slab on grade or Mat? 7

[pelelo]

Engineers,

I have a confusion in terminologies, please help.

I have a structure which will be supported on a mat foundation. The mat is 3 ft thick and will be built 3 feet below ground surface (not at grade).

My question is: is this considered truly a mat foundation or a slab on grade?. I think there are confusions about both terminologies.

In case it helps, the loads on the mat foundation will be 300 tons and it has a dimension of 25 x 15 ft.

Please advise. Thanks.

 

[Ron]

That's a mat foundation. A slab on grade is exactly that and is not intended to support the loads you have given.

 

[pelelo]

Thanks Ron.

So what is the maximum load required in order to consider this a "slab on grade" and of course, to be on grade?

 

[r13]

The mat foundation carries structural loads from structure above, it is usually placed at/below frost depth. Slab on grade, in general, is a floor covering mainly for service purpose, though it sometimes integrated with other structural components that carry structural loads.

 

[GeoEnvGuy]

I think the sketch that retired13 provides a lot of the story the only other similar variation is a thickened edge slab on grade, where the edge is thickened to carry the load of a strip foundation and is typically less than 2 m wide. This is commonly used in soft soils with a high water table.

 

[JoshPlumSE]

Retired13's post / sketch is great.

I would emphasize that a "slab on grade" is generally non-structural. You might have some lightweight furniture or equipment resting on it. But, you don't normally want to "Anchor" any equipment to it.

 

[Obinna1992]

Raft foundations are designed to carry relatively heavy load transmitted by columns or walls of structures on weak soils. Slab on grade can be described as mass or reinforced concrete slab on a prepared subgrade which carries relatively lighter loads which interact directly with it (for example vehicle wheel load or machines) without the use of columns. Slab on grade is expected to interact with the soil directly without the influence of beams or other structural elements.

 

[JoshPlumSE]

Obinna -

That's a good point. I think the term Raft Foundation is somewhat regional. I've heard engineers from other parts of the world (India?) use that term rather liberally.

Here in California, we would normally refer to it as a "Mat Foundation". But, it's essentially the same thing to me.... at least when compared to the term "slab on grade".