Bearing capacity is the strength of the foundation soil to hold forces from the superstructure without shear failure or excessive settlement. Foundation soil is that part of the ground that is subjected to additional stress when building foundations and superstructures on the ground. The term bearing capacity refers qualitatively to the supporting strength of a soil or rock. But to define or quantify it one must pay attention to the prefixes defined below.

The following are 7 interrelated definitions used to express the bearing capacity of the foundation.

Table of Contents

1. Gross Pressure Intensity

Gross pressure intensity (Why) is the intensity of pressure at the base of the foundation due to the load from the super structure, the foundation’s own weight and overburden, if any.

2. Net Pressure Intensity

net pressure intensity (Why_n) At the base level of the foundation before excavation, the gross pressure intensity minus the heavy load.

Why_n = q – d

Where,

= effective unit weight of foundation soil

d = depth of foundation

3. Final Bearing Capacity

final bearing capacity (Why_F) is the minimum gross pressure intensity at which the soil at the base of the foundation fails by shear.

4. Net Final Bearing Capacity

Net final bearing capacity (Why_NF) is the minimum net pressure intensity at which the soil at the base of the foundation fails by shear.

Why_NF = Q_F – D

5. Net safe bearing capacity

Net safe bearing capacity (Why_ns) is the maximum net pressure intensity to which the soil at the base of the foundation can be subjected without the risk of shear failure.

Why_ns = Q_NF / F

Where,

F = factor of protection against shear failure.

6. Safe Bearing Capacity

safe bearing capacity (Why_s) is the maximum gross pressure intensity to which the soil at the base of the foundation can be subjected without the risk of shear failure.

Why_s = Q_F / F

or more properly,

Why_s = Q_ns +d

or, Why_s = (Q_NF / f) + d

7. Allowable Bearing Pressure

allowable bearing pressure (Why_a) is the maximum gross pressure intensity to which the soil at the base of the foundation can be subjected without the risk of shear failure and excessive settlement damaging to the structure.

Why_a = Q_No +d

Where,

Why_No = net allowable bearing pressure.