Friday, August 3, 2012

Frost Heave

Frost heave is a process during freezing that causes ice lenses to be formed. Water is drawn to the freezing front. This process required three conditions; a slowly advancing freezing front, a soil that is permeable enough, yet has capillary rise, and a water table within the capillary rise. It is even worse it freezing occurs below the water table. This is all extra to the simple phase change volume increase.

Simple phase change can cause volume changes of 4 percent of the freezing depth; however, 2 percent is more typical. Frost heaves on the other hand can much large, often 150 mms to 300 mms on rural roads, and similar earth structures. Note that the frost penetration is much less in ice lens as the energy loss required to freeze water is greater. Normal soil has no laden heat, and its thermal capacity is only 10 to 15 percent of water, but the soil density is 1.5 to 2.2 time that of water, so a unit volume of soil has typically 15 to 20 percent thermal capacity plus that of contained water. For this reason, frost penetration is greater in dry soil, and typically greater in sand than clay due to the water retention properties of clays.

Silt sized material, with fine sand and small amount of clays is about the worst material for frost heave. This is usually delta sorted material or river settlements. These frequently occur in low land where water is also found. Road fills also reduce the likelihood of frost penetration, which is one thing that can help. Insulation under the pavement structure can also help.

Frost heaves, in the City's, where no frost heave has previously occurred are often indicators of water leaks, as these provide a source of water.  When opening these to expore the cause in clay tills, frequently, a water bearing crevasse infilling sand seam in east Edmonton / Sherwood Park area.  

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