API: drilled shaft

Computes the axial capacity of a drilled shaft (bored pile / caisson) with the FHWA method of O'Neill & Reese (FHWA-IF-99-025, 1999): α-method side resistance in clay, β-method (SPT-based) in sand, rock-socket resistance in rock, base bearing, belled-base uplift, and optional service-load settlement. The method and every input are explained in Drilled shafts; the same analysis runs interactively at the drilled-shaft tool.

Endpoint

The request mirrors the axial endpoint — a pile and a layered soil profile — and requires an explicitmethod, constructionMethod, and analysisMode. The released slice is fhwa-if-99-025 for a dry-stable-open-hole shaft. Cohesionless layers require sptN (the SPT blow count that drives the β correlation), not phi. Cohesive layers need cohesion (or qu); rock needs qu. Add bellDiameter and bellHeight together for a belled base. Rock layers require an explicit rockJointCondition; jointed layers also requirerqd from 20 through 100, and jointed rock at the base is rejected. Service mode also requires an explicit service load and numerical controls for soil t-z/q-z. For a complete homogeneous massive-intact rock socket, supply rockSocketService with method: "kulhawy-carter-1992", shaft/side-rock/base-rock Poisson ratios, side/base rock moduli, and interface cohesion, friction and dilation angles. Omit loadSettlementPoints and maxTipSettlementfor this closed-form model. Moduli and cohesion use kPa in SI or ksf in US; angles use degrees.

Authentication, rate limits, and the error envelope are shared by every endpoint — see the API overview. Requests are unit-agnostic: any self-consistent unit system works, and results come back in the same units. The examples below use SI (m, kN, kPa, kN/m³).

Request fields

This table is generated from the same schema that validates the request, so it cannot drift. Where a field appears once per variant row, it belongs to that variant only.

Response fields

Code examples

A complete, runnable request. Replace YOUR_API_KEY (or set PILECALC_API_KEY in your environment).

curl https://api.pilecalc.com/v1/drilled-shaft \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
  "unitSystem": "si",
  "method": "fhwa-if-99-025",
  "constructionMethod": "dry-stable-open-hole",
  "analysisMode": "capacity-and-service",
  "pile": {
    "kind": "uniform",
    "length": 22,
    "diameter": 0.9,
    "elasticModulus": 28000000,
    "unitWeight": 24
  },
  "soil": {
    "layers": [
      {
        "type": "cohesive",
        "top": 0,
        "bottom": 8,
        "gamma": 18,
        "cohesion": 60
      },
      {
        "type": "cohesionless",
        "top": 8,
        "bottom": 16,
        "gamma": 19.5,
        "sptN": 28
      },
      {
        "type": "cohesive",
        "top": 16,
        "bottom": 24,
        "gamma": 19,
        "cohesion": 120
      }
    ],
    "waterTableDepth": 4,
    "waterUnitWeight": 9.81
  },
  "serviceLoad": 1000,
  "segments": 240,
  "loadSettlementPoints": 160,
  "maxTipSettlement": 0.12,
  "bellDiameter": 1.8,
  "bellHeight": 0.45,
  "fsTip": 2.5,
  "fsSide": 2,
  "fsWeight": 1.5,
  "fsUplift": 2.5,
  "convergenceTolerance": 0.05
}'

The GET variant returns allowableDown,allowableUplift,service.settlement as a plain-text CSV line — made for Excel's WEBSERVICE() and Sheets' IMPORTDATA() (see Use PileCalc in Excel). Omit fields to get the full JSON response.

Interpreting results

• allowableDown applies fsTip and fsSide separately (side resistance mobilizes at much smaller displacement than base bearing).
• With a bell, tip and tipPressure are computed on the bell base area, and bellUplift adds the inverted-bearing uplift contribution.
• The FHWA five-foot top exclusion is applied only for surface clay; the bottom one-diameter (plus bell-height) exclusion is applied only for cohesive base soil.
• service is a t-z/q-z load-transfer result at serviceLoad, not an ultimate-capacity settlement estimate. The separate rockSocketService response uses FHWA-NHI-18-024 Appendix C.3.1: an elastic/full-slip bilinear approximation for a uniform circular non-displacement shaft beginning at the rock surface, with full base contact. The request must contain exactly one massive-intact rock layer, no bell or tip-area override. Service load above nominal resistance or a side/base service load exceeding its static nominal component resistance is rejected. The result describes applied head-load response only; construction/self-weight settlement and preload are excluded. Head displacement is continuous at the branch intersection, but the separate asymptotic load partitions can have a small jump. Layered/jointed rock and partial base contact remain unsupported.
• The bearing stratum is the layer immediately below a tip interface. The response records that convention and the fine/coarse convergence check. Jointed-rock side resistance uses the declared Table 11.4 column, but jointed-rock base resistance fails closed.