Active and Passive Pressures for Bedrock

[garrettk]

We are working on foundation design recommendations for spread footings that will bear directly on bedrock. Sliding is a concern of the structural engineer and would like recommendations on active and passive pressures if a shear key is installed.

I have lots of references which deal with soil, but none that deal with rock for this problem. Any suggestions on where to look?

 

[Panars]

With rock, I don't think you really have active and passive earth pressures develop. You should be looking at the shear strength of the rock along a wedge in front of the shear key. However, a shear key is not usually required if the concrete footing is cast-in-place against the rock. The friction angle recommended by NAVFAC for this is 35 degrees.

 

[GeoPaveTraffic]

There should be no active pressure from rock. The sliding resistance would be based on the frcition between the concrete and the rock, as given by Panars; however, this value will be limited by the shear strength of the rock OR the shear strength of a weak plane in the rock. The later is more likely to be the case.

 

[dgillette]

Unless the rock is weathered beyond recognition, the resistance from the rock (I wouldn't call it passive pressure.) is governed by jointing and bedding planes. Unless you have adversely oriented joints that daylight out of a slope below the footing, and/or extremely large loads, it wouldn't take much of a key to keep the footings from sliding.

 

[garrettk]

Ok, this at least gets me started down the proper path. Is there a method to use RQD or RMR to develop a shear strength for a rock mass?

Going from memory, the rock I'm dealing with is moderately fractured with steeply dipping fracture planes. I could do tests on individual pieces of rock, but I don't believe that gives me an accurate value for the entire rock mass.

Any more ideas?

 

[dgillette]

If the rock is only moderately fractured and the fractures dip very steeply, you probably have a pretty good situation. For a Q&D check, pretend it's soil with phi' >= 40 degrees and see how much of a key you need to resist the lateral force. That will likely give you a solid upper bound on the size of the key.

If it's still an issue, try to find "Rock Slope Engineering" by Evert Hoek and John W. Bray, Institution of Mining and Metallurgy, 3rd edition, 1981, pp. 107 - 110. There is an empirical relationship for rock-mass strength based on jointing, weathering, rock type, confining stress, etc. It's in the form of equations and a big table for the parameters, so I won't try to reproduce it here.

You are correct that testing individual pieces of the rock won't tell you a whole lot about the strength of the rock mass on a gross scale.