Bearing Capacity

What Is Bearing Capacity and Why It Matters

Bearing capacity is the ability of the ground to support foundation loads without shear failure or unacceptable deformations. For designers, the goal is not only to avoid collapse but to control settlement and rotation so the superstructure performs as intended. This page gives a practical, code-aligned overview: core definitions, classical equations, correction factors, soil-specific guidance, and field verification. We also link to adjacent topics like Foundation Design, Settlement Analysis, and Site Characterization.

The ultimate bearing capacity defines the shear failure threshold; the allowable bearing pressure limits contact stresses to satisfy both safety and serviceability. Because near-surface soils can be variable and moisture-sensitive, your design should be anchored in a defensible ground model and verified in the field. For stable public references, see FHWA and hazard guidance from FEMA Building Science.

“Allowable bearing” is a project-specific value that satisfies both ultimate shear and serviceability (settlement/rotation) limits.

Core Concepts and Definitions

• Ultimate bearing capacity, \(q_{ult}\): Contact pressure at foundation failure (general shear, local shear, or punching).
• Net ultimate, \(q_{nu}\): \(q_{ult} – \gamma D_f\); subtracts overburden at foundation level.
• Allowable pressure, \(q_{allow}\): \(q_{nu}/\text{FS}\) limited further by settlement criteria—often the governing condition.
• Modes of failure: General shear (dense/stiff soils), local shear (medium), punching (loose/soft).
• Serviceability link: Even with adequate \(q_{ult}\), differential settlement can crack superstructures—see Settlement Analysis.

Classical Bearing Capacity Methods

Classical formulations combine contributions from cohesion, surcharge, and unit weight using dimensionless bearing capacity factors that depend on effective friction angle \( \phi’ \). For design, apply code-consistent factors and corrections for shape, depth, and load inclination.

For undrained clays under short-term loading, use an undrained shear strength \(s_u\) approach with appropriate factors. For sands (drained), rely on effective stress methods. Always compare several methods (e.g., Terzaghi, Meyerhof, Hansen) and use the line of evidence most consistent with site data and code guidance. See our overview page on Geotechnical Modeling for numerical refinement of complex foundations (rafts, adjacent excavations).

Shape, Depth, and Load Inclination Factors

Real foundations rarely match “strip footing at the surface with vertical load.” Correction factors adapt the classical formula to real geometry and loading.

• Shape factors \((s)\): Square and circular footings develop higher capacities than strip footings. Example: \(s_q \approx 1 + (B/L)\tan^2\phi’\) for rectangular footings (indicative form).
• Depth factors \((d)\): Embedment increases confinement; typical forms scale with \(D_f/B\).
• Inclination factors \((i)\): Lateral loads or load eccentricity reduce capacity; factors typically decrease with \(H/V\) or \(M/VB\).
• Ground/ base slope: Footings on or near slopes require additional reductions and stability checks.

Bearing Capacity by Soil Type

Soil behavior dictates both strength and deformation response. Use the correct framework for each.

• Dense to medium-dense sands (drained): Capacity rises strongly with \( \phi’ \) and footing width \(B\). Check settlement with elastic solutions and modulus correlations (e.g., CPT/SPT). Beware local shear for medium densities.
• Soft to stiff clays (short-term undrained): Use \(s_u\)-based expressions; check long-term using effective stress parameters. Sensitivity and layering control punching risk.
• Silts & fills: Evaluate compactness and drainage; collapsible loess or uncontrolled fill warrant caution and often ground improvement.
• Rock: Capacity is often governed by blockiness and discontinuities; perform bearing and sliding checks on rock mass parameters rather than intact strength alone.
• Expansive or collapsible soils: Strength is only part of the story—moisture-driven heave or collapse can dominate; coordinate with Expansive Soils.

Eccentric Loads & Contact Pressure

Off-center loads, moments, and lateral forces reduce effective footing area and create nonuniform contact pressures. Keep resultants within the “kern” (e.g., middle third for rectangles) to avoid tension zones and edge overstress.

For edge-bearing control, adjust size/geometry or group footings into a raft. Coordination with the structural engineer via Soil–Structure Interaction (SSI) avoids over-conservatism and improves constructibility.

Groundwater, Buoyancy & Seismic Considerations

Groundwater reduces effective stress (lowering capacity in sands), drives uplift on basements/tanks, and can trigger liquefaction in susceptible deposits. Embed below frost depth in cold regions and detail drainage to limit seasonal moisture swings. Review regional geology and aquifers with USGS resources.

• Effective stress check: Use submerged unit weights where the water table intercepts the bearing zone.
• Uplift/buoyancy: Verify safety against flotation; add ballast or tie-downs if needed.
• Seismic: Assess strength loss and lateral spreading; consider mitigation or shift to deep foundations if required.

Investigation, Parameters & Field Testing

Bearing capacity predictions are only as good as the ground model. Plan explorations at actual footing elevations, capture variability, and select lab tests aligned with your governing limit states.

• Borings/CPT: CPT provides continuous profiling (tip \(q_c\), sleeve \(f_s\), and normalized indices) excellent for sands; use SPT with energy corrections where CPT is impractical.
• Lab testing: Oedometer for consolidation (settlement), UU/CU triaxial for clays, and drained triaxial or direct shear for frictional parameters; see Geotechnical Soil Testing.
• Plate load tests: Direct measure of stiffness and bearing for small footings; correlate to prototype with care; see Bearing Capacity Test.
• Reporting: Summaries should include recommended \(q_{allow}\) and settlement estimates, plus construction monitoring—see Geotechnical Reporting.

If Capacity Is Low: Options That Work

When calculated capacities or settlements don’t meet criteria, you can: (1) enlarge or deepen the footing, (2) switch to a raft to spread loads, (3) adopt deep foundations, or (4) improve the ground.

• Ground improvement: Densification (dynamic compaction, vibro), reinforcement (stone columns, geogrids), drainage (PVDs + preload), soil mixing, or grouting—see Ground Improvement Techniques.
• Shallow foundation tweaks: Thicker footings, combined footings, or rafts to control contact pressure gradients—see Mat Foundations and Raft Foundations.

Practical Design Workflow (Step-by-Step)

• 1) Build the ground model: Layering, variability, and groundwater using borings/CPT and mapping—see Site Characterization.
• 2) Choose footing type: Pads, strips, or raft based on column grid and tolerances—see Shallow Foundations.
• 3) Compute \(q_{ult}\): Use classical method(s) with appropriate factors and check slope/base effects if present.
• 4) Determine \(q_{allow}\): Apply factors of safety or LRFD resistance factors as per code and limit by settlement criteria.
• 5) Check settlements: Immediate and consolidation; consider differential limits and SSI—coordinate via SSI.
• 6) Detail drainage & durability: Edge drains, capillary break, frost protection, sulfate resistance—see Groundwater.
• 7) Specify QA/QC: Subgrade proof-rolling, compaction criteria (see Standard Proctor Test), and observation records.

FAQs: Quick Answers on Bearing Capacity

What controls more often—capacity or settlement?

For most shallow foundations on typical soils, settlement governs. After computing \(q_{ult}\), size for contact pressure that meets settlement and differential limits at service loads.

How do groundwater changes affect capacity?

A rising water table reduces effective stress and \(N_\gamma\) contribution in sands, lowering capacity. It also increases buoyancy and can drive heave in excavations—manage with drainage and embedment checks.

Are plate load tests required?

Not required universally, but invaluable for sensitive projects or reclaimed/heterogeneous fills. Combine with CPT/SPT and lab data for a robust, multi-evidence design.

Where should I go next?

Explore Settlement Analysis, Shallow Foundations, and Ground Improvement Techniques.

Conclusion

Bearing capacity is a foundational check—yet the best designs integrate shear resistance, settlement control, drainage, and constructibility. Use multiple lines of evidence (classical methods + in-situ/lab data), apply shape/depth/inclination factors consistently, and cap allowable pressures by serviceability. Where capacity or settlements fall short, consider rafts, ground improvement, or deep foundations. Anchor your decisions with stable resources such as FHWA and FEMA Building Science, and connect to internal guides on Foundation Design, Groundwater, and Geotechnical Reporting. With this workflow, your bearing checks become reliable, constructible, and resilient.