Foundation-Purpose and Types

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Foundation-Purpose and types

In simplest words,

Foundation is the lowermost part of structure which is in direct contact with soil and does the work of transmitting load to soil below it.

Purposes of Foundation:

  • to transmit the load from super-structure to soil below it (foundation).
  • Foundation provides a hard and even base to spread load uniformly over the soil.
  • It provides lateral stability to the structure (against earthquake and wind).
  • They minimize settlement of structure.

Type of Foundations:

Depending on the depth below ground, a foundation can be a shallow foundation, deep foundation or special foundation.

1.  Shallow Foundation.

Foundations laid at shallow depth (depth closer to ground level) are the Shallow foundation.
As per Karl von Terzaghi ‘s ( father of soil mechanics) theory foundation is shallow if its depth is less than or equal to its width.

Types of Shallow Foundation

i. Isolated footing:- 

An isolated footing support single column.
An isolated footing is a most economical type of footing.
When columns are spaced at relatively long distances, provide Isolated footing.
Isolated is provided when hard strata are available close to ground level.

ii. Wall footing / Strip footing:

Wall footings support structural walls that carry loads.
They support non-structural walls

(Non-load-bearing also called a panel wall, carry only their own weight. This type of wall is used to close in a steel or concrete frame building.)

iii. Combine footing: 

Combined footing usually supports two columns or three columns (not in a row).
The combined footing is provided when two columns are so close that their isolated footing would overlap (trench pit would overlaps).
For columns located near the property line, the combined footing is provided.

iv. Strap footing:

Strap footing consists of two-footing connected by a beam or a strap and it supports two single columns.
This type of footing can replace combined footings.
Strap footings are economical.

v. Raft footing:

Raft footings is a single footing for entire building area.
When Soil Bearing Capacity is low, column loads are heavy, Raft foundation is provided.
When single isolated footings are not economical, provide raft footing.
Raft footing is provided to reduce differential settlement of structure.

2. Deep Foundation

As per Karl von Terzaghi ‘s theory (1943), a deep foundation is that foundation whose depth by width ratio is more than 1.
Foundation having depth by width ratio more than 1 but less than 15 is called moderate deep foundation.
Foundation having depth by width ratio more than 15 is called deep foundation.

 

Pile Foundation:

Pile foundation is the best example of deep foundation.
Deep foundation resists load due to skin friction and end bearing.
Pile foundation is provided when hard strata are not available near the ground surface.
Pile is also provided when soil below the foundation have properties of expansion and contraction.( like Black Cotton Soil).

3. Special Foundation:

When the Black cotton soil is available below the ground for huge depth, then the special foundation is provided called under-reamed pile.

 

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