DPSH / DPL Calculator — Correlation with SPT, φ and Dr
The super-heavy dynamic penetrometer DPSH and the light DPL are rapid and economical tests for characterizing superficial granular soils up to 20 m depth: a conical tip is driven with a 63.5 kg (DPSH) or 10 kg (DPL) hammer and the number of blows N10 per 10 cm of penetration is recorded. Since the DPSH does not have the same standardization as the SPT, a correlation factor α = N_SPT / N_DPSH is required, which depends on soil type and depth. This calculator converts N10_DPSH to equivalent N_SPT, and from there estimates friction angle φ, relative density Dr, allowable bearing capacity and compactness class, useful for preliminary prospecting in housing projects, secondary roads and feasibility studies in typical terrains.
What is it and when is it applied?
The DPSH (Dynamic Probing Super Heavy according to ISO 22476-2) uses a 63.5 kg hammer with a 75 cm free fall on a 90° conical tip. The DPL (Dynamic Probing Light) is its reduced version: 10 kg and 50 cm drop, typical for the first 6-8 m. The test provides the N10 vs depth curve, where N10 is the number of blows to penetrate 10 cm. Its main advantage is speed: one rig can perform 5-8 profiles per day compared to 1-2 for traditional SPT, at 3-4 times lower cost. It is applied in dense prospecting campaigns for geotechnical zoning of urban developments, quality control of compacted fills, correlation with SPT boreholes to extrapolate results between points, and verification of soil competence under raft foundations and light shallow foundations. It does not replace the SPT for major foundation design — it is always used in combination with at least one in-situ calibration SPT borehole.
Applied Formulas
DPSH → SPT Conversion (local calibration):
N_SPT ≈ α · N10_DPSH
where α = correlation factor dependent on soil type
Typical α values (Cassan 1988, Cestari 1990, regionally verified):
Fine sand above water table α = 0.5-0.8 · Dry medium sand α = 0.8-1.0 · Coarse sand α = 1.0-1.2 · Clean gravel α = 1.2-1.5 · Silty fines α = 0.6-0.9 · Hard clay α = 0.4-0.6
DPL → SPT Conversion:
N_SPT ≈ 0.5 · N10_DPL (general rule for the first 6 m)
Friction angle (Peck, Hanson & Thornburn 1974 from corrected N_SPT):
φ = 27.1 + 0.30·N1_60 − 0.00054·N1_60² (sands)
Relative density (Meyerhof 1957):
Dr = √(N1_60 / 60) · 100 %
Dynamic point resistance (Cassan):
qd = (M·g·h / (M + Mp)) · M / (A·e10/10) [kPa]
with M hammer mass, Mp inert element mass, A tip area, e10 = 100 mm
Calculation example
| Parameter | Value |
|---|---|
| Equipment type | DPSH (M = 63.5 kg, h = 0.75 m) |
| Depth of interest for foundation | z = 2.0 m (strip footing level) |
| N10_DPSH recorded 1.90 - 2.00 m | N10 = 14 blows |
| Identified soil type (adjacent test pit) | Gray medium sand, dry, Gs = 2.67 |
| Correlation factor α (dry medium sand) | α = 0.9 (average range 0.8-1.0) |
| Water table | WT = 8.5 m (well below foundation) |
Conversion to equivalent N_SPT: N_SPT = α · N10 = 0.9 × 14 = 12.6 ≈ 13 blows. Energy correction (assuming equipment with 60% standard efficiency): N60 = N_SPT × CE = 13 × 1.00 = 13. Confinement correction CN = (100/σ'v)^0.5 with σ'v = 18·2 = 36 kPa: CN = (100/36)^0.5 = 1.67 (limited to 1.70). N1_60 = CN·N60 = 1.67 × 13 = 21.7 ≈ 22. Relative density: Dr = √(22/60)·100 = 60.5% → medium dense sand. Friction angle: φ = 27.1 + 0.30·22 − 0.00054·22² = 27.1 + 6.6 − 0.26 = 33.4°. Compactness class according to Terzaghi-Peck: N = 13 → medium dense (11-30). Preliminary allowable bearing capacity for footing B = 1.2 m, Df = 1.5 m in sand with φ = 33.4° and γ = 18 kN/m³: qa ≈ 0.11·N1_60·B·Kd = 0.11·22·1.2·1.1 = 3.2 kg/cm² = 320 kPa. Verify with at least one calibration SPT at 2 m to confirm assumed α.
Result: Equivalent N_SPT = 13 · N1_60 = 22 · φ = 33.4° · Dr = 60.5% · qa ≈ 320 kPa · Compactness: medium dense
Interpretation of results
The DPSH does not provide a sample, so the quality of interpretation depends entirely on the α calibration with a nearby SPT borehole. Without calibration, results have ±40% uncertainty. For routine housing projects, α = 1.0 is accepted as a first approximation, but in soils below the water table or with plastic fines, α can drop to 0.5 — a serious error if α = 1.0 is taken by default. The N10 vs z curve also detects strata: a sharp increase indicates a change to denser material; a localized drop detects organic pockets or loose fills. For important foundations (buildings >3 stories, industrial structures), DPSH is used to densify prospecting between SPT boreholes, not to replace them. In soils with boulders larger than 10 cm, the DPSH jams and gives false refusal (N10 > 50) — rotary drilling is necessary.
Reference standards
- ASTM D6951 — Standard test method for use of the dynamic cone penetrometer
- ISO 22476-2:2005 — Geotechnical investigation and testing. Dynamic probing
- ASTM D1586 — Standard test method for standard penetration test (SPT) and split-barrel sampling of soils
- ASTM D6951 — Standard test method for use of the dynamic cone penetrometer
- Cassan, M. (1988). Les essais in situ en mécanique des sols, Eyrolles
- Cestari, F. (1990). Prove geotecniche in sito, GEO-graph
- Highway Design Manual — dynamic penetrometers for subgrade
Frequently asked questions
What is the difference between DPSH and SPT?
The SPT uses a split-barrel sampler (51 mm) that recovers a disturbed sample and records N in 30 cm (blows 15+15+15, the last two increments are counted). The DPSH uses a blind conical tip without sample recovery and records N10 per 10 cm. The DPSH is faster and cheaper but does not provide a sample for classification; it must rely on test pits or nearby boreholes.
Why is DPSH not used to design important foundations?
Because the α correlation with SPT has 30-40% scatter even in well-classified soils, and because the stratigraphic profile is not known with certainty. DPSH is recommended for dense prospecting and extrapolation between SPTs; any structural decision must be anchored to at least 1 SPT per 3-5 DPSH in the campaign and visual test pits at the critical depth.
What is refusal in DPSH?
Refusal is considered when N10 > 50 blows or when penetration per blow is less than 2 mm sustained over two consecutive increments. It may indicate: competent very dense soil (good news), an isolated boulder (bad — false refusal), or rock/cemented layer. Always verify with rotary drilling if refusal occurs before the projected foundation depth.
Does DPSH work in saturated soils?
Yes, but with an α reduction of 0.5 to 0.7 in saturated fine sands due to the development of excess pore pressure during driving (partially undrained behavior). In saturated clays, the DPSH loses sensitivity and should not be used — the field vane or CPTu are more appropriate. In central areas, the water table often appears at 2-4 m; use a reduced α in the below-water-table section.