Bearing Capacity Test

What Is a Bearing Capacity Test?

A bearing capacity test establishes the stress that soil or rock can safely sustain under a foundation without ultimate failure (shear or punching) and while meeting acceptable settlement limits. Results inform the selection and sizing of Shallow Foundations (spread footings, mats) and, by extension, the choice to transition to Deep Foundations when near-surface soils are inadequate.

Engineers combine in-situ tests (e.g., SPT, CPT, plate load, pressuremeter) with analytical models (Terzaghi/Meyerhof/Bearing capacity factors) and lab testing to derive ultimate and allowable bearing pressures or factored resistances for LRFD. For fundamentals and terminology, see Soil Mechanics, Site Characterization, and our overview of Geotechnical Soil Testing.

For stable, long-lived external specifications and procedures, reference FHWA, USACE, and testing standards from ASTM International.

Safe bearing depends on both shear capacity and serviceability (settlement). Test, analyze, and cross-check—then choose the controlling limit state.

When Do You Need a Bearing Capacity Test—and Why?

• Foundation selection: Decide between spread footing, mat, or transitioning to piles/caissons.
• Risk management: Projects with variable fills, soft/loose layers, high loads, or seismic demands.
• Optimization: Increase allowable bearing with ground improvement; verify gains in the field.
• QA/forensics: Validate design assumptions and troubleshoot differential settlements.

Site Data, Sampling & Test Planning

A bearing capacity test is only as good as the ground model. Start with desktop geology, utilities, and historical fills; then plan borings/CPTs to capture stratigraphy and groundwater. Use Sieve Analysis, Atterberg Limits, and Standard Proctor Test to classify and anticipate compaction behavior for bearing pads and mats.

• Borings & CPT: Profile key layers; position tests under critical footings.
• Sampling: Undisturbed tubes for clays (triaxial/consolidation); bulk for index tests; reconstituted sands for CD triaxial or direct shear—see Triaxial Test.
• Groundwater: Measure seasonal high; see Groundwater in Geotechnical Engineering.
• Risk-based scope: Tie investigation density to Geotechnical Risk Assessment.

Field Test Methods That Inform Bearing Capacity

No single method fits all sites. Combine screening tools (SPT/CPT) with direct load testing where precision is required or ground is variable.

• SPT (Standard Penetration Test): Correlate N₆₀ with φ for sands and undrained strength for clays (with caution). Correct for energy, overburden, and fines.
• CPT (Cone Penetration Test): Continuous q_c, f_s, u₂; excellent for φ, OCR in clays, and liquefaction screening. Provides profiles to support bearing factor selection.
• Plate Load Test (PLT): Directly loads a plate at foundation level; yields load–settlement curve and an apparent allowable bearing and modulus for near-surface soils. Apply scale effects when extrapolating to large footings.
• Pressuremeter (PMT)/Dilatometer (DMT): In-situ stress–strain; useful for deformation-based bearing checks and mats.
• Test Pits & Hand Penetrometers: Useful in shallow fine-grained soils for quick checks and bearing stratum verification.

Analytical Framework: From Tests to Allowable Bearing

Analytical models combine soil strength (c′, φ′ or undrained s_u), footing geometry, embedment depth, load inclination, and base roughness. For simplicity, the classical Terzaghi expression for a strip footing is shown below; Meyerhof, Hansen, and modern codes add shape, depth, and inclination factors for 3-D footings and non-vertical loading.

For undrained clays under short-term loading, bearing capacity often simplifies to \( q_{\text{ult}} \approx N_c s_u \) with \(N_c\) ≈ 5.14 (strip). Allowable bearing is then obtained via factor of safety (e.g., FOS 2.5–3) or resistance factors for LRFD. Always check settlement in parallel; serviceability commonly governs spread footings on sands and compressible clays.

Groundwater, Drainage, and Settlement Limits

Groundwater reduces effective stress, triggers seepage effects, and can soften fine-grained soils, lowering both shear and stiffness. In sands, allowable bearing is often set by settlement rather than shear; in clays, short-term undrained capacity can be high but consolidation settlement may control long-term performance.

In clays, compute primary consolidation and secondary compression; in sands, use elastic methods or modulus from PLT/DMT/PMT. Manage water via subdrains, over-excavation and replacement, or dewatering during construction—see Groundwater.

QA/QC, Reporting, and Using Results in Design

Quality assurance ensures bearing data are defensible. Calibrate SPT energy, verify CPT zeroes, use proper seating and increments for PLT, and document groundwater at test depth. In the report, include soil logs, lab summaries, interpreted parameters, bearing checks (ultimate and allowable/LRFD), settlement predictions, and construction recommendations.

• Data integrity: Record corrections (SPT N₆₀, overburden), strain rates (triaxial), and scale factors (PLT).
• Multiple lines of evidence: Reconcile CPT/PMT/DMT with lab strengths; explain differences.
• Earthwork links: Specify subgrade prep, moisture control, and compaction targets—see Geotechnical Earthworks and Compaction Test.
• Software: Maintain a consistent design soil profile across Geotechnical Design Software to avoid parameter drift.
• Standards: Anchor procedures/specs to ASTM, with agency guidance from FHWA and USACE.

FAQs: Quick Answers on Bearing Capacity Tests

Which test gives the most reliable bearing capacity?

For general design, CPT provides continuous, repeatable profiles that correlate well with φ and stiffness. For direct footing behavior and settlements at shallow depth, a Plate Load Test adds value—provided scale and depth effects are accounted for. Use multiple methods where risk is high.

How do I account for footing shape and depth?

Apply shape (s_*), depth (d_*), and inclination (i_*) factors to classical equations or use code-based formulations. Depth increases confinement (higher N_q contribution) but may also raise effective stress sensitivity to groundwater.

What controls—shear or settlement?

On sands and granular fills, settlement often governs. On clays, short-term undrained shear can be high but long-term settlement typically controls. Always check both limit states.

When should I switch to deep foundations?

If enlarging footings or improving ground is impractical, or if differential settlement limits are tight (e.g., equipment pads), consider piles/shafts—see Deep Foundations.

What internal pages should I read next?

Explore Bearing Capacity (theory), Settlement Analysis, Shallow Foundations, and Geotechnical Modeling.

Conclusion

A bearing capacity test is more than a single number—it is a set of defensible parameters derived from coordinated field and lab work, interpreted through transparent analytics, and verified against settlement limits. Start with a risk-based investigation, collect high-quality SPT/CPT/PMT/DMT data, and deploy plate load testing where localized performance must be validated. Convert results using modern bearing equations with shape/depth/inclination factors, apply appropriate FOS or LRFD resistance factors, and always check settlements. Manage groundwater and earthwork to preserve design capacity, and maintain a consistent design soil profile across Geotechnical Design Software. For authoritative procedures and durable references, see ASTM International, FHWA, and USACE. If shallow options still fail serviceability or capacity checks, pivot to Deep Foundations with confidence, using the same disciplined approach.