A Discussion on Mat Foundation and Its Types
The Mat foundations can be defined as a thick concrete slab which is placed on the ground and works as the structure’s foundation. Mat foundations are also known as raft foundations. In building construction, bridge construction, tower construction and others mat foundations are constructed.
Raft foundations is probably the last option for shallow foundation, if there is a need for shallow foundations.
There will be a need to increase the area of footings (be it isolated, combined, strip footing) if there is an increase of the axial loads on the structure or even in the case of poor ground conditions.
The increase of the dimensions of the footings further and further, cause overlapping the stress bulbs each other create the weak zone. In this backdrop, we select the raft foundations.
How can Mat foundations be defined?
It is not fair to simply put that mat foundation is a flat plate place on the ground and it supports the superstructure. There are numerous designs which can be based on the way the load is applied.
There is a need to construct a flat plate when small loads are applied on the mat foundations. In case of increase in loads there are a number of methods mentioned in this article which can help to enhance the stiffness of the slab. Raft foundations can be used even to support the buildings up to the height of about 10 floors.
Also, the rise in axial loads can lead to increase in the cost of construction work. There is also a chance of exceeding the construction of a pile foundations that is beyond a certain level.
The different types of Mat Foundation
Mat foundations are categorized according to the modifications made to the flat plate:
The rafts are designed to improve the flexural stiffness of the foundation.
It is mostly at the column positions that the depth of the raft foundation is made to increase to be able to carry the high bending moments and also for the shear forces.
Given below are the detailed discussions of the different types of foundations:
- Flat raft is a thick concrete slab which is used to cast as the foundation.
- The way to stiffen the mat foundations is through the concrete shear walls.
Under the column the flat plate foundation is thickened
- There can be an increase in the bending and shear reinforcements with the increase of column axial loads.
- There is also an increase in the cost of construction. After a certain level the thickness of the mat foundation has to be increased.
- Also, we must remember, that an overall increase in thickness of the mat foundation will not be economical at all.
- Therefore, it is best to increase the thickness of the mat foundation right below the columns. Construction can be difficult as the projection is right below the flat plate.
- It will not be easy to place the reinforcement, also waterproofing etc.
The projection above and the projection below the flat plate is the same
- It is much easier to construct the projection from the raft above its surface. Though this can only be done if there is no usage of the raft slab or the space remaining is enough for the purpose it is to be used.
Beam and Slab Raft foundation
- The flat plate cannot bear if there is a further increase of the column axial load. For such situations beams are used to stiffen the foundation.
- There is a reduction in the thickness of raft slab with the introduction of beams.
Cellular Raft foundations
- A further development in the beam raft is the cellular raft foundation. A top slab is placed in this type of foundation.
- It will help in further increasing the stiffness of the mat foundation.
Pile Raft foundations
- In high rise buildings and also in situations where the pile is not possible to be socketed in the rock, also simply end bearing of the pile is not enough the Pile raft foundations are constructed.
- It is a complex process to design and construct the pile raft foundation.
In the beginning the pile takes on the load and then with time the load is shared with Initially, the pile takes the load, and then it starts sharing the load with the raft foundation.
Upon the complete mobilization of the piles the raft starts taking the load completely. After which the raft takes on the complete load.
The different designs of Mat Foundation
There are two methods to design the raft foundations.
- Conventional methods – Here there is the usage of manual calculations and charts
- Finite element analysis methods – Here a computer package is used to solve the design
Here is how you can design Mat Foundations with Conventional Rigid Method
You can follow the steps below when designing the mat foundation from the conventional rigid method.
Firstly, you have to calculate the total applied load into the mat foundation
Then you have to calculate the pressure under each column keeping in mind the eccentricity of the loading. To determine the pressure under each column there is a need to consider the axial stress and bending stress due to the eccentricity of the load center.
Then you have to check if the total allowable press is more than the applied pressure.
After this, the mat has to be divided into the strips according to its arrangement.
Then the bending moment and shear forces has to be determined.
The effective depth of the footing also has to be determined. This can be done based on the diagonal tension shear near various columns.
Then the bending moment diagrams have to be formed according to the calculations, then the positive and negative bending moments per unit width has to be determined.
Then you have tocalculate the reinforcement area per unit width of the section.
Along with this method, there are other processes for example approximate flexible method which can be used to analyze and design the raft foundations.
Finite Element Analysis Methods
When the soil flexible behavior is considered in the structural analysis it is called the finite element analysis method. The soil is the model and its behaviors are utilized into the analysis and design in this method.
Given below are the various ways you can model the soil.
The soil can be modelled under the foundation with its material properties. For this purpose, software such as plaxis can be used. Selecting the right kind of material model for the soil is extremely important in this kind of analysis. If the correct idealization is not considered then there will be wrong answers.
Software such as SAFE foundation analysis and design can be used for the bending moments and shear forces.
The model of are springs can be soil. The area springs can be calculated according to the way mentioned in the Bowels Foundations analysis and design book.
The subgrade reaction of the soil is what are spring is. There are a number of methods to calculate the subgrade reaction. The simplest method is discussed in the article:
Area Spring = SF x 40 x BC – for 25mm settlement of raft foundation
Here, SF – safety factor is taken into considerationso that the allowable bearing capacity and BC is the allowable bearing capacity can be calculated.
The above-mentioned equation is for a settlement of 25mm in the raft foundation. Any variation beyond this value can present wrong answers.
So, based on the specified settlement in the geotechnical investigation report to determine the allowable bearing capacity or based on the calculated settlement, the equation mentioned previously is modified:
Area Spring = SF x (1000/settleemnt ) x BC
After the calculation of the soil area springs or soil subgrade reaction, it can be used for the computer model created with suitable software.