Frost protection for foundations on bedrock

[legner]

Do spread footing foundations which bear directly on bedrock (unweathered) need to be extended down to frost depth? Specifically, we have a case where bedrock is, say 24" below grade, and minimum frost depth of 42" below grade. Is it necessary to excavate the rock to achieve the 42"?

 

[TimC]

The criteria for for setting spread footings below the frost depth relates principally to development of ice lenses within frost succeptible soils. Three conditions are necessary for the formation of significant lenses: A ready supply of moisture; a soil with a relatively high capillary tension capacity (silt) and; a static or slowly advancing freezing front. In the case of a foundation on competent bedrock, logic suggests that there is little potential to develop ice lenses at the interface between the concrete and the rock. If there is some concern on the part of the local building inspector, consider using rock anchors to tie into your new concrete.

 

[legner]

Thanks for your thoughts, TimC. About the rock anchors, they might make the building official feel better, but I'm wondering about their real ability to restrain frost. It's hard to come by a numerical value for frost heave, but Joseph Bowles gives a value of around 40,000 psf. That would be difficult to resist.<br><br>However, it seems logical to me that frost is not an issue in this case.

 

[TimC]

The comment about using rock anchors was meant to be a means of placating those who might demand that you 'go by the book'. There would effectively be no difference between the rock anchors and dowels at the cold joint between the footer and the poured wall (or pier). Thus, if normal dowels are ok then the rock anchors should be ok too.

 

It is not uncommon for us in northeastern Ohio to leave footings above conventional frost depth if the rock is sound and massive.&nbsp;&nbsp;Shales and other laminar rock may not be ok where sound sandstone most likely is.&nbsp;&nbsp;Positive drainage is always a feel good addition to help assure no readily available water.&nbsp;&nbsp;Rock type and condition are important factors.

 

[jayw]

Mdrotar is exactly correct. It is necessary to specifically evaluate the actual nature of the rock formation at those locations where it will support the shallow foundations. It is possible to have a relatively porous rock condition which might permit the penetration of water into the foundation zone, at a shallow depth that would be impacted by the penetration of the freeze line. In that case, you could develop frost heave. Unfortunately, it may not be easy to asess this potential.&nbsp;&nbsp;