General Observation1. Soil does not posses a unique or linear stress-strain relationship.2. Soil behavior depends up on the pressure, time and environment.3. Soil at every location is essentially different4. Nearly in all the cases, the mass of soil involved is underground and cannot be seen entirely, but must be evaluated on the basis of small size samples, obtained from isolated locations.5. Most soils are very sensitive to disturbance from sampling and thus the behavior measured by a lab test may be unlike that of in situ soil.
The foundation engineer should posses the following information Knowledge of soil mechanics and background of theoretical analysis Composition of actual soil strata in the field. Necessary experience-precedents-what designs have worked well under what designs have worked well under what conditions-economic aspects Engineering judgment or intuition - to find solutions to the problems.
Definition of foundationThe lowest part of a structure is generally referred to asfoundation. Function of foundationTo transfer load of the superstructure to the soil on which it isresting. Requirements (Functional)A properly designed foundation is one that transfers thestructural load throughout the soil without overstressing of soilwhich can result in either excessive settlement or shear failure,both of which can damage the structure.
Classification of Foundations Shallow foundations Deep FoundationsShallow foundations located just below the lowest part of thesuperstructure they support; deep foundations extendconsiderably deeper in to earth.
Loads on foundationDead Load : Refers to the overall weight of the structure. Includesweight of the materials permanently attached to the structure (such asflooring) and fixed service equipment (such as air conditioning)Live load : Refers to the weight of the applied bodied that are notpermanent parts of the structure. Applied to the structure during part ofits useful life (e.g. people, warehouse goods). Specified by code.Wind loads : Acts on all exposed parts of the structure. Calculated usingbuilding codes.Earthquake Forces : Building code is consulted.
Depth and location of foundation Depth and location of foundation depends on1. Zone of significant volume changes in soil.2. Adjacent structures and property lines.3. Ground water4. Underground defects
Depth and location of foundationZone of significant volume changes in soil :Clays having high plasticity shrink and swell considerably upon drying and wetting respectively.Volume change is greatest near ground. Decreases withincreasing depth. Volume changes usually insignificant belowa depth from 1.5-3.0 m
Depth and location of foundationAdjacent structures and property lines. Structures may be damaged by the construction of new foundations, as a result of vibrations, undermining by excavation or lowering of the water table. After new foundations have been constructed, the (new) loads they place on thePart extending soil may cause settlement of previously existingproperty line Property line structures as a result of new stress pattern in the surrounding soil. New FootingIn general, deeper the foundationsand closer to the old structure, 450 Existinggreater will be the potential for Limit for bottom of Footingdamage to old structures. deeper Footing
Depth and location of foundationGround waterPresence of water reduces soil bearing capacity, larger footing sizemore cost. During construction pumping is necessary – adds to the costof construction.Underground defectsFooting location affected by underground defectsFaults, caves, mines, sewer lines , underground cables and utilities.