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Mat Slab Foundations: Everything You Need to Know

  • October 25, 2023

Depending on your foundation’s purpose, you have a few options to choose from. One strategy for commercial structures is to go with a mat slab foundation. 

This article will explain everything you need to know about mat slab foundations, including their pros and cons, different types, their uses, and how they are constructed. 

Contents:

1. What Is a Mat Slab Foundation?

2. Mat Slab vs. Slab on Grade

3. Pros and Cons of Mat Slab Foundations

4. Types of Mat Slab Foundations

5. Uses of Mat Foundations in Commercial Construction

6. Mat Slab Foundation Design Methodologies

7. How a Mat Slab Foundation is Constructed

8. Conclusion


What Is a Mat Slab Foundation?

The term “mat slab foundation” in civil engineering is defined as one thick concrete slab placed directly on the soil.

This spreads the heavy loads created by high-rise buildings across the soil, reducing contact pressure on any one area of the ground underneath. 

A mat slab foundation is part of the shallow foundation and is also known as a “raft foundation.”


Mat Slab vs. Slab on Grade

Two types of foundations are common in commercial structures: mat slabs and slab on grade. What determines which is better for a particular project, and how do these foundations differ?

Slab on Grade

Slabs on grade are meant to be used for light loads. They would not work to hold a superstructure or if the column load is high. 

Slab on grade foundations have no reinforcements added to bear the building load. The support comes from individual footings placed around the slab. 

Mat Slab

Mat slabs, on the other hand, can handle heavier loads. The mat foundation design includes added reinforcements and distributes the load across the soil underneath.

Mat slabs are like slab on grade foundation upgrades. They’re great for building construction over uncompacted soil because of the larger spread footing they create. 


Pros and Cons of Mat Slab Foundations

Like any decision you make in the construction process, you will find both benefits and limitations in choosing a mat slab foundation to support your commercial structure — although the good far outweighs the bad.

Let’s take a look.

Pros

First, here are the benefits of choosing a mat slab for your next commercial project:

Reduced Construction Time

Since the foundation and floor slab are poured together in mat slab foundation creation, you can see a significant reduction in the construction time needed.

This can also save construction costs for the floor slab, so it’s a win-win. 

Less Excavation Necessary

A deep foundation requires extraneous excavation to remove the soil and make room for the foundation. 

The slab thickness of mat slabs is much thinner, requiring little excavation for installation. 

Loads Transferred Over a Wider Area

Mat slabs can handle heavier loads than other types of foundations because the slab extends across the entire building footprint, distributing the entire load across a wider soil area. 

Due to the nature of the slab, this foundation can be used to construct over weak or poor soil conditions, even controlling differential settlement or shifting during a flood.

Cons

Now, let’s discuss some of the drawbacks:

Requires a Lot of Material

Because mat slabs go across the entire building’s footprint, they require more cement concrete to complete. More steel is also involved in the reinforcements added to the slab than in some other foundation types. 

Skilled Laborers Needed

Unique measurements are often necessary to complete a mat slab foundation, especially when there are areas of concentrated loads. This requires a skilled and specially-trained team to construct the foundation properly.

Prone to Edge Erosion

Mat slab foundations are sturdy but have one weakness: edge erosion. This is most likely due to the weak soil conditions under the slab. Careful attention must be paid during the treatment process to ensure edges are well-treated. 


Types of Mat Slab Foundations

There are several versions of mat or raft foundations, each useful in unique circumstances. Below, you will find the various types of mat foundations and a brief description of what sets them apart from the rest.

Flat Plate Raft

A plate foundation is built with uniform thickness across the slab. This will distribute small, equal loads across the columns.

Plate mat slabs are the generic form of mat slabs used when there are no special concerns of concentrated weight or other environmental issues, and the structure is fairly lightweight. 

Piled Mat

This foundation is built with a slab supported by piles in the ground. Pile raft foundations reduce the potential for settlement while increasing load capacity. This type of mat foundation is generally used when the water table is high or near ground level. 

Cellular Mat Foundation

A cellular mat foundation, also known as a rigid frame mat or box type raft, is made with deep beams in the L-section, creating boxes above the ground-bearing slab. These are good for projects with very high bending stresses and loose soil or uneven settlement.

Two-Way Slab and Beam

A two-way slab has beams added across the slab laid in perpendicular directions. The columns are placed above where the beams intersect, providing extra support for each column. 

Thickened Plate Under Columns

As the name implies, these foundations have a portion of the slab thickened under the columns. Buildings with heavier and uneven loads may cause a diagonal shear and negative bending moment. The extra thickness of the RCC slab under the columns will help to counteract this extra load. 


Uses of Mat Foundations in Commercial Construction

Mat slab foundations were designed out of necessity. Certain circumstances require a mat foundation because all other types would fail. 

These conditions are as follows:

When the Soil Requires It

Uses of Mat Foundations in Commercial Construction

Depending on where you are building, the soil can have very different load-bearing capacities. 

For soil that is primarily sedimentary rock, the load-bearing capacity is high. Silt or clay, however, are weak and have a low bearing capacity.  For this reason, stress on the soil needs to be reduced, which can be done with a mat slab foundation. 

As we’ve already discussed, the slab distributes the load evenly throughout the soil. Mat slabs are also utilized for foundations where soil compaction is unpredictable.

Spacing Issues

Mat slabs are used whenever there are spacing issues for the foundation of a structure. These often include columns near the property line and walls or columns too close together for footings.

Since any other type of foundation would take up too much room, mat slab foundations are the usual solution to spacing problems. 

When Loads Require It

There may be issues that require load distribution to be expanded outside of soil conditions. This could be due to the construction of a superstructure. 

Sloped Sites

Sloped sites pose a unique problem for foundations, but mat slabs are the solution. Contractors can build a pad into the hillside with no footings or pier drilling required.


Mat Slab Foundation Design Methodologies

There are three main methods you can use to design your mat slab foundation:

Conventional Rigid

The conventional rigid technique uses a distinct formula to determine the necessary design of and reinforcement placements within the mat slab foundation. 

This method calculates the total applied load to the foundation and divides it by each column to ensure the pressure does not exceed the safe bearing pressure. The shear and bending moments must be calculated and considered as well. 

Finite Element Analysis

Finite element analysis uses sophisticated software to create a model of the soil and calculate the loads on each column, bending moments and shear forces, as well as the needed reinforcements.

Some of these software programs include:

  • ANSYS
  • Risa Foundation
  • ABAQUS V6.8

Approximate Flexible

For this design method, the mat is divided into strips, each with a row of columns. The pressure is calculated between the load of each column and the resistance of the soil below.

This strategy is out-of-date and not exact, so it is rarely used. 


How a Mat Slab Foundation is Constructed

Now that we’ve learned why mat slabs are so helpful and the different types you can choose from, let’s learn about the construction process of a mat slab foundation. 

1. Excavation

The first step is to prepare the site for a foundation. This will require some soil excavation to make room for the concrete being poured.

The construction crew will measure the depth of the slab necessary and extract enough soil from the surface of the site for it. They will then level and compact the soil to create a sturdy bed.

Read more: What Is Involved in Sitework Construction?

2. Waterproofing

The next step would be to protect the site from water erosion. This occurs in two parts.

The first is to install a drainage system on site which may include temporary stormwater management and permanent curbs and gutters. The second is to apply waterproofing products to the foundation itself. 

Depending on the environment and preference of the project owner, the chosen products can include simple plastic sheeting, deep penetrating spray, or a thick liquid membrane.

3. Pouring the Flat Layer

Once your pad is prepped and the waterproofing is complete, you are ready to pour your first flat layer of concrete. This preliminary layer needs to be about 7 centimeters or 3 inches deep of a concrete/sand mixture, providing a level base for the rest of the foundation. 

4. Adding Reinforcements

After the flat layer is poured, it is time to install reinforcements.

Steel mesh is added in two layers over equal-distanced spacers, one at the bottom and one at the top. This double layer helps to ensure that both upward and downward bending forces are balanced. 

If the slab is thicker than the minimum due to heavier load bearing, rebar can be added to provide more reinforcement. 

5. Retaining Wall 

Construction of Retaining Wall

For sloped sites, a retaining wall must be installed against the side cut into the soil. This will ensure the soil does not cave into the structure while constructing the foundation.

To make the retaining wall, formwork, along with vertical rebar, is installed and backfilled with stone. The formwork is then filled with concrete for a smooth wall. 

6. Pouring Concrete

The number of pours is determined by the size of the slab, which will be calculated by the dimensions of the structure’s footprint as well as the necessary thickness. 

While the concrete is being poured — and between pours — it is imperative that the temperature of the concrete itself is controlled. Otherwise, the foundation can crack. 

Adding fly ash to the concrete mix can make this easier, but it can also help to pour at night or use cold water for mixing. 

7. Curing

Lastly, you will want to vibrate the concrete to let out any air pockets that may have accumulated, and then start the curing regimen.

Curing the concrete will ensure that it reaches its maximum compression strength. The exact time needed for proper curing is dependent on the:

  • Weather
  • Concrete type
  • Size of the pour

Initial curing will require the concrete to maintain its moisture for at least seven days. There are a number of curing methods.  


Conclusion

Structures are only as strong as their foundation, so it is imperative that your foundation is made with the utmost care and dedication. It’s also crucial that you choose the correct type of foundation to support your commercial building come what may. 

Not sure if a mat slab is the best choice for your commercial project?

Contact FMP Construction today to determine the best type of commercial structure foundation and ensure that it holds up for many years to come.

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