Footings on Granular soil – In cohesion less soil, the allowable bearing pressure is governed by allowable settlement. The allowable bearing pressure in sands is governed by shear failure criteria only when the footing is very narrow and below water table and the sand is loose.
In sands, obtaining undisturbed sample is difficult even not impossible. Laboratory preparation of sand samples will not simulate the stress – strain relationship pertaining to situation in a representative manner. The effects of over-consolidation, the lateral stress and structural arrangement of soil grains are not reflected by prepared specimens. As a result, the strength and compressibility characteristics of sands are to be obtained from in – situ tests. Computation of allowable bearing pressure is made according to the procedure described in plate load and penetration tests.
Footings on Cohesive Soil – The ultimate bearing capacity of footings in clay soils is computed either by Terzaghi’s theory or Skempton’s theory or Hansen’s theory. For light structures, computation of settlement is not required and a factor of safety of 2.5 to 3 will ensure the expected settlement within permissible limit. In heavy structures, estimation of settlement is necessary and can be computed by methods, which will be discussed in subsequent sections.
To determine the bearing capacity of soils, estimation of shear strength is required. The estimation is done by carrying out shear tests on undisturbed samples. Unconfined compression test is the most widely used shear test for routine works. Seasonal climatic Variation and effect of vegetation generally affect soils up to a depth of 1.2 to 1.5 m. The undisturbed samples are taken from below the depth not affected by seasonal climatic variation and effect of vegetation. The samples are taken at depth intervals of every 15 cm up to depth equal to the width of the foundation below the base. For design, average value is taken. If several boreholes are made, the borehole giving minimum value of average strength is used in design.