Bearing on Rock without frost depth 1

[Berniedog]

Need help. I designed a retaining wall that will bear on rock. My plans require that the footings be placed at frost depth. The contractor does not want to excavate 3'-6" of rock to make this happen. Could we dowel into the rock to prevent frost heave? Or should I make them remove the rock?

 

[ACtrafficengr]

To get frost heave, you need three things: water, cold, and frost susceptible soil (silt).

What kind of rock is it? Sound rock is by definition not frost susceptable soil, so traditional frost heave should not really be a problem. On the other hand, shale has habit of degrading into silt. If it's porous at all, maybe you should check with a geotech engineer.

 

[msquared48]

As long as the rock is bedrock and not seriously fractured, you should be ok, especially if any loose rock is scaled off and any cracks pressure grouted.

If this rock is a loner sitting above the frost line, it should be removed.

Mike McCann, PE, SE (WA)

 

[Berniedog]

This wall is in a creek bed. Does it have to be below the flow line of the water?

 

[BigInch]

Yes, and below scour depth, unless you like undercutting the wall's support from fast current flows. Velocities get higher around obstructions.

 

[msquared48]

If you are bearing directly on sound bedrock, scour may not be that much of a problem, except to remove silt from any silted-up fissures in the rock. However, I would be skeptical about being able to see any cracks in the rock below the water level. Many cracks may be silted in.

Does the creek freeze completely in the winter? If it doesn't, then there may be enough latent heat in the water to prevent freezing as long as the water level is maintained. No guarantees though.

Mike McCann, PE, SE (WA)

 

[BigH]

ACtrafficengr made the good point. Some rocks, mainly sedimentary fine grained, might be susceptible to frost heave given that there could be a source of water that can migrate upwards - we had the Queenston shale in southern Ontario heave 50 mm when the contractor did not placed footings and then not keep them insulated. In you are in hard rock - such as the Canadian shield I would think that scour would be more crucial that frost . . . ability of water to be drawn into the zone of frost, sufficiently cold weather and frost susceptibility.