PROJECT REPORT ON DESIGN & CONSTRUCTION OF OPEN AND WELL FOUNDATIONS

INTRODUCTION

1.1. FOUNDATION:

A Foundation is the element of an architectural structure which connects it to the ground, and transfers load from the structure to the ground.

There are different types of foundation for building construction and their uses depend on soil condition and loads from the structure. It is advisable to know suitability of each types of foundation before making any decision for their selection in any construction project.

1.2. TYPES OF FOUNDATION:

1. SHALLOW FOUNDATION
2. DEEP FOUNDATION

1. SHALLOW FOUNDATION: Shallow foundations are constructed where soil layer at shallow depth (up to 1.5m) is able to support the structural loads. The depths of shallow foundations are generally less than its width.

Design of Shallow Foundations:

If the soil conditions immediately below the structure are sufficiently strong and capable of supporting the required load, then shallow spread footings can be used to transmit the load. On the other hand, if the soil conditions are weak, then piles or piers are used to carry the loads into deeper, more suitable soil.

Design Considerations:

• Must not settle excessively.
• Must be placed at depth sufficient to prevent damage from surface environmental effects (frost, swelling and shrinkage, erosion and scour).
• Must not cause failure of supporting soil (Bearing Capacity criteria).

Advantages of using shallow foundation

• Cost (affordable)
• Construction Procedure (simple)
• Materials (mostly concrete)
• Labor (does not need expertise)

Disadvantages of using shallow foundation

• Settlement
• Irregular ground surface (slope, retaining wall)
• Foundation subjected to pullout, torsion, moment.

Different types of shallow foundation are:

1. Strip footing
2. Spread or isolated footing
3. Combined footing Strap or cantilever footing
4. Mat or raft Foundation.

1. Strip Footing:

A strip footing is provided for a load-bearing wall. A strip footing is also provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. In such a case, it is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing.

2. Spread or Isolated Footing:

A spread footing (or isolated or pad) footing is provided to support an individual column. A spread footing is circular, square or rectangular slab of uniform thickness. Sometimes, it is stepped or haunched in order to spread the load over a large area.

3. Combined Footing:

A combined footing supports two columns. It is used when the two columns are so close to each other that their individual footings would overlap. A combined footing is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when kept entirely within the property line. By combining it with that of an interior column, the load is evenly distributed. A combined footing may be rectangular or trapezoidal in plan.

4. Strap or Cantilever footing:

A strap (or cantilever) footing consists of two isolated footings connected with a structural strap or a lever. The strap connects the two footings such that they behave as one unit. The strap is designed as a rigid beam. The individual footings are so designed that their combined line of action passes through the resultant of the total load. a strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and the distance between the columns is large.

5. Mat or Raft Foundations:

A mat or raft foundation is a large slab supporting a number of columns and walls under the entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other.

Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is a large variation in the loads on individual columns.

Deep foundation is required to carry loads from a structure through weak compressible soils or fills on to stronger and less compressible soils or at depth, or for functional reasons.

Deep foundations are founded too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level.

Deep foundation can be used to transfer the loading to deeper, more competent strata at depth if unsuitable soils are present near the surface.

Types of Deep Foundation

The types of deep foundations in general use are as follows

1. Pile foundations
2. Well foundations