Slab on Grade Foundation versus Isolated Shallow Footings for a House

[AymanRaouf]

Hello all,

I wonder on the decision choice between a Slab on Grade foundation versus a shallow foundation footing for a house. Why is it in some countries they use a slab on grade whereas others they use isolated foundations. The latter have a ground type of simsima limestone (strong ground greater than 300 kPA.

 

[1503-44]

What I don't want to see is a pier block on expansive or poor soils. House loads are small and completely at the mercy of soil movements. Soil movements are almost always never uniform and you wind up with wavey floor and roof lines.

Slabs on grade can counteract those differential soil movements quite a lot. They may go up and down if you have expansive soils, but the whole slab and house and roof go up and the same amount. On those occasions where they result in a tilt, at least its a more uniform tilt; you don't get waves lines. And you can usually fix tilts by pumping mud under the slabs, on the low side.

Limestone is relatively strong to begin with, but their best characteristics are that they are not subject to differential settlement and any water tends to lock up fine grained sand and even recement them into the original matrix. Limestone are also porus, so they won't hold seasonal water too long; water drains through the pores. Limestone is subject to desolution, if there is a permanent high water level. That's not good because they sometimes result in cavern formation, but that's their only serious problem.

So, slabs are great to combat expansive soils that result in differential settlement. Where that is not of concern, you can use piers, even better when they are founded on rock.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[AymanRaouf]

Thank you for your time in responding to my query.

I still don't get why they put the isolated shallow foundations. Are they cheaper than doing a monolithic raft foundation?

What if there is groundwater, is that when you go for a raft foundation?

 

[1503-44]

Block piers are much cheaper, but you do need good soil.

High groundwater never helps. Foundation loads may squeeze some water out, then the soil compacts to replace the water and subsidence may occur. You need to know how your soil reacts to changes in water content.

A ground slab will tend to somewhat dry the soil below it, at least a little bit. It may affect the water table slightly, but maybe not.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[AymanRaouf]

Do ground slabs need excavation though?

I know that with the bearing capacity formula there's a depth component. Hence the reason for making the shallow isolated foundations reach a certain depth.

I noticed you use the term "block piers". Is this the same as a shallow isolated foundations?

 

[1503-44]

Usually "slabs on grade" only require a minimum amount of excavation, which is generally limited to removal of the topmost, unconsolidated soil layer containing organic debrie, live vegetation and roots. Often no more than 300 to 400mm is removed. Slab thickness will be the removal depth plus an allowance to extend above the original surface elevation to permit backfill around the slab to drain rainfall away from the structure. Total thickness of the slab may be as thin as 300mm. 450mm is often the maximum thickness, 150mm or so, above the original grade elevation. If good soil bearing is found lower than 450mm, the slab thickness is usually kept at the required thickness, say the 300mm, then the excavated depth will be built back up with compacted fill, usually sand, to the bottom of the slab's thickness, approximately the original surface elevation, then only the edge of the slab, 300 to 450mm or so, will be dropped down to soil bearing depth, making a "turned down slab on grade" type foundation. The turn down portion is provided to retain the compacted fill under the slab and keep it from being washed away from under the slab's edges.

Block pier, is a short column of CMU blocks resting on a minimum concrete pad embedded in the soil below grade where the soil is hard enough to provide the required bearing capacity. The CMU blocks could equally be made as a concrete pier, resulting in the typical pier, or pedestal, or plinth type foundation constructed on top of a embedded concrete footing a meter or two below ground surface.

It is also common to see both slab on grade and pedestal-footings being used together when there are usually light bearing wall and floor loads placed on the slab with heavier column loads placed on the pedestals.

You might want to read Chapter 4 on foundation Design here

Download the
Residential Structure Design Guide

https://www.huduser.gov/portal/publications/RSDG.html

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[AymanRaouf]

I see. What I am comparing with is an isolated shallow foundation. Heres an image attached. I take thst the beloew is not a pier foundation.

 

[1503-44]

Yes, the Column, when made of concrete and cast above the footing has several names. A column, a pier or pedestal (USA), or plinth (UK).

If the column there is made of CMU blocks, that's my "pier block on pad", where pad = footing, where the footing is not usually large, often quite insufficient in many DIY constructions.

As you can see in the picture, there are several design styles that can all form a combination of slab on grade and pier/footing designs.

A few others here

https://duckduckgo.com/?q=slab+on+grade+and+pier+footings&t=brave&iax=images&ia=images

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

In addition to soil moisture changes, I should mention frost heave movements, where freezing of the soil is of concern.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Jrit]

I have been told by contractors in my area (Southeast US) that slab-on-grade is cheaper than doing a crawlspace (exterior shallow spread footings and interior piers with wood framed floor). They will even build up exterior walls a few feet, backfill behind the walls with soil and pour a slab on the fill over crawlspaces sometimes. They say its cheaper.

 

[1503-44]

Concrete placed on dirt is about as cheap as it gets.
Just a matter of how much concrete.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."