Finding elusive frost line is important to many projects
Knowing in advance where the "frost line" is for your area can save time when placing water lines, sewer lines, foundations and footings and to some extent, electrical ground rod length.
But the location of the "frost line" can vary, according to Ed Browning, natural resource engineering specialist, University Outreach and Extension.
"There is an imaginary line above which the soil may freeze due to cold temperatures and below which heat from the earth will keep it warm to prevent freezing. We call that the frost line," Browning said.
Technically, the frost line location will vary by types of soils and their heat transferring properties, which are influenced by tillage management, ground cover, slope, local temperatures and other characteristics.
"Water and sewer line location is critical because we don't want them to freeze and burst. The placing of footings and foundation walls is important because the ground heaves from freezing and thawing and could literally heave a structure out of the ground," says Browning.
According to the Rural Electrification Administration, earth resistively increases 300 to 400 percent as the earth freezes so the resistance to electrical energy flow from a fault through a ground rod is increased when the earth is frozen. That means the system isn't grounded as well as in unfrozen earth.
Browning says a local city engineer says the frost line in Jasper County is located at about 18 to 24 inches. In central Missouri, the frost line is considered to be 30 to 36 inches. But, central and part of southwest Missouri are in the same USDA growing zone.
A University of Missouri climatologist has reported that soil temperature records for sod kept at Columbia at depths of four, eight, 20 and 40 inches since 1969 show the frost line has never reached the 20-inch depth during this time.
The coldest temperature at the 20-inch depth was 35 degrees Fahrenheit in 1996, and the coldest 40-inch temperature was 42 degrees in 1978.
By comparison, the coldest eight-inch temperature was 28 degrees in 1982 and 1996.
According to Browning, a local Natural Resource Conservation Service soil scientist has been recording soil temperatures at six, 12 and 20 inches northeast of Carthage since 1996. That data shows the 20 inch depth has never reached freezing and the 6 inch depth has only reached freezing a couple of times.
"You might want to guard against that one time in 100 years, or some such frequency, that the frost line drops, so to be safe, water lines should be a minimum of 24 inches and preferably 30 inches in southwest Missouri," Browning said.