Foundation Engineering

A foundation is defined as that part of the structure that supports the weight of the structure and transmits the load to underlying soil or rock. Foundation engineering applies the knowledge of soil mechanics, rock mechanics, geology, and structural engineering to the design and construction of foundations for buildings and other structures. The most basic aspect of foundation engineering deals with the selection of the type of foundation, such as using a shallow or deep foundation system. Another important aspect of foundation engineering involves the development of design parameters, such as the bearing capacity of the foundation.

Aims of Foundation Engineering study

To be able to calculate the bearing capacity of shallow and deep foundations.

Super Structure:

Part of the structure which is above ground level

Foundation:

It is the lowest part of the structure which supports the structure by distributing its load on the soil and keeping it less than the bearing capacity of soil. Usually factor of safety in foundation is 2.5 – 3.0. Foundation can also be said to as the interface between the super structure and the soil.

Why do we use foundation?

If foundation is not provided the stress on the soil due to the building may exceed the bearing capacity of the soil and will fail. Thus, the foundation keeps the load on the soil in allowable range by distributing it on a vast calculated area.

Who is a foundation Engineer?

The one who has enough knowledge and experience to design the foundation. A foundation engineer must have:

1. Enough knowledge of scientific principles2. It comes by taking specialized courses e.g. Geology, Soil Mechanics, Foundation Engineering, Structural Engineering etc.3. Experience and courage to take engineering judgments

Requirements of a foundation

Safety

The foundation must be stable and safe in the first place. Safety of a foundation is ensured:

1. By keeping a factor of safety.
2. By avoiding structural failure of foundation itself.
3. By avoiding the excessive settlements (Total settlement + Differential settlement)

Depth requirement:

• Depth should be such that foundation is below seasonal volumetric changes caused by freezing and thawing.
• Depth of foundation should be up to a point where changes of moisture do not occur frequently and abundantly. Erosion should also be prevented and any loose soil should not be underneath foundation e.g. Peat

Spacing requirement:

When laying a foundation the surrounding (already existing foundations of adjacent buildings) should also be considered and the new foundation should be built at a distance from it so that the previous one is not affected by digging or excavation.

Economy and functionality:

The project of foundation designing must be economical and should fulfill all the requirements, necessities and standards.

Design Soil Parameters

• Soil Tests
• Scientific Procedures
• Engineering Judgment

Steps in designing foundation

Loads:

The type, quantity and amount of loads that the structure is subjected to. These loads are sorted out to the foundation engineer by a structural engineer working on the same project, by the client or he may have to estimate them himself.

Site visit and Soil Report:

The foundation engineer must visit the site and should observe the topographical and geological features of the area where foundation is to be built. Drainage system of the area and water table information should also be studied. By observing the buildings previously built in the area, the engineer can come to know about the log term effects of soil on the buildings.

Sampling and Testing:

Samples of soil should be collected from the area and should be tested for all the required properties and parameters that are required.

Design of foundation:

At the end, now, foundation is designed with consultance with the client, structural engineer of the same project, contractor and architect.

Types of foundations

1. Shallow Foundations
   1. Spread footing foundation
   2. Strip foundation
      1. Combined footing
         1. Rectangular footing
         2. Trapezoidal footing
         3. Strap footing
   3. Matt footing
2. Deep Foundations

Bearing capacity of shallow foundations

Ultimate bearing capacity:

It is the least pressure that would cause shear failure of soil underneath the footing. Denoted by qult

Allowable Bearing capcity:

It is the maximum pressure that would ensure factor of safety i.e 2.5 – 3 of soil and limiting settlement to tolerable amount Bearing capacity of shallow foundation can be found by

1. Analytical method
2. In correlation with field test
3. In situ determination
4. Presumptive value