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Friday, March 31, 2023

Types of Building Foundations-Shallow & Deep

Table of Contents for types of building foundations-

  1. Introduction
  2. Types of building foundations- Shallow & Deep
  3. Shallow Foundations
  4. Deep Foundations

1. Introduction

The foundation is the bottom-most part of any structure that connects it to the ground. The foundation transfers the load of the building to the earth. The selection of the type of foundations depends upon the load of the building, bending moment generated due to it, uplift pressure of soil, safe bearing capacity of the soil, space constraints etc.

The basic requirement of a foundation of a structure is to distribute the load over an area to avoid failure. The foundation should be made with durable materials as it is subjected to corrosive attack by soils, frost, water table changes, swelling and shrinkage of soil etc.

The change in length, width, depth and shape leads to different types of building foundations that have distinguished behaviour from each other. The foundations are made after carrying out the excavation works.

2. Types of Building Foundations

In general, the load-carrying capacity of the soil increases with the increase in depth. The soil strata become firm with the depth. The foundation type is such selected that it can transfer the load to the sufficiently hard strata. Thus, the soil having lower strength require foundations to go deeper. This requirement leads to the classification of foundations on the basis of their depth.

The foundations are broadly classified two into heads, shallow foundation and deep foundation.

Criteria to decide the type of building foundation. Width to depth ratio.
Criteria to decide the type of building foundation

The foundation that has width to depth ratio lesser than or equal to two is called shallow foundation.

Similarly, the foundation having width to depth ratio greater than two is called deep foundation.

Deep foundation generally carries higher loads for the same plan area.

3. Shallow Foundation

A shallow foundation is cheaper to construct as it rests on the soil near to the surface. It can carry only smaller loads. It is suitable for soils with sufficiently good safe bearing capacity. The shallow foundations are of the following types-

Types of Shallow Foundation
Types of Shallow Foundation

3.1 Isolated Footing

Isolated footing is an individual footing to a column. It is made where the soil strata are sufficiently hard and non-expansive. The isolated footing is one of the most common types of footing made in small scale construction works especially residential buildings.

Isolated Footing
Isolated Footing

The isolated footing can be made of following types such as simple rectangular, stepped and trapezoidal.

The isolated footing is not recommended where the soil settlement is higher.

3.2 Combined Footing

A combined footing is used where the column is spacing is lesser and the individual footings would foul with each other. It is also an economically feasible option for the nearby columns. It accommodates more than one column on a single base slab. The combined footing can be made either rectangular or trapezoidal.

Combined Footing
Combined Footing

The combined footing has lesser settlement than the isolated footing.

3.3 Strip Footing

The strip footing has a greater length than the width of the foundation. This footing is made under the walls or closely spaced columns. The strip footing can be made of concrete or masonry. Generally, the stone masonry wall has stone masonry strip footing and the brick wall has brick masonry strip footing.

Types of Building Foundations- Strip Footing
Strip Footing

3.4 Strap Footing

When two individual footings are joined together by a rigid beam, it is called strap footing. The strap footing is an alternative to combined footing when the column spacing is larger. The strap footing is an economical option.

Strap Footing Plan
Strap Footing

The beam connects the individual footings and makes the structure behave as a single unit. The beam centreline aligns with the centreline joining the two columns.

3.5 Mat Foundation

When the load is heavy or the column’s individual footings are overlapping, the mat foundation is made. The mat foundation contains a big slab supporting the columns. The mat foundation as the name suggests covers the entire built-up area of the building. The mat foundation is bulky and very expensive as the concrete quantity and excavation are more.

Types of Building Foundations- Mat Foundation
Mat Foundation

The mat foundation is very much resistive to the differential settlement of the soil.

3.6 Raft Foundation

The raft foundation is an economical version of the mat foundation. The concrete quantity is lesser than the mat foundation for the same built-up area. The raft foundation is also useful in resisting the differential settlement of the soil.

Raft Foundation
Raft Foundation

The raft foundation is very commonly made for big residential, commercial and industrial buildings.

4. Deep Foundation

The deep foundation is made to carry heavier loads or where the safe bearing capacity of soil is lesser than the required value or for both of the reasons. A deep foundation is also useful in expansive soils. The types of deep foundation are as follows-

Types of Deep Foundation
Types of Deep Foundation

4.1 Pile Foundation

The piles are vertical slender columns supporting a raft called a pile cap. The pile cap takes the load of the building and transfers it to the piles. The piles can be precast, cast-in-situ, driven and drilled.

Types of Building Foundations- Pile Foundation

The mode of load transfer from pile to the earth can be any of the following three types-

  1. Friction
  2. End Bearing
  3. Combination of Friction and End Bearing

The piles are very common type of deep foundation used in residential, commercial and industrial building.

The piles are circular in cross-section with having up to 600 mm diameter and are made of timber, steel or reinforced concrete etc.

Read more about pile foundation here.

4.2 Pier Foundation

The piles having a diameter of more than 600 mm are called pier foundation. The pier foundation can also be of any of the friction pier, end bearing pier or combination of both. Pier foundation is generally drilled and cast-in-situ.

Piers of Pier Foundation
Pier Foundation

4.3 Basement

Basement is constructed where space is required below the ground level. They support the entire building by connecting the base slab and ground-level slab by columns or load-bearing walls or both.

The basement has walls on the outer periphery that support the lateral earth pressure. The ability to support vertical loads as well as lateral pressure and space availability make the basement a desirable foundation type. The basement is often used in residential and commercial buildings for parking.

Basement Foundation
Basement Foundation (Image Courtesy-The Spruce)

Basement is constructed in open excavation.

4.4 Well or Caisson Foundation

A well or caisson foundation is made by digging a circular cross-sectional area and sinking a precast unit into it. The precast unit is made near the site and lowered into the pit.

Caisson Foundation
Caisson Foundation (Image Courtesy-Stock Adobe)

Caisson foundation has a huge load-carrying capacity. They are generally used in heavy construction such as bridge construction, oil rigs etc.

4.5 Buoyancy Foundation

The buoyancy foundation is a hollow box type foundation that is designed to decrease the load intensity on the ground beneath. It is made on silts, soft sands and alluvial deposits.

Buoyant Foundation
Buoyant Foundation (Image Courtesy-Researchgate)

The buoyancy foundation is expensive and thus rarely used.

4.6 Cylinder

Cylinder is small sized caisson.

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Read more about excavation here and foundations here.

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