Isolated columns or normally supported by different types of foundations e.g. spread footing, combined footing, pile foundation, etc. The design of the foundation is a challenging task. A foundation engineer must be careful during designing the building foundation.

Foundation thickness should be determined from one-way and two-way shear requirement calculations. However, there are code-specified minimum values for different types of foundations. Following are two code references (ACI and BNBC) for the minimum thickness value of concrete footing.

Depth of footing above bottom reinforcement shall not be less than 150 mm for footings on soil, nor less than 300 mm for footings on piles.

Type of Footing	Minimum Thickness	Remark
Masonry	250 mm; twice the maximum projection from the face of the wall	Greater of the two values shall be selected
Plain concrete	200 mm, or twice the maximum offset in a stepped footing
Reinforced concrete (depth above bottom reinforcement)	150mm 300mm	Resting on soil Resting on pile

N.B. Two storeys or less in Occupancy category A, B, C and D.

In conclusion, understanding and implementing the specified minimum thickness values from reputable codes like ACI and BNBC are vital steps in the foundation design process. Engineers must exercise diligence in incorporating these standards to guarantee the stability, durability, and safety of structures.

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Compaction and consolidation are two different processes which cause volume reduction of soil. These two processes are totally different. The main differences between soil compaction and consolidation process is given below.

Foundation is one of the essential parts of the structure. It is defined as that part of the structure that transfers the load from the structure constructed on it as well as its weight over a large area of soil in such a way that the amount does not exceed the ultimate bearing capacity of the soil and the settlement of the whole structure remains within a tolerable limit. Foundation is the part of a structure on which the building stands. The solid ground on which it rests is known as the foundation bed.

Why a Foundation is Provided

Foundation should fulfill the following objectives:

• Distribute the weight of the structure over a large area of soil.
• Avoid unequal settlement.
• Prevent the lateral movement of the structure.
• Increase structural stability.

Why There are Different Types of Footing

As we know that there are different types of soil, and the bearing capacity of the soil is different for each type of soil. Depending on the soil profile, size, and load of the structure, engineers chose different kinds of foundations.

Types of Foundation

In general, all foundations are divided into two categories, - shallow and deep foundations. The terms Shallow and Deep Foundation refer to the depth of the soil at which it is placed. Generally, if the width of the foundation is greater than the depth, it is labeled as the “Shallow Foundation”. If the width is smaller than the depth of the foundation it is called a “Deep Foundation.” However, deep foundation and shallow foundation can be classified as shown in the following chart.

The main aspects of different types of foundations, along with their images, are given below. As economic feasibility is one of the main factors in the type selection, it is also discussed briefly with each type. To know other factors that affect the range of foundations read: Factors Considered for Selection of Foundation.

Shallow Foundations

As the shallow foundation depth is low and it is economical, it is the most popular type of foundation for lightweight structures. Several types of shallow foundations are discussed below.

Types of Shallow Foundation

The followings are the types of shallow foundations.

1. Isolated Spread Footing

This is the most widely recognized and most straightforward shallow foundation type, as this is the most economical type. They are typically utilized for shallow establishments to convey and spread concentrated burdens caused, for instance, by pillars or columns. They are generally used for ordinary buildings (Typically up to five stories).

Isolated footing comprises a foundation directly at the base of the segment. Generally, every section has its footing. They straightforwardly transfer the loads from the column to the soil. It might be rectangular, square, or roundabout. It can comprise both reinforced and non-reinforced material. For the non-reinforced footing, however, the stature of the footing has to be more prominent to give the vital spreading of the load. They should possibly be utilized when it is sure beyond a shadow of a doubt that no differing settlements will happen under the whole structure. Spread footings are inadmissible for the orientation of large loads. It is given to lessen the twisting minutes and shearing powers in their primary areas.

The size of the footing can be roughly calculated by dividing the total load at the column base by the allowable bearing capacity of the soil.

The followings are the types of spread footing.

1. Single pad footing.
2. Stepped footing for a column.
3. Sloped footing for a column.
4. Wall footing without step.
5. Stepped footing for walls.
6. Grillage foundation.

To decide when to use shallow foundations, it is necessary to know when it is economical. It is economical when:

• The load of the structure is relatively low.
• Columns are not closely placed.
• The bearing capacity of the soil is high at a shallow depth.

2. Wall Footing or Strip footing

Wall footing is also known as continuous footing. This type is used to distribute loads of structural or non-structural load-bearing walls to the ground in such a way that the load-bearing limit of the soil isn't outperformed. It runs along the direction of the wall. The width of the wall foundation is usually 2-3 times the width of the wall.

The wall footing is a continuous slab strip along the length of the wall. Stone, brick, reinforced concrete, etc. are used for the construction of wall foundations.

• On account of block walls, the footing comprises a few courses of bricks, the least course being generally double the expansiveness of the wall above.
• On account of stone masonry walls, the counterbalances could be 15 cm, with the statues of the course as 30 cm. Along these lines, the size of the footings is marginally more than that of the block divider footings.
• If the heap on the wall is substantial or the soil is of low bearing limit, this reinforced concrete foundation type can be given.

Wall footing is economical when:

• Loads to be transmitted are of small magnitude.
• It is placed on dense sand and gravel.

3. Combined Footing

The combined footing is very similar to the isolated footing. When the columns of the structure are carefully placed, or the bearing capacity of the soil is low and their footing overlaps each other, combined footing is provided. It is fundamentally a blend of different footings, which uses the properties of various balances in a single footing dependent on the necessity of the structure.

The foundations which are made common to more than one column are called combined footings. There are different types of combined footing, including slab type, slab and beam type, and rectangular, raft, and strap beam type. They may be square, tee-shaped, or trapezoidal. The main objective is the uniform distribution of loads under the entire area of footing, for this is necessary to coincide with the center of gravity of the footing area with the center of gravity of the total loads.

Combined foundations are economic when:

• The columns are placed close to each other.
• When the column is close to the property line and the isolated footing would cross the property line or become eccentric.
• Dimensions of one side of the footing are restricted to some lower value.

4. Cantilever or Strap Footing

Strap footings are similar to combined footings. The reasons for considering or choosing strap footing are identical to the combined one.

In strap footing, the foundation under the columns is built individually and connected by a strap beam. Generally, when the edge of the footing cannot be extended beyond the property line, the exterior footing is connected by a strap beam with the interior footing.

5. Raft or Mat Foundation

Raft or Mat foundations are used where other shallow or pile foundations are not suitable. It is also recommended in situations where the bearing capacity of the soil is inadequate, the load of the structure is to be distributed over a large area, or the structure is subjected continuously to shocks or jerks.

A raft foundation consists of a reinforced concrete slab or T-beam slab placed over the entire area of the structure. In this type, the whole basement floor slab acts as the foundation. The total load of the structure is spread evenly over the entire area of the structure. This is called a raft because, in this case, the building seems like a vessel that floats on a sea of soil.

Raft foundations are economic when:

• The soil is weak and the load has to be spread over a large area.
• The structure includes a basement.
• Columns are closely placed.
• Other kinds of foundations are not feasible.
• Differential settlement is to be prevented.

Deep Foundations

Several Types of Deep Foundations Are Discussed Below.

Types of Deep Foundation.

The followings are the types of deep foundations. 

1. Pile Foundation

Pile is a common type of deep foundation. They are used to reduce cost, and when as per soil condition considerations, it is desirable to transmit loads to soil strata that are beyond the reach of shallow foundations.

The followings are the types of pile foundations.

1. Based on Function or Use
   1. Sheet Piles
   2. Load Bearing Piles
   3. End Bearing Piles
   4. Friction Piles
   5. Soil Compactor Piles
2. Based on Materials and Construction Method
   1. Timber Piles
   2. Concrete Piles
   3. Steel Piles
   4. Composite Piles

Pile is a slender member with a small cross-sectional area compared to its length. It is used to transmit foundation loads to deeper soil or rock strata when the bearing capacity of soil near the surface is relatively low. Pile transmits load either by skin friction or bearing. Piles are also used to resist structures against uplift and provide structures stability against lateral and overturning forces.

Pile is a slender member with a small cross-sectional area compared to its length. It is used to transmit foundation loads to deeper soil or rock strata when the bearing capacity of soil near the surface is relatively low. Pile spreads load either by skin friction or bearing. Piles are also used to resist structures against uplift and provide structures stability against lateral and overturning forces.

Pile foundations are economic when

• Soil with great bearing capacity is at a greater depth.
• When there are chances of construction of irrigation canals in the nearby area.
• When it is very expensive to provide raft or grillage.
• When the foundation is subjected to a heavily concentrated load.
• In marshy places.
• When the topsoil layer is compressible in nature.
• In the case of bridges, when the scouring is more in the river bed.

It can again be classified based on its material and its mechanism of load transfer or function. Several types of pile foundations are shown in the following chart.

2. Pier Foundation

Pier is an underground structure that transmits a more massive load, which cannot be carried by shallow foundations. It is usually shallower than piles. The pier foundation is generally utilized in multi-story structures. Since the base region is determined by the plan strategy for the regular establishment, the single pier load test is wiped out. Along these lines, it is increasingly well-known under tight conditions.

Pier foundation is a cylindrical structural member that transfers heavy load from the superstructure to the soil by end bearing. Unlike piles, it can only transfer load by bearing and by not skin friction.

Pier Foundation is economic when:

• Sound rock strata lie under a decomposed rock layer at the top.
• The topsoil is stiff clay that resists driving the bearing pile.
• When a heavy load is to be transferred to the soil.

Pier foundation has many advantages:

• It has a broad scope of assortment with regard to structure. There are different materials we can here to build a stylish view, and it stays in our spending limit.
• It sets aside cash and time as it doesn't require the broad removal of a ton of cement. 
• Bearing limits can increment by under-reaming the base.

Along with the advantages, it has a few disadvantages as well:

• If one post or dock is harmed, it can prompt critical harm to the general establishment.
• It can be vitality wasteful if not protected appropriately. 
• Floors must be intensely, vigorously protected, and shielded from critters.

3. Caisson Foundation

Caisson foundation is a watertight retaining structure used as a bridge pier, construction of the dam, etc. It is generally used in structures that require a foundation beneath a river or similar water bodies. The reason for choosing the caisson is that it can be floated to the desired location and then sunk into place.

A caisson foundation is a ready-made hollow cylinder depressed into the soil up to the desired level and then filled with concrete, which ultimately converts to a foundation. It is mostly used as bridge piers. Caissons are sensitive to construction procedures and lack construction expertise.

There are several types of caisson foundations.

1. Box Caissons.
2. Floating Caissons.
3. Pneumatic Caissons.
4. Open Caissons.
5. Sheeted Caissons.
6. Excavated Caissons.

Caisson foundations are economic when:

• The pile cap requirement is to be minimized.
• Noise and vibration needed to be reduced.
• It has to be placed beneath water bodies.
• Highly lateral and axial loading capacity is required.

To conclude, the foundation is a structural supporting member that transfers the total load from slab, beam, column, wall, etc. The main objective of the foundation is to provide stability to the total structure and safely transfer the total load from the structure to the soil at an optimum cost.

Economic feasibility is one of the most important factors for selection of foundation system. We have discussed the economic suitability of different type of foundation on another article: Types of Foundation – Pick the Best One. for a Project. There are many other important factors that one should consider before choosing a foundation system for a particular structure. Those factors are briefly noted below:

• Importance of the Building
• Life of the Structure
• Loads from superstructure
• Type of construction materials to be used.
• Water table level.
• Type of adjoining structure. 
• Soil condition.
• Location of building

The raft foundation is a very commonly used type of foundation system. Raft foundation is also known as Mat foundation. Definition of raft foundation along with working principle, when to choose raft, types of raft foundation, materials of raft foundation, raft construction steps etc are discussed below.

What is the Raft Foundation?

Raft foundation is actually a thick concrete slab resting on a large area of soil reinforced with steel, supporting columns or walls and transfer loads from the structure to the soil. Usually, mat foundation is spread over the entire area of the structure it is supporting.

Raft foundation is generally used to support structures like residential or commercial buildings where soil condition is poor, storage tanks, silos, foundations for heavy industrial equipment etc.

Working Principle of Raft Foundation

To get a better idea about when to use raft foundation, it is important to understand how raft foundation works. Let’s get a quick review of its working principle.

Raft foundation transmits the total load from the building to the entire ground floor area. Stress distribution mechanism of raft foundation is very simple. Total weight of the structure and self-weight of the mat is calculated and is divided by the total area of the foundation it is covering to calculate the stress on the soil.

As in case of raft foundation the contact area of the foundation with soil is much more than any other type of foundation, so the load is distributed over a larger area and thus the stress on soil is lesser and the possibility of shear failure of soil is also reduced.

When to Choose Raft Foundation

For foundation design, one of the most important aspects is choosing the right type of foundation. Raft foundation is preferred when-

• The soil has a low bearing capacity.
• Load of the structure has to be distributed over a large area.
• Individual or any other foundation area would approximately cover 50% of the total ground area beneath the structure.
• The columns or walls are placed so closely that the individual footings would overlap.
• Stress on soil needs to be reduced.
• There is a possibility of differential settlement in case individual footing is used.
• When soil strata are unpredictable and contain pockets of compressible soil.
• Basement is to be constructed.
• Any other type of footing cannot be used advantageously.

Types of Raft Foundation

Several types of Raft foundation may be used depending on the condition of soil and the load imposed on the foundation.
Followings are the different types of raft foundation used in construction:

1. Flate plate mat
2. Plate thickened under the column
3. Two-way beam and slab Raft
4. Plate raft with pedestals
5. Plied raft
6. Rigid Frame Mat or Cellular Raft Foundation

Different types of mat foundation are briefly discussed below.

1. Flat Plate Mat

This is the simplest form of raft foundation. This type of mat is used when the columns and walls are uniformly spaced at small intervals and the subjected loads are relatively small. Reinforcement is placed in both directions and more reinforcement is required at the column locations and load-bearing walls. The thickness of this types of raft foundation is generally restricted within 300mm for economic reason. A thicker slab would not be economical.

2. Plate Thickened under Columns

When the columns and load bearing walls are subjected to heavier loads, the slab is thickened under the columns and walls and extra reinforcement is provided to resist against diagonal shear and negative reinforcement.

3. Two-way Beam and Slab

In this type of raft, beams are cast monolithically with the raft slab connecting the columns and walls. This type of raft is suitable when the columns are placed at a larger distance and the loads on the columns are variable.

4. Plates with Pedestals

In this type of mat, a pedestal is provided at the base of the columns. Purpose of this type of foundation is same as flat plate thickened under columns.

5. Piled Raft

This type of raft foundation is supported on piles. A piled raft is used when the soil at a shallow depth is highly compressible and the water table is high. Piles under raft help in reducing settlement and provides resistance against buoyancy.

6. Rigid Frame Mat/Cellular Raft Foundation

In this type of raft, the foundation walls act as a deep beam. Rigid frame mat is referred when columns carry extremely heavy loads and the connecting beams exceeds 90cm depth. Here two concrete slabs are placed, one on top of another and connected with foundation walls in both directions and thus forms a cellular raft foundation. This type of raft is very rigid and is economical when the required slab thickness is very high.

Materials Used for Raft Foundation Construction

Here are some materials used for the erection of Raft foundation-

• Formwork
• Spacer
• Reinforcement
• Concrete.

Construction Process of Raft foundation

Key steps of Raft foundation construction are noted below.

1. Identify the desired depth at which foundation is to be provided.
2. Excavate soil up to the required depth.
3. Compact the soil.
4. Provide a waterproofing membrane.
5. Pour 3” of plane cement-sand paste.
6. Lay reinforcement maintaining the required spacing using spacers.
7. Pour concrete to the desired depth.
8. Curing.

To conclude, mat is one of the most common and popular type of foundation system due to its simple construction process and its effectiveness where soil condition is poor at a shallow depth. One must investigate the soil condition and analyze the loading condition of the building for an optimum use of the foundation and necessary precautions should be taken for a safe construction.

The raft foundation or mat foundation is a very popular type of foundation. Before selecting raft foundation for a project, its advantages and disadvantages must be considered.

Advantages of Raft Foundation

Followings are the advantages of raft foundation:

• Mat foundation can be provided where the shallow foundation is necessary but soil condition is poor.
• It resists differential settlement. • Mat foundation requires less earth excavation.
• Mat foundation itself can be considered as a floor slab. So it reduces the cost of constructing floor slab.
• Distribute loads over a larger area. 

Disadvantages of Raft Foundation

Followings are the disadvantages of raft foundation:

• Prone to edge erosion.
• Special measurements are needed when mat foundation is subjected to the concentrated load.

The above limitations of mat foundation can be avoided by taking necessary steps.

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While designing and constructing a raft foundation one must take all the necessary precautions which are applicable for any type of construction.

Besides those precautions, one must take the necessary steps to provide shore protection. It is a common term used before installation of a mat foundation. Shore protection is usually provided by shore piles. They support the surrounding loads and prevents surrounding soil from breaking at the time of construction.

While constructing a raft foundation one needs to understand the concept of uplift. When the soil outside the mat becomes saturated or the water table rises up, the water exerts huge upward pressure on the foundation. In such a case, the foundation tends to move up. This force is usually counterbalanced by strong downward force, like loads from the structure. But if the water table rises up while construction of the mat and no necessary steps are taken then there could be serious damage.

Isolated Pad Foundation is Selected When-

• There is high bearing capacity at a shallow depth.
• There is no chance of differential settlement.
• Loads from superstructure are relatively low.
• For cost-effective solution.

Factors Affecting Pad Foundations

• Accessibility
• Condition of subsurface
• The magnitude of the load to be transferred
• The proximity of other structures
• Water table depth

Types of Pad foundation

Followings are the types of pad foundation used in construction

1. Plain Concrete Pad Foundation
2. Reinforced Concrete Pad Foundation
3. Combined Pad Foundation
4. Continuous Pad Foundation
5. Pad Foundations with Ground Beam

Different types of pad foundations are briefly discussed below-

Plain Concrete Pad Foundation

Pad foundations can be reinforced or non-reinforced. Plain concrete pad foundations are a non-reinforced foundation. For non-reinforced pad foundation, the thickness of the foundation should be equal or more than the required width of the foundation to safely distribute the load. This type of foundation can be used when the loads from the superstructure are low.

Reinforced Concrete Pad Foundation

Reinforcement in pad foundation reduces the thickness required for the foundation. For the ease of construction, they are usually designed to be square plan area. Rectangular pads are also used, especially when there is eccentric or inclined loading.

Combined Pad Foundation

Combined pad foundations are usually used for the columns placed near to the property boundary or in the case where the pad cannot be extended to a particular direction due to other reasons like the existence of service or utility lines. In this type of foundation, the pad of external column is connected with the pad of an internal column to counterbalance the loads.

Continuous Pad Foundation

This type of pad foundations is used when the columns are closely placed and the individual pad foundations overlap each other. The reinforcement in continuous foundation ensures longitudinal stiffness. This type of foundation can also be used to resists differential settlement.

Pad Foundations with Ground Beam

This type of foundation and its purpose of use is similar to combined pad foundation. In this case, instead of continuous foundation, isolated pads are connected by ground beams. This ground beams provides integrity and counterbalance the eccentric loads.

Pad foundations are one of the most common and economic foundation systems when loads from superstructure are relatively low. The size of the pad depends on the safe bearing capacity of the soil and the load from the structure. The shape of the foundation depends on the arrangements of columns and other site conditions such as available space for construction.

Advantages of Pad Foundation

Advantages of pad foundations are written below-

• Economic
• Shallow pad foundation requires less excavation.
• Size and shape can be varied depending on site condition.

Disadvantages of pad foundation

Disadvantages of pad foundations are written below

• Foundation size may become very large depending on the loads.
• Weak against uplift force and lateral forces.

Foundation is one of the most important parts of the structure. It transfers the total loads from the structure to the soil and provides stability to the structure. Foundation can be primarily classified into two parts, such as Shallow Foundation and Deep Foundation. They are basically classified depending on the depth at which the foundation is provided.

Shallow Foundation vs Deep Foundation

Shallow foundation and deep foundation have several differences. Sources of main differences between deep and shallow footings are definition, depth of foundation, cost, feasibility, mechanism of load transfer, advantages, disadvantages, types, etc.

In the following table the main differences between shallow and deep foundations are given: