Potential Range of Modulus of Subgrade Reaction 1

[phamENG]

I'm working on the design of an industrial floor slab subject to general loading of extremely heavy stacks of material (4000psf give or take in loaded areas). I'm going in with a reinforced concrete slab on grade, but obviously the critical piece of the puzzle is what's happening underneath. The current slab that's being replaced has deflected more than 6" over an 80' "span" resulting in ponding of rain water that makes its way in through doors and serviceability problems for equipment. So we'll be removing the better part of 18 to 24" of clayey muck from under the slab and replacing it with compacted steel furnace slag (have access to large quantities on the cheap, so that's the material of choice).

My question is this: what kind of values for modulus of subgrade reaction can I expect from properly compacted furnace slag? I'll have it tested and set a limit on final material storage numbers based on the final conditions, but I want to make sure it's feasible to hit my target.

Thanks for any input.

 

[fattdad]

the settlement performance of a large areal load cannot be evaluated using only modulus of subgrade reaction. If the areal load is 40 ft by 80 ft, the load distribution would extend to depths of likely 100 ft or more! How will the loads that attenuate with depth trigger settlement of these much lower-depth layers? What method will you use to gauge such performance? It will not be subgrade modulus! Just get better data. The soils may consolidate (those clays under the water table). The soils may be subject to elastic compression (i.e., those sands that are above the critical state).

This is not a structural engineering problem!

f-d

p.s., I worked on a newspaper job and we had to preload the paper storage area for these exact same reasons.

ípapß gordo ainÆt no madre flaca!

 

[phamENG]

Thanks, fattdad. I agree the biggest concern is at the geotechnical level. I'm not aware of the time dependent characteristics of that deep settlement (if there are any), but I will say that these loads aren't usually sustained for more than a day or two. Material is stored temporarily while it waits for a truck. The issue is localized to one particular area - identically constructed and loaded sections of slabs are performing well up and down the facility. That is what leads me to suspect it's more an issue with the material near the surface than down really deep. Perhaps my logic there is flawed.

I ask about subgrade reaction as that's a key component in all slab on grade design methodologies I'm familiar with. Even if I can get a guarantee that no deep settlement will occur, I'll still need k to work out the elastic behavior of the slab and verify the concrete strength and reinforcing is correct. That is the structural issue I'm attempting to solve at the moment. Once things get opened up and we can actually collect more data at depth, I'll have a geotechnical engineer to bounce things off of. For now, though, I need to make preliminary assumptions that have a prayer of holding up in the face of better information later on.

 

[oldestguy]

Subgrade modulus likely well above 400, but that is of less importance than the slab thickness and strngth. A pavement design, as for airports or highways, should be looked at specially with repetitions yearly and of course that load. I'd also look at the subgrade modulus of the soil left below the undercut as getting you into the design field range for the site..

 

[Mad Mike]

I would estimate the CBR of the slag material and then relate it to the guideline correlations provided in FDOT and other pavement design manuals- I would guess your steel slag has a soaked CBR of say 15% and an elastic modulus in line with what oldestguy notes - 400psi with some variation.

You could then have a geotech. give you an estimate of long-term settlements beneath that area of the slab, if it is the geotechnical conditions which have led to the problem.

It's uncanny how many sites I've seen where one corner of the platform has been filled out over soft ground, and it is always that one corner that gives problems...

All the best,
Mike

 

[phamENG]

Thanks, guys. I'm in that tricky spot where I can't get better geotech info until the existing slab is gone, but we can't pull the slab until we have an approved and funded plan, and for that I need to provide a prelim and cost estimate. Your info is quite helpful in getting me there. I'm hoping a few decades of loading has done its worst to the deeper subgrade, but we'll find out when we start digging.

Thanks again, everyone.

 

[fattdad]

I'd say the range for natural soils (of any value - i.e., CBR>5) is 125-400 pci. If you thicken the subbase, you can throttle up the soil value. So, if the soil has a value of 150 and you construct a 10-in thick subbase, the design value would increase to about 200 or a bit better.

I consider the subgrade modulus a structural value for point loads - something to help you design the shear and moments in the slab cross section. When applying areal loads, it doesn't address all that's going on below the ground.

f-d

ípapß gordo ainÆt no madre flaca!

 

[phamENG]

Thanks, fattdad. It's useful for determining shear and moments in the slab when considering area loads, too - but I agree that the area loads have a much more important impact in the deeper soils. To consider only the first without thinking about what's happening in the lower soil layers is a mistake - one that seems to have been made by the original designers here and is leaving me with my current situation.

 

[fattdad]

haha! I used to work in Panama (Colon Free Trade Zone). Those old warehouses are all constructed on soft ground. No clue how the slabs were designed, but they have all settled dramatically owing to the areal loading of the warehouse.

It's all solvable.

f-d

ípapß gordo ainÆt no madre flaca!

 

[jdonville]

phamENG,

Be careful with that slag. Some slags are expansive.

I worked on a site where the original contractor "saved" money by using a cheap slag as backfill around the process pits. Later on, the floor slabs and pit walls were all heaving making the cart runs unusable. The original contractor ended up building a new facility for the owner for free and I heard paying for lost profits in the interim. Original savings were on the order of $20,000. Final liability was reportedly 10s of millions.

 

[oldestguy]

per Jdonville, I'd run a sample to a geotech lab for tests on potential swelling, possibly related to take=on of water, etc. The least one should do is check where the stuff has been in use for at least a few years.

 

[fattdad]

regarding slag: jdonville brings up the correct point. Especially true if somebody tries to stabilize it with cement, lime or other such basic stuff. Remineralization can/likely-will occur. Hotel Roanoke (Virginia) had such troubles.

f-d

ípapß gordo ainÆt no madre flaca!

 

[SlideRuleEra]

Excerpt from "Properties and Uses of Iron and Steel Slags" published by the National Slag Association.

http://www.SlideRuleEra.net

 

[phamENG]

Thanks, guys. Very timely information. I'll look into other end uses and see how the performance rates. I know it has been used under slabs in the facility before - I'll see if I can get my hands on any testing that may have been done then.

 

[SlideRuleEra]

I know it has been used under slabs in the facility before...

Could be good luck, with slag expansion "down" into known poor underlying soil.

http://www.SlideRuleEra.net

 

[phamENG]

That was my thought exactly. I just checked, and indeed there have been some swelling issues for lightly loaded areas where it was used. When used in heavily loaded areas, there's no measurable swelling effect. So it seems that if the load is high enough to cause significant settlement, it's heavy enough to force the swelling into the native soils. If it isn't, then it's not heavy enough to do so and the swelling is pushing it up.

Wrapped up my prelim this morning and should get the go ahead to bring the local geotech in next week. Thanks again, everyone.