2. Foundation
�Part of a building which is in direct contact with the
ground and which transfers the load imposed up on it to
the subsoil
3. Functions
� Transfer of load from super structure to ground
� Reduction of load intensity
� Not to exceed SBC
� Even Distribution of load
� Non-uniform load of superstructure evenly to the subsoil
� Provision of level Surface
� Provide levelled and hard surface over which superstructure can be built
� Lateral Stability
� Anchors the super structure against seismic, Wind and waves
� Safety against undermining
� Burrowing of animals and scouring
� Protection against soil movements
� Expansion and contraction
12. Basic Definitions
� 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.
� NET PRESSURE INTENSITY
Net pressure intensity (qn) is gross pressure intensity minus the over burden pressure at the level of
base of foundation prior to excavation.
qn = q – ϒD
Where,
ϒ = Effective unit weight of foundation soil
D = Depth of foundation
� ULTIMATE BEARING CAPACITY
Ultimate bearing capacity (qu) is the minimum gross pressure intensity at which the soil at the
base of foundation fails by shear.
13. NET ULTIMATE BEARING CAPACITY
Net ultimate bearing capacity (qnf) is the minimum net pressure intensity at which the
soil at the base of foundation fails by shear.
qnu = qu − ϒD
NET SAFE BEARING CAPACITY
Net safe bearing capacity (qns) is the maximum net pressure intensity to which the soil
at the base of foundation can be subjected without risk of shear failure.
qns = qnu / F
Where,
F= factor of safety against shear failure.
14. SAFE BEARING CAPACITY
Safe bearing capacity (qs) is the maximum gross pressure intensity to which the soil at the base of
foundation can be subjected without risk of shear failure.
qs = qu / F
or more appropriately,
qs = qns + ϒD
or, qs = (qnu / F) + ϒD
ALLOWABLE BEARING PRESSURE
Allowable bearing pressure (qa) 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.
qa = qna + ϒD
Where,
qna = net allowable bearing pressure.
15. Bearing capacity Theory
� Zone 1: State of elastic equilibrium, act as part of footing
� Ac and bc acts as retaining wall pushing zone 2
� Zone 2: Zone of radial shear
� Zone 3: Rankine’s Passive zone
