Foundation Construction: Purpose, Functions, Depth and Process of Foundation

What is foundation in construction?

Foundation is the essential part of the structure which helps to transfer the load of the structure from footing to the soil and it provides the stability of the structure. It is a part of below the ground level where we can provide levelled surface to transmit load in large area of the soil. 

Purpose of Foundation:

The primary purpose of foundation to make the structure stable and It is essential to bear the load of small and high-rise buildings for following purpose:

1. Foundations provides the stability of the structure and their wider size provide the more stability to the foundation. If foundation will be stronger than stability will increases.

2. Foundation structure provides proper load distribution of the structure into large area of the soil and the transmission of the load should not exceed the allowable bearing capacity of the soil.  

3. It helps to protect the structure from lateral movement and provide the levelled area to the substructure for building a superstructure.

4. Foundation provides the uniformly load distribution of the structure which helps to secure the structure from unequal settlement and also decrease the differential settlement.

5. It provides the protection to the structure from scouring due to borrowing animals and water flood etc.

Functions of Foundation in construction:

The major function of foundation is transferring the all loads from the structure to the soil. the following are important of the foundation:

1. It provides the lateral stability to the structure from overturning and sliding.

2. Foundation helps to provide the levelled surface to build the substructure.

3. It is evenly distributed non-uniformly load into the sub-soil.

4. It is distributed the loads into the large of the soil but the load intensity or pressure should not be exceeding the allowable pressure. It is the main function of foundation.

5. It helps to prevent to decrease the cracks during soil movement.

Process of Foundation Construction . Image Source - Pixbay
Process of Foundation Construction

Types of foundation:

There are different types of foundations used for construction of foundation structure for building depends on the load of the structure and bearing capacity of soil. Foundation has been classified in two categories:

1. Shallow Foundation

2. Deep Foundation 

Shallow foundation used where the bearing capacity of soil is good and load of the structure is less. If load of the structure is heavy or bearing capacity of soil is low then deep foundation will be preferred. In foundation construction, if soil condition is not good for construction of shallow foundation, soil reinforcement is required to provide the strength of the soil such as soil replacement or to provide stones below P.C.C. etc. are used for increasing the load capacity of soil. If soil condition is weak for heavy structures then foundation will be deep until hard strata is not come. Foundation generally made of concrete, stones and bricks.  Footing provides wider because it transfers the load uniformly into the soil. For sky-scrapper or high-rise buildings foundation will be heavy, strong and deep to provide the stability of the structure. In foundation construction of high-rise buildings safety should be taken during the time of trench drilling and to take caution to prevent the settlement of nearby structures due to vibration.  

To know more about Types of foundation in construction and its uses, check our article on Types of Foundation (PDF): Definition, Diagram, Uses or Applications where pdf file also available for download. 

Procedure of Foundation Construction:

Foundation is the lower and essential part of the structure. It is constructed below the ground level and some part above the ground surface. It is the part of sub-structure and super-structure constructed above the sub-structure. It helps to transmit the load of the structure to the sub-soil. there are following steps for foundation construction:

1. Before foundation construction, we need to calculate the depth, and width of the foundation, then we will make the layout of the foundation.

2. After that marking of the foundation for excavation on proposed site will start but before marking of foundation layout you need some equipment such as Auto level, Theodolite, Thin Cotton Thread, Bricks, Cement, Screen Sand, Lime Powder Long Nails, Hammer, Right Angle, Steel Tape. 

3. In marking process, we will generally be using the 3-4-5 method and using auto level for marking of foundation in right angle and check their diagonal as well.

 4. Pegs will fix at centreline and with the help of pegs we will construct the pillars at 1.5 to 2 m distance from the pegs and mark the centreline to plaster top surface of pillars with the help of thread then, marking of foundation layout will start.

5. After that we will excavate the trench according to the calculation of depth of foundation.

6. We will level the surface for footing casting and mark the centreline of foundation then footing will provide as per structure load and bearing capacity of the soil.

7. Footing is made with brickwork, stones, steel and concrete. if soil condition is not good so, before casting of the footing we will provide the layer of gravel or stone for soil strength then above this layer P.C.C. will provide and after final setting of P.C.C., footing mesh will lay over the cover blocks and

8.  Column cage will attach with footing mesh then casting of the footing will start with the help of formwork but remember formwork will in right angle and cover will provide as per drawing. 

9. Formwork will remove within 24 hrs for O.P.C. cement and it varies with the weather condition but the removal days of formwork for other type of cement is different. 

10. After removal of formwork 10 days curing of footing is required to achieve the proper strength. It will better to provide 28 days curing to the footing to achieve 100 percent strength 

11. After that foundation trench will fill with the soil and compacted well after column construction till ground surface. 

12. After all these process above structure will be construct.  This process will help for foundation building in proper way.

Foundation Laying Process. Source: Pixbay
Foundation Laying Process

Depth of Foundation:

To construct the foundation structure, need to know about the bearing capacity of soil, soil density, angle of repose, water table and load of the structure. These parameters show the geotechnical report of the foundation. Forfoundation construction, it is very important to know about soil condition for construction foundation. To calculate the minimum depth of foundation Rankine’s Formula used. The depth of foundation calculates with the help of geotechnical report or soil report which provides the soil bearing capacity below the ground level at different depth and different location. It also provides the soil dry and wet density and ground water table. This the Rankine’s formula to calculate the minimum depth of foundation.

Depth of Foundation formula

Where,

Df = Depth of foundation

p = Bearing capacity of soil

ꝩ = Density of soil

Ø = Internal friction or angle of repose of the soil

From the help of above formula, you can calculate the foundation depth in different level of soil depends on soil property but this formula can does not consider the structure for calculating the depth of foundation of building. It provides the minimum depth of foundation but it varies with the type of structure and its load. 

 Example: If bearing capacity of soil (p) = 300 KN/ m2

  Density of soil (ꝩ) = 20 KN/m3

  Angle of repose Ø = 30° 

Then,

Depth Calculation . Source: Eye on structures.
Depth Calculation
Image: Depth Calculation of a building
Image: Depth Calculation of a building

Width and length of the Foundation:

It depends on the load of the structure. Designer will design the structure as soil investigation report and type of structure. Generally, we will take the minimum width and length of foundation 1m for small structures like 1-2 storey buildings, if design is not available at site. It should not be less than 1 m.

Precaution during foundation design:

There are some precautions should be taken during the time of design and construction of the foundation are:

1. Foundation should be design to bear the dead load, live load and to transmit the load from structure to the sub-soil which provide the stability of the structure.

2. It should be designed as the settlement of the building is in under limitation. Differential settlement can be ignored if foundation base is rigid. Generally, when super imposed load is not distributed evenly.

3. Foundation for construction of building should be built in proper depth and strong to protect the structure from cracks, shrinkage or swelling of sub-soil.

4. Foundation structure should be built at place where it is not affected or could carry any unexpected future influence.

Precaution during Foundation Excavation:

1. First of all you will check the length, width and depth of the foundation is according to the centreline diagram or not.

2. If design sheet is not available then remember the depth of the foundation should not be less than 1 m.

 3. Trench soil surface compacted properly to provide hard surface to the footing. It helps to prevent the foundation structure from settlement. 

4. Foundation trench should be clean during the pouring time and if roots of trees are in trench then cut it properly and remove from there because it affects the base of the foundation.

Hope this information on foundation in construction clears the concepts related to foundation. If you have any doubts in solving problems of structure or doubt in fundamental topics related to structural analysis, then feel free to write to me! 

And if you like my articles and efforts, please tell us through commenting below. We will be pleased to get your valuable feedbacks. Eye on structures will try to bring you concepts of structural engineering, structural design through software and real-life structure issues and possible solutions for them. So get ready to build up your strong structure knowledge with us. You can also join us through our social media handles on Instagram and Facebook for getting daily doses of information related to impressive structures and their concepts. Happy reading!

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