Relative Compaction In-situ?

[fattdad]

Here's the nature of the cunundrum: If you have a site where the natural soils are firm to compact clayey sand (i.e., N=10 to 30) and very favorable for reuse in earthwork (i.e., correct moisture content, etc.) how many cubic yards of excavation would it take to yield the equivalent amount of fill (based on 95 percent Standard Proctor)? I would think that the in-situ soils (seemingly stable for cut slopes, foundation bearing, etc.) would be at a favorable level of compaction. I would also think that one cubic yard of soil in-situ would provide more than one cubic yard in the back of the dump truck. I would also think that the yardage in the dump truck would be more than the yardage in the newly-placed fill. My question is just what is the relationship between the excavation volume and the fill volume?

I just had an earthwork contractor tell me that he uses 12 percent, which would make some sense in going from the dump truck to the newly-placed fill. But is this even close to the "shrinkage" factor in going from the in-situ volume to the compacted volume?

I considered an ideal soil: gammaDmax@117, gammaD95@111.15 and gammaD85@99.45 pcf. Solving the weight-volume relationship, going from 85 percent compaction to 95 percent compaction is a factor of about 12 percent. Does anybody out here really believe that compact in-situ soils are sitting at 85 percent relative compaction? I don't. . . .

Comments?

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

You're on the right track, papa gordo.

A QUICK look at some of our old projects with very large qties (100,000 cy up to several million) shows everything from over 5% net gain (borrow to embankment) to 15% net loss. That would have included some spillage, over-build that got trimmed off, etc. On a smaller project, I'd expect a higher percentage of spillage and such. Pulling numbers from my...uh...hat, I'd look for anything from 5 to 10 percent net loss.

85% sounds a bit light for material that is compact in the ground and has N up to 30.

 

[fattdad]

Here is an idealized case for discussion. For a soil with a maximum dry density of 117 pcf and specific gravity of 2.68 typical compaction in the field would be at 95 to 100 percent (let’s use RC=98). Correlations by Lee and Singh suggest that “relative compaction” (RC) is related to “relative density” (Dr) by the equation RC= 80+0.2Dr. DM 7.1 (page 7.1-87) presents correlation between SPT N-value, vertical effective stress and Dr. For soils within 7 ft of the ground surface, I was able to estimate Dr at zero feet and at 7 ft. Knowing Dr, I was then able to calculate what the volumetric strain would be for soils of varying N-values for these two depths (I could do this at one-foot intervals as well. . . ).

Here’s the correlation in the form of “shrinkage factors” versus N-value. Now we just have to figure out what it means.

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f-d

¡papá gordo ain’t no madre flaca!

 

[fattdad]

p.s., how do you get the image to appear in the post (rather than the link)?

¡papá gordo ain’t no madre flaca!

 

[dgillette]

If I understand this right, the looser material is shrinking (densifying) by 6 to 10 per cent (net, borrow to fill), and the denser material is fluffing by 1 to 2 percent.

In general concept, that looks appropriate, but I can't say on the specific ratios. I'd be very hesitant to apply a correlation from Dr to clayey material where Dr can't be determined. Lee and Singh's correlation data presumably came from the band of finer granular soils with small amounts of silt where either Proctor-type impact test or the vibrated max test can be used. (The data used in the correlation in DM7 came from granular soils.) For one thing, the stress normalization of SPT (like Cn used by HB Seed for liquefaction analysis) doesn't really work the same in granular and clayey materials.

Regards,
DRG

 

[fattdad]

DGR, I hear you on all counts. Regarding stress normalization, in the "typical" cut to fill range, we are in the 0 to 1/2 tsf range, so I could (not that I will) factor that variable out of this type of assessment. Regarding sandy versus clayey soils this type of analysis just isn't valid. Matter of fact, as a quantifiable exercize, I'm not sure for sandy soils this is valid. Qualitativelly, I do find this somewhat interesting.

I welcome further comments. At this point I'm faced with local earthwork contractors just using 12 percent "shrinkage" and then telling us the site doesn't balance. Then our civil department regrades the site and there's excess to deal with. Not a problem on a big site - big problem on a small site (i.e., where there is no place to put the excess).

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

12% net loss (borrow to fill) just sounds high to me, based on how you've described the materials and what I've seen on other projects, mostly large dam embankments.

Your local dirt movers may be mixing up NET loss with 12% loss from truck volume to compacted volume (a believable amount), or maybe they are used to measuring by load count, and what's actually in the truck is lower than the full struck or heaped capacity they are assuming. (I'm shocked!) Short of doing a lot of density tests in both borrow and fill, and detailed 'before' and 'after' surveys of both (as are sometimes done when the C is being paid for exc in borrow separately from placed embankment fill), I don't know how to prove that.

 

[fattdad]

So, here's the question. Should I let this inquiry out to the earthwork forum? My initial thought was to direct it here to get a better peer perspective.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Sure, but I think it would be good to put it out in terms of asking for people's EXPERIENCE with shrink and swell factors. Otherwise, you might get just more discussion along the lines of "Well, it really ought to..."

I'll cast around here to see if anybody has ever compiled our shrink and swell experience. We have often, not always, paid for excavation in borrow area as a separate item from embankment fill because we often have to reject portions of the borrow material as being unsuitable for dam core or whatever. That might complicate the interpretation. (We figure the contractor isn't responsible for what's in the borrow area, so handle that with explicit separation of exc in borrow and placed fill. Probably get a better price, and definitely fewer claims, if the bidders know they will get paid for exc even if the load is rejected and they don't have to figure that into their bid price for embankment fill.)

DRG