Shallow Foundation
- A shallow foundation transmits structural loads to the soil strata at a relatively small depth.
- As an approximate criterion, Terzaghi's (1943) definition that a shallow foundation is one that is laid at a depth Df not exceeding the width B of the foundation, is generally Df/B≤1.
- Foundations with a Df/B ratio greater than 1 but less than 15, are moderately deep. Deep foundations such as pile foundations have a Df/B ratio greater than 15.
- For reasons of economy, shallow foundations are the first choice of a foundation engineer for a structure unless they are considered inadequate.
- The various types of shallow foundations are:
- Strip footing or Continuous footing with its length much greater than its width (L >>B), commonly used below walls.
- Spread footing is commonly known as Isolated footing, square or circular in section, commonly used below the column.
- Combined footing- constructed due to space limitation and support two or more columns. Commonly rectangular or trapezoidal in plan.
- Raft/Mat foundation- are large continuous footing that supports all columns and walls of a structure and are constructed when soil is weak. It covers the entire area of a structure.
- Pile Foundation- These are used to transmit heavy column loads to a group of piles joined at the top by a pile cap.
- These piles transmit the structural loads to the underlying soil through friction and bearing.
- Such type of foundation system is usually adopted when the material below footing is too weak to support the structure and it becomes essential to transfer loads to better start underlying weaker strata.
- These foundations are very expensive.
The Choice of a particular type of foundation depends on:
a) Magnitude of Loads
b) Nature of Subsoil strata.
c) Nature of superstructure and its specific requirements.
General requirements of the Foundation.
For satisfactory performance, a foundation must satisfy the following three basic criteria:
- Location and depth criteria:- A Foundation must be properly located and founded at such a depth that performance is not affected by Seasonal volume changes of soil due to swelling and shrinkage and also by the presence of adjoining structures.
- Settlement Criteria:- The settlement of the foundation especially the differential settlement must be within the permissible limit. Excessive settlement may affect the utility of the structure.
- Shear failure or Bearing Capacity criteria:- A foundation be safe against shear strength failure or soil rupture. An adequate factor of safety is provided to preclude bearing capacity failure, as soil rupture is commonly referred to.