Backfill lift thickness with jumping jack 1

[bighouse]

I will be performing density testing for backfill being placed behind foundation walls and I have been fighting with the contractor over lift thickness. The contractor wants to place the backfill in 12 inch lifts; however, I don’t want the contractor to do so because the excavations are so narrow the only compaction equipment that will fit is a small jumping jack (like a Dynapac LT 6000 – static weight 150 lbs, impact force 3,300 lbs) so I don’t think he will be able to meet the 95% Modified Proctor density requirement. Does anyone have experience as to how thick of a lift you can actually get compaction on in this situation? The backfill soil is generally sand with a lot of silt (USCS classification SM). I am hesitant to even let the contractor attempt a thicker lift, because it will be extremely difficult to remove the soil once it is in place without significantly widening the excavation. Anyone experience of advice that anyone has would be greatly appreciated.

 

[DDBPE]

You'd be surprised what you can get out a jumping jack, especially in a confined area.
Do your specifications call for a specific maximum lift thickness? If there is, refer to it, if not, you do not want to get into a position to be directing a contractor on how to perform the work.
If he desires to try a 1 foot lift and there are no requirements on thickness, let him, and if it works, great, if it does not, then he has to remove it, but you are protected, just make sure you document the conversation.

If it were me (the contractor), I would try a 1 foot thickness with sandy soil in a small area and have you test it, then if it did not work, remove the small area and change my depths. If you are using a density gauge, this would not take long to do this test section.

 

[Mercycity]

It is possible to lift to 300mm(12in) and still compact to required density. If you are using nuclear gauge you can measure density at 12 in depth and 6 in depth at the same time.

 

[fattdad]

this is a means and methods matter. As said above, if the contract specifies maximum lift thickness, then the contractor has no choice but to oblige. If not, he could try a 2-ft thick left thickness! That'd put the burden on you to dig down a foot or so to acertain whether the lower depth of the 2-ft thick lift met the specification requirement. It'd be a lot of rework if it failed, so the contractor is unlikely to do that. I'd certainly be o.k. with the 12-in lift, but I'd make sure my tests reflected the level of compaction in the lower depth of the lift.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

I had a job once where the beckfill was 4 feet deep and the full thickness was hit with a "jumping jack". It tested OK the full depth. Maybe it's time to get practical.

 

[aeoliantexan]

This is not the question you asked, but I would be concerned that the compaction needed to get 95% of Modified may generate high lateral soil pressures against the wall. If it is a foundation stem wall to be backfilled on both sides, bringing up the backfill uniformly on both sides can probably limit the movement adequately. If this is a basement or retaining wall, compaction may deflect or crack it. Good temporary bracing may be needed.

This is the Contractor's problem, but avoiding damage is in everyone's interest.

Why is 95% Modified needed on a 1-foot wide backfill?

 

[oldestguy]

Texan brings up an important point. Many a wall has been forced in by compaction of backfill. If a retaining type wall, such as a basement, it likely is not designed for the high pressures that such compaction will produce. I have come to the point that no compaction is applied to backfill within 2 feet of a retaining type wall due to that effect. At the very top one can compact some. Below that there is limited settlement due to the "silo effect". Common sense is needed when doing inspection on any work, since not everyone is on the same page during design work. The "book" can't hope to cover all job situations. New inspectors need to know these facts before they get into trouble enforcing specs.

 

[jjesik]

Having the upper floor in or some bracing before back fill is compacted helps prevent basement walls from moving or cracking, but then you run the risk of rain or snow getting into the excavation also.

 

[mikeofBelAir]

As DDBPE said, You don't want to be in a position of directing work. Inspection is NOT directing. Once you tell the contractor what to do, you become responsible for his work.
Our profession was once described to me using the analogy of a football ref. The ref doesn't call the plays, he doesn't huddle with the team, he doesn't coach--His job is simply to watch for an infraction of the rules, and then throw the flag. Same for our job--we test and report. Now, there may be some special situations where we do actually have control--and that is a different animal. In general, we test and report.

As for compromising the foundation wall, that is more a function of moisture. Wet soil will transfer more force to the wall. Even a tracked loader running near a wall that was loosely backfilled with wet soil can cause it to fail.

 

[bb29510]

i have been able to get compaction on two foot lift sand with jumping jack and about ten inches with a sandy clay, but remember the nuke cannot read correctly on a close excavation due to pulling moisture out of the side wall, so moisture correction will be needed on nuke testing

ICC Master Special Inspector, Structural Masonry, Reinforced Concrete, Soils, Structural Bolted Joints, Structural Welding, AWS CWI D1.1