Compacting Frozen Soil 1

[claytill]

Background: I am overseeing a project in which we have encountered freezing air temperatures and recommended to our client that the imported fill must not contain frozen soil. The client decided to continue to immport frozen material and instructed us to continue with the 95 % compaction spec - my final report will qualify this. The future land use is unknown at this time and likely to be just landscaped. However, this can change as we all know.

Question: We plan to continue placing the frozen material and compacting to 95%, but I am still concerned with settlement issue. Does anyone know of any literature/paper that discusses these issues?? Any comments welcome.

 

[GeoPaveTraffic]

Every soils book I've ever seen addresses this issue as DON'T PLACE FROZEN SOIL. How much clearer can you get. If the owner continues to place frozen soil, I would send him a fax/letter stating that the fill is not an engineered fill, is an uncontrolled fill, and that testing has no meaning. I would also stop taking density tests and issuing test reports.

Unless the owner changes their mind, removes the frozen material, and stops placing additional frozen material, this can only end badly for you. I would attempt to minimize the exposure and get way from the project.

 

[claytill]

Thanks for your opinion, which parallels mine - the letter was send to the client immediately after they informed of their plan and they have been informed that this is not acceptable. The client is bound to backfilling this area by a contract and I have informed them that there is a possibility tthat they may have to remove and re-engineer this material.

From a technical standpoint, I am still looking for some technical literature to back this up (from a settlement standpoint).

 

[eric1037]

It is in the International Building Code. See 1805.2.1.

If they ever plan on putting a structure there, you shouldn't leave frozen soil in place.

 

[dgillette]

Can you even accomplish the 95% spec??? If you measure 95%, you are probably seeing some very dense material interspersed with icy material with low density. Once the ice thaws, settlement is likely.

Let me add one more voice to the chorus of naysayers. Get the client to send you a letter acknowledging that he is proceeding against your recommendations. In embankment dam construction specs, placing frozen material is strictly and explicitly forbidden, and any material that gets frozen after already being placed is to be removed, often at the contractor's expense. (Contractor's responsibility to keep fill covered.) Public floggings are generally not authorized, however.

 

[TDAA]

A portion of the settlement you will see will be partly from the expansion of the water in the solid form (about 9% from the published value I saw). Add that to the poor mixing and compaction of the frozen particles.

 

[DRC1]

I would sed the owner a letter saying that you are proceeding at his direction to fill with frozen material, and that as lng as it has no intended use, thats okay. However, if he dicides to locate a structure there in the future, so remedial work may be necessary. Since the space is not going to be used for anything and the material will probably settle upon thawing, I would question he need for 95% compaction. Relaxing the spec will at least get the operation over a little quicker.
Depending on the client's situation, it may be necessary to complete the site work to either get his CO or meet his obligations to a tennenat. Therefore, he may feel it is better to get it done and redo it later if the need arises.

 

[boffintech]

GeoPaveTraffic is correct. Stop taking density tests and stop issuing test reports. At a later date the client is going have selective amnesia and insist that the fill was correctly compacted and as evidence he is going to cite your test reports that show 95%.

 

[claytill]

I agree with you boffintech - I would love to relax the spec and stop testing, but the client insists we continue to test. And again, you are right that the client will likey have selective amnesia and say "well we achieved 95% on all the fill...it should fine right?". I have recommended settlement monitoring as well.

Thanks for all your input - much appreciated.

 

[fattdad]

I'm just seeing a technician with a nuke gauge and it's not a pretty sight. . . . . . I'm not sure a nuke gauge gives reliable moisture content data on ice, but then again, I'm not sure what the knowledge base is for this.

f-d

¡papá gordo ain’t no madre flaca!

 

[TDAA]

The gauge should read fine. It is the same number of hydrogen atoms, right? Interpreting the results to show “passing tests” is the real debate.

If you continue to test, because your client tells you to, make sure your certification letter describes the situation and states something like: Although the density testing indicates the fill was compacted at 95%, the material was frozen and did not meet the intent of the specifications. Thawing of the material may lead to settlement and/or destabilization of the fill.

Maybe add in some lines about removal for structures or pavements, depending on how conservative you want to be. Also, if you seal each page of testing, and not just a cover letter, be sure the disclaimer appears where applicable.

 

[VAD]

Construction in freezing weather is not unusual in some parts of the world. Air temperatures and ground temperatures are different. What temperatures are you working with and values. There are methods of working fill that would allow achieving close to 95 % compaction. Ground temperatures and material types are factors as well.

From my personal experience with construction in freezing weather some 24 years ago along with studies by others, the following is relevant. Below an air temperature of -2.2C tremendous effort is required to achieve 95 % compaction. Below - 9.4 C compaction to 95% is not feasible. Desired compaction can be achieved from +1C and above. There can be a differece between ground and air temperatures varying by as much as 14 C points. An air temperature of -20 C would translate in a ground temperature of -6 C. The material was a CL-ML soil class with 14 12-14 % OMC Std Proctor and Dry densities varying from 1863 - 1920 kg/m3. field moisture ranged betwwn 18 and 22 %. Field density tests done by Volutester. Stopped when holes became harder to dig.

For the project some 24 years ago, winging off the top 300 mm and recompacting the fill the following spring allowed a good roadbed to be achieved. Of 47 tests done during the freezing period 14 of these achieved 95 % compaction. Air temperatures were as low as - 22 C. Needless to say the Contractor did not like the removal and recompaction of this depth of material. However, he had no choice as he was pinned down as soon as he attempted to continue work as temperatures declined. He has never forgotten this to today.

As said before by others, you need to write some letters and have the backing of the client. However your Client liks the idea, perhaps he is not the owner, I would think.

Hope this helps.

 

[looksatstars]

On frozen soil, the densometer will show no moisture. Thats how i always know if the soil was frozen or not.

Claytill, if the owner is hiring you to do compaction testing on frozen ground, then you will have to complete the tests but i would make it clear on the report (like I am sure you are doing) that the material is frozen.

if you are getting 95%, the soil will hold up quite a bit. How deep is the area you are placing the fill?

 

[unsrtn]

As VAD points out, air temps and ground temps dictate whether compaction is feasible. As well, soil type and moisture content will influence feasibility of compaction, as well as spring time settlement. If the fill is very thick and winter is long and cold where you're at, it could take months to years for the frozen soils to thaw. I have seen clay fills of 4 m thick placed over frozen peat in northern B.C. and the peat is still frozen 2 years later when drilling for geotech investigation.

Hoarding and heating of reasonably sized sites is an option worth considering.

If fill is a crushed sand and gravel with moistures at or below Optimum, settlement won't be a big deal upon thawing, and re-excavation and recompaction in critical areas should be undertaken. Clays, silts with higher moistures can settle big time, as you are just placing frozen lumps. Settlements in wet clays and silts could easily reach 30 % of fill thickness.

In spring time, you could dig test pits with 'undisturbed' bases and run some density tests. I would run multiple test pits and multiple test types (take samples for Procters from the tested soils). Multiple test types include nuke, sand cone, balloon. Fill your boots.

Definitely write a letter emphasizing the problem of placing frozen soil and disclaiming that a 'successful' nuke test does not demonstrate that a quality product has been constructed.

Have fun, been there in minus 25 Celsius with a contractor trying to subexcavate clay and replace with too-wet gravel for a bulk fuel tank farm. You can guess my comments when the warmed soil was placed and packed in 10 minutes but already frozen solid to the base of the lift. The moisture results on the nuke were wonky. Only slight settlement of the tanks was observed. Fill thickness was about 6 feet.