Min. ftg Depth for Exterior Slab 1

[Ibeam]

My question is about frost depth requirements for an outdoor industrial slab on grade. Minimum ftg depth for frost penetration is 24 inches. Slab size is 70 x 80 ft.

I have numerous pieces of equipment – tanks, pumps, platforms, pipe supports, vessels, all light to moderate loadings in that the highest pressure under the slab from any one item may be 1000psf.

I would like to support all of this with a thickened slab and pedestals (heights of 12 to 18 inches for the pedestals). The slab perimeter will only be inches above grade, and the thickened edge will rest on a separately poured stem wall (10”x 2’ depth). The base requirements for such a situation per the soils report calls for 3-ft. of type-a aggregate material compacted to 95% std. Proctor. The existing soil is uncontrolled fill material and expansive. The fill depth reaches down to 18 ft.

Using this method, the Client does not think the minimum ftg depth requirement due to frost has been met and that the slab could heave at the inner areas. My thinking is that the base material will prevent the water from penetrating to form ice lenses in addition to the 3’ concrete depth at the perimeter keeping surface water from getting under the slab. Is this a sound theory?

 

[concretemasonry]

The moisture for the ice lenses usually comes from below as I recall. You seem to have a great deal of "unknown" in your 18' of fill material that may hold significant moisture.

 

[jike]

Water for the formation of ice lenses may be transmitted by the capillary action of the soil beneath the slab. By using a fill material that can act as a capillary break, you will prevent the ice lenses from forming. Soil that can act as a capillary break is defined by the amount of particles passing a no. 200 sieve (generally less than 5 or 6% although this number can often vary depending on who you talk to). This type of soil is often called "Non-frost susceptible fill"

Check the gradation of your fill to know for sure.

Also make sure that your excavation has not created a bath tub where water will collect.

Good luck!

 

[BigH]

Ibeam: There are a number of unknowns that may/would have an effect on the most suitable solution. What are the soils beneath the slab and to what depth? Where is the groundwater level beneath the slab?

To have frost problems you need a frost susceptible material which may expand when frozen (there are a number of texts out there that discuss this (classes F1 to F4 - developed by the US Navy I believe). Clean sands or sands and gravels are not considered frost susceptible - so if these materials are beneath the slab, then you shouldn't have to worry. Clays, believe it or not, are also not judged severly frost susceptible as the coefficient of permeability is so low that insufficient water can migrate to freeze into lens and that. If the groundwater table is deep, there would not be any chance of water to "rise". I once did a job in northern Ontario where the depth of frost penetration was about 9ft. The groundwater level was down 12 ft and the soils were clean sands - hence we founded footings at 3ft.

 

[civilperson]

I agree with BigH.
Frost is not always destructive to industrial structures. Many transformers slabs rise and fall every freeze and thaw without distress to the components, (connected by flexible conductors or resilient rigid aluminum bars). Capillary flow of moisture can be stopped by a gap graded gravel sub-grade, (no fines or fine sands). If the client wishes frost proof, then provide depth to below frost but show the alternatives and the associated cost savings.

 

[Ibeam]

Type-A base material here is a well graded 1.5-inch max aggregate with 4-12 percent passing the No. 200 seive. It compacts tight.

The ground water is approx. 10 to 13 feet. There were 4 borings to rock (avg. 50 ft.) within the footprint.

Unlike civilperson's example, this slab cannot have rise and fall to excess. There are several very large pieces that require pier groups to rock. These have an isolated cap that penetrates the slab. Differential movement more that 3/4-inch can have some adverse effects on pipe and nozzle loads for equipment.

The soils according to the geotech are not suitable for re-use due to the high PI in some of the layers (high was 39). The soils are also deemed contaminated so disposal of spoil is not a small factor.

Thank you for the comments so far.

 

[dik]

IBeam... 12% passing 200 is too much... shouldn't have any more than about 8% if frost is an issue.

Stiffened slab edges is fairly common construction in many areas and it greatly depends on the soils type and then on the amount of freezing... can be mitigated by using perimeter insulation on the flat as well as proper drainage of the sub slab granular material and the perimeter granular material.

Dik

 

[BigH]

The groundwater level is 10 to 13 ft deep - how is it going to rise up to and cause ice lensing. Ground 3 ft and below are not freezing! I don't know what Type A aggregate is - but it seems that is should be a crushed stone or so. You have already, in my view, handled the frost issue by removing the three feet of natural soil and replacing it with the Type A aggregate base. Make sure you have drainage about the sides of the situation so that water cannot infiltrate from ground surface. I don't see a problem or need for other special measures. However, you need to do (1) what you think is best and (2) what the client wants so long as he is aware of any/all cost ramifications.

There is an article by Konrad in Cdn Geo Jour (2005). He gives a table of %fines for base-course materials:
Asphalt Institute 7%
Newfoundland 6
Japan 6
Alaska 6
Colorado 5 - 10
Kansas 15
Maryland 12
Massachusettes 10
Minnesota 10
New Hamshire 8
Ohio 15
Vermont 10
Washington 10
Wisconsin 5
His study found problems only when > 15% (Quebec soils with fines being kaolinite).
See:

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd182_e.html

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd026_e.html

http://books.google.com/books?id=b6...y3X&sig=nz_QQxtwyZ3KdVMTPaJ2-f0ni34#PPA182,M1

 

[Ibeam]

All are interesting comments -

I realized that 12% fines may be an issue on one hand, but the other is the impermeability after compaction. I didn't want the bathtub effect. The geotech on the job says that he deems the type-A base material as "non-frost" susceptible fill.

A friend here at work had the following experience just lately with his home that is on a cmu stemwall with crawl-space. This is somewhat/remotely related to what BigH wrote. His backyard is about 10-inches higher than the dirt floor of the crawl-space. According to him, the base of the wall is about 12-inches below the crawlspace floor. We had some really heavy rain and his backyard was getting flooded. The water made it underneath the bottom of the wall and was filling up the crawlspace to about the elevation of the backyard. His garage floor (in the front of the house)is only a few inches above the crawlspace floor. He had a 3-ft wide opening in the wall that's exposed in the garage for access to the crawlspace. Water was pouring around the sides of the plywood he had over the opening. And yes, he decided to take it off to look inside. It flowed like a river through his garage and down the driveway.

The base of my stemwall will extend about 3' below grade, but I figure that theoretically water could be forced up thru the compacted fill if the hydro-head outside is well above the base. So it needs to be impermeable.

In any event, it turns out that the client's real problem is the design concept of using slab-on-grade for supporting the loads instead of the more traditional spreadfooting with pedestals; then a thinner slab as a pavement on top. After several meetings, they have now seen the light about differential settlement and the ability of the slab to better distribute the concentrated forces.

I've compromised on the constructibility end and agreed to have the pedestal dowels (vertical rebar)post-installed with adhesive versus the original sketches of cast-in-place hooked bars sticking out of the slab.