Subexcavation in frozen soil near existing foundations 1

[dBasement]

Background:

I am an independent third-party consultant being asked to supervise an excavation and fill operation in frozen soils.

Scenario:

Over-wet (significantly above optimum), clay-rich soils were used as backfill around a number of exposed foundations on an oilfield site in a northern region where frost penetration is typically 5 or 6 feet. Fill was placed around the foundations last winter in a likely frozen state and significant problems were noted after the backfill thawed in spring (not surprising). The backfill has been rejected by the owner and now the contractor is faced with having to remove this frozen muck in winter (there are time-related penalties in effect). After removal, the contractor will be placing and compacting thawed material.

Concerns:

I expect the foundations will be frozen to this muck and my concern is related to the excavation contractor causing damage to the foundations by using a ripper close to the foundations.

I am also concerned about frost penetration in exposed cuts.

Questions:

Are my concerns valid and, if so, should we ensure all frost is thawed out of the ground with heating blankets before excavation proceeds?

Does anyone have any additional tips on what to watch for?

 

[civilman72]

Realistically you will have to thaw material and work in small areas along the foundation wall. Trying to thaw all the material at once will not be efficient and will probably prove to be very expensive.

You also want to protect the foundation from the frost, so exposing as litte foundation at a time as possible would also be prudent.

I have seen contractors successfully thaw and re-compact frozen material, but it's usually a granular material. I can see issues with trying to thaw out wet clay - getting out the clumps (of ice) and monitoring/controlling water content when re-compacting would seem like some major challenges to consider.

Sorry, can't help you with the material sticking to the foundation and whether this is usually an issue??

 

[oldestguy]

Your concerns are very valid.

Thawing stuff like that with steam injections has been done IF REALLY MANDATORY. This sounds like an unnecessary construction make-work operation. However, is there any way to remove the mud from the site and not re-use it? Granular fill, not frozen, would be the best replacement. If frozen, it won't compact. If frozen, you won't know that unless you use a thermometer.

I trust your contract has excellent release clauses in case you are accused of doing some action that is related to damage, etc. If it were me, I'd let some other engineer take this one.

 

[dBasement]

The mud was a poor choice from the start. I am coming in after the site work was done and there is the possibility that the contractor or subcontractor received bad advice from the project geotech.

It is unnecessary, except the contract stipulates a significant penalty clause for late delivery. Hence the reasoning.

As to liability, I am definitely approaching this with my eyes wide open. I am having my lawyer review my contract. I'm not an engineer and my contract is just to provide technical support and supervision. The method used and the liability is in the hands of the contractor and sub.

Thanks for the suggestions.

 

[darthsoilsguy2]

seriously doubt the geotech there before said "dump mud over the footings" or "dump loose fill in gigantic lifts over the footings". i don't see what they would have to gain from it. the contractual involvement of a testing company doesn't mean things are good. ultimately, if the contractor doesn't schedule to have the soil tested as it's backfilled, who can say what's there at all. But who knows... more speculation anyway.

On another note, i know a contractor who would use thermal blankets which ran hot water through them which he heated. Worked pretty well to pour slabs on previously frozen subgrades in Vermont. for a deep freeze, it might need to be done in portions.

 

[Dozerman56]

I'd strongly suggest you take a good look at the project specifications as well.

 

[dBasement]

We have begun to remove the material from two of the pad areas. It actually comes out relatively well without thawing. There is about 2 feet of frost and once the crust gets broken, the 200 excavator has an edge he can chip away at. The fill that was used previously was silty (reworked glacial till), but inorganic. Obviously very poor compaction. The subcontractor has rented 2 ground thaw units that generate heat through long rubber hoses that are laid out on the frozen ground. Diesel heaters send warm liquid (probably anti-freeze) through the hoses and keep things warm under tarps. The rental on each is about $400 a day and about $350 in diesel fuel.

Air temperatures are a fairly constant -12. We begin filling and compaction today. We have opened up about 300m3 to fill.