Specifying compaction testing 1

[rnjroles]

I have a project where I'm being asked to specify compaction testing - type, when, where and how many for the backfill over a newly installed sewer (10-16' deep). We need to insure 95% compaction. What is the best/practical way to do this?

Thanks for any input.

 

[cvg]

need much more data:

are you responsible for quality control or quality assurance? ie: are you working for the contractor or the owner?

also, is this in right of way, existing or new street, what type of soils and what type of bedding and backfill material? what type of pavement? installed in trench or posative projecting? what type of equipment is doing the compaction? is this a local street or a highway? what country? what type of pipe? what does your permit require?

 

[rnjroles]

Additional info.

Project is in Northeastern USA.

Installation is new sewer under existing local street (private). The testing agency is responsible for quality assurance and will be hired by the contractor who is responsible for quality control. My spec. is for the contractor with the purpose of giving the owner what they want. The pipe used will be 10" SDR-35, bedding material is 3/8" stone, backfill material will be the excavated material which is sand w/ trace to little silt. Compaction equipment is up to the contractor.

Hope this helps.

 

[civilperson]

Test once per lift, (maximum lift thickness 12" loose), per thousand square feet of ditch area until ten passing tests in a row and then decrease frequency to once per every other lift per thousand square feet and so on. When failing test is encountered, revert to original frequency. If testing by nuclear density gauge, then must calibrate it with comparison sand cone compaction test daily. Number of proctors is a hard item to quantify but should be run in conjunction with one point compaction tests that prove correct M-D curve is applied.

 

[Stillerfan]

From my experience, contractors generally struggle with utility trench backfill. Therefore, I recommend a maximum loose lift thickness of 8 inches. As far as testing frequency, I would say every lift and at least every 200 feet or every time the backfill material changes. Also, sand cone calibration is a good idea, however, if the same gauge is used everyday, one sand cone per 10 tests is fine even if it takes several days to record 10 tests. Number of Proctor tests, one for every material used (United Soil classification System, CL, ML, SC, SM, GC, or GM). A gradation test in conjuction with Atterberg Limits can be used to classify the soil. Also, color change does not mean the soil classification has changed. Hope this helps.

 

[GoldDredger]

Our specifications for 'Source Quality Control' in the Trench Backfill section calls for gradation analysis in accordance with ASTM C136 which is for Standard Test Method for Sieve Analysis of Fine and Course Aggregates. (though I have not read that ASTM section personally)

Also, the lift of trench backfill thickness (Backfill above pipe zone, pipe zone backfill being separate) we have dependent upon the class of backfill (ie Class A, C, D and E). They seem to vary from 6" to 8" lift thickness max.

Don't allow free fall of backfill into trench until at least 2 feet of backfill has been placed.

Do not allow power driven impact type compactors for compaction unless at least 4 feet of backfill is placed over pipe.

Anyway, there are a whole slew of considerations in the trench backfill specifications that I didn't include, but here were just a few.

 

[PhxCivilPE]

You want hand compaction tools, ie. wacker, jumping jack, etc for the first two feet after you've shaded the pipe. Typically you'd bury the pipe to 1' above top of pipe with aggregate material which will achieve compaction with little to no effort.

Backfill after that point with native material, 1 test per 1000 sf, every 6". Sand cone every day is excessive, but it's up to you.

 

[msucog]

if you've got a geotech on the project, consult with them. don't be very stringent specifying the parameters if the owner isn't going to pay for the necessary testing. i've seen some folks put very specific evaluation/testing at high frequency in the specifications but then only want to pay for someone to be onsite to test for 2 hours per day...it can't be done.

if you're really concerned about compaction, the testing should accompany onsite observations of the fill placement procedures. and contractors being the "fine" folks they are (i.e. sneaky), i'd suggest having someone onsite full time. some budgets don't allow for this so part time testing is all you get. so if the tech is onsite say 2 hours per day, then you've got test results to make someone feel good about what is there. in reality, you're most likely not seeing the less than desireable stuff going in place.

for part time testing, i suggest budgeting for half a day each day at a minimum. again, i suggest full time testing. either way, make sure it is in the specs that the contractor is responsible for scheduling the testing firm as needed and that evaluation/testing should be performed at least every 2' vertical lifts for every 150' or so. lifts using small mechanical hand tamps should be 4" thick max and lifts using larger equipment can be 6"-8". you might get by with slightly thicker lifts as mentioned by others but i personally wouldn't spec thicker (the contractor will always manage to sneak a few extra inches in to each lift unless you've got a someone standing on top of him watching). i'd also suggest including a remark about moisture content (generally +/- 3% of optimum typically works well but might needs adjusting depending on your specific soil conditions). also note that safety and means and methods are the sole responsibility of the contractor. i quite often seen deep trenchs with no trench box so how the heck is anyone going to compact much less test such a scenario? even if they have a compactor attachment for their trackhoe, someone still has to test the stuff so safe access is essential.

good luck

 

[sodengineer]

What about lean grout?
About a 2-3 sack mix, +- 150psi sets up in a couple of hours, look at the batch tickets, done! Pave!

Yes, you will pay a bunch of $$ for the mix, especially since this is a sewer and it is deep. But, run the numbers, you may be better of financially this way. As a side benefit, you will get to know your local concrete producer very well.

 

[oldestguy]

95 percent compaction by what standard? Why? For full depth? Why? What is the basis for the 95?

A common spec for trench fill in streets is something like 90 % up until the last 5 feet and then 95 %, and it may be difficult to get, especially if Modified Proctor is your standard.

No one said anything about how many lab Proctors to do? One? One per ten FDT?

I'd sit back and see what you really need for the site. Is there some guarantee of filling of depressed pavement in case the testing is skimpy?

If it is sand fill, some communities have done jetting of the backfill and no tests and it has worked out fairly economically, with some subsequent pavement settlement. But who is paying the bill. A lot of items need to be considered.

 

[MALLAN]

project specifications shoulh dictate density testing frequency. the soils report would make reccomendations to the design engineer.