Fill For Undermining Utility Trench 1

[EngineerKojak]

A contractor that I work with has apparently cut a 10" wide by 26" deep trench for a domestic water line, just outside of the foundation for an existing house. There's pretty much a vertical cut right beside the footing. It's obviously too late to stop them. How should they fill this trench? My initial thought was a flowable fill, but my concern is that the water content may cause the vertical cut to collapse. I'm not sure if sand is a reasonable solution, or, if it may be possible to do a sand/portland cement mix, and place that in the trench dry? Any thoughts?

 

[jayrod12]

Why not just free draining backfill with a 12" clay cap? 10"x26" straight side cut it fairly typical where I am. But it's thick, stiff clay

 

[EngineerKojak]

Thanks Jayrod12, I'd think that would be fine above the footing, but this trench is 26" below the bottom. My concern with a free draining fill would be that as it drains, it could transport soil particles from below the loaded footings, which may allow the foundation to settle.

 

[oldestguy]

I'd fill full length with clean sand or sand with low silt to just above footing grade. Then, use a mechanical tamper on that 26 inches of fill. Mechanical tamper must be more than just a button headed, hand held. A good walk behind is likely too wide. If you can, the jumping jack tampers are likely to compact the full depth. Do only short sections of trench, say 4 ft. long. As each ares is compacted, immediately add fill to just above footing grade and compact until no further lowing of that area occurs. This will put compactive effort into that zone below the footing also. Digging likely has loosened that material and it needs improvement. If the contractor has a bucket width to do this job, he likely can find a tamper that fits on the back-hoe to do the job also.

 

[hokie66]

Cement stabilized sand or flowable fill would be my choice. Just do it quickly. I don't think the water content of the fill is an issue, unless the existing soil is sand. You could also just use concrete.

 

[jayrod12]

I missed the whole part about below the footing. My bad.

I would be comfortable with oldestguy's response. Some form of compactable fill (depending on area crushed limestone works well) and the jumping jack style compactor. Those jumping jacks do a great job of compacting it however you might want to consider doing it in lifts because if you ran it on 26" thick of loose fill it would bury itself in the first two jumps.

 

[EngineerKojak]

Thank you all. The debate in my mind has been whether or not a self-compacting fill would be better, or if it would be better to compact fill. I agree with Oldguy, that the compaction would/could restore some of the value lost, and if it can be compacted in one 26" lift, that would be ideal. I'm wondering how much of a risk there is in over-compaction. I think the building is pretty light (single story, gable end, wood-framed). In my mind, the self-compacting fill would stabilize, but the compacted fill could repair. At this point, I'm not aware of any foundation damage, but it could occur over time .

 

[cvg]

you cant compact a 26 inch thick lift, you will need to compact two lifts

 

[jayrod12]

At least 2. I would lean towards 3 or even 4. Our standard specification is 6" lifts

 

[EngineerKojak]

Flowable Fill it is.

 

[oldestguy]

As to how thick a lift to compact, I've seen a case or two where someone dumped in fill, no compaction and the jumping jack then was applied to the top of it. In one case 4 feet of compaction took place, and was verified with a test pit. I've also seen where loose fill under a new footing was compacted by hitting fill alongside it and that effect was transmitted underneath horizontally, in clean sand. A one story school. It does not hurt to give it a try and check the first few lengths done that way and then decide. If no test equipment, push a 1/2" dia. rod, an amazing tool used by many a tech for a quick check.

 

[BUGGAR]

I like the flowable fill. Mechanical vibration may disturb the existing soils under the existing foundation?? What is the concern with the moisture? We use "wet" drilling mud (bentonite) to retain soils in the hole when drilling. Also used frequently for cutoff walls. Flowable fill is diggable if you keep the cement content low.

Bob