Pad Foundation on Rock

[ONENGINEER]

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.

 

[MiketheEngineer]

What about frost heave.

Also - if you are supporting a 60' tower - it would have to be a very big pad to prevent overturning!!??

 

[ONENGINEER]

MiketheEngineer
Can you explain what you mean by Frost-Heave. May be something that I am missing. Thank you.

 

[molerat2210]

Frost Heave = expansion of soil due to frozen water in the pores. Just as important is loss of soil strength during the thaw. Not an issue for rock. However, local building codes likely dictate foundation embedment below frost zone.

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?

 

[ONENGINEER]

Molerat2210

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?

 

[BigH]

If you think it important to put this below frost depth, then put 24 inches of soil above the foundation. I would think that you will have uplift problems that must be taken into account - anchoring like was said would be one way - another that was recently used around here (South Sulawesi) is to place a ballast above the foundation to provide for the uplift and potential sliding problems. The foundation would have to be sufficiently wide to ensure that the weight of the material placed above would counter the uplift forces.

 

[molerat2210]

ONENGINER:

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.

 

[ONENGINEER]

BigH

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.

 

[fattdad]

or you can post-tension some rock anchors to hunker the concrete footing onto the rock (and resist overturning).

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!

 

[BigH]

In my view, the concept of foundations and frost depth where it is "taken" that foundations MUST go below is a codal issue that has woven its way into the books without justification. If I have a well graded clean sand and gravel and the frost level is 3 ft, why is it necessary to put a footing below frost depth? The sand and gravel is not frost-susceptible so it is a non-issue. Yet I am sure 95% of the geotechs and civil designers will do it anyway.

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://www.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-26.html).

Massive clay soils of low permeability are not necessarily "frost susceptible" in their natural state because water would not be able to move quick enough during the frost months to reach the footing to cause lensing.

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 . . .

 

[ONENGINEER]

Thank you again BigH. If a layer of soil is going on top of bedrock, practically the bond between the soil and rock seems risky especially in a seismic prone area. Is my concern right? Thank you.

 

[dgillette]

How about excavating only as far as you need to get decent rock for bearing, then place a blanket of insulating soil over the area? That gives you more surface elevation to work with for getting drainage away from the pad. The link to the picture didn't work, so I don't know what your tower actually looks like (so can't tell whether it could be built that way).

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.

 

[fattdad]

Just for the record, I completly agree with BigH.

f-d

¡papá gordo ain’t no madre flaca!

 

[ONENGINEER]

I have uploaded a rock mass picture. Hope it can be downloaded.

Hope I am right to understand that the intermediate soil level is just to facilitate site drainage around the foundation.