Fiberglass box pad and Frost Heaving – Myth versus Realities

Frost heaving in the spring of the year in the upper Midwest and New England areas is a common occurrence. It can occur at depths of 48”, sometimes lower. When properly installed, a Nordic Fiberglass box pad will not heave to one side due to the phenomenon of frost heaving. The key is transferring the weight of the transformer to the bottom flanges of the box pad, thus preventing soil from being pushed up due to rising frost. Instead, the frost freely moves up inside of the box pad, and around the outside box pad flange perimeters where there is no downward pressure to push against the frost.  In essence, the weight of the transformer is acting to level the box pad below grade to counteract the movement of the soil.

Nordic single-phase box pads generally range in heights of 15” through 32”, with recommended above grade height of 6” to allow for grade changes.  Therefore, one can subtract 6” from the height of a box pad and the difference would be the burial depth.  We also recommend digging the installation hole 6” deeper than the burial depth, and 12” wider than the bottom flange dimensions of the box pad.  The next step is to add 6” of gravel to the bottom of the hole, machine tamp and level it.  This level gravel bed provides the base for the box pad’s flanges to rest on throughout the entire year and through frost cycles as well.  Even though the box pad installations rest above the frost line, using this method will avoid instances of frost heaving.

Please contact Power Sales Group for recommendations on box pad selection and installation!

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