Backfill Material Gradation 2

[NewNorthernGeotech]

Another question. This one pertains to backfill material. The building is again a large industrial building, requiring fill depths to slabs of 3 m or more in some places. The problem; scheduling requires that materail be placed outside during temperatures well below zero and also, once things start to thaw this excavation is going to see a fair bit of water flowing through the lower portions.

As a result, it has been suggested that a coarser, cleaner material be used to get the backfilling process out of this soon to be saturated area during these temperatures below freezing. In my mind this works; it would allow the contractor/project to move forward, allow water to flow through the lower areas without sweeping away all our finer materail and if placed correctly (foot lifts when compacted using large vibratory roller) it should serve as a good solid base for the finer materails we'd place on top. I may be a little concerned about slippage between rocks, but am hopeful that good packing of reasonable lifts should help these rocks find a good secure home.

My questions; is there any magic gradation that would be perfect for this application. We are currently producing material with an onsite crusher. Or is something like 8" to 2" with minimal fines, just as good as something like 3" to 1/2" with minimal fines. Also, if we placed a well graded 2 1/2" minus material (placed with water) on top, how crucial would the application of filter cloth be to future settlement?

Any comments are appreciated as always!

 

[MRM]

Will this backfill material be supporting foundation elements, or under slabs only?

 

[NewNorthernGeotech]

This backfill will be supporting slabs only, as backfill around foundation elements. Foundation elements were taken to bedrock. Naturally settlement is still a concern and I'm wondering if anyone would like to voice any opinions regarding the use of cleaner materials beneath building interiors. There has been some debate onsite and I'd appreciate more perspectives on the idea. The full plan right now is to use this coarser/cleaner material to within 1.5 m of the bottom of the slab. The rest would be backfilled using well-graded 2 1/2" or 3/4" inch material placed in the warmer months.

 

[MRM]

I'm still not sure of the main problem yet.

When you say "...once things start to thaw this excavation is going to see a fair bit of water flowing through the lower portions," are you talking about water within the soil, which was at some point frozen, thawing and migrating downward, potentially causing some soft soil conditions for the slab? Something similar to the spring thaw for roads? Or are you talking about a seasonal groundwater fluctuation you're expecting to occur? Where is the water coming from that you're concerned about?

Also, which of the following are you concerned about?
Engineered fill placed and later becoming saturated with normal groundwater levels? Not wanting engineered fill freezing after placement? Not wanting engineered fill freezing during placement? Wanting to create a backfill that doubles as an underdrain system?

Help me narrow down the main problem.

 

[NewNorthernGeotech]

MRM:

First; the groundwater flow I'm talking about will be due to melting snow and ice up gradient of the excavation. The site mainly consists of fractured bedrock and once thawing kicks into high gear (over the next couple of weeks) this excavation will gather quite a bit of water. It would be easiest to get up above these low spots so the water will flow beneath while we work. So that would fall under your "backfill that doubles as an underdrain system". I just don't want all my fines to wash away over time and cause settlement.

Second; the main concern which lead to the selection of a coarse/clean material was the need to place in below zero temps. So that would fall under your "not wanting engineered fill freezing during placement". We plan to place the coarse/clean material without water.

Does that clarify? My main question is about a "magic answer" to gradation for dry packing material? I don't see there being one, but would like some more perspectives on the topic. I tend to think that so long as there's minimal sand and fines involved, and a decent gradation from cobbles to gravels then it should pack up good and solid. But, is it better to have no fines what so ever; is it better to have 8" to 1" gradation; is it better to have 4" to 1/4" gradation? You see what I mean?

Thanks

 

[fndn]

The following doument has a comprehensive filter design criteria:

http://www.water.az.gov/dwr/content...on/Dam_Safety_Docs/1994_SCS_Filter_Design.pdf

 

[MRM]

There's no "magic" gradation, although the info in fndn’s post, or some other similar resource would be applied in some way in terms of evaluating the potential for loss of "fines." In this context, "fines" is relative. You don't want P200 washing out of conventional backfill as much as you don't want P4-R20 washing out of a coarser rock fill. I'd imagine the final product would be considered a well-graded rock fill.

I’d like to say that backfill (especially sandy fills) can be placed in just about any temperature. The key is to properly protect it from the cold weather when it is left alone. That might include using insulating blankets, blankets with heating elements, or placing a "sacrificial" loose fill layer over the engineered fill. It gets trickier and probably more costly, but we routinely place backfill in subzero degree days.

Concerning the material itself, the reason I was interested in whether or not conventional sand backfill could be used is because, in general: it is more readily compacted (sand size particles respond more readily to a compactor than cobble size particles, which require more energy to move), you can perform quality control tests on it during placement, and slight particle shifts over time (secondary compression) are less likely to adversely affect the slab subgrade (if one or two cobble-size rocks settle or move, the slab subgrade may shift noticeably. Crushing of larger particles such as rock fill particles may be subject to an additional mode of secondary compression; crushing. Crushing potential would depend on the final thickness, rock type, shape, and other factors. Maybe in this case it would be minimal with only 1.5 m.

I was wondering if you’ve considered a more conventional underdrain system instead of relying entirely on the rock fill itself transmitting enough water through. You could reroute the groundwater around the foundation area. The smaller-sized backfill could then be used.

 

[DRC1]

We do a lot of witer site work. We only use 3/4 in clean crushed stone. Will drain and will not hold mosture and freeze. Water should not be flowing across site during base placement. Perimeter drains and swales should be built to intercept and move water away from the slab area.

 

[egmlca]

A 3/4 cushed agg. would be a better choice below the slab. Still though make sure to place a fabric over the rock to prevent leaching away of fill soil. Lastly, the rock should come up at least 1 foot or so over the highest anticipated seepage level. Just for clarification, are there only spread/colunm footings and not a perimeter foundation? Also, the above comments are correct in that a subdrain system should be employed.