Backfill in sub-zero Temprature 1

[QC73]

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?

 

[beton1]

Cover with straw bales. As for the gravel, it will clump together if frozen. You may have to pick your days/times when you can perform this work.

 

[Dozerman56]

I'd suggest insulated blankets for keeping the surface from freezing. You need to be mining the backfill material as you place it, digging it from deep enough that the frost hasn't penetrated it. If you're getting it from a commercial quarry, you have to get them to work with you to provide non-frozen material. If the material is being transported any significant distance you'll need trucks with heated bodies.

 

[oldestguy]

Which zero? centigrade or Fahrenheit?

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).

 

[QC73]

It's C. And the temprature for this week will be between -5 and -11C.
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).

 

[beton1]

I believe that asphalt haulers have heated boxes.

 

[oldestguy]

Frozen gravel may not look frozen, since it may not be clumped together. However, there is enough frozen ice on particles that compaction will not work. Come spring and you will have settlement or what ever loose stuff does.

A thermometer will tell what you have, even if your eyes think otherwise

 

[DRC1]

The only we have been able to do this in a reasonable manner is to excavate to unfrozen material and backfill right away with clean crushed stone. Generally 3/4 inch works pretty well.

 

[newsoil]

I am an inspector for a backfilling operation right now where the temperature range from 21 F to 32 F during the day. The pile of dirt removed for the excavation of the footings has frosted on the surface but underneath the soil which is mostly sandy is not frozen. Sometimes though there are chunks of frosted soil but I let it go since the jumping jack can break it up. All the replies on this thread are correct and idealistic. But in real life situations I wonder if inspectors like myself, a new one, should require the contractors strict adherence to protocol even if this will cause delay in their operation. After all the compacted soil passes our inspection.

 

[dgillette]

How much frosty material, what does the spec say, is the fill already placed being protected overnight, and what are the consequences of frozen fill thawing and settling?

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.

 

[newsoil]

The backfilling is on both sides of 3 feet deep footings and foundation walls. How much frosty material? About 5%. Fills already in place are not protected overnighted. But contructors fill and compact to subgrade level. No partial backfill.

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.

 

[dgillette]

So do the footings go on the frozen ground? That could be bad news, and I believe it would violate all accepted standards of practice, and building codes, too.

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.

 

[newsoil]

The footing subgrade was not frozen when the concrete was poured.. And it is below the frost line. That is correct, the footing and walls do not bear on the backfill. Eventually , 4 inches of gravel and on top of it 4 inches of concrete will bear on the backfill but the edge of the finished floor will sit on part of the foundation wall. I should have made that clear at the onset of my post.

 

[concretemasonry]

Since the specification is written and a part of the construction documents, you are never wrong if you enforce then to the "T" and can document the conditions are to properties, temperatures and time. Shutting down a project can be a very big and attract a lot of attention from your firm's pricipals, the contractor and the owner, so have to make sure you are right.

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.

 

[DRC1]

Wether or not the sugrade was frozen at the time of pouring the footing or wall is inmaterial. It is also hard to estimate the percent of frozen materials, because small chunks effect the backfill as much as big chunks. In many granular materials it is difficult to get compaction with out water. Once the material thaws foundation movement can result. You should not be in this postion. Most specs say backfill shall not be frozen, nor should backfilling proceed in freezing temperatures. If this needs to be done, your bosses and the contractors should decide on the parameters. Do yourself a favor, don't be a hero.

 

[newsoil]

I asked the project engineer if there was a specification for the backfilling operation. He was not aware of it. I was told that the contractors follow an industry practice.

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.