Knowing how to find the depth of footing is the first step in shallow foundation design. A geotechnical investigation report usually supplies a recommended founding depth, but the designer must understand the reasoning behind that figure before accepting it. The same evaluation procedure applies to strip footings, raft foundations, strap footings, and more complex layouts such as the combined footing design, where two columns share one base and the depth decision affects the whole transfer of load. An incorrect depth at this stage forces expensive rework later, because excavation volume, soil preparation, and concrete quantity all depend on the level settled here.
Factors That Control the Depth of Footing
The depth of a foundation is not an arbitrary number taken from a table. Several site-specific conditions set the lower limit, and the designer weighs each one before fixing the final founding level. The factors below appear in almost every footing depth calculation.
Bearing Capacity of the Supporting Soil
The footing must sit on ground that carries the imposed loads without excessive deformation. Bearing capacity comes from the geotechnical investigation, but site judgment still matters because test borings sample only a few points across a large site. Where the top layer is weak, the footing goes deeper until it reaches a stratum with adequate capacity. Even simple footings follow this rule; a stone masonry footing relies on the same bearing-capacity check to spread a wall load safely over the ground below.
Settlement and Compressible Soils
Foundations must avoid soil that settles under load. Weak ground conditions such as low-capacity clays, loose sands, and compressible deposits like peat produce both immediate and long-term movement. Differential settlement between adjacent footings is the most damaging case because it cracks walls and distorts frames. Where such soils cannot be avoided, the founding depth increases or the design switches to a piled solution, whichever the cost comparison favors.
Position of the Groundwater Table
Water changes the economics of a footing excavation. A high groundwater table reduces effective bearing capacity, requires dewatering during construction, and adds waterproofing cost. The rule is simple: keep the base of the footing above the water table whenever the site allows it. Where that is impossible, the design must account for uplift, buoyancy, and the effect of water on soil strength.
Organic Matter and Disturbed Soil
Soil containing organic material is unstable by nature. Decaying vegetation and buried topsoil break down over time, leaving cavities that the footing eventually drops into. Loose or disturbed ground from previous excavation behaves the same way, because it has not been compacted and continues to settle under its own weight. The chosen depth must therefore reach below the zone of organic content and loose fill.
A Practical Minimum
Practicing engineers commonly set a minimum founding depth of 0.9 to 1.2 m below finished ground level for light structures, regardless of the calculated value, to stay below frost depth and the active zone of surface soil movement. Local building codes override this default where climate or soil conditions demand more.
How to Calculate Minimum Depth with Rankine’s Formula
Rankine’s formula is the most widely used equation for estimating the minimum depth of a shallow footing. It links the required depth to the bearing capacity of the soil and its angle of internal friction, and it is simple enough for a hand calculation during preliminary design.
The Rankine Equation
The equation reads D = (q / γ) × [(1 – sin φ) / (1 + sin φ)]², where D is the minimum depth of footing in metres, q is the bearing capacity of the soil in kN/m², γ is the unit weight of the soil in kN/m³, and φ is the angle of internal friction in degrees. The formula assumes a homogeneous soil mass and a uniform spread of load from the base. It produces a conservative starting point that the designer adjusts for groundwater, sloping ground, and neighboring foundations.
Worked Example for a Strip Footing
Consider a strip footing with a soil bearing capacity q of 150 kN/m², a unit weight γ of 18 kN/m³, and an angle of internal friction φ of 30 degrees. The sine of 30 degrees is 0.5, so the ratio (1 – 0.5) / (1 + 0.5) equals 0.333, and the square of that ratio is 0.111. The minimum depth works out to 150 / 18 × 0.111, which is 0.93 m. The design rounds up to 1.0 m and applies the code minimum where that is higher.
- Take q, γ, and φ from the soil investigation report.
- Calculate sin φ and the ratio (1 – sin φ) / (1 + sin φ).
- Square the ratio.
- Multiply q / γ by the squared ratio.
- Round up to the nearest 0.1 m and compare with code minimums.
When the Formula Needs Adjustment
The calculation assumes dry, homogeneous ground. Adjust the computed depth for the water table position, layered soils with differing strengths, and surcharge loads from adjacent structures or traffic. Strip and strap footings respond differently to these adjustments because a strap footing connects two pads to balance an eccentric column load, while a strip footing runs continuously under a wall; the differences between strip footing and strap footing behavior change how the depth formula applies to each.
Soil Investigation and Ground Conditions
Every depth calculation is only as good as the soil data behind it. A proper investigation samples the strata that will actually carry the load and reports the parameters the formula needs, so the depth decision rests on measurements rather than assumptions.
What the Geotechnical Report Should Provide
- Bearing capacity at various depths in kN/m²
- Soil classification and unit weight
- Angle of internal friction and cohesion
- Groundwater level and seasonal variation
- Presence of organic or contaminated material
With these values, the designer can plot depth against allowable bearing pressure and select the shallowest level that satisfies the structural requirement.
Field and Laboratory Testing
Standard penetration tests, cone penetration tests, and plate load tests give field measurements of soil strength. Laboratory work on recovered samples adds classification, compaction, and consolidation data for compressible clays. The program should cover enough points to capture variation across the footprint rather than a single borehole at one corner. The same report feeds the reinforcement design downstream, because bar sizes, spacing, and development lengths depend on the bearing pressures and soil classification recorded during the investigation; the reinforcement detailing of the footing proceeds from the same site data.
Design Checks After the Depth Is Fixed
Setting the depth is not the end of the design. The footing must still pass bearing pressure, shear, and flexure checks before it can be built, and these checks sometimes send the designer back to revise the depth.
Bearing Pressure and Footing Size
The allowable bearing pressure must exceed the actual pressure from the column or wall load divided by the footing area. Where the pressure exceeds the allowable value, the footing widens rather than deepens, because spreading the load over a larger area reduces the pressure at the base. Widening also changes the excavation volume, so the depth and plan size are finalized together.
Thickness, Shear, and Flexure
The footing thickness must resist one-way and two-way shear, usually without shear reinforcement. Flexural bars at the bottom carry the bending moment produced by the upward soil pressure. Width-to-depth proportions follow standard practice, and for individual pads the isolated footing design guidelines based on ACI 318-14 set out the full sequence of sizing, shear, and reinforcement checks.
Depth Rules for Different Footing Types
Different footing geometries place different demands on depth, although the same underlying factors govern every type. The table below lists typical starting ranges for common shallow foundations.
Typical Depth Ranges by Footing Type
| Footing type | Typical depth range | Main depth driver |
|---|---|---|
| Isolated pad footing | 0.9–1.5 m | Column load and soil capacity |
| Strip footing | 1.0–1.5 m | Wall load and frost depth |
| Combined footing | 1.2–1.8 m | Eccentric load and soil capacity |
| Raft foundation | 1.5–2.5 m | Total building load and water table |
| RC wall footing | 1.0–1.5 m | Wall thickness and bearing stratum |
Adjusting Depth for Site Conditions
The ranges in the table shift with site conditions. A high water table pushes raft and combined footings deeper or adds dewatering, while rocky ground lets the design stop at the first sound stratum. Frost depth controls exterior footings in cold climates, where the base must sit below the freezing line. For walls, the analysis and design of RC wall footing based on ACI 318-19 shows how a continuous footing under a wall is proportioned with the same bearing-capacity and shear checks as a pad.
Construction Checks Before Pouring Concrete
A correct depth on paper means nothing if the excavation does not match it. Field checks during construction close the gap between the design and the finished foundation.
Excavation and Foundation Bed
The excavation is trimmed to the designed level and the foundation bed is compacted or blinded before reinforcement is placed. Loose soil at the bottom is removed rather than buried, and water that collects in the pit is pumped out before concrete work starts.
Verifying the Level and the Soil
The surveyor checks the excavated level against the design depth at several points, and the engineer inspects the exposed soil to confirm it matches the investigation report. Where the exposed stratum differs from the report, work stops until the design is revised.
Pouring the footing and the foundation wall in one continuous operation cuts construction time and reduces cold joints, a practice covered by the footing and foundation in one pour method for monolithic concrete construction. Whichever method is used, the depth, the bed preparation, and the concrete placement are checked together before the next stage of the structure begins.