Well Foundations

Types of Foundations: Selection, Advantages & Disadvantages

In this article, we will discuss types of foundations.

Foundation is the substructure that transfers the load from the superstructure to the soil beneath. It is responsible for the stability of the whole structure.

Foundation is an important component of any engineering work that greatly determines the strength and stability of the entire structure. Foundation acts as a medium to transfer the load from the superstructure to the underlying ground.

 

  1. Types of Foundation  

In general, foundations can be classified as:

a. Shallow Foundation

b. Deep Foundation

 

Types of Foundation

 

  a. Shallow Foundation  

A shallow foundation is also called an open or wide foundation.

Those, foundation which transfers the loads to subsoil at a point near the ground floor of the building such as strips and rafts are called shallow foundations.

For this type of foundation, the depth of the foundation is less than or equal to the width of the foundation.

The features of a shallow foundation are as follows:

a. Depth is less or sometimes equal to its width.

b. It is placed immediately beneath the lowest part of the superstructures.

c. It is spread more horizontal than vertical.

d. It transfers the loads to subsoil at a shallow depth, close to the ground level.

 


 

1. Types of shallow foundation  

There are 4 types of shallow foundations. They are:

a. Spread footing

b. Strip footing

c. Mat foundation

d. Grillage foundation

 

a. Spread Footing

This footing is also known as pad foundation. In this type of foundation, the base is made wider than the top to distribute the load from the superstructure over a large area.

This type of foundation is suitable for walls and masonry columns.

These foundations are constructed after opening the trenches to the required depth.

It is economical for a maximum depth of 3 m.

While constructing this type of footing trenches are opened to the required depth and the soil is rammed well. Then a plain concrete mix 1:4:8 is placed. Its thickness differs from 150 to 200 mm. The stone-masonry footing is constructed over this bed. It is constructed in courses and each course is projected 50 to 75 mm from the top course and the height of each course is 150 to 200 mm.

In the case of wall footing, the projections are provided only in one direction while in the case of columns, they are provided in both directions.

The projection of bed concrete from the lowest course of foundation masonry is usually 150 mm.

 

Spread Footing  

 

Types of Spread Footing

There are 3 types of spread footing. They are:

1. Independent Footing / Isolated

2. Combined Footing

3. Continuous Footing

 

1. Isolated Column Footings / Isolated Footings

Separate footings are built for each column in isolated column footings.

For distributing the load of the columns safely and uniformly over the soil, the size of the footing is kept according to the area required.

Such type of footings is usually constructed over 100 or 150 mm concrete bed.

Design engineers evaluate the required reinforcement and thickness of footing.

Footing thickness may be uniform or sometimes varying.

 

2. Combined Footings

In this type of footings, two or more columns are supported by a single base.

This type of footing is required when a column is extremely close to the boundary of the property and hence it is worthless to provide footing much beyond the column face.

This footing may be rectangular or sometimes trapezoidal.

The footing should be designed and constructed to transfer loads from both columns safely to the soil.

A strap beam is provided for connecting two columns.

 

3. Continuous Footings

In continuous footings, a footing is common for more than two columns in a row.

In case the columns in a row are nearer or if the SBC of soil is low, the continuous footing will be more applicable.

 


 

b. Strip Footing  

Strip footing is the independent footing of two columns connected by a beam. It is of four types.

They are:

1. Wall footing

2. Inverted arch footing

3. Eccentrically loaded footing

4. Offset and strap (cantilever) footing

 

1. Inverted Arch Footing

This type of foundation is applicable for the areas where the SBC of the soil is incredibly poor and the load of the structure is through walls.

Thus, inverted arches are built between the walls.

End walls should be able to withstand the outward horizontal thrust due to arch action. So, it should be sufficiently thick and strong.

The outer walls may be provided with buttress walls to strengthen them.

 

2. Eccentrically Loaded Footing

As far as practicable, the foundation should be so shaped and proportioned in such a way that center of gravity of the imposed loads coincides with the C.G of the supporting area of the base.

But the footing which is so shaped that the center of gravity does not coincide with the C.G of the supporting area of the base is known as eccentrically loaded footing.

 

Strip Footing

 


 

c. Mat footing / Raft footing  

Whenever the load on the column is extensive (multi-story column) or when the SBC of the soil is low, the foundations overlap each other.

In such a situation, it is beneficial to provide common footing to several columns and this footing is called mat footing.

Load distribution is uniform in this footing.

It is also called raft footings.

The raft foundation in which the beams are built in both directions over the footing slab for connecting columns may be called a grid foundation. The settlement is uniform in this type of footing and hence unnecessary stresses are not developed.

 

Mat footing / Raft footing 

 

Types of mat footing:

a. Slab (solid)-up to 30 cm

b. Slab and beam-slab > 30 cm

c. Cellular-slab > 90 cm

 


 

d. Grillage Footing  

Most high-rise buildings are constructed with steel columns encased in concrete. Such types of columns carry a very heavy load and hence it requires special foundations for spreading the entire load to a larger area of soil.

So Grillage foundation is one such special foundation that is used where the load of the structure is excessive and the bearing capacity of the soil is poor and a deep foundation is not possible.

It has one tier or more tiers of I-section steel beams.

The top tiers consist of fewer numbers but large steel sections while the lower tier consists of larger numbers but smaller size steel sections.

Through the baseplate; column load is transferred to the top tier.

The unpainted grillage beams are enclosed in concrete beyond the edge of steel sections with a minimum cover of 100 mm.

A minimum clear space of 75 mm is required to be kept between the flanges of adjacent grillage beams which ensures proper concreting.

Pipe separators are used to maintain spacing.

 

Based on the material of the foundation, the grillage foundation is of two types.

1. Timber grillage:

It is mostly used for masonry wall foundations. It avoids differential settlement.

2. Steel grillage:

It is used to carry heavy loads from steel columns and distribute them into the soil having low bearing power.

 

Grillage foundation

 

 


 

  b. Deep Foundation  

The type of foundation whose depth is more than that of width is known as a deep foundation. Deep foundations are used to bear loads from a superstructure through weak compressible soils or fill onto stronger and less compressible soils or rocks at a certain depth, or for functional causes.

 

Deep Foundation

 

Types of Deep Foundation

The types of deep foundations in general use are as follows:

a. Pile Foundations

b. Well Foundations

c. Pier Foundations

 

a. Pile foundations

A Pile foundation is defined as a type of deep foundation that comprises a long, slender column used to transfer the load coming from the superstructure to the lower-lying ground thereby providing overall support to the structure.

In general, a pile foundation is a long cylinder usually made up of steel or concrete and driven deep into the ground to act as a base or support to the structure built on it.

Pile foundations transfer the load through skin friction or end bearing mechanism.

Sometimes, pile foundations made up of timber are also used.

Pile foundations are mostly used for large structures and when the upper soil layer is weak and does not have sufficient bearing capacity to withstand the imposed load.

 

Suitability of Pile Foundation  

It is desirable to use pile foundations in the following cases:

1. The surface soil layer is weak and cannot resist the load of the structure and undergoes excessive settlement.

2. When the structure to be constructed consists of very heavy concentrated loads such as in the construction of high-rise buildings, water tanks, bridges, etc.

3. When the groundwater table is relatively high.

4. When the construction has to be done on the riverside, river bed, or seashore where there is an increased risk of scouring.

5. When other types of foundations are not feasible or expensive.

6. When a canal system or deep drainage structure exists nearby the construction site.

 

Pile foundations

 


 

Types of Pile Foundation  

Pile foundation may be classified based on various factors which are discussed below:

 

A. Based on Function or Use  

Depending upon the function or purpose served, pile foundation may be classified as follows:

 

i. Sheet Piles  

Sheet piles are the type of piles that are mostly used for providing lateral support and resisting the lateral pressure generally from loose soil, the flow of water, etc.

Mostly, such type of piles is used for the sheeting of trench, protection of shore, cofferdams, etc.

Sheet piles are commonly used for the following purposes:

1. Retaining the loose soil around the foundation trenches.

2. Constructing retaining walls.

3. Constructing erosion protection structures.

4. Confining the soil and isolating the foundation from the adjacent
soil layers.

One of the limitations of sheet pile is that it cannot be used for providing vertical
support to the structure.”

 

  ii. Load Bearing Piles  

Unlike sheet piles, load-bearing piles are the type of pile foundations that transfer the vertical loads to the underlying soil.

Based on the load transfer mechanism, load-bearing piles can be further classified into the following:

 

a. End Bearing Piles

End bearing piles are the type of piles that are based on the load transfer mechanism and load is transferred through the bottom tip of the pile itself.

The end-bearing piles are driven into the ground such that the bottom end tip of the pile rests at the intermediate layer between the weak soil layer and the strong soil layer. On account of this, end bearing piles develop most of the bearing capacity at the bottom tip itself.

Thus, end-bearing piles act as a column and transfer the load coming from the superstructure to the underlying soil.

The bearing capacity of such a pile can be easily determined by multiplying the area of the bottom tip of the pile and the bearing capacity at that particular depth of soil where it rests.

Then, considering a certain factor of safety, the diameter of the pile can be determined.

 

b. Friction Piles

A friction pile is the type of load-bearing pile that transfers the load to the soil by the friction mechanism between the surface of the pile and the surrounding soil layer.

Friction force can be developed along the entire length of the pile or a certain length of the pile depending upon the strata of the soil.

Unlike, end bearing piles, in friction piles, the entire pile surface functions to transfer the loads from the superstructure to the soil.

The capacity of friction piles can be determined by multiplying the surface area of the pile by the friction force developed per unit area. A certain factor of safety must be considered while designing friction piles.

 

  c. Soil Compactor Piles  

Sometimes, it may be necessary to compact the soil to increase its bearing capacity of the soil.

Piles used for such purpose of increasing the bearing capacity of the soil are known as soil compaction piles.

The use of soil compactor piles is one of the potential methods for increasing the bearing capacity of the soil and improving the overall stability of the soil.

 

 


 

  B. Based on the Construction Method/ Construction Material Used  

Depending upon the method of construction of the pile foundation and the materials used pile foundations may be classified into the following:

 

  i. Timber Piles  

Timber piles generally refer to the type of pile foundation that comprises timber as the material of construction.

Timber piles are mostly placed under the water level and can be rectangular or circular.

They have a long life span of approximately 30 years.

During the design of timber piles, commonly the diameter is kept between 12 to 16 inches and the length is taken as 20 times the top width.

Such piles can withstand loads as high as 15 tons to 20 tons.

Usually, fish plates are attached or bolted on the side of the timber piles to gain additional strength.

 

Advantages of Timber Piles

Some of the advantages of timber piles can be listed as follows:

1. Timber piles are economical because of the easy availability of timber.

2. The installation process is relatively simple and easy.

3. The timber piles can be cut to the desired length even after installation and it has a lower possibility of damage.

4. In case, timber piles have to be removed, they can be easily pulled out.

 

Disadvantages  of Timber Piles

Some of the disadvantages of timber piles can be listed as follows:

1. Timber piles cannot be used as end-bearing piles.

2. The driving of timber piles is extremely difficult in the hard soil layer.

3. If proper care is not taken, defects may occur in timber. Treatment of timber such as the application of preservatives must be carried out.

 

timber pile

 

 

  ii. Concrete Piles  

Concrete piles are the type of pile foundation that essentially consists of concrete as the chief element.

Concrete piles may be further classified into Pre-cast concrete piles and cast-in-place concrete piles. They are briefly described as follows:

 

a.  Pre-Cast Concrete Piles

Pre-cast concrete piles are one of the most commonly used pile foundations. Such piles are directly cast in the pile bed and transferred to the location of the foundation.

Usually, rectangular piles are cast in horizontal form whereas circular piles are cast in vertical form.

Pre-cast concrete piles may also be reinforced. Once, the piles are cast they have to be cured for at least 28 days.

 

Advantages of Pre-Cast Piles

Some of the advantages of pre-cast concrete piles can be listed as follows:

1. Pre-cast piles have high strength and stability.

2. They can withstand a load of high intensity.

3. Pre-cast piles have high resistance to corrosion and biological defects and remain unaffected by groundwater.

4. Such piles can be used in almost all types of construction.

5. Pre-cast concrete piles have high durability and are cost-effective.

 

Disadvantages of Pre-Cast Concrete Piles 

Some of the disadvantages of pre-cast concrete piles can be listed as follows:

1. The change of length of such piles and mobilization is difficult.

2. Driving the piles requires heavy and advanced equipment.

3. There is a high risk of breakage of piles during the handling or driving operation.

 


 

b. Cast-in-Place Concrete Piles

Cast-in-place concrete piles are the type of concrete piles that are constructed by boring the soil up to a certain desired depth, filling it with freshly mixed concrete, and allowing it to dry on the site itself.

Generally, an outer metallic shell is first driven into the ground and filled with a fresh mix of concrete.

It is then allowed to be set and cured.

Finally, the outer metallic shell is removed or pulled out.

 

Advantages of Cast-In-Situ Concrete

Some of the advantages of cast-in-situ concrete piles can be listed as follows:

1. They are more flexible in terms of length variation.

2. Handling such piles is easier.

3. Additional piles may be provided easily.

4. The installation process is simple with no possibility of breakage.

 

Disadvantages of Cast-In-Situ Concrete

Some of the disadvantages of cast-in-situ concrete piles can be listed as follows:

1. Since the pile is cast in situ, a proper storage place must be arranged for storing the construction materials safely.

2. Quality control must be ensured during installation.

3. The difficulty may arise in casting the concrete where the underground flow of water is relatively heavy.

 


 

  iii. Steel Piles  

Steel piles are the type of pile foundation that is made up of steel sections. Commonly used steel piles include I-section piles and hollow steel piles.

This type of pile is mostly used as end-bearing piles. They are usually designed in smaller sections with a diameter ranging between 10 inches to 24 inches.

 

Steel Piles 

 

Advantages of Steel Piles

Some of the advantages of steel piles are listed as follows:

1. Due to the smaller sectional area, such piles are easy to handle.

2. Steel piles can be driven easily through hard strata of soil and can be taken up to deeper depths.

3. Steel piles can withstand heavy loads.

 

Disadvantages of Steel Piles

Some of the disadvantages of steel piles can be listed as follows:

1. Steel piles are comparatively expensive.

2. Such piles are prone to corrosion.

 


 

  iv. Composite Piles  

Composite piles generally consist of two or more different materials usually one mounted over the other such that they function as a single unit.

For example, composite piles may be constructed of steel and timber or steel and concrete, etc.

Such piles have a very high capacity.

 

Composite Piles  

 

b. Well Foundations

Well, a foundation is a deep foundation that is built below the water level for bridges. Cassions or well have been in utilizing for foundations of bridges and other structures since the Roman and Mughal periods. The term ‘Cassion’ is advanced from the French word Caisse whose real meaning is box or chest.

Types of Foundations: Selection, Advantages & Disadvantages

 

c. Pier Foundations

A pier foundation is a type of deep foundation that is a collection of huge diameter cylindrical columns to hold the superstructure and transmit huge super-imposed loads to the firm strata underneath. It stood many feet above the ground. It is also known as “post foundation”.

Pier Foundations

 

  2. Selection of Foundation Depends Upon  

The selection of foundation depends upon these factors that are listed below:

a. Loads from building

b. Type of soil

c. Type of structures in neighborhood

d. Type of foundations

The types of foundations such as isolated foundations, combined footings, pile foundations, raft or mat foundations, etc. depending on the types of soils and loads from the buildings can be chosen based on the situation and needs.

 


 

  3. Advantages of Foundations  

The advantages of a foundation can vary with the type of foundation, so the major advantages of a foundation are noted below:

a. Support Super Structure.

b. Provides Stability

c. Reduce the unequal settlement of the building

d. Transmits the load from the superstructure to the ground.

 

  3. Disadvantages of Foundations  

The disadvantages of a foundation can vary with the type of foundation, so the major disadvantages of a foundation are noted below: 

a. Increase the construction cost.

b. Maintenance is needed

c. Disturb the soil mass.

 

Read Also: Wall Cladding