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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />with >3% by weight of grains finer than 0.02 millimeter in diameter (silts, silty sands, and clays) <br />form frost lenses more easily and have a very high susceptibility to frost heaves. <br />4.1.3.11.5. Moisture content: Soil moisture effects the initial freezing of soil because of the <br />increased heat capacity and thermal conductivity of the soil surface. The initial freezing point of <br />soil is usually delayed with increasing amounts of soil moisture. As winter progresses, the soils <br />that have started with greater amounts of water filling pore spaces experience greater overall <br />frost depths due to increased thermal conductivity since air is a less efficient conductor of heat <br />than water. Water tables within 10 feet of the surface are a contributing factor for frost heaves. <br />4.1.3.11.6. Snow cover: The insulating effect of snow cover is a key factor in slowing the <br />penetration of frost into the soil. Each foot of undisturbed snow cover typically reduces the depth <br />of soil freezing by an equal amount. Snow cover is a function of the amount of snowfall received <br />at a location, along with the type of surface material at that location. Darker colored surfaces also <br />tend to help accelerate snow melting and help remove the insulating effect of snow (see albedo). <br />Snow removal on paved surfaces helps to push frost deeper by not allowing insulating snow cover <br />to accumulate. <br />4.1.3.11.7. Vegetative cover: Like snow, vegetation acts as an insulator to slow frost penetration <br />into the soil. Loose grasses or leaves can form insulating air pockets that reduce the depth that <br />frost can penetrate. <br />4.1.3.11.8. Geographic location: In general, in Minnesota the average initial soil frost date is <br />earlier with higher latitudes and more westerly longitudes. More northerly latitudes have longer <br />overall frost seasons on average. In Minnesota the change in average freezing date is about 3.3 <br />days per degree of latitude. <br />4.1.3.11.9. Infrastructure condition. In general, older buried infrastructure such as service lines, <br />pipes and conduits are in a more deteriorated condition than newer infrastructure and are more <br />susceptible to damage from deep frost. <br />4.1.3.12. Prevention <br />Unknown, pending conclusion of the Hennepin County Emergency Management assessment in 2020. <br />4.1.3.13. Mitigation <br />4.1.3.13.1. Frozen water lines. Water lines can be protected against deep frost by ensuring they <br />are buried to the correct depth. Lines which are already installed can resist freezing by ensuring a <br />constant flow of a small amount of water (pencil -diameter stream from a faucet) flowing in from <br />the service line. Typically, water utilities will request that customers maintain running water at <br />addresses that have had freezing problems in the past. <br />4.1.3.13.2. Buildings, roads, and infrastructure. When it occurs, typical vertical ground <br />movement due to frost heaves and melt collapse is between 4 to 8 inches. Extreme movement <br />can be up to 24 inches. These ground movements are enough to cause significant damage to <br />human -made structures. Various mitigation measures can protect structures against frost heave <br />and melt collapse. Buildings which are heated rarely experience frost heave problems because of <br />27 <br />