# How are the structural properties of intentionally frozen soil determined?

Soil freezes naturally all of the time. It is at this point that dirt work usually stops.

Occasionally soil is intentionally frozen to improve its structural properties. This process has been used notably in Boston's Big Dig and the Fukushima Daiichi nuclear plant.

When soil is intentionally frozen so that tunnels or vertical shafts can be excavated, the structural properties of the frozen soil are important. It would seem logical that the frozen water would help to stiffen the soil, but how are the actual structural properties of the soil determined?

• Do all frozen soils end up with the same properties? i.e. the presence of ice controls the properties.
• Are the frozen soil properties related to the unfrozen soil properties in any way?
• I don't know enough about this to write an actual answer, but I'll hazard an educated guess. Soil fails in shear. When you freeze the water in the soil, the shear strength transitions from the soil shear strength to the shear strength of the ice. It is more complicated, though, because the ice causes the pores to expand, which effects the soil portion of the strength. I believe you would determine these properties the same as in any other soil: SPT testing in the field (not sure what correlations exist for SPT data of frozen soil, though), triaxial testing in the lab. – Rick supports Monica Apr 1 '15 at 17:20
• The freezing would also increase the short term "strength" (actually decrease the applied stress) because you no longer have pore pressure increases. Effective stress is $\sigma'=\sigma-u$, where $u$ is the pore pressure. When pore pressures drop to zero (frozen water), the effective stress (confining stress) increases (well actually, it remains stable when the load is applied instead of dropping), which increases soil shear strength. – Rick supports Monica Apr 1 '15 at 17:24
• @RickTeachey I think that you have captured why it isn't as straight forward as it would initially seem. – hazzey Apr 1 '15 at 18:01

## 2 Answers

I don't believe there is any fundamental difference between intentionally frozen and naturally frozen soils. Other than control of the temperature.

I'm afraid the mechanics of soils (frozen or otherwise) is complex and depends on many factors such as the soil type, water content, grain-size distribution, compaction, etc. The soil properties used for design purposes are generally obtained through a combination of testing and empirically derived relationships. I don't think it would be appropriate for me to attempt to summarize the entire subject here. If you would like to know more details, this MIT OpenCourseware course may be helpful: MIT - Soil Behavior

With regards to your question specifically about frozen soils. Every soil will have different properties when frozen depending on the unfrozen properties as well as the temperature, ice content, porosity, etc. Your intuition that the frozen soil will be stiffer than the unfrozen soil is correct. Frozen soil can be 10-100 times stiffer than the unfrozen soil 1.

This book deals entirely with the design of frozen soils, including a chapter on Construction Ground Freezing, which was what you were inquiring about:

• Frozen Ground Engineering (Second edition). Orlando B. Andersland and Branko Ladanyi. Chichester: John Wiley & Sons. 2004.

1 Yershov, E. D.. General Geocryology. Ed. Peter J. Williams. 1st ed. Cambridge: Cambridge University Press, 1998.

• While it's undoubted that frozen soil is stiffer when it remains frozen, I wonder if that still hold true when preshear stress plains exists within the frozen ground (kind of like the equivalent of a slickenslide). Would be interesting to know - I just thought of that while reading your response, and I don't really know enough about that subject tbh. – Isa Jun 19 '20 at 16:27
1. take core sample
2. stick in freezer
3. run tests
4. ?
5. profit!

One important factor is the water content of the soil, freezing dry sand won't do much compared to freezing a waterlogged riverbank.

• I'm not the downvoter, but I can see their reasoning. This is more of a comment than an answer to the question. – Chris Mueller Mar 18 '15 at 14:37
• How is it not an answer? The question is 'how are the properties of frozen soil determined', the answer is 'do tests on the frozen soil' (or the soil, frozen). – achrn Mar 19 '15 at 14:33
• At the very least, properties of the soil will surely be different freezing it after a sample is taken than before - water will migrate as it freezes, there will be frost heaves, etc. There's also a whole array of various geotech tests, some of which I'm sure are valid with this methodology, and some of which will not be. – Ethan48 Mar 19 '15 at 19:22
• @acrhn also not a downvoter, but I would suggest that ratchet freak at the very least give examples of what tests would be run. – Rick supports Monica Apr 1 '15 at 17:30