Pad Foundation on Rock
Pad Foundation on Rock
(OP)
I am designing a pad foundation for a 60 ft tower resting on igneous rock on the top of a hill. If it was soil I would put the foundation beyond the frost depth (about 20 in).
Shall I put the foundation on the external side of the rock? Do I need to excavate the rock to combat with frost depth? If yes how much?
I saw that a nearby foundation on the same rock was underpinned slightly and some of rock-concrete contacts were replaced by degraded rock or soil, which may be due to the frost action.
Shall I put the foundation on the external side of the rock? Do I need to excavate the rock to combat with frost depth? If yes how much?
I saw that a nearby foundation on the same rock was underpinned slightly and some of rock-concrete contacts were replaced by degraded rock or soil, which may be due to the frost action.





RE: Pad Foundation on Rock
Also - if you are supporting a 60' tower - it would have to be a very big pad to prevent overturning!!??
RE: Pad Foundation on Rock
Can you explain what you mean by Frost-Heave. May be something that I am missing. Thank you.
RE: Pad Foundation on Rock
Not sure what you mean by underpinning? Maybe dowels or anchors to resist overturning/sliding?
Frost wedging may have degraded concrete/rock interface in the picture and subequently in-filled with debris. This would not have much effect on bearing capacity of the rock.
Check base for sliding as well as overturning. No embedment means less resistance to sliding unless you are doweling to the rock. What about stability of the rock mass? Near a slope?
RE: Pad Foundation on Rock
Hello. Regarding slope, decreased the bearing capacity from 750 kPa to 250 kPa to account for the footing, if they build it near the crest of a slope. Used a US D o N curve from 1982. Would this be sufficient?
RE: Pad Foundation on Rock
RE: Pad Foundation on Rock
Reducing bearing pressure may improve stability of the rock mass. It is difficult to say without any knowledge of the rock. I would move away from the crest but you may have no choice. I am not a rock mechanics expert but perhaps some investigation of the joint pattern, strike/dip of the rock from outcrops to feel more comfortable with the global stability.
RE: Pad Foundation on Rock
Thank you for your comment which was helpful. However, Is it a standard to apply frost depth on rock foundations? I have seen many foundations on the ground surface over the rock with no frost depth considerations.
RE: Pad Foundation on Rock
I'm not so sure about frost heave on rock, but I'd likely blast away some rock to get sufficent depth anyway.
f-d
¡papá gordo ain't no madre flaca!
RE: Pad Foundation on Rock
For the rock, some rock, say granite and volcanic without joints would not be frost susceptible and in my view would not need, from a technical reason, to extend below frost depth. Shale, on the other hand, in my experience does frost heave when there is a source of water that can be drawn up into the shale, so again, I would be prudent and make the frost correction.
For frost to be critical, one needs (1) a frost susceptible soil (clean sands and gravels are not; (2) in a frost susceptible soil, a source of water at or near the ground surface or which may be drawn up by capillary and heat transfer action so that the effects of ice lensing, etc. can occur; and (3)a freezing plane (i.e., cold weather to freeze the soil/rock. (see: http:
In your case, if you have massive rock (little to no jointing or fracturing) a low water table (say 2B below the footing level) I do not see a need to provide frost protection from a technical point of view - BUT you may run into bureaucrats who insist on it because of the "code" - so this is why I suggested a blanket of soil over the footing rather than blasting rock to lower the foundation - clearly, in my mind, a layer of soil placement is less costly than blasting.
But, these are only my views . . .
RE: Pad Foundation on Rock
RE: Pad Foundation on Rock
The contact between the soil and rock isn't likely to be much weaker than the soil itself.
Big H - I think I would be in the 95% most of the time because a landscaper or somebody will find a way to foul up the surface drainage in a way that it saturates the foundation near the ground surface.
RE: Pad Foundation on Rock
f-d
¡papá gordo ain't no madre flaca!
RE: Pad Foundation on Rock
Hope I am right to understand that the intermediate soil level is just to facilitate site drainage around the foundation.