Allowable Bearing Pressure Calculator – Shallow Foundation Design Tool

Allowable Bearing Pressure Estimator

This tool calculates the permissible pressure that can be applied to the ground from foundations without causing shear failure or excessive settlement.

Basic Parameters

Design Code

Units

SI (kPa, kN/m³) US (psf, pcf)

Factor of Safety (FOS)

Typical range: 2.5-3.0

Pressure Type

Calculate Net Pressure

Calculate Gross Pressure

Foundation Type & Dimensions

Foundation Type

Width (B)

Length (L)

For rectangular footings only

Depth (D_f)

Soil Properties

Cohesion (c)

kPa

Friction Angle (φ)

Unit Weight (γ)

kN/m³

Bearing Capacity Factors

Water Table Effects

Water Table Depth

Submerged Unit Weight (γ')

kN/m³

Used when water table is at or above foundation base

Settlement Check (Optional)

Perform settlement check

Results

Ultimate Bearing Capacity (q_ult)

0.00

kPa

Allowable Bearing Pressure (q_a)

0.00

kPa

Theory

Terzaghi's Bearing Capacity Theory

The ultimate bearing capacity of shallow foundations is calculated using Terzaghi's general bearing capacity equation:

For strip footings:
q_ult = cN_c + γD_fN_q + 0.5γBN_γ

For square footings:
q_ult = 1.3cN_c + γD_fN_q + 0.4γBN_γ

For circular footings:
q_ult = 1.3cN_c + γD_fN_q + 0.3γBN_γ

Where:
c = cohesion of soil
N_c, N_q, N_γ = bearing capacity factors
γ = unit weight of soil
D_f = depth of foundation
B = width of foundation

The allowable bearing pressure is then calculated by dividing the ultimate bearing capacity by a factor of safety (typically 2.5-3.0).

Interactive Guide

Select the foundation type (strip, square, circular, or rectangular)
Enter the foundation dimensions (width and depth)
Input the soil properties (cohesion, friction angle, unit weight)
Specify water table conditions if applicable
Set the factor of safety (default is 3.0)
Click "Calculate" to get the allowable bearing pressure
Optionally, enable settlement check for additional analysis

Ultimate Bearing Capacity (q_ult): The maximum pressure the soil can withstand before shear failure occurs.

Allowable Bearing Pressure (q_a): The safe pressure that can be applied to the soil, considering a factor of safety.

Net Allowable Pressure (q_net): The allowable pressure excluding the overburden pressure from soil above foundation level.

Estimated Settlement: The predicted settlement under the calculated bearing pressure (if settlement check is enabled).

Soil Type	Cohesion (kPa)	Friction Angle (°)	Unit Weight (kN/m³)
Soft clay	0-25	0-10	15-18
Stiff clay	25-100	10-20	17-20
Loose sand	0	25-30	16-18
Dense sand	0	35-45	18-21