Foundations · Pile testing & NDT
Parallel seismic: unknown pile length calculator
Fits straight lines to the first-arrival times above and below a trial break depth; the intersection, corrected for the borehole offset, estimates the pile toe.
Standard: ASTM D8381-type parallel seismic · two-line least-squares fit
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 |
|---|---|---|
| Borehole offset from pile | 1 | m |
| Trial break depth (last point on the upper line) | 12 | m |
| Readings: depth (m), first arrival (ms) | 1, 0.930 2, 1.192 3, 1.455 4, 1.715 5, 1.982 6, 2.243 7, 2.509 8, 2.774 9, 3.036 10, 3.302 11, 3.562 12, 3.825 13, 4.100 14, 4.645 15, 5.265 16, 5.903 17, 6.557 18, 7.214 19, 7.873 20, 8.537 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Upper slope | 0.263 | ms/m |
| Upper intercept | 0.665 | ms |
| Lower slope | 0.650 | ms/m |
| Lower intercept | -4.48 | ms |
| Pile velocity 1000/m1 | 3,796 | m/s |
| Soil velocity 1000/m2 | 1,538 | m/s |
| Line intersection | 13.31 | m |
| Toe estimate zi − x·Vc/(Vc − Vs) | 11.63 | m |
- ✓ Pile faster than soil (method valid)
Method, formulas and assumptions
- Upper slope = SLOPE([t.yu],[t.xu]) = 0.26347 ms/m
- Upper intercept = INTERCEPT([t.yu],[t.xu]) = 0.66456 ms
- Lower slope = SLOPE([t.yl],[t.xl]) = 0.65011 ms/m
- Lower intercept = INTERCEPT([t.yl],[t.xl]) = -4.4813 ms
- Pile velocity 1000/m1 = 1000/0.26347 = 3795.6 m/s
- Soil velocity 1000/m2 = 1000/0.65011 = 1538.2 m/s
- Line intersection = (-4.4813-0.66456)/(0.26347-0.65011) = 13.309 m
- Toe estimate zi − x·Vc/(Vc − Vs) = 13.309-1*3795.6/(3795.6-1538.2) = 11.628 m
- Points within about one metre below the break are excluded from the lower fit. Check the pick against the shape of the data near the break.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.