Foundation, Concrete and Earthquake Engineering

Selection of Foundation and Structural System for Expansive Soil

Any one planning to construct foundation for new buildings on expansive soil, some special considerations should account in design and constriction which are somewhat different from construction on normal soils. In the preliminary design of a building to be constructed on expansive soil, the swelling potential should be considered and the acceptance limit of cracking, may be structural or nonstructural, should be set at this time.


Objective of foundation design


• Foundation should be designed such that no unexpected foundation and structural distress are observed throughout its service life

• Foundation should be selected based on availability of building materials, both construction skills and equipment.

• The moisture content below foundation should be made and maintained constant. In this regard, if possible the foundation construction should be followed by wet season and proper drainage should be provided to avoid ponding water. The moisture content of the excavation should be kept constant as far as possible.

Special consideration for bearing capacity, foundation system and superstructure system must be taken and a tolerable a angular distortion have to set during planning and design. To avoid excessive distress to the buildings, necessary flexibility to the structural system must be provided.

a. Evaluation of bearing capacity


Pressure from foundation loading should be more than swelling pressures of expansive soil, if possible; sometimes this foundation pressure may be less than swelling pressure even a proper deign is done. This is due to long construction period while loads are exerted gradually in case of multistoried buildings. The foundation pressure should be less than the predicted bearing capacity to avoid unexpected foundation displacement.The factor of safety is decided to keep safe value of foundation pressure sufficiently lower than bearing capacity.

Current theoretical assessment and empirical relationship furnishes a reliable estimation of ultimate capacity, but which is not applicable for evaluation of differential deformation of the foundation. So factor of safety is used for ultimate bearing capacity to evaluate allowable or safe working loads should be consistent with allowable settlement.

b. Selection of Foundation Systems


A well designed foundation should fulfill all functional requirements of the building and limit differential movement of different parts of the building that may be subjected to damage at optimum cost. A perfect foundation should effectively transmit maximum allowable distortion. The allowable distortion that can be said tolerable depends on design and use of the building.

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