General ObservationGeneral Observation 1. 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.

The lowest part of a structure is generally referred to as foundation.

Definition of foundationDefinition of foundation

To transfer load of the superstructure to the soil on which it is resting.

Function of foundationFunction of foundation

A properly designed foundation is one that transfers the structural load throughout the soil without overstressing of soil which can result in either excessive settlement or shear failure,

both of which can damage the structure.

RequirementsRequirements (Functional)(Functional)

Classification of FoundationsClassification of Foundations

Shallow foundations located just below the lowest part of the superstructure they support; deep foundations extend considerably deeper in to earth.

Shallow foundations Deep Foundations

Shallow FoundationsShallow Foundations

PLAN

ELEVATION ELEVATION

PLAN

Combined Rectangular Footing

Distributed Load

Concentrated Load

Shallow FoundationsShallow Foundations

Combined Trapezoidal Footing

PLAN

ELEVATION

ISOMETRIC VIEW

Wall Footing

Shallow FoundationsShallow Foundations

Raft Foundation

Dead Load : Refers to the overall weight of the structure. Includes weight of the materials permanently attached to the structure (such as flooring) and fixed service equipment (such as air conditioning)

Live load : Refers to the weight of the applied bodied that are not permanent parts of the structure. Applied to the structure during part of its useful life (e.g. people, warehouse goods). Specified by code.

Wind loads : Acts on all exposed parts of the structure. Calculated using building codes.

Earthquake Forces : Building code is consulted.

Loads on foundationLoads on foundation

Depth and location of foundation depends on 1. Zone of significant volume changes in soil.

2. Adjacent structures and property lines.

3. Ground water

4. Underground defects

Depth and location of foundationDepth and location of foundation

Zone of significant volume changes in soil :

Clays having high plasticity shrink and swell considerably up on drying and wetting respectively.

Volume change is greatest near ground. Decreases with increasing depth. Volume changes usually insignificant below a depth from 1.5-3.0 m

Depth and location of foundationDepth and location of foundation

Adjacent structures and property lines.

Depth and location of foundationDepth and location of foundation

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 the soil may cause settlement of previously existing structures as a result of new stress pattern in the surrounding soil.

In general, deeper the foundations and closer to the old structure, greater will be the potential for damage to old structures.

450 Existing Footing

New Footing

Limit for bottom of deeper Footing

Part extending

property line Property line

Ground water

Presence of water reduces soil bearing capacity, larger footing size more cost. During construction pumping is necessary – adds to the cost of construction.

Underground defects

Footing location affected by underground defects Faults, caves, mines, sewer lines , underground cables and utilities.

Depth and location of foundationDepth and location of foundation