Frost Depth Deeper Than Normal This Winter

I was in my home town of Osseo this weekend to do some computer work for my mother. As I drove through town, I noticed many signs of water system maintenance (see Figure 1). This is a sign that the winter this year was a hard one − the soil froze so deep that the water pipes froze and burst.

Figure 1: Osseo Road Work.

In my region, state regulations require that we bury water pipes 48 inches deep. Since the frost depth in the southern counties of Minnesota is stated as 42 inches, I have never seen problems like this before. However, this year the frost depth went down to 5 feet, hence the problems with pipes freezing in Osseo. The northern counties of Minnesota do have a 5 foot frost depth specification.

The Osseo water pipes that froze were all in the streets. We had a massive amount of snow this year, which insulated the pipes that run under the lawns and into homes. However, the streets were kept clear of snow and there was no insulating layer of snow on the streets to prevent the frost from going very deep.

Figure 2: Wikipedia Picture of an ONT.

I frequently have to deal with issues related to frost depth and fiber optic deployments. In clay soils, I see as much as 9 inches of frost heave. I have seen this frost movement tear the fiber out of the Optical Network Terminal (ONT).

Frost heave can damage much more substantial structures than an ONT. I had a conversation with a building inspector who said that he had seen Sonotube foundation piers split in half by frost heave. Figure 3 shows my sketch of the scenario he described. The frozen soil grabs the upper section of the pier and pulls it up, while the footing stays in place. This tears the pier in two.

Figure 3: Illustration of How A Pier is Broken By Frost Heave.

I have included an old US government frost depth (or line) graph in Figure 4 so that you can see the frost depth in your area.

Figure 4: Old Weather Bureau Contour Map of Frost Depth.