Geotechnical Allowable Bearing Pressure Calculator

What Is an Allowable Bearing Pressure Calculator?

The allowable bearing pressure of soil is the maximum pressure that a foundation can apply on the ground without causing bearing failure or excessive settlement.

Your Geotechnical Allowable Bearing Pressure Calculator is a smart tool that:

• Estimates ultimate bearing capacity of soil
• Converts it into allowable bearing capacity using a factor of safety
• Checks settlement-limited pressure based on soil stiffness and footings
• Chooses the governing allowable pressure
• Even suggests the type of structure suitable for that soil (light, medium, heavy)

Instead of juggling multiple formulas and charts, the calculator packs them all into one clean interface.

Why Allowable Bearing Pressure Matters

When you design a foundation, you are trying to avoid two main problems:

1. Bearing capacity failure
   • The soil shears and collapses under excessive load
   • This is sudden and dangerous
2. Excessive settlement
   • The soil compresses too much over time
   • The building tilts, cracks, or becomes unusable

A good foundation design must satisfy both:

• Strength safety (no shear failure)
• Serviceability (settlements within tolerable limits)

Your calculator respects both these criteria by computing:

• Ultimate bearing capacity → then divide by safety factor
• Settlement-limited pressure based on soil modulus and footing size
• Final governing allowable bearing pressure = lesser of the two

This is exactly how geotechnical engineers are trained to think.

Inputs: What the Calculator Asks From You

The calculator is designed around real-world geotechnical thinking. Let’s walk through each input in plain language.

Soil Type (Preset Profiles)

You can choose from soil types like:

• Dense Sand
• Medium Sand
• Loose Sand
• Stiff Clay
• Medium Clay
• Soft Clay
• Silt
• Gravel
• Rock

Each soil type has default values for:

• Cohesion, c (psf)
• Friction angle, φ (degrees)
• Soil modulus, Es (psf) – measure of soil stiffness
• Typical allowable settlement (inches)

These presets make the tool fast for conceptual design. You can still override them manually if you have site-specific data from investigation reports.

Foundation Type

You can select:

• Strip footing
• Square footing
• Rectangular footing
• Circular footing
• Mat foundation

Each type comes with shape and depth modification factors, reflecting how footing shape affects bearing capacity.

Footing Geometry

• Footing Width, B (ft)
• Footing Length, L (ft)

These determine the contact area and influence the bearing and settlement performance.

Embedment Depth, Df (ft)

Depth of foundation below ground level. A deeper foundation usually:

• Increases confinement of soil
• Increases overburden pressure
• Improves bearing capacity

The calculator uses depth factors to reflect this behaviour.

Water Table Depth (ft)

Water table reduces effective stress and bearing capacity. The tool:

• Considers whether the water table is:
  • Below the footing influence zone → no effect
  • At or above footing level → reduces effective unit weight
• Calculates an effective unit weight (γ’) depending on the position of the water table

This is crucial to avoid overestimating soil capacity in saturated conditions.

Safety Factor

You can choose:

• 2.0 (Conservative)
• 2.5 (Standard)
• 3.0 (High Risk)
• 3.5 (Very High Risk)

Ultimate capacity is divided by this factor to get allowable bearing capacity. Higher factor → more conservative design.

Allowable Settlement (inches)

You can specify:

• How much settlement the structure can tolerate
• For example:
  • 1.0 inch for typical buildings
  • Smaller values for sensitive structures

If left unchanged, the calculator also knows typical settlement limits from the soil type.

Soil Density, γ (pcf)

Total unit weight of soil (pounds per cubic foot). Important for:

• Overburden pressure
• Weight of soil above footing level
• Contribution to bearing capacity components

Cohesion, c (psf) and Friction Angle, φ (degrees)

You can either:

• Manually input c and φ if you have lab results
• Or leave them at 0 to let the calculator use the soil-type defaults

These are the key parameters in most bearing capacity formulas.

Soil Modulus, Es (psf)

• This represents soil stiffness
• Higher Es → smaller settlements for the same load
• Used to calculate settlement-limited pressure

Again, you can use defaults from soil type or input your own.

How the Calculator Thinks: Bearing Capacity Side

The calculator uses a semi-classical bearing capacity formula, combining:

• Cohesion term:
  ( c × N_c × s_c × d_c )
• Overburden term:
  ( γ × D_f × N_q × s_q × d_q )
• Surcharge / width term:
  ( 0.5 × γ’ × B × N_γ × s_γ × d_γ )

Where:

• (N_c, N_q, N_γ) = bearing capacity factors based on φ
• (s_c, s_q, s_γ) = shape factors
• (d_c, d_q, d_γ) = depth factors
• (γ’) = effective unit weight based on water table

The internal functions:

• calculateBearingFactorNq, Nc, Ng – compute classical bearing factors
• calculateShapeFactorSc, Sq, Sg – adjust based on footing shape and dimensions
• calculateDepthFactorDc, Dq, Dg – adjust based on foundation depth and width
• calculateEffectiveUnitWeight – modifies unit weight depending on water table position

From all this, the tool calculates:

Ultimate Bearing Capacity (qult) in psf

Then:

Allowable Bearing Capacity = qult / Safety Factor

How the Calculator Thinks: Settlement Side

Strength is not the only control. The calculator also checks settlement limitations.

Using:

• Allowable settlement (in inches)
• Footing width, B
• Soil modulus, Es
• Influence factor based on footing type

It estimates the maximum allowable contact pressure that would not exceed the target settlement.

This is the settlement-limited pressure.

The key idea is:

• Larger footing → more settlement for same pressure
• Softer soil (low Es) → more settlement
• Different shapes have different influence factors

So the calculator uses:

Settlement-limited pressure = f(settlement, Es, B, influence factor)

Now we have two pressures:

1. Allowable from strength: qult / FS
2. Allowable from settlement: q_settlement

The governing allowable bearing pressure is simply the lesser of these two.

Outputs: What You See After Clicking “Calculate”

Once the user hits “Calculate Allowable Pressure”, the calculator displays:

Ultimate Bearing Capacity

• In psf (pounds per square foot)
• Indicates the theoretical maximum pressure the soil can resist before shear failure

Allowable Bearing Capacity

• In psf
• Obtained by dividing ultimate capacity by chosen factor of safety
• Reflects strength-based allowable pressure

Settlement-Limited Pressure

• In psf
• Based on soil stiffness and settlement criteria
• Ensures that long-term settlements stay within limits

Governing Allowable Pressure

• The final design value
• Defined as the minimum of:
  • Allowable capacity from bearing
  • Settlement-limited pressure

The calculator also colors the governing pressure:

• Green (≥ 8000 psf) → Very strong ground, suitable for heavy structures
• Orange (≥ 4000 psf) → Good ground, usable for medium structures
• Red (< 4000 psf) → Weak soil, design needs attention or improvement

Maximum Column Load

Using:

Governing pressure × footing area / 1000

The calculator gives the maximum column load in kips (1 kip = 1000 lb) that the footing can safely support.

This is extremely practical for structural engineers sizing columns and footings during preliminary design.

Safety Margin

Shows how close strength and settlement criteria are:

• Positive margin → settlement pressure is higher than capacity-based allowable
• Smaller margins indicate critical or sensitive conditions

Design Recommendation

Based on the governing allowable pressure and soil type, the tool suggests:

• SUITABLE FOR HEAVY STRUCTURES
• SUITABLE FOR MEDIUM STRUCTURES
• SUITABLE FOR LIGHT STRUCTURES
• SOIL IMPROVEMENT REQUIRED
• Or CONSIDER DEEP FOUNDATIONS (especially for soft clays)

This gives users an immediate, intuitive sense of what kind of building the soil can realistically support.

Practical Use Cases of This Calculator

This Geotechnical Allowable Bearing Pressure Calculator is ideal for:

• Preliminary foundation design
• Conceptual stage comparison between soil improvement vs deeper foundations
• Quick checks during structural modelling
• Teaching soil mechanics and foundation design
• Field decisions, such as:
  • “Can I increase footing size instead of going for piles?”
  • “Is this soil good enough for G+3 or only for a single-storey structure?”

It does not replace a full geotechnical report, but it gives a fast and realistic estimate that guides smarter decisions.

Limitations and Good Practice Notes

While the calculator is powerful and thoughtfully designed, it comes with some natural limitations:

• It assumes general shear failure conditions (not punching or local shear separately).
• It does not directly cover:
  • Layered soils
  • Sloping ground or inclined loads
  • Seismic effects
  • Time-dependent consolidation settlements for compressible clays

You should always:

• Verify inputs against geotechnical investigation data
• Follow local design codes and standards (e.g., IS, Eurocode, ACI, AISC, etc.)
• Consult an experienced geotechnical engineer for critical or complex projects

The calculator complements, not replaces, full design.