Pipe Zone Compaction

[durtaholic]

Have a situation where as a technician am not looking at the bottom of trench excavations full time . Have residual water from existing system draining into trench as new 8" pvc is being installed. Contractor has made some effort to pump water out and place 3/4" gravel in bottom in spots. Then placing 12" of sand over pipe. Spec calls for 95% at top of pipe zone. (+1 over T.O.P.) They tried running a small vibra plate over sand with no luck . Suggestion was made to use hand tamper on sides of pipe but they do not want to do this. In areas were we did observe the gravel go on bottom they were able to place approximately 12" of processed base material over sand and compact with sheep foot wheel. We would dig back to sand and were getting decent tests. The sand is a dirty sand with 17% - 2oo. Now we have an area where we can not get above 87 % on sand. Just wondering if anyone has any ideas to get that pipe zone area compacted, and is it possible that the material under the pipe is crap causing a problem getting compaction ? Also, has anyone used that method of placing a lift of agg base over sand and compacting that in an effort to compact sand underneath ? Thanks for any help in advance .. BG

 

[Ron]

recheck your Proctor. 87% not likely

 

[molerat2210]

I am not sure about the subgrade condition from your post. It's not clear if sand boils or sloppy bottom have been rectified. Cannot get compaction if the trench bottom is not stable.

If you have the bottom stable and the water under control then you should be able to compact the material. And, you cannot obtain compation in the hauches unless you have adequate width to the side of the pipe to run a trench roller or jumping jack on the side (not likely for 12" pipe). Not a good idea to run compaction equipment over the pipe and less than 2 feet above the pipe (typical for most specs). Compaction at 1 foot over the pipe may crush the pipe. Compaction in the pipe embedment zone is a nice though but is difficult to achieve (and usually not required to be tested).

You cannot compact a material in lifts more than 1 foot thick (especially in the pipe haunches). So, don't expect any compaction of your dirty sand below 1 foot (6 inches for jumping jacks and small trench rollers).

 

[Dima7]

What is the Lab max density ( MD) and optimum moisture content (OMC) of this sand? What results densities/moisture content (D/M C) do you get in situ test?

If the mosture content of the sand on the site is too high you cannot compact this material.

The only solution in this case:
- excavate again,
- pump and
- put back this sand in drier condition and compact.

If you give me the values 9 MD-OMC and D/M C) I can assess the situation better.

 

[durtaholic]

Thanks for the replies... A check point was ran on the sand and results were noted about 6 pounds lower that the first max of
131 @11.5. In place results were noted to have d/d in the 113-116 range between 8.9 % and 12.4%. The requirements for the compaction was lowered to 90% which helped. It appears the sand material was achieving 90-94% compaction, and being only an 8" pipe with sand on the wet side of optimum, the engineer felt compaction around the haunches wouldn't be a problem. Just taking good notes on test locations and elevations, as not being on the site full time as a tester puts a great amount of responsibility on the contractor to do their job correct.

 

[Dima7]

Optimum Moisture content 11.5% looks fine. How many fine in this sand? around 10-15%? The maximum lab dry density (131),on my own opinion, is too high. Field densities look more reasonable.

I would say that the better compaction could be reached with moisture content 1-2% less than optimum MC. Look at your field compaction results I think higher compaction was with MC lower than OMC.

It is the best way to check out the compaction on field and force a contractor to do his job better.

By the way, checking a maximum density and optimum MC for sand ( less than 10% of fines) by using standard/modified Proctor is not so good way. Proctor was not designed for clear sand.

Better to carry out max and min densities according to ASTM D4254 and D4253, respectively, and deal with a relative densities.

 

[dBasement]

Be really careful about using a nuke close to the pipe! We had issues with that and ended up using a sand cone for anything within a foot of the pipe. If anything, your nuke will read lower than the actual compaction due to the proximity of the void in the pipe.

 

[BigH]

Nukes are, in my view, notorious for having issues . . . need correlations of specific materials against sand cone.