What Is a PDA Test and What Does It Measure?

A PDA test (Pile Driving Analyzer test) determines pile load capacity by the dynamic method (dynamic load test) in accordance with ASTM D4945 and DPT 1252-51. A drop weight strikes the pile head while sensors measure force and acceleration at the pile head. The wave signals are then converted into load capacity and other check values, which the report lists as abbreviations such as RMX, EMX, FMX and CSX. For a detailed description of the field procedure, see Dynamic Load Testing of Piles (Dynamic Load Test / PDA)

What Is the Main Equipment for a PDA Test?

  • At least 2 strain transducers mounted on opposite sides of the pile head: measure strain to compute force (large piles may use 4)
  • At least 2 accelerometers mounted on opposite sides: measure acceleration, which is integrated to velocity and displacement
  • Pile Driving Analyzer (PDA): records and processes the signals in real time
  • Drop weight: approximately 1-2% of the ultimate capacity to be proven, e.g. a 2.5-5 t weight to prove 250 t
SPN field engineer bolting a strain transducer and signal cable to the head of a square driven pile before a PDA test SPN technician installing sensors and routing signal cables at the head of a driven pile on site, next to the pile-driving rig, before a dynamic load test
SPN on site — sensors are installed at the pile head before testing. They must be bolted firmly and mounted in opposite pairs so that the signals from both sides are symmetric, as required by the signal-quality criteria in the next section

Why Check PDA Signal Quality Before Reading the Values?

Signal quality is the first gate when reading a PDA report, because every computed value, including RMX and EMX, is derived from the measured force and velocity signals. If the signals are poor, none of those values can be trusted. DPT 1252-51 (clause 4.5) clearly illustrates signals that should and should not be used:

Examples of usable and unusable PDA signals per DPT 1252-51 — consistent strain and velocity signals and symmetric signals, compared with noise and drift
Examples of usable signals (a, b) and unusable signals (c, d) — Source: Figure 7, DPT 1252-51 Standard for Pile Load Capacity by Dynamic Method, Department of Public Works and Town & Country Planning
  • a) Good signal: the strain (force) and velocity curves peak together (peaks match) in the early part of the wave
  • b) Good signal: signals from the same gauge pair (mounted on opposite sides of the pile) must be similar and symmetric
  • c) Do not use: heavy noise — usually caused by loose sensors or faulty signal cables
  • d) Do not use: peaks do not match, or the signal does not return to zero (drift) — usually caused by eccentric impact or a faulty sensor

If the report shows signals like c) or d), the field engineer must fix the sensor installation or correct the impact alignment and retest. That blow should not be used for further analysis

🔬 Why the force and velocity curves must overlap at the start

In the early part of the record, before the wave is reflected back from the pile toe or from soil resistance, force and velocity are proportional according to the wave equation F = Z·v, where Z is the pile impedance = EA/c (E = elastic modulus, A = cross-sectional area, c = wave speed in the pile). When velocity is multiplied by Z and the two curves overlap exactly, no resistance has been encountered yet. When the force curve and the Z·v curve separate, the wave has started to meet resistance from the soil (skin friction) or the pile toe — this is the basic principle that the Case Method and CAPWAP use to convert signals into load capacity

The diagram below combines this principle in a single chart. Point 1 is the early part where the force curve overlaps the Z·v curve, point 2 is the gap after the peak caused by shaft resistance, and point 3 is the toe response at time 2L/c. The box on the right is a quick map of the report values explained in the next section

Conceptual chart of the force signal F and velocity times impedance Z·v at the pile head during one blow, with a summary table of what each PDA report value tells you and what to check it against Two-panel diagram. The left panel is a conceptual example chart, not measured data. The horizontal axis is time and the vertical axis is force. The solid dark-blue line is force F computed from the strain transducers; the dashed orange line is velocity from the accelerometers multiplied by the pile impedance Z, which equals EA divided by c. Point one: from the start up to the peak, the two lines overlap and peak together, meaning no resistance has been met yet and the signal is usable. Point two: after the peak, the force line separates above the Z·v line; the growing green-shaded gap is shaft resistance. Point three: at a time of 2L divided by c after the peak, the wave reflected from the pile toe returns to the sensors; the force line rises and the Z·v line drops, shaded light blue, showing the toe carrying load. Below the chart is a bar marking the 2L/c interval and a horizontal sketch of the pile: the ram strikes the pile head, strain transducers and accelerometers are mounted in opposite pairs, and the wave travels down a distance L and reflects back a distance L, a round trip of 2L taking 2L divided by c, where c is the wave speed in the pile. A lower box states that the Case Method uses the signal from the peak to the peak plus 2L/c to compute RMX on site, while CAPWAP uses the whole record to separate shaft and toe. The right panel is a table of PDA report values, color-coded into a capacity group, an energy and force group, and a displacement and stress group: RMX, mobilized capacity, checked against at least 2.5 times the design working load; RAU and RA2, automatic total capacity, used to cross-check RMX; Shaft and Toe, shaft and toe resistance separated by CAPWAP; EMX, energy transferred to the pile, checked as the ratio of EMX to ram energy; FMX, maximum force at the pile head, which must be high enough to mobilize resistance; DMX and DFN, maximum displacement and permanent set, with DFN expected to be close to the set measured on site; CSX, maximum compressive stress, not exceeding 0.85 fc for concrete or 0.9 fy for steel; and TSX, maximum tensile stress, checked against the project specification Reading PDA signals: F vs Z·v in one blow Conceptual example · Not measured data Pile-head signals in one blow Force F vs velocity × impedance (Z = EA/c) F / Z·v → Time → Force F Z·v 1 2 3 2L/c Peak Ram Head Pile toe 2L round trip Time 2L/c c = wave speed in pile 1 Early: F overlaps Z·v, peaks match No resistance yet (F = Z·v) — signal usable 2 After peak: F separates above Z·v Growing gap = shaft resistance (skin friction) 3 At 2L/c after peak: toe reflection returns F rises · Z·v drops = toe resistance (end bearing) Case Method: F & Z·v from peak → peak + 2L/c gives field RMX CAPWAP: signal matching over full record to split shaft / toe What do PDA report values tell you? Value → meaning → check against Capacity Energy/force Motion/stress RMX Mobilized capacity (Case Method) Check: ≥ 2.5 × Design Working Load RAU/RA2 Automatic total capacity Check: cross-check with RMX Shaft/Toe Shaft / toe resistance split Check: CAPWAP · Ru = Rs + Rt EMX Energy transferred to pile Check: EMX ÷ ram energy FMX Max force at pile head Check: enough to mobilize resistance DMX/DFN Max disp. / permanent set Check: DFN ≈ field set CSX Max compressive stress Check: ≤ 0.85 fc · ≤ 0.9 fy TSX Max tensile stress Check: per project spec Criteria per this article · actual values per project spec
Principle of reading a PDA signal for one blow — early on, the force curve F overlaps the Z·v curve and the peaks match; after the peak the curves separate because of shaft resistance, and at time 2L/c the wave from the pile toe returns to the sensors. The right-hand box summarizes the report values (RMX, RAU/RA2, Shaft/Toe, EMX, FMX, DMX/DFN, CSX, TSX): what each tells you and what to check it against — the chart is a conceptual example, not measured data (Image: SPN Soil Engineering)

What Do RMX, EMX and CSX Mean in a PDA Report?

RMX is the mobilized load capacity, EMX is the energy delivered to the pile, and CSX is the maximum compressive stress in the pile. They are part of a set of 8 abbreviations that summarize pile behavior for each blow, in 3 groups: capacity (RMX, RAU/RA2, Shaft/Toe), energy and force (EMX, FMX), and displacement and material stress (DMX, CSX, TSX). This table is a complete reference for all of them:

← Scroll to see the full table →

Summary of the 8 key values in a PDA test report
ValueUnitMeaningCriterion / reference value
RMX
Maximum Mobilized Capacity		kN or tMobilized capacity — the most important value, compared with the design working load≥ 2.5 × Design Working Load (FS = 2.5 under the 2023 Foundation Regulation)
RAU / RA2
Automatic Case Method		kN or tTotal capacity from the automatic Case Method, independent of the damping factor (J)RAU: low shaft friction · RA2: low to moderate — used to cross-check RMX
Shaft / Toe
Resistance (from CAPWAP)		kN or tSeparates shaft friction from end bearing by CAPWAP analysisRu (total capacity) = Rshaft + Rtoe
EMX
Maximum Transferred Energy		kN·m (kJ)Maximum energy actually transferred from the hammer into the pile in each blowTypical energy transfer ratio ~50-70% of ram energy — below 30%, investigate the cause (a practice guideline, not a fixed criterion)
FMX
Maximum Force		kN or tMaximum force at the pile headMust be large enough to mobilize soil resistance up to the level to be proven
DMX
Maximum Displacement		mmMaximum pile-head displacement, including elastic reboundRead together with DFN (permanent set), which should be close to the set measured on site
CSX
Maximum Compressive Stress		MPa or kscMaximum compressive stress in the pile material during impact≤ 0.85 × fc (concrete piles) · ≤ 0.9 × fy (steel piles), or per project specification
TSX
Maximum Tensile Stress		MPa or kscMaximum tensile stress — important for long piles where a tension wave may develop during drivingMust be within the material's safe limits, per project specification

Capacity Group — How Does RMX Differ from RAU/RA2 and Shaft/Toe?

RMX is the total load capacity computed in real time on site by the Case Method, so it is a preliminary estimate — the value used for the final conclusion must be confirmed by CAPWAP Analysis. RAU/RA2 are automatic total-capacity values that do not depend on the damping factor; engineers use them to cross-check RMX when soil conditions are uncertain. Separating shaft friction from end bearing cannot be read directly from the field values; it comes from CAPWAP analysis, which is especially important for piles that rely mainly on end bearing

← Scroll to see the full table →

Criteria for comparing RMX with the design working load
RMX vs Design CapacityInterpretation
RMX ≥ 2.5 × Design✅ Passes the 2023 Foundation Regulation criterion (FS = 2.5)
RMX < 2.5 × Design❌ Fails the dynamic-method criterion — investigate the cause and consider a confirmatory static load test

Note: The FS = 2.5 criterion follows the 2023 Foundation Regulation, which allows values from the dynamic method (dynamic load test) to be used up to no more than 40% of the ultimate resistance

Energy and Force Group — What Do EMX and FMX Tell You?

EMX indicates how efficiently the driving system delivers energy. For a free-fall drop hammer, the energy transfer ratio (EMX ÷ ram energy) is typically around 50-70%; below 30% means the system is losing a lot of energy. These figures are common practice guidelines — the criteria actually applied should follow the project specification

Common causes of low energy transfer are a worn or damaged pile cushion, a hammer out of line with the pile axis, insufficient hammer stroke, or friction in the hammer guide system; these must be corrected before retesting. FMX must be high enough to mobilize soil resistance up to the level to be proven, but whether the pile can withstand the impact without cracking is checked from the stress CSX against the material strength, not from FMX directly

Displacement and Stress Group — How to Read DMX, CSX and TSX?

DMX is the maximum displacement and includes elastic rebound, so it is not the permanent set. For the permanent set, look at DFN (final displacement), which should be close to the set measured on site. CSX and TSX are used to check the safety of the pile material: compressive stress should not exceed 0.85 × fc for concrete piles or 0.9 × fy for steel piles (or per project specification), and tensile stress needs particular attention in long piles

Many PDA reports also show BTA (β), an integrity index of the pile cross-section estimated from wave reflections arriving before the toe reflection. A value of 100% means a uniform cross-section; a lower value means there may be a change in section or damage, and an engineer should investigate further, e.g. with CAPWAP

How Do You Read a PDA Report Value by Value?

Read a PDA report one value at a time, in order: compare RMX with the design load first, then check EMX, CSX, FMX and DMX/DFN. The example below uses illustrative numbers to explain the reasoning; it is not the result of any particular project

📋 Example test result

Bored pile Ø60 cm, 24 m long, concrete fc = 30 MPa · Design load = 100 t · 5 t ram, 1 m drop height · Results: EMX = 18 kJ, FMX = 280 t, DMX = 12 mm, RMX = 260 t, CSX = 12 MPa

Value-by-value analysis

  • RMX = 260 t vs design = 100 t → FS = 2.6 ✅ passes the FS = 2.5 criterion of the 2023 Foundation Regulation
  • EMX = 18 kJ from a 5 t ram dropped 1 m (energy ≈ 49 kJ) → energy transfer ratio = 18/49 ≈ 37% ⚠️ above the 30% level that requires investigation but below the typical range (50-70%); check the cushion condition and the driving alignment as well
  • CSX = 12 MPa vs 0.85 × 30 MPa = 25.5 MPa → ✅ compressive stress is below the material limit
  • FMX = 280 t > RMX = 260 t → the impact force exceeds the level to be proven; use it together with the set (DFN/Set) to confirm that enough soil resistance was mobilized
  • DMX = 12 mm → this displacement includes elastic rebound; for the permanent set, compare DFN with the set measured on site

Example summary: passes on both load capacity and stress

This pile has FS = 2.6, which is not less than 2.5, and its compressive stress is below the material limit. However, RMX is a Case Method value, so the value reported in the conclusion should be confirmed by CAPWAP, and the design engineer should review it together with the results of the other test piles in the project

What Is CAPWAP Analysis and Why Is It Needed?

CAPWAP (CAse Pile Wave Analysis Program) is a program for in-depth analysis of PDA signals using signal matching. After the field signals are obtained, engineers run CAPWAP to:

  • Separate the load capacity into shaft and toe components
  • Simulate a load-settlement curve for comparison with static load test results
  • Assess pile integrity along the full length and detect defects
  • Adjust damping and quake values to match actual soil conditions
Actual CAPWAP analysis output — measured/computed force, force/velocity, simulated static load-displacement, and shaft resistance distribution along the pile
Example CAPWAP output from an actual SPN project — top left: measured vs computed force (signal matching) · top right: force/velocity · bottom left: simulated load-displacement vs static test · bottom right: shaft friction distribution along the pile

This article gives only an overview of CAPWAP — for an in-depth look at signal matching, its outputs, and a comparison of CAPWAP vs Case Method vs static load test, see What Is CAPWAP? Why PDA Test Results Need CAPWAP Analysis

The 3D view below traces the data path from the sensors at the pile head, to the PDA unit that computes the Case Method on site, through to CAPWAP, which produces the shaft/toe resistance distribution and a simulated load–settlement curve. The box on the right is a 7-point checklist for reviewing a report before drawing conclusions

3D model of the PDA test data path, from the sensors at the pile head and the on-site PDA unit to CAPWAP analysis, producing the resistance distribution and a simulated load–settlement curve, with a report-review checklist Conceptual 3D view of a soil block cut open to show layers of soft clay, stiff soil and sand. The test pile is shown full length on the cut face, with its head protruding above the ground and a guide frame and drop weight above it; the weight is about one to two percent of the capacity to be proven. Point one: just below the pile head, orange strain transducers and light-blue accelerometers are mounted in opposite pairs, with signal cables running to point two, the PDA unit placed on the ground on site, which immediately computes Case Method values such as RMX, EMX and CSX. A data arrow leads to point three, a CAPWAP signal-matching box in which the measured force (solid line) and the computed force (dashed line) overlap. The results split into point four, resistance distribution bars along the pile on the cut face, where green bars are shaft resistance growing longer with depth and a light-blue bar below the toe is toe resistance; and point five, a simulated load–settlement curve for comparison with a static load test, a conceptual example rather than measured data. The box on the right is a seven-point report-review checklist: signal quality, with force and Z·v peaks matching and symmetric; RMX at least 2.5 times the design working load under the 2023 Foundation Regulation; CAPWAP results, with the computed curve overlapping the measured curve and shaft and toe separated; stresses, with CSX not exceeding 0.85 fc or 0.9 fy and TSX within limits; EMX energy divided by ram energy, with the cause to be investigated if below 30 percent; DFN close to the measured set, because DMX includes elastic rebound; and test timing, considering set-up in clay and relaxation in sand. A closing note says that if the pile fails, investigate the cause, run CAPWAP and confirm with a static load test 3D model: pile head → PDA → CAPWAP → report Not to scale 3 CAPWAP: Signal Matching Tune soil model until computed matches measured Measured Calculated Conceptual example 5 Load–Settlement Simulated vs static Load → ← Settles Signal cable Data / results Crane drop weight ≈ 1–2% of capacity to be proven 1 Sensors Strain Gauge + Accelerometer in opposite pairs 2 On-site PDA unit Computes RMX · EMX · CSX instantly (Case Method) 4 Resistance distribution from CAPWAP Green bars = shaft Blue bar = toe Section: soft clay / stiff soil / sand PDA report checklist Signal quality F & Z·v peaks match Symmetric, no noise / drift RMX vs design load ≥ 2.5 × Design Working Load FS 2.5 · Foundation Reg. 2023 CAPWAP result Computed matches measured Shaft / toe separated Pile stresses CSX ≤ 0.85 fc · ≤ 0.9 fy TSX within spec limits EMX energy EMX ÷ ram energy Below 30%: find the cause Set DFN ≈ measured set DMX includes elastic rebound Test timing Clay: wait for set-up Sand: watch for relaxation Fails the criteria? Find cause → CAPWAP → confirm by static load test Criteria per article · follow project spec
3D model of the PDA test data path — sensors below the pile head → on-site PDA unit (Case Method: RMX, EMX, CSX) → CAPWAP signal matching → shaft/toe resistance distribution and a simulated load–settlement curve, with a 7-point checklist for reviewing the report (conceptual view, not to scale; the chart is a conceptual example · Image: SPN Soil Engineering)

When Does a PDA Test Pass — How Many Times the Design Load?

A PDA test passes on load capacity when RMX is at least 2.5 times the design working load. In Thailand the acceptance criteria follow the Ministerial Regulation on foundations B.E. 2566 (2023) together with DPT 1252-51, considering both load capacity (RMX) and stresses in the pile (CSX/TSX), as follows:

  • RMX ≥ 2.5 × Design Working Load: ✅ Pass — Clause 24 of the 2023 Foundation Regulation allows values from a dynamic load test to be used up to no more than 40% of the ultimate resistance, equivalent to FS = 2.5
  • RMX < 2.5 × Design Working Load: ❌ Fails the dynamic-method criterion — investigate the cause and consider a confirmatory static load test (the static method allows up to 50%)
  • Values from the dynamic method must be calibrated against static load test results
  • Number of piles to test: the regulation does not fix a number. Common practice is about 1–2% of all piles, and not less than 1 pile (a practice guideline, not a requirement; it depends on each project's specification), and the selected piles must be representative of the whole project — read the reasoning in How Many Piles Should Be Tested?
  • CSX and TSX within safe limits: ✅ stresses during impact do not exceed the material limits
Dynamic load test equipment on site with a project sign stating design load 40 t, FS 2.5, capacity to be proven 100 t, square pile 0.40 m, 8.50 m long
SPN on site — the project sign states design load 40 t × FS 2.5 = capacity to be proven (Qu) 100 t, per the 2023 Foundation Regulation criterion explained in this section

For the background to the 40/50/40 criteria, see How Much Load Can a Pile Carry by Law? The 40/50/40 Criteria in the 2023 Foundation Regulation

What Should You Do If PDA Results Fail?

A PDA result below the criterion does not always mean the pile is unusable. Investigate the cause systematically before deciding, following these steps:

  1. Check the cause: Does the signal fail the quality criteria? Was the impact energy too low, so the pile set too little? Is the damping unsuitable? Were the sensors installed incorrectly? Is the pile too short?
  2. Run CAPWAP analysis for an in-depth re-analysis
  3. Wait for set-up and retest: in clay, skin friction increases with time, so a later retest may give a higher RMX. Read more in What Is a Restrike Test?
  4. Run a static load test: measures pile behavior under real load and serves as the reference to confirm the PDA result (values usable up to 50% under the Ministerial Regulation)
  5. Consult the design engineer: if the pile still fails after confirmation, the design engineer will consider revising the foundation, e.g. adding piles

What Are the Limitations of a PDA Test?

A PDA test estimates capacity indirectly from wave signals, so the result depends on the impact energy, the soil type and the test timing. The main limitations are:

⚠️ Points to watch

RMX is the capacity mobilized during the test. If the impact energy is not enough to make the pile set sufficiently, resistance is not fully mobilized, so RMX may be lower than the actual load capacity. In clays with high damping, the value may be either higher or lower than the true value, depending on the damping value selected

  • Soft clay: damping is high and the damping factor (J) is hard to choose, so the Case Method RMX can be significantly off; confirm with CAPWAP
  • Set-up effect: skin friction increases with time in clay; testing too early gives low values, so wait for the period specified by the engineer
  • Relaxation: in some dense sands, resistance (usually at the toe) can decrease over time, so results from testing immediately after driving may be higher than the long-term value
  • Bored piles: the cross-section and concrete quality are less uniform than in precast driven piles; the pile head must be well prepared and the drop weight heavy enough, and interpretation must be supported by CAPWAP. Read more in High Strain Testing of Bored Piles

Summary

Reading PDA test results takes 4 steps: check signal quality (F and Z·v peaks must match) → compare RMX with the design working load → check CSX/TSX to confirm pile stresses are safe and EMX to confirm enough energy was delivered → confirm with CAPWAP. The pass criterion under the Ministerial Regulation on foundations B.E. 2566 (2023) is RMX ≥ 2.5 × Design Working Load (FS = 2.5)

SPN Soil Engineering has more than 20 years of experience providing pile testing services by PDA test per DPT 1252-51 and ASTM D4945, with CAPWAP analysis, nationwide across all 77 provinces of Thailand. Reports are certified by licensed civil engineers (COE). Contact us to discuss your project