Frost depth consideration in existing rock layer
Frost depth consideration in existing rock layer
(OP)
If I propose to install a strip footing of reinf. concrete
for live and dead loads such that design allowable soil
bearing capacity is 8000 psf ...do i have to be concerned about the fact that the existing rock layer is
above the frost line?
for live and dead loads such that design allowable soil
bearing capacity is 8000 psf ...do i have to be concerned about the fact that the existing rock layer is
above the frost line?





RE: Frost depth consideration in existing rock layer
Yes. But what do you mean by rock? Gravel with fines, or sandy gravels with some silt and clay?
There are many factors which will impact frost heave, such as load, soil type, and water table to mention a few. To just put a footing on gravel above the frost line and say should it be OK is not good enough. You will need to complete a soil assessment by a local geo who should then assess site conditions and whether a shallow footing will perform satisfactory for the entended use of the building.
By the way, 8000 psf under curtain soil conditions is not satisfactory to prevent frost action from lifting the building.
Regards
RE: Frost depth consideration in existing rock layer
RE: Frost depth consideration in existing rock layer
RE: Frost depth consideration in existing rock layer
In the instance of the case study BigH references, and he can correct me if I'm wrong, to my recollection it was water under the slab freezing and heaving the slab and then "defrosting" to return it. Don't underestimate the destructive forces of ice by itself. Another good example is the loads the ice applies to dams by lateral squeezing. These loads are induced by thermal expansion and can be on the order of 7.4 x 10^4 to 2.6 x 10^5 lbf/ft.
I would suggest if the foundation is not founded below the local frost line to drain and insultate it throughly. But would suggest founding it below the frost line.
Keithe J. Merl
RE: Frost depth consideration in existing rock layer
RE: Frost depth consideration in existing rock layer
RE: Frost depth consideration in existing rock layer
Note that in the posts, there are "structural" reasons for rock to be frost susceptable - layered limestone, layered shales/schist, etc. Doubt in massive granite would be.