Foundations · Pile testing & NDT
Low-strain pile integrity test (PIT): length and impedance calculator
Pile length from the toe echo time, depth of an anomaly, and the impedance ratio and equivalent diameter implied by the anomaly's reflection amplitude.
Standard: ASTM D5882 · velocity reflection r = (Z1 − Z2)/(Z1 + Z2)
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Wave speed c | 3800 | m/s |
| Nominal diameter D | 0.6 | m |
| Design (as-built record) length | 15 | m |
| Toe echo arrival time | 7.9 | ms |
| Anomaly echo arrival time | 3.7 | ms |
| Anomaly reflection ratio (+ same sign as input = reduction) | 0.08 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Measured length c·tL/2 | 15.01 | m |
| Length deviation | 0.067 | % |
| Anomaly depth c·td/2 | 7.03 | m |
| Impedance ratio Z2/Z1 = (1 − r)/(1 + r) | 0.852 | |
| Equivalent diameter D·√(Z2/Z1) | 0.554 | m |
| Impedance change | -14.81 | % |
| Check: |length deviation| | 0.067 | % |
- ✓ Measured length within ±5 % of record
- ✓ Impedance reduction no worse than 20 % (screening)
Method, formulas and assumptions
- Measured length c·tL/2 = 3800*7.9/2000 = 15.01 m
- Length deviation = (15.01/15-1)*100 = 0.066667 %
- Anomaly depth c·td/2 = 3800*3.7/2000 = 7.03 m
- Impedance ratio Z2/Z1 = (1 − r)/(1 + r) = (1-0.08)/(1+0.08) = 0.85185
- Equivalent diameter D·√(Z2/Z1) = 0.6*SQRT(0.85185) = 0.55377 m
- Impedance change = (0.85185-1)*100 = -14.815 %
- Check: |length deviation| = ABS(0.066667) = 0.066667 %
- Same material assumed on both sides of the anomaly (impedance change = area change). Soil resistance and soil impedance changes also cause reflections; interpret with the installation records.
- Echoes from deeper than about 30 pile diameters, or below a strong anomaly, are often too weak to read.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.