Foundation Engineering Chapter One and Chapter two Note

    Chapter -1                                   Introduction

1.1 Foundation Engineering, Importance and Purpose

Every building consists of two basic components; the superstructure and the sub-structure or foundation. The superstructure is usually that part of the building which is above ground and which serves the purposes of intended use. The substructure or foundations is the lower portion of the building, usually located below ground level, which transmits the load of the superstructure to the sub soil.

The lowermost part of superstructure which acts as media to transfer loads coming from the structure to the underlying soil is known as foundation

.

Function of foundations:

The foundation serves the following purposes:

·         Reduction of load intensity:

It distributes the loads of the superstructure to a larger area so that the intensity of the load at its base doesn’t exceed the bearing capacity of the sub soil.

·         Even distribution of load:

It distributes the non-uniform load of super structure uniformly on the sub soil.

·         Provision of level surface:

It provides levelled and hard surface over which superstructure can be constructed.

·         Lateral stability:

It anchors the super structure to the ground thus imparting lateral stability to the super structure.

·         Protection against soil movements:

It prevents the distress in the super structure due to expansion or contraction of soil.

·         Safety against undermining:

It provides the structural safety against undermining or scouring due to burrowing animals and flood water.

Foundation Engineering

The branch of engineering which deals with the design, construction and maintenance of foundation investigation of the site for foundation purpose is called foundation engineering.

Importance of foundation engineering:

·         For selection of appropriate type of foundation for a structure including its design.

·         To get familiar with parameter like bearing capacity permissible settlement.

·         To investigate foundation soil improvement technique.

·         To investigate effect of groundwater table on foundation and bearing capacity of soil.

1.2 Classification of Foundation

Foundations are of two types based on the transmission of load. They are namely;

1.       Shallow Foundation

2.       Deep Foundation

1. Shallow Foundation

Shallow foundation is one which is laid at the depth ‘D_f’ not exceeding the width of foundation. It transmits the load laterally to the soil strata at a relatively small depth. They are located just below the lowest part of super structure. They are different type of shallow foundation.

a)      Spread footing

Spread footings are commonly used for reinforcement cement concrete column. It is used to support single column. They are provided for single footing and used when columns are not closely spaced, loads on footings are less and safe bearing capacity of soil is high.

b)      Wall footing (strip or continuous footing)

The strip footing is employed in case of load bearing wall. The strip footing is used for the row of columns that are held very closely and spaced such that their spread footing overlaps or tends to nearly touch each other. In such cases it is more economical and effective to use a strip footing rather than to use a number of spread footing held in a single line. Thus, a strip footing is also called as continuous footing.

c)       Combined footing

Combined footing is designed to support two parallel columns. It is used when two columns are close to each other and individual footing would overlapped. The combined footing is constructed when the property line is so close to column that spread footing gets eccentrically loaded, if kept within the property lines. The use of combined footing will distribute load uniformly.

d)      Strap footing

The strap footing comprises of two isolated footing connected by strap and works as single unit. Strap doesn’t resist any soil reaction. The individual footings are designed such that their combined line of action passes through the resultant of the total. The strap footing becomes economical to combined footing when permissible soil pressure is greater and distance between two columns is greater.

e)      Mat foundation

Mat foundation is big slab supporting numbers of columns and walls. It is efficient when the permissible soil pressure is smaller or columns and walls are very close such that individual footing gets overlap or nearly touches each other. It is efficient to eliminate differential settlement on the non-homogenous soil or where there is variation in loads on the individual column.

2. Deep Foundation

Deep foundation is one which is laid at a depth such that the depth exceeds the width of foundation (D_f /B> 4). They transmit the load vertically to soil strata at a greater depth. Deep foundation is extended considerably into the earth. There are different types of deep foundations.

     a) Pile foundation

   b)  Pier foundation    

   c) Well or cassion

General Requirements

The general requirements of foundations are:

1.       Location and depth criterion

A foundation should be located and founded at such a depth that its performance is not adversely affected by factors such as later expulsion of soil from beneath of foundation, seasonal volume change causes by freezing and thawing and pressure of adjoining structure.

2.       Bering capacity criterion

A foundation must be safe against shear strength failure (soil rupture). An adequate factor of safety is provided to forbid bearing capacity failure.

3.       Settlement criterion

The settlement of a foundation, especially differential settlement should be within permissible limit. Excessive settlement may affect the utility of structure, spoil appearance of structure and in some cases may even damage the structure.

1.3 Factor affecting choice of a foundation

Factors affecting choice of a foundation are:

·         Type of structure

·         Probable load that foundation has to transfer

·         Existing subsurface condition which includes soil properties, effect of ground water table, bearing capacity, settlement

·         Cost/ Budget

Beside these other factors that affect the choice of foundation are:

·         Constructability

·         Service life

·         Need of client or owner

·         Margin of safety

·         Risk level

1.4 Selection of type of foundation

Following steps are essential for the selection of type of foundation:

1.  Approximate information regarding nature of the superstructure and load to be transmitted to foundation is obtained.

2.     Subsurface condition is determined.

3.     Each of customary type of foundation is judge regarding load capacity, settlement on the existing condition and eliminate the unsuitable types.

4.   More detail study and tentative design of most promising type is made and additional information regarding load and subsurface condition is obtained to determine approximate footing or piers or length and number of pile. It may be necessary to predict the behavior of the structure.

5.  Evaluate the cost of each promising type of foundation and choose the type of foundation that represent most acceptable compromise between performance and cost.