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Why don’t the water mains underground freeze?

Did the recent cold temperatures have you wondering why the water mains in the ground didn’t freeze? Well, you are in luck. We asked Gabe Sasser our Water Quality Specialist and Clemson Alum (Go Tigers!) that exact question, and here is what he said:

The North Carolina Department of Environmental Quality (NCDEQ) requires water mains to be buried below the frost line or at a depth providing at least 30 inches of cover, whichever is greater.

The frost line is relative to the climatic conditions of a geographic region and is the depth to which the groundwater in soil is expected to freeze. So the frost line in northern states would be deeper than here in the south. For example, the New England region has a frost line of 48 inches (that’s a lot of digging)! The frost line in Mecklenburg County is approximately 12 inches (significantly less digging).  Soil depths below the frost line are relatively constant in temperature.  Positioning mains below the frost line provides thermal insulation to the water line that helps prevent water from freezing within.

During the winter months when air temperatures in the Charlotte area may dip below freezing, source and treated water temperatures trend consistently higher by comparison.  In addition, a substantial amount of kinetic energy is generated from the flow of water through underground pipelines as a result of pumping and customer usage which also aids in preventing water from freezing within the distribution system.  So basically the more the water moves the less chance it has to freeze.

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