huangyhg

non-expansive frost free fill and its depth question
the foundation footing is requested a minimum of 6" below frostline unless it is placed on non-expansive frost free fill. i have 2 questions here:
1. say, the frostline is 60" below grade, the footing is requested to be flush with the grade by the client, so i have to put non-expansive frost free fill beneath the footing. my question is how deep should i put those non-expansive frost free fill below grade?, do i need to put 66" depth non-expansive frost free fill beneath the footing?
2. what kind of soil or material can i specify as non-expansive frost free fill?
thank you very much for your help.
1) yes
2) you want to minimize the amount of material passing a no. 200 sieve since this is the material that draws water thru capilliary action into it and then it freezes and heaves. in simple terms this means no silt or clay. in practical terms this is generally less than 6% passing a no. 200 sieve although a few percent more will probably not make any difference.
3) please note that water is an important ingredient in whether the foundation will heave. i suggest that you be careful not to create a bathtub with your excavation because water will accumulate and heave whether you have non-frost susceptible material or not. if this is the case, then it should be drained.
1. i do not believe the depth of frost is known in any location with great precision. in my area, central alberta, frost can go as deep as ten feet under a cleared road. in the middle of an open field, it may extend to a depth of six to eight feet depending on the depth of snow cover (snow acts as an insulator). adjacent to a heated building, frost is thought to be about five feet deep but this is an approximation, not to be taken too literally.
2. coarse sands or gravels are considered to be non frost acting fill because the voids are too large for water to migrate upward due to capillary action.

best regards,
ba

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