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Slab-on-grade Settlement

Slab-on-grade Settlement

I was curious of how you all calculate settlement below a slab-on-grade including depth of significant influence (vertical stress increase) and soil improvement depth (if necessary).

Would the Hough Method be an acceptable approach? I have read that this method can over-predict the settlement by a factor of 2 or more. Or, maybe using one of the charts on the Theory of Elasticity or Westergaard's Theory (depending on the subsurface conditions) to calculate the increase in vertical stress beneath a corner of uniformly loaded rectangular area, and multiplying by 4 for the center of the slab. Is the Schmertmann Method an acceptable method? I was told by one of the old PE's that used to work here that it should not be used for mat foundations. I know the analyses of immediate settlement and consolidation settlement; my questions pertains more to cohesionless soils.

I guess my main question is, do you treat the slab as a mat (one large load)? Does a slab-on-grade behave like a mat? I would think the mat would be much stiffer. I feel like I'm losing my mind on this probably not so difficult analysis. I can't find any references that specifically address slab-on-grade settlement.

RE: Slab-on-grade Settlement

I'd use the Hough method mainly to ease calculations. However, it is pretty easy to set up a spread sheet for these types of area loadings, such as for an earth fill. My method of spreading is very simple and for these rough calculations, I take the width of load plus the depth as the width dimension down there for the calculation for a unit of depth (such as a foot). I worked for Hough when he came forth with this approach and wrote a paper (ASCE SM division August 1959, #2153. "Compressibility as the Basis for Soil Bearing Value". Later for his text, he Decreased his recommended Bearing Capacity Index as given in the ASCE paper . However, one discussor of his method from California checked the method for settlements of over 40 earth fills and footings using those ASCE paper numbers. The agreement was very good except for a high water table. There it underestimated the settlements (like 30-=40% of the measured value in sands ).

My spread sheet calcs compute for one foot layers below the loads and so one can go down as far as he wants, but usually only am concerned only to the point where added pressure is more than 10% of existing. A spreed sheet method is easily modified to fit ones job details. You can play with pre-coompression also very easy.

RE: Slab-on-grade Settlement

Poulos and Mayne wrote a paper for ASCE about elastic settlements. I found that paper interesting. Their method consider the stiffness of the structural element and other influence factors.

Here is the paper:

RE: Slab-on-grade Settlement

A question. What is this slab used for? What loads and what reinforcing? Seldom have I ever had to worry about a slab settling due to it's loads. It usually is the fill it sits on may just take it along as that fill settles due to its own weight . However, on very weak support they have been reinforced so as to not bend excessively.

RE: Slab-on-grade Settlement

No particular slab. I just routinely see a couple of my coworkers recommend undercutting slabs-on-grade down to 4 to 5 feet below finished grade when the soils are pumping during a proofroll without knowing (or requesting) loading information and running a settlement analysis. I know the soils are saturated when they are pumping and may consolidate or shear if they are loaded. Sometimes it just seems a little excessive to me when dealing with small structures such as a single story home.

RE: Slab-on-grade Settlement

JMCC: Instead of getting into settling, it would seem those guys need a little educating. Just because a soil pumps and seems "weak" to remove it isn't always needed. Weak in shear strength isn't necessarily meaning compressible. I once had a call from a well experienced contractor experiencing that "unstable" condition for a basement area. Somehow he had never seen it before. I told him to get his front end loader out of there and wait two days. He was surp0rised that I knew he had a front end loader there (a very common machine that caused disturbance). He poured his slab after those 2 days and all was well. No subsequent problem either.

Your statement "may consolidate" is a guess. By drying, yes, but not usually due to its own weight. Slabs re amazing at what crap they can traverse and not have any lasting effect.

RE: Slab-on-grade Settlement

OG, I also consider that SOG will work okay for most of the cases. But I have to deal some times with sites on very thick layers of uncontrolled fill (concrete/construction debris). What can be your approach for the SOG in that case? I had one site very close to an slope (the site was on top of the slope) on 2~4 meters of uncontrolled fill. We did some soil improvements for the fondations and structural slabs.

RE: Slab-on-grade Settlement

OKIRYU: For uncontrolled fill and structures later plced there later, a common way I have done it is run a rolling surcharge across the site. That is done with a windrow maybe 10 feet high and 10 feet wide on top. At various places along there a settlement platform is first set and elevations are recorded every so often, as well as before the surcharge comes along. I only consider the width of the top of the pile as acting, when figuring stress spreading in the fill. In these dumped fill sites (one was about 50 feet deep) I wait until the platform elevations show no movement for about a week. Then move the pile to the next location. Design pressures for footings can be increased some by undercutting (usually 4 feet)and placing compacted fill there (along column lines). At the bottom of that undercut, the increased stress from the footing is taken as no more than the earlier surcharge pressure. These sites usually the get one story buildings, such as stores or small warehouses. The fills usually contain concrete slabs and a occasional organic, but no garbage dumps (that I know of.... sometimes snuck in I suppose). No reports of any problems, even if used where some peat was there first.

One caution if rolling surcharge is used. That material usually had to be hauled in. The contractor does not want to haul it off site ater. They may want to use if for raising the grade at the site some place. Never allow it near any proposed or future building area if there is any question of that area settling!!!!

RE: Slab-on-grade Settlement

OG, thanks much for the response. My only concern is that if we use surcharge, it will work fine for a while but if there are large water content changes in these fills (cohesive soils) settlement may occur. However, appears that you have not experienced such problems. Also, I noted that if the increased stresses (allowable net bearing pressure) from the footings are taken as no more than the earlier surcharge pressure, the allowable increased stresses may be low (about 400-450 psf, considering the bottom of the footing at GL-4ft) which as you mentioned it may be good enough and work okay for small 1-story buildings and small warehouses. So, if you allowed for placing footings in uncontrolled fills (as long as some surcharge is conducted) I assume that you consider that placement of SOG is not an issue. Anyway, I always try to do some settlement checks for allowing SOG in uncontrolled fills assuming low soil modulus. Depending on the fill conditions, we also recommend additional support (small grade beams).

RE: Slab-on-grade Settlement

After surcharge loads the site much more than any slab, I don't get concerned and never have heard of any such possible slab problem later. The fills can be any types of soil and they all seem to work fine. Even situations like slabs with possible voids work OK. If any doubt or course the foundations can be made to allow for differential support.

RE: Slab-on-grade Settlement

Thanks OG again for your input...

RE: Slab-on-grade Settlement

An interesting description of rolling surcharge . . . basically oldestguy is looking at the big site and surcharging it it small area bits. I've used a rolling surcharge too (along the lines of that presented in Tschebotarioff 1951 book "Soil Mechanics, Foundations and Earth Structures" page 598). In this, you are displacing the soft soil - in my case it was tailings slimes by ramping up a mound of sand to a point that under the bulldozer load (and ramp load), it would cause a planar slope failure thereby pushing out the very soft soils - and you will see a mud wave develop. It was effective, with only a 5 to 6 ft high ramp and the D8 dozer, to displace up to 39 ft (about 13 m) of the tailings slimes (tailings dam developed in upstream method). At times, too, you can use blasting. Matich wrote a paper years ago about blasting a silt for a causeway - thereby remoulding the soil so that a rock fill "rolling surcharge" would displace the silt. But, I digress a bit on the OP's original question as the fill is likely too "stiff" to be able to displace as I have discussed.

RE: Slab-on-grade Settlement


Your displacement method has been used along with excavation in front in Michigan DOT for many years crossing peat marshes. About 1960 I managed to sneak that into a project for a Wisconsin road job. It was not in the plan. It worked so well it also was adopted here. That treatment would leave a road fill that later did not settle (noticeably)ever.

RE: Slab-on-grade Settlement

OldestGuy - had forgotten to say that Tschebotarioff did note that it was Michigan that he was describing. It is an effective method - one that many, today, probably would not know of.

RE: Slab-on-grade Settlement


I have the second edition, 1973. He shows it there also, but doesn't name Michigan. I learned about it from a PHD student at UW who had worked in Michigan. I think their Soils Manual had that spelled out. On that first job, we (at the Wis DOT) took some test borings through the fill and found complete displacement to 30 feet.

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