The Short Answer
Allowable soil bearing pressure is the maximum load per unit area that soil can safely support beneath a foundation without excessive settlement or a bearing failure. It is expressed as a pressure — force divided by footing area — and it sets the limit a structural engineer must design to. The value is established by a geotechnical engineer based on the actual soils at a site, including a factor of safety against failure. Footings are then sized so that the load they impose stays at or below the allowable bearing pressure.
How Allowable Bearing Pressure Is Determined
Soil Investigation and Testing
The value starts with knowing the soil. Borings and lab tests reveal the type, strength, and compressibility of each layer beneath the foundation. Strong, dense soils support higher pressures; soft or compressible soils support less. The soils that matter are those within the depth influenced by the footing loads.
Two Limits: Strength and Settlement
Allowable bearing pressure must satisfy two conditions. First, the soil must not fail in shear — a bearing capacity failure — with a factor of safety applied. Second, the settlement under load must stay within tolerable limits for the structure. Often settlement, not strength, governs the allowable value, especially on compressible soils where a strong soil could still settle too much.
Foundation Type and Geometry
The allowable value can depend on footing size and depth. Deeper and, in some cases, larger footings behave differently than shallow, small ones. The geotechnical report typically provides values applicable to the foundation types and sizes anticipated for the project.
How Structural Engineers Use It
The structural engineer takes the allowable bearing pressure from the geotechnical report and sizes each footing so the imposed load divided by the footing area does not exceed it. A lower allowable pressure means larger footings; a higher value allows smaller ones. This single number links the geotechnical investigation directly to the structural foundation design, which is why the two disciplines must coordinate.
When It Applies
Allowable bearing pressure applies to virtually every project with new shallow foundations — new buildings, additions with new footings, and many retaining structures. On sites where shallow foundations cannot achieve adequate bearing or acceptable settlement, the geotechnical engineer may instead recommend ground improvement or deep foundations, which use different design parameters.
Frequently Asked Questions
Can I assume a bearing pressure without a report?
Some codes permit conservative presumptive values in limited cases, but structural engineers and building departments often require a site-specific value from a geotechnical report, especially for larger or heavier structures.
Why is my allowable pressure lower than expected?
Settlement limits or compressible soils frequently govern, holding the allowable value below what soil strength alone might suggest.
Who is responsible for the value?
The geotechnical engineer establishes and takes responsibility for the allowable bearing pressure; the structural engineer designs footings to it.