Foundation design

Definition: 

  • The lowest artificially built part of a structure which transmits the load of the structure to the ground.
  • The foundation of a structure is always constructed below ground level to increase the lateral stability of the structure.


Purposes of Foundations:

  • Foundations are used for the following purposes:
  • To distribute the load of the structure over a large bearing area so as to bring the intensity of loading within the safe bearing capacity of the soil lying underneath.
  • To load the bearing surface at a uniform rate so as to prevent unequal settlement.
  • To prevent the lateral movement of the supporting material.
  • To secure a level and firm bed for building operations.
  • To increase both the overturning and sliding stability of the structure as a whole. 
Main types of foundation:
Foundations or ‘footings’ come under two main categories:
  • SHALLOW
  • DEEP

Shallow Foundation:

A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation.

The guidelines for a shallow foundation are :

  • They are founded near to the finished ground surface.
  • The depth is generally less than the width of the footing and less than 3m.
  • They are used when the surface soils are strong enough to support the load imposed upon them.

Shallow foundations are further classified into the following types:

Spread or Isolated footing
Isolated footing is used when:
  • the soil has sufficient strength within a short depth below the ground level.
  • the soil has adequate bearing capacity.
  • Super structural load is very small
The major advantages of selecting the Isolated footings are:
  • Economical when columns are placed at longer distances.
  • Ease of Constructability- Excavation, Form-work, Reinforcement placement, and placing of Concrete are at ease.
  • Workmen with little or no knowledge can easily construct an Isolated Footing.


Combined footing
Combined footing is used when:
  • When columns are very near to each other and their individual footings overlap.
  • When bearing capacity of the soil is less, requiring more area under the individual footing.
  • When the end column is located at or near the property line and its footing cannot be extended on the side of the property line.
Types of Combined Footing:
  • Rectangular
  • Trapezoidal: Trapezoidal footing is provided when the load on one of the column is larger than the other column.

Cantilever or Strap footing
Strap footing consists of two isolated footings connected with a structural strap. The strap connects the footing such that they behave as one unit.

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.
  • A normally combined footing is impractical due to the required large excavation.


Continuous (strip) or wall footing
A strip footing is another type of spread footing which is provided for a load-bearing wall. A strip footing can also be provided for a row of columns that are so closely spaced that their spread footings overlap or nearly touch each other. In such cases, 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.



Raft (Mat) foundation
Raft foundation is required on the soil of low bearing capacity, or where structural column or other loads areas are so close in both direction that individual pad will nearly touch each other. The function of raft foundation is to spread the load over as wide an area as possible, and to give a measure of rigidity to the sub-structure to enable it to bridge over local areas of weaker or more compressible soil. The degree of rigidity given to the raft also reduces differential settlement. It is useful in reducing different settlement on variable soils or there is a wide variation in loading and adjacent column or other applied loads.

Typical use :
  • Rafts are used to bridge over soft spots if the spots are very localized and to reduce the average pressure applied to the soil. Raft foundation can be used as a matter of constructional convenience in a structure supported by a grid of fairly closed spaced columns.
  • In such case, an overall raft will avoid obstruction of the site by a number of a individual excavation with their associated heaps of spoil.
  • Some designer work on the rule that if more than 50% of the area of the structure is occupied by individual strip foundation it will be more economical.
  • Normally built at for support construction at low bearing capacity such as abandon at the slopping site which is refilled or not.
Where constructed:
  • In made-up ground
  • Soft clay
  • Marshy areas
  • Uncertain the behavior of subsoil water condition
  • Pile foundation cannot be used advantageously.
  • Independent column footing is impracticable.


                          


Deep Foundation:

Deep foundations are used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil. Deep foundations are distinguished from shallow foundations by the depth they are embedded into the ground. 

The most common types of deep foundations are piles, piers and caissons.

The guidelines for a deep foundation are :

  • They are unsuitable in weak or highly compressible soils.
  • These foundations include piles, pile walls and piers.
  • They are usually at depths deeper then 3m.
  • They are used to transmit the loading to a deeper, more competent strata.
Pile
Pile is a long (slender) vertical load transferring member made of timber, steel, or concrete. 

                

Pile foundations are divided into two types they are:-
  • Pre cast Piles: Precast piles are cast at the factory and transported to the site. This kind of pile is readymade and used where there is less place to cast pile. Precast piles are not economical and require more money to transport piles to the site.
  • Cast-in-situ piles: The piles which are cast on-site. And don’t require any transportation is called cast-in-situ piles. The cast-In-situ concrete piles are cast in position inside the ground and need not be reinforced in ordinary cases. 
Pier  
A pier is a vertical column of relatively large cross-section than a pile. 


Caisson (Drilled shafts)
A caisson is a type of foundation of the shape of hollow prismatic box, which is built above the ground and then sunk to the required depth as a single unit. 
  • A pier and caisson differs basically only in the method of construction. The caissons have an action similar to pile foundations but are high capacity cast-in-situ foundations. It resists loads from structure through shaft resistance, toe resistance, and/or a combination of both of these. The construction of drilled shafts or caissons is done using an auger.
                     


Advantages and Disadvantages of Caissons

Advantages of Caissons:
  • Economics
  • Minimizes pile cap needs
  • Slightly less noise and reduced vibrations
  • Easily adaptable to varying site conditions
  • High axial and lateral loading capacity
Disadvantages of Caissons:
  • Extremely sensitive to construction procedures
  • Not good for contaminated sites
  • Lack of construction expertise
  • Lack of Qualified Inspectors

Loads on Foundation:
Dead Load: Refers to the overall weight of the structure. Includes weight of the materials permanently attached to the structure (such as flooring) and fixed service equipment (such as air conditioning)
Live load: Refers to the weight of the applied bodies that are not permanent parts of the structure. Applied to the structure during part of its useful life (e.g. people, warehouse goods). Specified by code.
Wind loads: Acts on all exposed parts of the structure. Calculated using building codes.
Earthquake Forces: Building code is consulted.
 

Depth and Location of Foundation
Depth and location of foundation depends on
  • Zone of significant volume changes in the soil.
  • Adjacent structures and property lines.
  • Groundwater
  • Underground defects

To know more about foundation design, subscribe to my Email alerts (which is on your right side of pc/mobile). You just need to put your email id and all my posts will be directly shared on your email. 

Also, you can connect with me on my social networking sites, Follow the icons it's just one click away.

To connect directly with me, you can email me at aabs.ashish@gmail.com

Helpful links:

You can buy a Photoshop tutorial book from here: https://amzn.to/3toch31

You can buy an AutoCAD tutorial book from here: https://amzn.to/2QY42xs

You can buy a Quad-Ruled Paper Pad from here: https://amzn.to/3b6xSa2

Stay Home Stay Safe