Frost depth consideration in existing rock layer 1

[ziontucky]

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?

 

[cdh61]

ziontucky,

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

 

[BigH]

In my view - a subjective yes. Not all rocks will exhibit heave if above the frost line - e.g., granite, rhyolite. But many will - e.g., shale, schist. Much depends on the nature of the rock. I gave a Case History in another thread, but one of the first jobs I had was a warehouse floor that was poured in the winter on sound Queenston Shale. It heaved 50mm (unheated) during the winter. After thaw, it went back down and since the building was heated thereafter, it was not a worry, but . . . You could use geofoam insulating layer. In this post I am assuming that you understand the difference in rock vs rocky soil.

 

[connect2]

The Geotechnical Engineers who provided you with the allowable bearing capacity of the 'rock' will be able to tell you if the rock and hence your foundation/structure needs frost protection. Depends on the type of rock. Many rock surfaces are fractured, layered as a result of frost action and do and will be susceptable to frost action unless removed or protected. Metamorphic vs. Sedimentary (but i suspect it's far more complicated than that). We're not Geotechnical types so we always look to them and defer to them for advice and direction in the design of our foundations. We recently completed a church founded on a sandstone bedrock. In the original design frost protection of the foundation was required. Subsequent inspection at the time of foundation construction ... the Geotechnical Engineers removed this requirement. While it surprised us and we requested clarification it was not our reponsibility nor did we have time to try to become the experts that they all ready were.

 

[kmerl]

In addition to the actual reaction of the rock is if water is allowed to pond below the foundation within the frost zone there is a possibility of direct frost heave casued by water expandion during freezing. Ice is very destructive and applies stresses alone.

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

 

[BigH]

Slab wasn't under the watertable per se, but certainly being a bit fractured in the upper zone, capillary rise provided "water" to the upper parts of the shale - causing ice lensing.

 

[Rowland33]

What sort of rock has a bearing capacity of 8000psf? Leonards gives presumptive bearing capacities for "Soft Rock, Sedimentary Layered rock (hard shale, Sandstone and Siltstone)20,000-30,000 psf. That being said, the soft rocks described are subject to frost heave if located above the frost line. I had experience some time ago with layered limestone frost heaving.

 

[BigH]

One, presumably, specified by a very conservative type - or a highly weathered rock (e.g., residual soil) that is basically soil.

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.