Allowable Bearing Pressure 1

[kggatarm]

Is there an acceptable correlation (i.e. tables, equations, etc.) between compaction test results and allowable bearing pressure? Specifically, I have a client who is planning on placing a CMU wall, single-story, slab on-grade building on about 15 feet of compacted fill. The fill, which consists of sandy lean clay (CL), has a low to medium plasticity and all compacted lifts (about 10" loose lifts) exceeded 95% of the stand. proctor (D-698).

Also, no test borings (or test pits) have been competed previously in this area.

 

[MikeDB]

The 95% just means that it is within 5% of it's optimum compaction. A soil at 80% might have a greater bearing capacity than another soil at 98%. Actual bearing capacity could be estimated with a pocket pentrometer, torvane or soil testing. If your loading is less than 1 ksf, it may not matter.

 

[bashabo]

The difference between 90% and 98% is a great deal of compaction effort on the soil. Effortwise, it is not a difference of 8%, rather a larger number. Normally, a stockpile of dirt accumulated after excavation would result between 80-85% as a result of minimum compaction effort, which is achieved by minimum wheel roll of construction equipment, like a small track dozer. I don't know how soil with 85% compaction can result greater bearing capacity than 98% compaction.

 

[Ron]

To answer your first question...not really.

Bearing capacity depends on several variables and soil density is only one. Overburden confinement has a much greater effect on bearing capacity than compaction; however, as density increases, so does the overburden efficiency, though not greatly.

The appropriate test for bearing capacity is the plate load test.

 

[fndn]

Take few soil borings and obtain shelby tube samples of the foundations soils. Then see what values you obtain for unconfined compressive strength. Two things come to memory-if the 15 ft fill has been placed recently, have a Geotech analyze if consolidation of the new fill is complete. When you excavate for the footings, use a cone or a hand penetrometer to verify in place bearing capacity.

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[casimmons]

It sounds to me like your real problem might be settlement, not bearing capacity. The natural soils will settle under the load of the fill mass, and the fill mass will settle under its own weight. In this case the weight of the structure is probably small but you still need to check settlement. To calculate the bearing capacity you need to estimate the shear strength properties of the soil (or determine by lab testing), find the appropriate bearing capacity factors, and use the bearing capacity equation.

The problem with plate load tests it that the "zone of influence" for the plate is typically not as deep as the foundation, and the results could be misleading. One thing you could do is monitor the settlement over a period of time at some set points to see what is going on with the fill. Having said all that, considering the type of fill and its compaction along with the relatively light structure, this discussion is probably academic unless the fill was placed over some soft compressible soils.

 

[DRC1]

Relative compaction does not corralte directly with strength. I would agree a few test would go along way to a better undertanding. Checking the degree of consolidation of the fill was also a good idea. I would also be conserned about swellig for a light structure. Two construction suggestions - keep water away from the foudation, including roof leaders. Pipe and drain far away. Second, estimate the foundation bearing pressure of the structure. Then assume a weight for the clay. For the sake of argument say it is 120 pcf. Say you have calculated your bearing pressureto be 500 psf, put your footing down at least 4 ft 2" (500 psf/ 120 psf)This will reduce the apparent surcharge to the soil and reduce potential settlement.
Focht3 has extensive experience with clay and foundations. Hopefully he can provide insight to this.

 

[BigH]

There are a number of papers out there that show trends of unconfined compressive strength (contours) on compaction test results. There is a good one in one of the Overseas Research notes - I'll try to find it; but in any event, you can use this to estimate the anticipated behaviour of your soil - knowing where your soil plots on the compaction curve plot. You might want to check out Bishop's classical Paper in the 1960 Clay Soils proceedings - (it was the Boulder Conference).

 

[VoyageofDiscovery]

Hi BigH,

Sorry everyone,

What occurred wrt the LRFD question, I missed all the responses and now the thread was removed. What happened?

Regards

VOD

 

[PSlem]

A couple responses were wiped from the moment reduction question, too. I suspect just housekeeping. I did get Tomlinson's Foundation Design and Construction but would still like a source to buy Eurocode 7.

 

[Focht3]

Thanks for the kind words, [blue]DRC1[/blue].

With 15 feet of fill - compacted or not - settlement is definitely the principal issue. That's 180 inches; at 2% shrinkage (typical for lean clays) you will get about 3[½] inches of settlement due to fill shrinkage alone.

And we haven't even discussed elastic and consolidation settlement, or bearing capacity for that matter.

Soil borings are clearly required, as well as a statistical evaluation of the compaction testing of the fill. The fill was tested at every lift, right? If not, the fill may not be usable...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"