Introduction
A soil bearing capacity chart is a preliminary reference for comparing foundation pressure with a code value or a value established in a geotechnical report. It is not a universal ranking of soil types: dense, well-confined sand can perform very differently from loose or saturated sand, and clay behavior depends on strength, drainage, plasticity, and consolidation. Related guidance: Can You Pour Concrete on Wet Clay Soil?.
This chart cannot by itself size a footing, approve a building site, establish a soil classification, or replace the locally adopted building code and a site-specific geotechnical evaluation where required. Bearing resistance, settlement, groundwater, frost, expansive soil, fill, slope stability, seismic hazards, and structural loads are separate issues that must be checked for an actual project.
Understanding Soil Bearing Capacity Charts and Their Significance
“Soil bearing capacity” can describe different values. Do not treat them as interchangeable:
| Term | What it means | How to use it |
|---|---|---|
| Ultimate bearing capacity, qult | The theoretical pressure associated with shear failure of the supporting soil. | It is an analysis result, not automatically a permitted foundation pressure. |
| Allowable bearing pressure, qallow | A design pressure limited by a safety factor and, commonly, acceptable settlement. In general, it may be expressed as qallow = qult / FS. | Use the basis and loading method stated in the geotechnical report and structural design. |
| Presumptive load-bearing value | A code-permitted default for a specified soil classification and stated conditions; it is not a measured site capacity. | Use only when the locally adopted code permits it and its conditions are met. |
A footing can satisfy a shear-bearing check and still perform poorly from total or differential settlement. Granular soils can settle immediately; fine-grained soils may consolidate or creep over time. Added fill, groundwater changes, shrink/swell, and variable layers can also move a foundation.
How Does a Soil Bearing Capacity Chart Function?
A chart associates a defined soil or rock category with a pressure value. Before using one, identify its source, code edition, units, soil-classification system, value type (ultimate, allowable, or presumptive), and limitations. A generic range for “sand” or “clay” is not enough for design because density or consistency, drainage, groundwater, foundation size and depth, layering, and settlement criteria matter.
The table below is specifically the vertical presumptive foundation-pressure table in 2024 IBC Section 1806.2 and Table 1806.2. The IBC is a model code: confirm the edition and amendments adopted by the local building department before relying on it. The classifications are USCS-based and cannot be established from appearance or a homeowner soil test. The code also restricts assumed values where classification, strength, moisture sensitivity, or compressibility is uncertain, and it does not allow unsubstantiated presumptive values for materials such as organic soils, peat, mud, or undocumented fill.
IBC Presumptive Load-Bearing Values (2024 Table 1806.2)
| IBC material category | Vertical presumptive foundation pressure |
|---|---|
| Crystalline bedrock | 12,000 psf / 574.8 kPa |
| Sedimentary or foliated rock | 4,000 psf / 191.6 kPa |
| Sandy gravel and gravel (GW, GP) | 3,000 psf / 143.7 kPa |
| Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) | 2,000 psf / 95.8 kPa |
| Clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) | 1,500 psf / 71.9 kPa |
These are not blanket approvals for all material with a similar name. They are presumptive code values subject to the table notes, site conditions, and the locally applicable code.
How to Read and Interpret a Soil Bearing Capacity Chart?
- Identify the value type and source. A geotechnical report’s allowable pressure and an IBC presumptive value have different bases. Do not substitute an internet range for either one.
- Confirm the soil classification at footing elevation. Soil can change over short distances and with depth. Do not infer a USCS classification from a surface sample alone.
- Keep pressure units consistent. Convert with kPa = psf Ă— 0.0479 or psf = kPa Ă— 20.885. For example, 1,500 psf is about 71.9 kPa, and 2,000 psf is about 95.8 kPa. Do not confuse psf (pressure) with psf/ft (a lateral-pressure gradient); 1 psf/ft equals 0.157 kPa/m.
- Calculate preliminary average foundation pressure. Divide the applicable service-level load by the loaded footing area. Do not compare factored loads to an allowable or presumptive service-level pressure unless the applicable design method directs that comparison.
- Check the limits beyond bearing. Settlement, eccentric loading, footing self-weight, frost depth, drainage, groundwater, adjacent excavations, slopes, and seismic conditions may govern.
Preliminary pressure-comparison example
Suppose a preliminary service-level wall-load estimate is 30,000 lb distributed over a 40-ft-long continuous footing. With a 2-ft-wide footing, the estimated average pressure is:
q = 30,000 Ă· (40 Ă— 2) = 375 psf
That is about 18.0 kPa. If the locally adopted code permits a 1,500-psf presumptive value for a properly classified soil, 375 psf is below that tabular value. This is only a screening comparison—not a footing design approval. It omits or simplifies footing weight, load combinations, eccentricity, variable layers, groundwater, frost, seismic effects, and settlement.
What are the key components of a soil bearing capacity chart?
A usable chart identifies the soil-classification system, units, whether the pressure is gross or net and ultimate, allowable, or presumptive, plus the conditions behind the value. If those items are absent, treat the chart as general background rather than a design input.
For site investigation, an SPT records penetration resistance that engineers interpret through correlations; it does not directly produce one universal allowable pressure. A CPT measures cone resistance and sleeve friction continuously with depth, also requiring engineering interpretation. They can be complementary, but neither is interchangeable with the other or with a homeowner soil test. The FHWA’s geotechnical reference guidance describes allowable bearing pressure as a strength-and-safety-factor issue that must also account for settlement and project conditions.
Can Soil Bearing Capacity Change Over Time?
What Factors Can Influence Soil Bearing Capacity?
Yes. Moisture and groundwater changes can alter effective stress and soil behavior. Freeze-thaw can affect frost-susceptible ground, and expansive soils can move with moisture changes. Excavation, uncontrolled fill, leaking utilities, drainage changes, vibration, added loads, and nearby construction can also change the support conditions.
Compaction can improve engineered fill only when it is specified, placed, and verified appropriately; surface tamping does not establish the capacity of deeper or variable native soil. Drainage and grading should follow the project design rather than being used to justify a higher chart value.
How Can You Ensure the Accuracy of Soil Bearing Capacity Over Long-Term Projects?
Use the site investigation and foundation design as the governing documents, then protect the assumed conditions during construction. Keep footing subgrades from being softened, disturbed, or replaced with unverified fill; verify excavation conditions when required; and avoid uncontrolled water discharge near the foundation. Where movement, groundwater, or slope risk is a design concern, the engineer may specify inspections or instrumentation.
Geophysical methods such as GPR, ERT, or seismic surveys can help map variability, voids, groundwater, bedrock, or anomalous zones. They generally supplement—not replace—borings, test pits, samples, laboratory tests, and in-situ geotechnical testing for foundation design.
Conclusion
Use a bearing-capacity chart to understand terminology and make a preliminary comparison only after confirming the source, code edition, units, soil classification, and value type. A final foundation decision requires a compatible load basis and separate checks for bearing resistance and settlement.
Stop and obtain geotechnical or engineering advice for undocumented fill, peat or organic soil, soft ground, visible settlement or cracking, wet areas or fluctuating groundwater, slopes or landslide-prone ground, expansive, collapsible, soluble, karst, or potentially liquefiable soils, basements or retaining walls, deep excavations, unusual or heavily loaded structures, or any design pressure above a permitted presumptive value. A building official, lender, insurer, or local code may also require a report.
FAQ
What should I do if my soil’s bearing capacity is lower than expected?
Do not simply increase the chart value. Have the condition evaluated. Possible engineered responses can include a wider or different foundation system, removal and replacement of unsuitable material, verified ground improvement, drainage work, or deep foundations. The appropriate option depends on the soil profile, loads, settlement limits, groundwater, and local code.
How can I test my soil’s bearing capacity on-site?
A homeowner test cannot establish design bearing capacity. A geotechnical professional may use borings, test pits, sampling, laboratory tests, SPT, CPT, vane shear, or other methods selected for the site and design question. The results must be interpreted with the proposed foundation geometry and settlement criteria.
What are the implications of poor soil bearing capacity for construction projects?
Low bearing resistance can require a different foundation solution, but settlement is often just as important. Unaddressed conditions can lead to excessive or uneven movement, cracking, drainage problems, and costly repairs.
Are there specific soil types that generally have better bearing capacities?
There is no reliable soil-type ranking without condition details. Dense gravel or sand may support higher pressures than loose, saturated material, while some stiff clays can perform well but may be controlled by consolidation or shrink/swell. Use a properly classified, site-specific value rather than the soil name alone.