Consolidating 3/4 " Gravel

[DirtDelight]

Hello all,

A project we are starting next week requires 95% compaction on a 9" section of 3/4 " crushed aggregate under the pipe as well as thru pipe zone section. The pipe is 24" h20. As a technician, I have never actually tested the rock for density. Usually would just recommend the contractor run a vibratory plate over the bottom rock a few times, back-fill to spring line, hit again and then once more at top of pipe zone. There will be approximately 4 feet of soil cover over top of pipe zone.

Any suggestions as far as testing rock, or for that matter achieving the 95 % requirement ?

Thanks for the help................

BG

 

[fattdad]

I think relative compaction on open-graded aggregate is stupid. There is no "Proctor curve" to go by either. There may be some benefit to doing relative density, but I don't have much experience with that other then to know that you need a vibratory table to obtain the max/min values in the lab. Whether you can then get a good value of density in the field using a nuke gauge is another question. I'm with you. Place it in measured lifts, hit it with a compactor and then move on.

Maybe I'm just too practical. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[theCorkster]

Dirt Delight;

ASTM has a procedure refered to as "sleeve density" to test such materials. The USBR developed this procedure to be able to reasonably provide some numerical means of assessing relative density in poorly-graded granular trench backfill that were not conducive to sand replacement or nuclear inplace density procedures.

http://www.astm.org/Standards/D4564.htm

The other item is that cohesionless soils are often tied to laboratory compaction procedures more appropriate for fine-grained soils. A poorly-graded granular soil with few fines should really be evaluated by the relative density mehtod, based on the maximum density by vibratory methods (D4253 - Maximum Index Density of Soil).

One approach I've used to communicate with a contractor on the degree of compaction in trench backfill was to use a concrete vibrator. We inserted a concrete vibrator when the backfill was at pipe springline and let it go; the vibrator quickly penetrated the backfill, as well as created a cone of depression around the vibrator. The contractor saw this and recognized that the backfill was not well compacted in the haunch at all (which was significant for the large diameter flexible pipe). Not wanting to revisit the project later, he had two laborers compacting the backfill until it resisted penetration by the vibrator.

Since your degree of compaction (95%) is relatively high, I would not expect a well compacted granular backfill to settle at all under vibration.

Good luck!

 

[cvg]

are we all sure that this is "open graded gravel" and that "relative density" is to be the controlling specification? from the original post it is stated "95% compaction". This would be difficult to achieve with "relative density" method. In fact, nearly impossible unless using "standard proctor"... Typically 70 - 75% "relative density" is all that is required for open graded gravels. I have to believe that this is 3/4" dense graded aggregate, ie: road base.

Dirtdelight, you need to verify if proctor or relative methods are to be used. If "relative", than verify they need 95%. If "Proctor", check with the state DOT manual to determine the appropriate test methods for aggregate base.

 

[msucog]

to me, it sounds like someone drew the plans that didn't know what they were putting on there. if someone has specific requirements, then they should point you in the direction as to what they want (ie. astm method specified). my guess is that they'll say d698 which doesn't make any sense. if they happen to throw out d4564, then maybe they do know what they're talking about. it all seems to me that someone is getting "wrapped around the axle" if they truly want testing on the 9" of 3/4" stone. i'm guessing they didn't specify seperation fabric or anything like that to prevent soil migration in to the stone. now if you happen to be referring to GAB/base course type aggregate, then density testing would be appropriate in my opinion and that could easily be done via sand cone. if this is a force main and compaction is critical, then i'd recommend GAB (i still would worry about 9" of 3/4" stone).

i've seen this on plans before and it turned out to be an unknowing civil drafter sticking "95% compaction" everywhere that had some form of fill/backfill. it sounds like the stone is for bedding purposes. the contractor should put in the bedding stone, bed his pipe, then backfill with a very thin layer of soil then compact with a jumping jack/wacky packer. being that thin, they shouldn't have a problem sufficiently densifying it with a jumping jack. if they're using a plate tamp, then they'd probably better put in the 1st 4-6", run the plate over it then put the rest...then throw the pipe in and backfill/compact from there.

 

[DirtDelight]

Thank for all the input. To clarify a few things. The material required originally was a Class 2 3/4 maximum aggregate base. Standard Caltrans specification Section 26. An rfi was submitted, and a 3/4 gravel was accepted as bedding. That material is a poorly graded 3/4 " crushed angular aggregate, very little fines. The specs are stating the want 95 % relative compaction. We are having a meeting tomorrow, I will try to clear up some confusion.

Thanks again

BG

 

[dgillette]

RC (i.e., percentage of vibratory max) of 95% would be in the ballpark of 60-70% RD for most materials, so it's not a totally unreasonable spec, and easier to work with than RD, especially with min density being hard to measure on coarse material. (You do still have to use the vibrating table to get max density.) It's usually not difficult to attain 95% RC. On large projects, we often spec a procedure (e.g., 2' lifts, each with 4 passes of a 10-ton smooth drum vibratory roller), with record tests only.

In this case, I'm inclined to believe that a couple passes with a walk-behind or backhoe vib plate is all you need. After all, it's pipe bedding, not a highway fill or the foundation for a skyscraper.

 

[cvg]

as msucog suggested, I would be a bit worried about soil migration into this poorly graded material. Geotextile wrap might be advisable. Or perhaps, seeing how this is the foundation material under the pipe which is arguably the most important layer of material in the trench - the material should be rejected in favor of something better.

 

[fattdad]

Somewhat off topic, but in response to dgillete: When using Standard Proctor, Lee and Singh (Journal of the Soil Mechanics and Foundation Division, ASCE, Vol 97, No. SM7, July, 1971) show the correlation of RC (relative compaction)=80+0.2Dr (relative density). This implication is that a relative compaction of 80 percent represents a relative density of 0 percent (i.e., soil at the minimum void ratio). The published work was performed on soils from well-graded silty sands to poorly-graded gravels. I've always considered a 5% drop in relative compaction to represents a 25 percent drop in relative density, based on this published correlation.

Don't ask me what to do with Modified Proctor data!

Irrespective of the specification, I'd agree witn dgillette, 2-ft lifts and a couple of passes with a vibratory roller would be o.k. by me.

If surface water infiltraiton is not an issue and there is no ground water to speak of, I'm not sure I'd use the separation geotextile.

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

I'm not particularly concerned with either surface or groundwater relative to the need for geotextile. However, this is a 24" waterline and assuming even at low pressure, even a small pipeline leak could send a considerable amount of water flowing through the gravel bedding material which could then cause piping of the adjacent native material or trench backfill into the gravel zone (assuming this gravel material is not filter matched to the adjacent soils). This could cause subsidence of the ground above.

 

[msucog]

i expect that for a 24" force main (assuming force main here) even a small leak will be a big hole...so i'm not sure it really matter when it comes down to it (regarding the fabric issue...i was more trying to prove a point that the spec might have been written without fully thinking about the scenario). for 9", i wouldn't give a big flip about it as long as it was densified to some degree. and maybe i'm off base here but i wouldn't get wrapped up trying to test the density...but for 9" of bedding stone simply have a rep on site to check that it's being compacted while they're testing the soil backfill along other parts of the pipeline. as far as crushed stone in general, i typically require 1' lifts thoroughly compacted because i typically see the contractor screw it up if you tell them anything more than that. i think there's other threads discussing this so i won't fill up this thread repeating my ramblings.

 

[rconner]

One of the threads per msucog's (good) memory may be

http://eng-tips.com/viewthread.cfm?qid=193256

.

 

[DirtDelight]

Thanks to all for the help. They decided to go with an aggregate base for bedding. Compaction will be to 90 % of the max density. Contractor not to happy with having to compact bedding, but it is what it is. Thanks again for all the input, I learned a few things about gravel backfill and compaction.

 

[dgillette]

Only 90 percent? With gravel, I think you would probably get that from dumping and spreading. By the correlation fattdad cites above, it's only 50% RD. Dump it, spread it 9" thick, and run the walk-behind vibratory plate over it once for good measure. Do a record test now and then (vibrated max density on this material, not Proctor).