Topic 5 - Bearing Capacity of Shallow Foundations

adequate factor of safety against shear failure of soil, tolerable settlement of soil underneath the foundation

requirements for foundation design

bearing capacity

loading intensity which would cause shear failure of soil below & adjacent to soil

general shear failure

1. bearing capacity assumes general shear failure

2. as load increases, failure surfaces develop, initially around edges of the footing, then downwards & outwards

3. soil mass above the bottom failure surfaces is in the state of failure

4. occurs in dense & stiff soils

local shear failure

1. failure surfaces develope

2. Strat from edges of footing, but end w/in soil mass [do not reach ground surface]

3. soft soils, large deformations

punching shear failures

1. shear failure occurs to the soil directly under the footing

2. large settlements

3. soft soils, foundation bases at depth

compressibility of soil, depth/breadth ratio of foundation

mode of bearing capacity failure depend on

Terzaghi

1943 - approximate theory for bearing capacities of strip footings

footings on clay

1. shear strength of clay increases w/ time (consolidation)

2. critical time is directly after construction

3. TOTAL stress analysis

footings on sand

1. bearing capacity closely related breadth of footing

2. all footings on sand wider 2m, design pressures are limited by consideration of settlement only