Backfill in sub-zero Temprature
Backfill in sub-zero Temprature
(OP)
We are about to start a culvert structure backfill in subzero temprature. The specification says the subgrade should not be frozen prior to backfilling and gravel for backfilling cannot be frozen either.
How do you keep the surface from freezing and how do you check the gravel for signs of frost?
How do you keep the surface from freezing and how do you check the gravel for signs of frost?





RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
If C, then usually no real trouble until temp gets below minus 6. Below that temp. I'd buy from a concrete readymix plant with no cement, but heated gravel (probably done with hot water).
RE: Backfill in sub-zero Temprature
The gravel pit is one hour away and I am worried that it will get frozen by the time it gets to the site even if it's warm when it leaves the plant. As for heated box, we are looking into it. We haven't heard about anything like it here (Canada).
RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
A thermometer will tell what you have, even if your eyes think otherwise
RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
Idealistic? No! It could be very important. For a dam or filter, we would never tolerate frozen material, and that would be clear in the specs.
If it's there, tell the contractor RTFS! (Read The F[ine] Spec.) If it isn't and the effects could be important, there may need to be a change order. Ask the designer, since an inspector should not be making that call.
RE: Backfill in sub-zero Temprature
The subgrade of the pad is frosted right now, I was told by our soil engineers that to pour or not to pour is the call of the concrete inspectors,
My job is to see to it that the compaction is satisfactory.
RE: Backfill in sub-zero Temprature
Or is your situation that the footings and walls don't bear on the backfill you are interested in, so thawing-induced settlement wouldn't affect the structure, pavement, or utilities? If so, that's very different from pouring the footings on frozen ground.
Make sure that final grading slopes away from the structure, in case settlement leaves any place water could pond.
RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
One of the big factors in cool weather construction. If the construction is done quickly and well organized, it can be with no problems. Even a 1600 sf lowly full height block basement can be started and built at -10F or 0F, if it is quick, planned and protection is provided. Usually it is a 4 day job from start of excavation to completion, but with no backfilling since it is preferred by the G.C. and owner to backfill and pour the slab later after other work is done. The key item is the timing, scheduling and protection of areas.
If your voids to be backfilled have been setting unprotected for a while, the frost "depth" is immaterial because freezing knows no direction and can freeze horizontally also. Even at 21 to 32 for daytime AIR TEMPERATURES, the exposed surfaces can freeze if exposed for extended period because evening temperatures are usually lower, plus there is more radiant heat loss.
Your situation is a matter of judgment (keeping in mind the accepted specifications) plus what the construction schedule will dictate for timing. If you have documented information on the conditions, any prior approval from your principals you can get for being a "hardnose" will help greatly if there objections or job delays.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Backfill in sub-zero Temprature
RE: Backfill in sub-zero Temprature
I am a retired analytical chemist by profession. so fairly new to geotechnical industry I ask a lot of questions. During my training I was taught about 8 inch lifts before rolling. I was also trained on modified proctor curves and watched a density/moisture determination by a nuclear device. the grain size distribution of soil show 95% fine to course sand. the maximum dry density is 112 pcf and opt. moisture at 10.4%
I inspected the lifting of the pad up to subgrade level, footing subgrade and now the backfilling. Pouring of concrete for the perimeter footing was done within hours of the excavation. then blankets were placed were placed on the poured concrete. Now that the masons are done with the foundation walls, 2 days were scheduled for the backfilling. Our firm recommended no less than 95% of modified dry density for the compaction above the footing founding levels. The depth of backfill will be about 2 feet to subgrade.
the problem I have is, it is impossible to fit the jumping jack to the side of the trench to do lifts. And the trench walls have also frosted. I asked the soil engineers what to do. They allowed to free fall the backfill up to subgrade then have the jumping jack pass through 7 times or until the subgrade passes the test. to have them scrape the horizontal frost out and then backfill by lifts would have taken them more than 2 days work. but was that right thing to do?
I wonder! I am fully aware that my job is to inspect. Not to tell the concrete contractors how to do their job. So No, I did not try to be a hero. But I did tell the workers to remove big junks of frost from the backfill soil. and they did not object.