Raft Or Mat Foundation: A Comprehensive Guide (For Constructors)

This is a comprehensive guide on mat foundation (Raft foundation).

In this guide, you’re going to learn:

• What is mat foundation?

• Their functions

• Uses of them

• Advantages and disadvantages

• Different types, and

• The construction process of mat foundations.

Let’s dive right in.

What Is The Mat Foundation?

It’s a type of shallow foundation.

Actually, it’s a thick reinforced concrete slab that is spread over the entire area of the structure it is supporting.

Raft foundation or mat foundation, both phrases indicate the same type of footing.

So, don’t be confused with the name.

Functions Of Mat Foundation

• Unlike any other footings, raft foundation transfers imposed loads to the subsoil and spread it over the entire area of the building footprint.

• It reduces the differential settlement of structures built over weak soil.

Uses Of Mat Foundation

Here are the common uses of mat foundation:

• This foundation is used for low-bearing capacity soil. So that loads of a structure can be spread over a large area to build a stable structure.

• Uses of these foundations are popular in areas where basements are common.

• Mat foundation is also used to reduce the differential settlement of a structure.

• This foundation is used where the soil layer is unstable.

Advantage And Disadvantages Of Mat Foundation

Advantage:

• If the soil condition is poor to provide the shallow foundation, we use a deep foundation there. But deep foundations are costly. In that case, mat foundation is extremely useful. So we can say that mat foundation is cost-effective.

• This foundation requires less earth excavation.

• If a building has a basement, the raft foundation itself can be used as a floor slab. And thus it reduces the construction cost of a floor slab.

• Mat foundation distributes loads over a large area. Thus it resists differential settlement of the structure.

Disadvantage:

• It’s difficult to repair this foundation.

• Mat foundation requires specific treatment for point loads.

• Sometimes, mat foundations need heavy reinforcement in certain areas, which can increase the foundation cost.

• It is prone to edge erosion.

Types Of Mat Foundation

Here are various types of mat footing:

• Flat plate type mat

• Thickened Flat plate mat

• Two-way Beam and Slab type mat

• Piled mat

• Rigid frame mat

Let’s discuss these types of mat footing.

Flat Plate Type Mat Foundation:

This is the simplest form of the mat foundation.

Reinforcement is placed in both directions in this type of mat. Normally, two mesh are reinforced at the top and bottom of the slab to balance upward and downward bending forces.

And, extra reinforcement is provided at the column locations and load-bearing walls.

The thickness of this mat is uniform and usually kept under 12 inches.

This type of mat is used when the columns and walls are uniformly spaced at small intervals and the subjected loads are relatively small.

Plate Thickened Under Columns:

This type of mat is designed when the flat plate type isn’t suitable due to heavy column loads.

To handle heavy column loads, the thickness of the mat slab must be increased.

But, instead of increasing the thickness of the whole slab, a portion of the slab under the columns is thickened.

It’s done to resist negative bending moments and diagonal shear into the slab which is created by the heavy loads from columns.

Sometimes, pedestals are provided under the columns instead of increasing the slab thickness at the top of the mat.

Two-Way Beam And Slab Mat:

In this type of raft foundation, Beams are laid in perpendicular directions.

And, columns are placed exactly on intersections of beams.

This type of raft foundation is designed when:

• The columns are carrying unequal loads

• There are large spaces between columns.

Piled Mat Foundation:

In this type of mat foundation, Piles are constructed first below the mat slab.

Unofficially, you can say that this type of mat is a combination of pile foundation and flat plate type mat.

This combination of foundation:

• Reduces the amount of settlement.

• Controls the effect of buoyancy.

• Increases the ultimate load capacity of the foundation.

• And, it is useful in high compressibility soil and in the high water table zone.

Rigid Frame Mat Foundation:

In this type of mat, some RCC boxes are formed.

And, the walls of each box act as beams.

These walls are connected by two slabs — one at the bottom and one at the top like the image below:

This type of raft foundation is also known as Box type raft foundation or Cellular raft foundation.

The Construction Process Of Raft Foundation

You’re going to learn a step-by-step process of constructing raft foundations below.

Step-1: Excavate Earth Up To The Desired Level

For that, get the structural design of the mat footing from your project’s drawing book.

Study the drawing carefully and calculate the excavation depth from the existing ground.

But before going for actual excavation you need to think about the excavation support.

Because most of the time, mat foundation is designed for structures with basements.

In that case, the excavation is going to be deep that you need supports for retaining side soil.

Most of the time, we use sheet piles for this purpose.

Step-2: Pour Plain Cement Concrete

Plain Cement Concrete (PCC) is poured to make a flat and uniform platform for the foundation.

To pour PCC:

First, prepare the subsoil.

The subsoil of the excavated base may be loose.

You may need to compact that as per guidelines specified by the structural designer.

Also, make the subsoil uniform and flat level.

After that, lay a waterproofing membrane.

Obviously, a waterproofing membrane is provided to make a water barrier between the PCC and the subsoil.

We normally use thin polythene sheets for this purpose.

Laying plastic sheets over the subsoil isn’t a big deal.

Just lay that as you lay a bedsheet over your bed.

And, make sure it covers the entire area below the PCC.

Finally, pour PCC.

PCC (Plain Cement Concrete) is just a mixture of cement, sand, and coarse aggregate of a specified proportion.

Normally, a 1:3:6 ratio is used for making PCC.

But you should check the structural notes of the project’s structural drawing book for the ratio and thickness of PCC for the mat.

The thickness of this is normally kept 3”.

Once you finalize these, just pour concrete over the polythene sheet.

And make sure the top surface of the PCC creates a perfectly flat and level base for the foundation.

Step-3: Do The Reinforcement Work

Once the PCC is cured and gain strength, you can carry on for reinforcement work.

I won’t discuss much about installing reinforcement here.

Just get the reinforcement details of the mat foundation from your structural drawing book.

Follow the drawing and install reinforcing bars.

[Note: Before installing reinforcements, a layer of polythene sheets may need to lay over the PCC in some specific zones. Check structural notes in your structural drawing book to find out this.]

Step-4: Pour Concrete Into The Mat

The foundation is almost ready to pour concrete.

To pour concrete:

First, make concrete forms.

The concrete formwork for mat foundations is simple.

You just need to make shuttering work all around the footing.

Next, get the thickness of the footing.

You’ll get this in the structural drawing sheet of the mat.

Mark this thickness on the concrete forms all around. So that you can maintain the footing’s thickness during concrete finishing work.

Finally, pour concrete into the footing.

Reinforcement work — check.

Formwork — check.

Now, it’s time to pour concrete into the form.

For that, you can order ready-mix concrete from a supplier.

Or, you can prepare concrete on site.

Before that, make sure, you’re pouring concrete with the desired strength or the mix proportion specified in the structural drawing.

And pour concrete into the mat foundation.

That’s it.

Conclusion

I hope this guide showed everything about mat foundation.

Now I’d like to turn it over to you:

Have I missed anything in this post?

Or maybe you have a question about something you read.

Either way, let me know by leaving a comment below right now.