Foundation bearing capacity assessment standards establish the technical criteria for determining whether soil and rock formations can safely support structural loads without shear failure or excessive settlement. This guide covers the governing specifications—including GB 50007, GB 50021, EN 1997 (Eurocode 7), and ASTM D1194—together with sampling requirements, core testing items, and internationally recognized evaluation methods. Engineers, contractors, and developers can use this overview to plan compliant geotechnical investigations and reliable foundation designs for projects of any scale.
Technical Overview
Foundation bearing capacity assessment is the systematic process of quantifying the resistance that a soil or rock stratum can provide against loads applied through a footing, raft, or pile. The assessment establishes two governing values: the ultimate bearing capacity, at which shear failure of the ground occurs, and the allowable (or characteristic) bearing capacity, obtained by applying appropriate safety factors and settlement limits for working-load conditions. Design and evaluation are governed in China by GB 50007 (Code for Design of Building Foundation) and GB 50021 (Code for Investigation of Geotechnical Engineering), in Europe by EN 1997-1 and EN 1997-2 (Eurocode 7), and internationally by ASTM standards such as ASTM D1194 for static plate load testing and ASTM D1143 for pile load testing. Japanese practice follows JIS A 1215 for plate load testing within the national geotechnical standards framework.
Shear failure beneath a shallow foundation may develop in three modes: general shear failure on dense sands or stiff clays, local shear failure in medium-density soils, and punching shear failure in loose sands or soft clays. Classical bearing capacity theory, expressed as qu = c·Nc + q·Nq + 0.5·γ·B·Nγ, is refined by shape, depth, load inclination, and base inclination factors defined in GB 50007 and EN 1997-1, and is verified against measured data from in-situ and laboratory testing. For most building foundations, global safety factors of 2.5 to 3.0 on the ultimate value, or the partial factor format of Eurocode 7, apply. Because bearing failure and excessive settlement are both service-limiting conditions, verification of both ultimate resistance and deformation is mandatory.
Testing Samples
Undisturbed Soil Samples
Thin-wall (Shelby) tube samplers, piston samplers, or block sampling are used to recover samples that preserve the natural fabric, moisture content, and strength of cohesive soils. Sampling procedures follow GB/T 50123, ASTM D1587, and ISO 22475, with sampler area ratios below 10 to 15 percent required to obtain Class 1 or Class 2 quality samples suitable for strength and consolidation testing.
Rock Core Samples
NX-sized (approximately 54 mm diameter) or larger cores recovered by rotary drilling are used to determine unconfined compressive strength, Rock Quality Designation (RQD), and discontinuity characteristics. These parameters support the assignment of bearing values for foundations on rock, with empirical reduction factors applied to intact rock strength to account for jointing and weathering.
Compacted Fill and Treated Ground Specimens
Where foundations bear on engineered fill or improved ground, samples are taken for compaction quality control, including degree of compaction, field moisture content, and maximum dry density, in accordance with GB 50202 acceptance criteria. Plate load tests performed on each compacted or treated layer verify that the specified bearing capacity has actually been achieved before the next construction stage.
Groundwater Samples
Water samples collected from boreholes are tested for pH, sulfate, chloride, and other aggressive agents so that concrete exposure classes and protective measures for buried foundation elements can be assigned correctly.
Testing Items
The scope of testing depends on the geotechnical category of the structure, but a complete assessment program typically includes the following items:
- Ultimate bearing capacity (qu) of the foundation stratum
- Characteristic or allowable subgrade bearing capacity (fak, fa, or qa) for design use
- Proportional limit pressure and failure pressure interpreted from load–settlement curves
- Deformation modulus (E0) of the subgrade
- Coefficient of subgrade reaction (k) for pavement, mat, and machinery foundation design
- Shear strength parameters: cohesion (c) and internal friction angle (φ)
- Index properties: moisture content, density, specific gravity, Atterberg limits, and particle size distribution
- Compressibility parameters: compression modulus (Es), consolidation coefficient, and oedometer behavior
- Predicted and measured settlement at design load, including differential settlement and angular distortion
- Corrosivity of soil and groundwater toward concrete and reinforcing steel
Testing Methods
Static Plate Load Test
The plate load test is the primary direct method for verifying shallow bearing capacity. A rigid steel plate—commonly 300 mm, 500 mm, 600 mm, or up to 1000 mm in diameter—is seated in a test pit at the proposed foundation level and loaded in increments while settlement is recorded at each stage. Tests follow ASTM D1194, ASTM D1196/D1196M, JIS A 1215, and GB/T 50123. At least three test points per geotechnical unit are generally required; the characteristic value is derived from the proportional limit, a specified settlement ratio (typically settlement-to-plate-width of 0.01 to 0.015 for moderately compressible soils), or one half of the ultimate pressure. The deformation modulus and coefficient of subgrade reaction are calculated from the initial loading branch, with size-correction factors applied when transferring plate results to full-scale footings.
Repetitive Plate Load Test
For pavements, crane runways, and machinery foundations subject to cyclic loading, repetitive plate load tests per ASTM D1195 determine the resilient behavior of the subgrade and provide the design value of the reaction modulus under repeated loading.
Standard Penetration Test (SPT)
The SPT, performed to ASTM D1586, GB 50021, and ISO 22476-3, drives a split-spoon sampler and records N-values that correlate empirically with relative density, friction angle, and allowable bearing pressure in granular soils. Corrections for overburden pressure, hammer energy ratio, and fines content are applied before published correlations are used.
Cone Penetration Test (CPT/CPTu)
Cone penetration testing per ASTM D5778 and ISO 22476-1 provides a continuous profile of cone resistance, sleeve friction, and pore pressure, enabling direct estimation of bearing capacity, soil stratification, and liquefaction susceptibility in sands and fine-grained soils without sample recovery.
Pressuremeter and Dilatometer Testing
The Ménard pressuremeter test (ASTM D4719, ISO 22476-4) and flat dilatometer testing measure in-situ modulus and limit pressure, from which bearing capacity is calculated using the pressuremeter method presented in EN 1997-2 and the associated national annexes.
Field Vane Shear Test
Vane shear testing per ASTM D2573 measures the undrained shear strength of soft saturated clays and provides direct input for short-term bearing capacity checks on cohesive foundations soils.
Laboratory Strength and Consolidation Testing
Direct shear (ASTM D3080), consolidated undrained triaxial (ASTM D4767), unconfined compression (ASTM D2166), and oedometer consolidation tests (ASTM D2435, GB/T 50123) define the strength and compressibility parameters used in theoretical bearing capacity and settlement calculations.
Pile Static Load Test
Where deep foundations are used, static load tests to at least twice the design (characteristic) load per ASTM D1143 and JGJ 106 verify the load–settlement behavior, shaft friction, and end bearing of test piles. Slow maintained load procedures with staged increments and defined holding criteria constitute standard acceptance practice.
Analytical Verification
Field and laboratory data feed into bearing capacity equations incorporating correction factors for footing shape, embedment depth, load inclination, and ground slope, combined with settlement analysis using layered or elastic methods. Verification of both bearing resistance and serviceability limit states is mandatory under GB 50007 and EN 1997-1.
Applications
Foundation bearing capacity assessments are applied across the full spectrum of construction and infrastructure engineering:
- Shallow footings, rafts, and mat foundations for residential, commercial, and industrial buildings
- Verification of ground improvement works, including dynamic compaction, stone columns, cement-fly-ash gravel piles, and surcharge preloading
- Bridge abutments, highway and airport pavements, and railway subgrades
- Tower and mast foundations, wind turbine gravity bases, and telecommunication structures
- Heavy machinery, compressor, and generator foundations subject to static and dynamic loads
- Tanks, silos, and storage structures sensitive to differential settlement
- Temporary works, crane pads, and working platforms on natural or prepared ground
- Due-diligence assessment of existing foundations during renovation, retrofit, or change of use
Accredited third-party laboratories operating under ISO/IEC 17025 quality management systems commonly perform the field and laboratory portions of these programs, providing independent data for design verification, contractor acceptance, and regulatory approval.
FAQ
What is the difference between ultimate and allowable bearing capacity?
Ultimate bearing capacity is the pressure at which the ground fails in shear or enters uncontrolled settlement, while the allowable (or characteristic) value is the ultimate value reduced by a safety factor—commonly 2.5 to 3.0—and further limited so that settlements remain within serviceability criteria. Only the reduced value may be used for working-load design.
When is a plate load test required?
Plate load tests are required for important or Grade A structures, whenever foundation data cannot be obtained reliably by indirect methods, when foundations bear on fill or improved ground, and for verification of compacted layers. GB 50007 and related specifications require no fewer than three test points for each bearing stratum under evaluation.
How deep must the investigation extend?
Exploration should penetrate all compressible strata within the stress influence zone of the proposed foundation—generally to a depth at which the induced additional stress falls to about 10 percent of the in-situ overburden stress, and at least 1.5 to 2.0 times the foundation width below foundation level for footings, with substantially deeper zones required for rafts and piles.
What settlement limits apply?
Typical serviceability limits are 100 to 200 mm total settlement for ordinary buildings and angular distortion of 0.002 to 0.004 radians, with stricter values for structures sensitive to differential movement. Project-specific limits are set by the structural engineer and must be verified during the settlement analysis.
Can existing foundations be re-assessed?
Yes. For renovations, load increases, or adjacent excavation, supplementary boreholes, in-situ testing, and load tests allow the bearing capacity of the existing strata to be recalculated and the original design assumptions to be verified against current codes and standards.