Soil Compaction

[ataman]

Hi

Just a few questions on compaction.

Is there any correlation between Impact Value (determined by site testing using a Clegg Hammer) and Density?

What are the methods used to determine whether compaction has attained 95% or 100% modified density.

What is the best way of determining if the bottom of a fill is compacted? Does excavating to the bottom disturb it enough that additional prep work has to be done before it is tested?

Thanks

 

[civilperson]

No correlation without more information.

A sample of soil, (in situ), is measured for volume and weight and is compared to the maximum density achieved by the proctor run in the lab per ASTM. Variety of methods to measure soil in place density; sand cone, nuclear gage, shelby tube, etc.

Bottom of the fill:
1) during construction, test and record the density of each lift at random locations per testing schedule.
2) after completion, drill to elevation in question and push a shelby tube to obtain a sample or excavate a pit for tester to check density.

 

[GeoPaveTraffic]

Got to say I've never heard of a Clegg Hammer.

As for methods to determine percent compaction, you have the lab determine the moddified Proctor density. You then determine the in-place field density via (sand cone, nucular densometer, drive tube, bulk sample, water ballon, etc.) and compare the densities.

As for methods to determine "...if the bottom of the fill is compacted?" you need to give us some more information. Are you saying that the fill was placed to some significant depth and no tests were taken and now you want to know if it was done correctly?

 

[ataman]

Thanks

The site has already been filled and no tests were taken.

Civilperson....are you saying that you can't excavate and then test?

If a fill has been compacted to spec...say 95% modified proctor and then exposed to rain, can this impact the compaction test results? If so, then it leads back to my previous question. How do you determine where the compaction is still acceptable so that we can establish how much material needs to be removied.

The Clegg hammer is a simple impact hammer that is commonly used to measure "stiffness and strength". They come in different weights and are dropped from a specific height and the decceleration of the weight is measured.

Thanks

 

[BigH]

GeoPaveTraffic: Clegg - a device from OZ (aka Australia)

 

[GeoPaveTraffic]

Thanks for the info on Clegg, sounds interesting. I doubt there is much of a correlation between Clegg results and percent compacation.

As for compacted soil exposed to rain, etc. Yes the percent compaction for the upper 1/2 to 1 inch will be effected. But deeper within the fill, there should be pretty much no effect. If the fill is only a couple of feet deep a trench could be excavated with tests taken either in the bottom of the trench as it is excavated or in the sides of the trench. However, if the fill is several feet deep, a drill rig pushing Shelby tubes would be the better way to go.

That said, if no one was watching the fill being placed and compacted; no one know what might have been burried in the fill. Could be ok, but could also have a car incorporated into the fill.

 

[msucog]

if compacted to the numbers you mention, rain will not effect more than the upper few inches of the surface. i sometimes perform test pits with density tests at regular intervals as you go down (say every 2'). be aware that there is more of a possibility of "missing" something bad. at least if the testing firm was there during fill placement, they could probe it and perform tests in the more questionable areas. if it's already filled, it's 99% pure luck finding the bad spots. instead of perform test pits (which leave big bad spots from the pits themselves unless put back properly), drilling could be performed with spt at 2.5' intervals or even continuous. you can go cheap with few test results and minimal findings or you can go far to the other end of the spectrum and perform many many thousands of dollars of sampling/testing. one choice has lots of risk (whether foreseen or otherwise) and the other has less risk...and then there's a wide range in between. depending on the proposed construction, one may be more appropriate than the other. heck, you could even perform hand augers (to depths of up to about 10' or so deep) and possibly retrieve tubes with a dcp setup for lab testing.
i'm not aware of Clegg so i can't comment on that. i hope this helps a little.

 

[BigH]

You can do a search of the internet. However, I have some newsletters I got off the net so I assume there is no copyright.

 

[BigH]

I meant copyright issues with posting.

 

[dirtsqueezer]

I'd say it depends on how deep your fill is. Like BigH said, you can start out with a test pit and do a density tests(sand cone, if you don't have the license to use a nuclear densiometer), but you can only go 4 feet in the states without danger of wall collapse. Using the N60 value with an SPT (drill rig) would be expensive, but if your fill is very deep; on the order of 10-20+ feet, it would be more feasible to just take the N60 value and try to correlate stiffness. You could at least compare the upper layer results (probably more compacted, or you could have them roll it) to your lower layers and see if there's any difference in the blow count. Just depends on the depth of your fill.

If you're wondering about methods of density testing go to the ASTM website and look up nuclear densionmeter and sand cone methods. That should pretty much cover your bases, if you don't have a local testing lab to call.

 

[msucog]

also, i believe astm has a work in progress for testing density via dcp. i'm guessing it'll take quite a while for this to become a standard since the whole hit it with a hammer and extract a number has so many variables and is hoaky at times. my recommendation is that if you really need a density number, use a drill rig and sample/test accordingly. if you're trying to get a general feel for what's there, use a drill rig and sample/test accordingly. a borehole and most exploration types are pin-hole pictures of what is there. test pits at least let you see if there's a bunch of crap mixed in the fill at the test pit locations. if it's a lightly loaded building that is not of a critical nature, and if the owner is willing to accept additional risk of the ugly unknowns that could happen, you could explore it as much or as little as someone wants. i urge that caution be used so that someone doesn't get sucked in to saying "yes, it appears okay for the purpose" which then gets turned around in to their (the engineer's) liability because they were trying to be the good little helpful engineer for the client. if the client doesn't want to take on the risk for the project conditions (known or otherwise), then they can always take all the old fill out and replace it while testing it. besides, with the way that ibc codes are written, the building official may require some rather detailed letters from the engineer at the end. so in other words, i've seen this exact situation turn in to the engineer being the one pressured to take the liability because they were trying to be helpful. i know that all this is not exactly what was asked in the original question but i believe it's important to the discussion (especially since i see this situation reappear rather often).

"no good deed goes unpunished"

 

[tds23]

Q: Can clegg hammer results be correlated with nuclear densometer results and CBR or is this not possible without lab tests.

 

[genomty]

IV (impact value) might be correlated to some soil parameters like strength, specific moisture content, unsoaked CBR, dry density (compaction), etc.. all this correlations can be established in laboratory or in field, for instance, lets say, specified compation for certain material is 95 percent, in laboratory you determine de maximum dry density as described by ASTM D698 or D1557, depending on your spec, now, some samples should be preapared following a procedure similar to the indicated in ASTM D 1883 (CBR test), using different blows for each sample, as result of this procedure, you will have several samples having a wide range of compaction values, then using the Impact hammer or clegg hamer a Target Impact Value is determined, now you have a value that should be reach in field,, this value is the measured in field by using the hammer.

In the other hand, in field, having or not the maximum dry density established in lab you can correlate th IV to certain field density and moisture content values by using any of the avaiable test methods to determine on-site density (nuclear gauge, sand cone, rubber ballon, etc). Impact Tests are performed in a place and then compaction or the field density is determined.

I'm not saying that test is flawless, some considerations should be made, for instanse, oversize particles, in any of the compaction methods you can determine a corrected density (field or lab) following the procedure described in ASTM D 4718, however, how these oversize particle may affect the IV, my best guess is doing something like nuclear gauges technicians commonly do, excavating by hand the testing point to observe the soils surronding the test hole, speecific information related to IV test is included within ASTM D 5874

Regarding to bottom of the fill is compacted, I have participated in some forensic jobs to determine if compaction was reached in some embankments and the procedure used was firstly established in a meeting with contractors, project owner, laboratory , if any other firm is participating, and commonly the agreement reach is, perform the first test on top, then partially excavate the layer, using a construction equipment (usually a backhoe)and complete the excavation by hand to the top of the next layer and perform the next test, this procedure is followed until bottom is reached (certain conditions requiere a safety plan for trenching, it is certaing disturbing degree, nevertheless, is there any sampling method in soil testing that not induce certain disturbace in samples?, generally speking, geotechnical engineers and cmt engineers have to work with soil samples as reasonbly undisturbed as can be obtained, so that is why a procedure should be agreed and signed by anyone involved before testing starts, just to avoid that if test results were different to those expected by any of the involved part, no one can come out and said that procedure used to reach certain depth is not adequate, or ask for a re-test or use it as argument in court.