Frost Penetration for Earth Grillages

[ljj2113]

Hi, I'm new to the forum. I hope to be part of this resourceful and helpful community.
I need some guidance on the topic of frost heaving. The following is the background:
Erecting a lattice type tower on an earth grillage foundation
- The earth grillage foundation consists of a base 2mx3m for a the base and a mast to 3.7m high
- There is instruction for the grillage to be buried below frost depth @ 3.75m. The nominal frost depth in the area is given to be 3.5m form grade.
- By burying the grillage below grade, 0.15m of the mast sticks out above finished grade and it is onto this that the assembled lattice tower is erected.
The above is what is given to the contractor from the engineer's office. However, on arriving on site, they face the challenge illustrated in the attachment.

I am particularly concerned with Case#2. I do not understand frost heaving enough to be certain that by placing the grillage above nominal frost depth but ensuring that more material is added on top there will be no issues.

Please share your thoughts.

 

[oldestguy]

Are you sure about the building code requiring such a very deep "frost depth"? Are you in a perma-frost area? Frost heave won't occur in clean granular soils. What is the soil type? Is bedrock close? More info needed.

 

[oldestguy]

Movement of energy (heat) takes the least resistance path, so in your case that "frost depth" is below final grade, not necessarily vertical. Also, as heat leaves the ground, that frozen front stalls or slows where water is present, since heat of fusion is released as that soil moisture freezes. In clay ground, the usual higher presence of moisture results in a lesser depth of freezing as compared to clean granular soil. Take the extreme where high moisture is present, depth of freezing is the least, all other conditions equal. Where is that? A lake.

 

[ljj2113]

Thank you greatly for the response.
This is in Northern Canada, Alberta. The Design Engineer had given the specification of 3.5m frost depth based on prior geo-technical evaluations. This is not a lake.
- The material used for backfill is typically sandy till that is native to the excavation or imported from nearby. It has a lot of cobbles and trace silt.
- I understand that frost penetration could occur from the side of the mounded material. If we had to go in the direction of raising the founding level of the grillage and mound material on top such that the mast is fully covered, what would it take to establish a distance from centre of grillage to edge of mound to ensure that frost penetration form the side is prevented. Is there a paper or any study related to this?
Please see attachment.

 

[oldestguy]

I couldn't open the attachment. If you have to follow the code there as to "depth of frost", then that is from the edge of your grillage (footing)out to that surface, in the shortest path, possibly inclined from vertical..

Consideration might be given to reduce the overall depth of the foundation by interposing a sheet of closed cell insulation between that grillage and the nearest surface. Under those circumstances the distance from the grillage edge (not its center) to that surface is a line following around the edge of that insulation and then up to that surface (your "depth of frost" dimension). Exactly how thick a layer of insulation and what depth reduction you can use, will depend on experience, or some form of heat loss calculation (which I think the Canadian highway departments may do from time to time for their projects).

In Wisconsin, I use this system regularly by placing the sheet (closed dell insulation) one foot under ground, around the perimeter of a building with shallow foundations. With a sheet four feet wide, and with a code required "frost depth" of four feet, that foundation is protected from heave. In this case the "frost depth" dimension is horizontal 4 feet under the sheet, plus one foot up to the surface at the edge of the sheet (5 feet of protection). We also place that insulation on the wall surface above that sheet.

In your case, you would surround the vertical section of the foundation with that same material. Concrete does conduct heat, as does earth.

One further protection that might be considered in making use of the heat of fusion in fine grained soil. If it is granular with a "trace" of silt, there is not much moisture there to give off that heat. Keeping boulders out will help, but instead using a fill high in clay content would be best. The more natural moisture in that fill, the shallow the depth of freezing.

Promoting growth of grass and snow on the surface adds some degree of insulation also.