static capacity used for conclusions must come from CAPWAP analysis (the bottom panels of the diagram in the steps section summarize this).

✅ Advantages

Much faster and more economical than a Static Load Test, several piles can be tested per day, it provides capacity, integrity, driving stress and energy data together, and it can handle high-capacity piles.

⚠️ Limitations

A reliable static capacity must come from CAPWAP analysis because the CASE Method is an estimate. The hammer must deliver enough impact energy to fully mobilize the soil resistance. Results are an indirect dynamic evaluation, so they should be calibrated against a Static Load Test on important projects and must be interpreted by experienced engineers.

What Standards Apply to Dynamic Load Testing?

Dynamic testing in Thailand refers to two main standards: ASTM D4945 and DPT 1252-51 for PDA testing:

  • ASTM D4945 (Standard Test Method for High-Strain Dynamic Testing of Deep Foundations; current edition D4945-26) — specifies gauge installation, impact energy, recording of force/velocity signals and analysis of results
  • DPT 1252-51 (Standard for Dynamic Load Testing of Piles) of the Department of Public Works and Town & Country Planning — referenced in project specifications and by relevant authorities in Thailand

The test report must state the standards used and be certified by a civil engineer licensed by the Council of Engineers Thailand (COE), for use in pile acceptance with the designer and relevant authorities.

How Does PDA Differ from a Static Load Test and a Seismic Test?

PDA evaluates load capacity quickly and can test many piles. A Static Load Test measures capacity directly with real load but is slower and costlier. A Seismic Test checks only integrity and length and does not evaluate capacity:

← Scroll to see the full table →

Test method / standardWhat it measures and key features
Dynamic Load Test — PDA (ASTM D4945 + DPT 1252-51)Load capacity + integrity + driving stress by high-strain impact; fast, economical, suits large numbers of piles
Static Load Test (ASTM D1143 + DPT 1251)Direct load capacity under real load; used as the reference for calibration, but slow and costly
Seismic / Low Strain (DPT 1551-51)Integrity and length only, by low-strain impact; does not evaluate load capacity

Typically, PDA is used to test the capacity of many piles, and a few are selected for calibration by Static Load Test. For an in-depth comparison, see Dynamic vs Static Load Test: Which to Choose; for how to read the results, see PDA Test Result Interpretation; and for “integrity” checks, see Seismic Test (Low Strain)

Examples of Dynamic Load Testing (PDA) from SPN's real sites on projects across Thailand, from gauge installation and the high-energy drop hammer to the Pile Driving Analyzer in the field

Overview of a prestressed concrete piling site with a pile driver and crane before PDA testing by SPN
Overview of a piling site before testing
Pile driving rig and drop hammer for PDA testing on site in Saraburi
High-energy drop hammer — Saraburi project
Pile Driving Analyzer set up on site with the P54 pile information board during a Dynamic Load Test
PDA unit on site — pile P54
Drop hammer striking the pile head to create a high-strain impact in a PDA test, Rayong project
High-strain impact — Rayong project
SPN engineer marking the pile location in the middle of the site before a Dynamic Load Test (PDA)
Marking the test pile location on site
Pile Driving Analyzer connected to signal cables at a pile testing site, TPP project
PDA unit with signal cables on site — TPP project
SPN team mounting signal gauges on a bored pile head in an excavated pit, preparing for a Dynamic Load Test (PDA)
Mounting gauges on a bored pile head in an excavated pit before PDA testing
SPN engineer drilling gauge anchors on a bored pile head on site before load capacity testing with PDA
Drilling gauge anchors on a bored pile head on site, ready for load capacity testing
📌 Note

This article is provided as engineering knowledge. The choice of test method, number of piles to test, damping factors and interpretation of results should follow the project specifications and the judgment of a licensed engineer.

SPN Soil Engineering provides full-service PDA testing nationwide, from field testing and CAPWAP analysis to engineer-certified reports — see the pile testing services page for details