Surcharge Loading for Soaked CBR 1

[BigH]

Quick question: What do most of you do for soaked CBR as far as surcharge is concerned? Do you use the nominal 5 to 10kg or do you actually put on a surcharge equivalent to the pavement structure even though commercially available moulds don't readily handle 30kg+ ??
and Thanks!

 

[Focht3]

We follow the ASTM standard...



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[VAD]

Hello BigH:

My experience with this test was based on the approach outlined in Soil Mechanics for Road Engineers by the Road Research Laboratory. In that book the surcharge to be placed was to be equal to or estimated weight of construction to be placed on the particular layer under consideration. It also went on to say that for design purposes the weight of surcharge should not be less than 10 lbs for test in a mould nor more than 30lbs for an insitu test. Each 5lb weight is approximately equivalent to 2.5 in of construction when the mould is used and 1 inch of construction when used with the 10 in plate in an insitu test.

Generally for heavy clay soils (70% natural water content), we found that the surcharge did not make much of a difference. For sandy soils there was some difference. Generally, I think we used a 10 lb weight in those days regardless of material type.

[Cheers]

 

[SirAl]

Hi BigH

We have always followed ASTM D1883. When designing a new structure, subgrade parameters are an input value so structure size and therefore surcharge loading is unknown. It would be interesting to retest at the calculated surcharge to compare. If the actual surcharge is higher then the swell/softening after soaking would be, seemingly, less providing a higher soaked CBR value and a conservative input value.

 

[BigH]

Thanks - I'll have to find ASTM D1883 - probably same as AASHTO which I have. The reason I ask is that at normal 5kg weight, CBR of a FASM (fly ash/sand mixture) is about 2% after soaking; if 35kg is put on, it is well above 6% which is the project minimum.
Any other thoughts?

 

[Focht3]

Hmmm,

Is the higher CBR due to cementation, or because of a temporary densification? I see the following effect in high PI clays: if you use D1557 instead of D698, you can raise the CBR from 2 - 2.5 to 6 - 8. But the clay swells over time until the conditions are closer to a standard Proctor - predicted density. One of my (unscrupulous) competitors tried to sneak this past the City on a commercial project; they didn't buy his arguments. (Neither did the other local geotechs. The City called us to see what we thought...)

One way to check and see whether the effect is a lab phenomenon, or a real behavioral change, is to do some field tests. After all, the goal is to predict actual field performance -



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[BigH]

I have a strong suspicion that it is due to the heavier weight not permitting &quot;swell&quot; to occur as it is a rigid lateral confining or, perhaps, some secondary/creep pheonomae causing densification - perhaps both happening at once. The FASM is not swelling type - with 5kg surcharge we get <1% swell in 5 days. With the higher surcharge we would get less, I would think but I will confirm this with the other contract (it isn't mine, yet). I doubt it will be due to fly ash cementation in just 4 days.

I'm ready to have the contractor try cow pies or dog pies just to see if that, too, will . . .