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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />41�Hazard Assessment: SOIL FROST <br />4.1.3.1. Definition. <br />Soil frost is caused when water, which is present as a <br />component of soil, freezes into pore ice. The depth to which <br />this freezing penetrates is called the deep frost. Some soils <br />are vulnerable to frost heaving, which is the vertical <br />displacement of the surface due to frost expansion or the <br />development of ice lenses. Melt collapse happens when the <br />ice lenses melt. These effects can damage roads and building <br />foundations and other infrastructure. Deep penetration of <br />frost can also have a devastating impact on critical buried <br />infrastructure, such as water and wastewater pipes. In <br />extreme cases, fire hydrants and fire sprinkler water supplies may freeze. Hard impervious frost layers in <br />the soil also can worsen springtime rain and snowmelt flooding by not allowing water to penetrate the <br />soil and increasing run-off. <br />4.1.3.2. Range of magnitude <br />Unknown, pending conclusion of the Hennepin County Emergency Management -sponsored soil frost <br />hazard assessment in 2020. <br />4.1.3.3. Spectrum of Consequences B211b <br />4.1.3.3.1. PRIMARY CONSEQUENCES <br />4.1.3.3.1.1. Water utilities: In Hennepin County, water service lines are typically buried <br />between 78 to 90 inches (198.1 to 228.6 centimeters) deep. This depth is usually <br />protecting these lines against freezing. When particularly deep frost is formed, however, <br />water service lines may freeze, cutting off water services to residences, businesses, and <br />government facilities. Bottled water delivery is often the response of choice while <br />awaiting water service restoration. Water service freezing not only stops the flow of <br />potable water to an address, it may also interrupt fire protection systems such as <br />sprinklers or standpipes. Water mains, which are buried deeper than service lines, are less <br />likely to freeze. If they freeze, then fire hydrant services also are interrupted. Thawing <br />frozen water lines is difficult and time consuming. It requires special equipment and <br />experience. Some methods may cause structural fires. In widespread instances of frozen <br />water lines, service may be cut for days to weeks. Without intervention, frozen water <br />service lines in Hennepin County would thaw by May. Service line freezing may be <br />prevented by keeping a pencil -sized flow of cold tap -water always moving through the <br />system. Prevention is usually done at the request of the local water utility. <br />4.1.3.3.1.2. Wastewater services: In general, municipal sewer lines have similar depth <br />requirements as water service lines to prevent frost damage or disruption. Sewer lines <br />typically have fewer freeze problems during deep frost events than water lines, however. <br />Rather than frost causing problems for municipal sewer systems, a bigger issue seems to <br />be impacts to household septic systems. <br />23 <br />