Determination of the Characteristic Value of Foundation Bearing Capacity: Principles, Methods, and E

The characteristic value of foundation bearing capacity is a fundamental geotechnical parameter that governs the safe and economical design of shallow foundations, subgrades, and improved ground. This article explains how the characteristic value is defined under recognized standards such as GB 50007 and ASTM, which soil and treated materials are typically tested, and the in-situ and laboratory methods used for its determination. Whether you are a geotechnical engineer, site investigation specialist, or construction quality reviewer, this guide provides the technical background needed to interpret and apply bearing capacity test results correctly.

Technical Overview

The characteristic value of foundation bearing capacity, commonly denoted as fak, is the reference soil resistance parameter used in limit-state verification of shallow foundations. It represents the pressure that the subsoil can sustain under working-load conditions while keeping both shear failure risk and settlement within acceptable limits. In the Chinese national standard GB 50007 (Code for Design of Building Foundation), fak is defined so that it does not exceed one-half of the measured ultimate bearing pressure, or corresponds to a plate settlement of 0.01 to 0.015 times the plate width on the load-settlement (p-s) curve, whichever is more conservative.

Determination of fak is inherently a statistical exercise. According to GB 50021 (Code for Investigation of Geotechnical Engineering), multiple test results at a site are evaluated so that the reported characteristic value carries a 95% assurance rate, typically computed as the mean value minus 1.645 times the standard deviation. The characteristic value is then converted into a design bearing capacity fa by applying empirical depth and width corrections, ensuring that site-specific foundation geometry is accounted for. This two-tier framework separates the intrinsic soil property from foundation-specific design adjustments and is conceptually aligned with international practice in ASTM and ISO geotechnical standards.

Testing Samples

Natural Cohesive Soils

Undisturbed clay, silty clay, and loess deposits are common testing targets. Cohesive soils exhibit gradually curving p-s curves, so both the proportional limit pressure and the settlement-based criterion are evaluated. Undisturbed sampling and careful pit preparation are essential because structural disturbance of sensitive clays can significantly understate the true bearing resistance.

Sandy and Gravelly Strata

Dense sands, gravel, and cobble layers typically show stiffer load response but may fail in a progressive punching mode. Because undisturbed sampling of coarse granular soils is impractical, in-situ penetration tests and large-diameter plate load tests are the preferred means of characterization.

Soft and Marine-Deposited Soils

Muck, mud, and soft silty soils with high natural water content and low shear strength are critical cases. The characteristic value in these strata is often small, and deformation control frequently governs the design rather than shear failure, making deformation modulus measurement equally important.

Treated and Improved Ground

Compacted engineered fill, dynamically compacted soil, cement-stabilized columns, gravel stone columns, and geosynthetic-reinforced platforms are tested after treatment to verify that the specified post-improvement bearing capacity has been achieved. Testing is normally scheduled after the specified curing or rest period.

Weathered Rock and Rock Sockets

For shallow foundations bearing on moderately to slightly weathered rock, plate load testing at the design bearing level is used to confirm the characteristic value, as theoretical methods tend to be overly conservative for intact rock masses.

Testing Items

A comprehensive bearing capacity determination program typically reports the following parameters:

• Proportional limit pressure (p0) at the transition from linear to nonlinear response on the p-s curve.
• Ultimate bearing pressure (pu) at failure or at the termination criterion.
• Characteristic value of subsoil bearing capacity (fak) derived with code-specified interpretation rules.
• Deformation modulus (E0) and compressed modulus derived from the linear segment of the load-settlement curve.
• Subgrade reaction coefficient for structural interaction analysis.
• Elastic and residual settlement components, and settlement recovery ratio after unloading.
• Corrected design bearing capacity (fa) after depth and width modification.

Testing Methods

Static Plate Load Test

The plate load test is the primary reference method for determining fak, specified in GB 50007 Appendix C, GB/T 50123, ASTM D1194, and ISO 22476-13. A rigid circular or square plate of 0.25 to 0.50 square meters bearing area is seated at the design foundation level in a test pit at least three plate widths wide. Load is applied in approximately ten equal increments using a hydraulic jack reacting against a kentledge or anchor system, with settlement recorded by dial gauges or displacement transducers to a precision of 0.01 mm. Each increment is held until the settlement rate falls below 0.1 mm per hour. Testing terminates when the plate settlement exceeds 0.06 times the plate width, when soil heave or extrusion occurs around the plate, or when the target maximum load, typically not less than twice the design pressure, is reached.

Interpretation follows strict rules: where the p-s curve shows a clear proportional limit, fak is taken as p0; where an ultimate pressure is identifiable, fak does not exceed one-half of pu; where neither feature is distinct, fak corresponds to the pressure at a relative settlement of 0.01 to 0.015 of the plate width, and in no case exceeds one-half of the maximum applied load.

Standard Penetration Test Correlations

For sands, silts, and general cohesionless to mixed strata, the SPT blow count N provides a rapid empirical route to fak using the correlation tables in GB 50007 and regional experience. The method follows ASTM D1586 and ISO 22476-3 and is widely used to extend plate load test conclusions across a site.

Cone Penetration and Dynamic Penetration Tests

The CPT and CPTU, standardized in ASTM D3441 and ISO 22476-1, deliver continuous profiles of cone resistance from which fak is estimated through calibrated regional correlations, particularly valuable in soft and layered soils. Heavy and super-heavy dynamic penetration tests (N63.5 and N120 counts) are applied to gravel and cobble deposits where sampling and static penetration are impractical.

Pressuremeter and Screw Plate Tests

The pressuremeter test per ISO 22476-4 and ASTM D4719 measures the limit pressure and pressuremeter modulus from which bearing capacity is derived, and it is especially useful at depths beyond the practical range of open pit plate tests. Screw plate tests extend the load test concept to greater depths in softer strata.

Laboratory Testing with Analytical Computation

Where in-situ loading is not feasible, undisturbed samples are tested in consolidated-undrained or unconsolidated-undrained triaxial and direct shear apparatus in accordance with GB/T 50123, ASTM D4767, and ASTM D3080 to obtain cohesion c and friction angle phi. Theoretical ultimate bearing capacity is then computed with the Terzaghi, Hansen, or Vesic equations, and the characteristic value is established by applying the code safety factor, commonly not less than 2.0 against shear failure, together with settlement verification.

Statistical Evaluation

All data from a single stratigraphic unit are grouped and evaluated statistically. The characteristic value is derived from the arithmetic mean minus 1.645 times the standard deviation, or, where the coefficient of variation is high, from the geometric mean and logarithmic standard deviation, ensuring the reported fak carries a 95% confidence level as required by GB 50021.

Applications

Characteristic bearing capacity values underpin a wide range of geotechnical and structural design decisions, including:

• Sizing and verification of spread footings, strip footings, and raft foundations for buildings.
• Subgrade strength acceptance for highways, airport pavements, railways, and port yards.
• Quality verification of ground improvement works such as dynamic compaction, stone columns, soil mixing, and compacted replacement fills.
• Bearing checks for base slabs of retaining walls, culverts, underground structures, and storage tanks.
• Foundations for machinery, transformers, and vibration-sensitive equipment requiring stiffness characterization.
• Feasibility studies, foundation scheme comparison, and value engineering during project investigation phases.

FAQ

Q: What is the difference between the characteristic value fak and the design value fa?

A: The characteristic value fak represents the intrinsic bearing resistance of the soil layer determined by testing with a 95% statistical assurance rate. The design value fa is obtained by modifying fak with empirical depth and width correction factors in GB 50007, accounting for the actual foundation dimensions and embedment, and it is the value used directly in bearing capacity verification.

Q: Why is the plate load test considered the reference method?

A: The plate load test applies a real stress path and boundary condition closest to that of a shallow foundation, capturing the combined effects of soil strength, stiffness, and heterogeneity. Correlation methods based on SPT, CPT, and penetration counts are calibrated against plate load test results, which is why it anchors the determination chain.

Q: How many tests are required for a reliable characteristic value?

A: GB 50021 requires that a stratigraphic unit be represented by at least six data points wherever practicable so that meaningful statistical reduction can be performed. Where fewer tests exist, conservative judgment values must be adopted and documented.

Q: What termination criteria signal the ultimate bearing pressure?

A: Testing is stopped when settlement reaches 0.06 times the plate width, when the plate settles rapidly under constant load, when visible heave or extrusion of soil appears around the plate, or when the p-s curve shows a pronounced drop indicating shear failure of the subsoil.

Q: Can penetration test correlations replace plate load tests entirely?

A: Correlations provide efficient coverage between load test locations and are acceptable for routine projects with well-established regional relationships. However, for important structures, unusual soils, or treated ground, accredited third-party laboratories and design engineers generally require direct load test verification of the adopted characteristic value.