Bearing capacity is how much pressure the ground can carry under a footing before it shears or settles too much.

Scope (bearing capacity)

This page covers ultimate bearing resistance of shallow foundations (strip, pad, raft concepts) from soil shear strength (c′, φ′) and soil weight. In South Africa, geotechnical limit states and partial factors are set in SANS 10160-5 (basis of geotechnical design and actions for building structures); allowable pressures follow from ultimate resistance with factors of safety or partial factors per that standard and the project’s geotechnical report.

Shallow footing check (concept)

  1. From investigation, select design c′, φ′, and γ (drained or undrained model as appropriate).
  2. Choose footing width B and embedment Df; compute overburden pressure q = γ Df at footing base (water table effects per your model).
  3. Evaluate ultimate bearing pressure qu with a classical formula (e.g. Terzaghi strip); apply FS or partial factors for allowable qa.
  4. Use the calculator below for a Terzaghi strip illustration; use geotechnical software for layered soils, water, and 3D effects.

Geotechnical Engineering — Bearing Capacity

Estimate ultimate and allowable bearing pressure for shallow foundations on soil or rock.

Introduction

Bearing capacity is the pressure the ground can sustain at the foundation level before shear failure or excessive movement. Classical bearing-capacity theory (Terzaghi, Meyerhof, Hansen, Vesic) links footing geometry, embedment, and soil strength to an ultimate value qu; design uses factors of safety or limit-state partial factors to obtain allowable or design bearing resistance.

Strip, square, and circular footings use different shape factors; layered soils, groundwater, and eccentric loads require more detailed analysis than a single formula.

Diagram of types of shallow foundations including strip and pad footings
Types of shallow foundations (concept layout). See licence on Wikimedia Commons
Diagram comparing shallow and deep foundation types
Shallow vs deep foundations (schematic). See licence on Wikimedia Commons

What bearing-capacity analysis produces

  • Ultimate bearing pressure qu (or net ultimate) for the chosen failure mechanism and soil model.
  • Allowable bearing pressure qa = qu/FS or design value per limit-state format.
  • Sensitivity to footing size and embedment for sizing and optimisation.
  • Inputs to settlement checks (bearing capacity alone does not control serviceability).

Standards (South Africa)

Structural actions and geotechnical design basis: SANS 10160 (including Part 5 for geotechnical design). Site investigation and testing: SANS 3001 series. Parameters must be consistent with the ground investigation report and groundwater assumptions.

Terzaghi (strip, rough base, general shear):   qu = c′Nc + q Nq + ½ γ B Nγ   with q = γ Df at footing base.

Notation (common)

  • qu — ultimate bearing pressure (kPa).
  • B — footing width (m); Df — embedment to base.
  • c′, φ′ — effective cohesion and friction angle (drained analysis).
  • γ — soil unit weight (kN/m³); clarify above/below water table in real design.
  • Nc, Nq, Nγ — bearing capacity factors from φ′.

Examples in practice

  • Pad footing under a column: size B × L from column load and qa.
  • Strip footing under a wall: check qu per metre run.
  • Raft: simplified bearing check plus differential settlement analysis.
  • Weak upper layer over strong stratum: punch-through or layered analysis may govern.

Calculator — strip footing (Terzaghi-style)

Illustrative strip footing with rough base and Terzaghi N-factors (φ′ > 0). For φ′ = 0°, the script uses Nc = 5.14, Nq = 1, Nγ = 0. Overburden q = γ Df when Df > 0. Verify shape factors and water effects for real footings.

qu = c′Nc + (γ Df) Nq + ½ γ B Nγ

Ultimate bearing pressure

Terzaghi ultimate bearing pressure qu for a strip footing from c′, φ′, γ, B, Df.

Key terms

Ultimate bearing capacity (qu)
Gross pressure at failure; divide by FS for allowable.
Bearing capacity factors (Nc, Nq, Nγ)
Dimensionless factors from soil friction angle.

Software and bearing-capacity workflows

Layered ground, groundwater, eccentric loads, and 3D effects are usually analysed with geotechnical FEA or specialist bearing-capacity tools. Examples (official sites):

  • PLAXIS — geotechnical finite-element limit analysis and deformation.
  • GeoStudio — slope stability, seepage, and foundation scenarios.
  • gINT — logs and parameters feeding design models.
  • Rocscience — foundation and slope tools.
  • Autodesk Civil 3D — site modelling with geotechnical extensions.

No product endorsement—bearing resistance must align with the approved ground model, groundwater level, and national standard. This calculator is a teaching aid for strip Terzaghi N-factors only.

Diagram sources

Educational schematics. Files in Images/bearing-capacity/ were downloaded from Wikimedia Commons into this repo (not copied from other topic folders). Confirm licence on each Commons file page before reuse.