Bearing Capacity Definitions

Bearing capacity is the power of foundation soil to hold the forces from the superstructure without undergoing shear failure or excessive settlement. Foundation soil is that portion of ground which is subjected to additional stresses when foundation and superstructure are constructed on the ground. The term bearing capacity qualitatively refers to the supporting power of a soil or rock. But to define it or quantify it one should pay attention to the prefixes as defined below.

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

1. Gross Pressure Intensity

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

2. Net Pressure Intensity

Net pressure intensity (q_n) is gross pressure intensity minus the over burden pressure at the level of base of foundation prior to excavation.

q_n = q – ϒD

Where,

ϒ = Effective unit weight of foundation soil

D = Depth of foundation

3. Ultimate Bearing Capacity

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

4. Net Ultimate Bearing Capacity

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

q_nf = q_f − ϒD

5. Net Safe Bearing Capacity

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

q_ns = q_nf / F

Where,

F= factor of safety against shear failure.

6. Safe Bearing Capacity

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

q_s = q_f / F

or more appropriately,

q_s = q_ns + ϒD

or, q_s = (q_nf / F) + ϒD

7. Allowable Bearing Pressure

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

q_a = q_na + ϒD

Where,

q_na = net allowable bearing pressure.