Bearing Capacity - Fractured Bedrock

[NewNorthernGeotech]

My question pertains to the bearing capacity of fractured bedrock. I'm dealing with spread footings placed on some pretty well fractured rock. I'm working in a new area and we don't have this problem back home. I've been giving it lots of thought, talking it over with my boss and experienced contractors onsite, so I am slowly gaining a better perspective of the issues, but I still feel as if I'm missing something.

If anyone could suggest some good references for reading material on the subject and/or simply provide some tips regarding the key issues to watch for when dealing with fractured bedrock in compression, I would really appreciate it. I imagine my question will grow from there.

Thanks

 

[fndn]

Give us more description of the rock, the site topography and proposed structure or loadings.

 

[NewNorthernGeotech]

The area is located in a northern climate, just above the tree line. I'm seeing evidence that frost shattered bedrock can continue to depths greater then 3 m. The building is a large industrial complex; columns supported on spread footings, with very conservative loadings of 1 MPa.

I guess my question is this; is the lower bearing capacity of fractured bedrock based only on the fact that once loaded from above, these fractured pieces may be pushed out along the fracture planes? If so, then are there any concerns if the excavated area is supported on all sides by undisturbed bedrock and backfill material?

 

[BigH]

John Franklin has a book out on Rock Mechanics and if I remember right, he will have covered this. Another one is Coates (Canadian - CASMET, I believe).

 

[BigH]

Thinking about it - the US Army Corps of Engineers has a manual on rock foundations. EM-1110-1-1908 "Rock Foundations" - is the manual number and designation.

For the USACE Manual Site:

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em.htm

For the Rock Foundations Manual:

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-1-2908/toc.htm

 

[fndn]

When our supporting materials are rock, an experienced and licensed LOCAL engineering geologist should be involved. This professional will oversee drilling investigation, measure the bedding planes and research the site geology. He or she will also compute the dip, analyse if the bedding planes are neutral or adverse, and will comment on further weathering of rock by water, soil or chemicals during the design life of the industrial building.

Based on those findings, other issues besides bearing capacity may govern. For example, if the fracture line is neutral, it may still affect the FS against sliding. Once you converse with the engineering geologist, then I would look at the book by leggett "Geology and Engineering" to help gap the link to geotechnical design. If the ground water is shallow, I would also do scour study.

 

[egmlca]

"Is the lower bearing capacity of fractured bedrock..." Just curious by what rational was the bearing pressure derived? Was there a prelim. geotech. investigation? Also,
if the excavated area is supported on all sides by undisturbed bedrock why aren't the foundation elements suuported likewise?

 

[Mccoy]

"Foundations on rock" by Duncan C. Wyllie is a recent reference. A subchapter, starting from page 117 is devoted to bearing capity of fractured rocks.
A basic issue in dealing with "pretty well fractured" rocks (where I am it's inevitably fractured) is that you have isotropic conditions, so failure planes are approximatively equal to failure planes in soils.

This makes things a lot simpler.

what I usually do is to start the Roclab freeware (

http://www.rockscience.com,

Hoeck's website), enter my input parameters, get my output phi and c', and use 'em in bearing capacity formulas.

The roclab freeware in the opinion of the (very qualified) authors yields more realistic values of resistance parameters than the conservative RMR method, in weak and fractured rocks.

I usually get bearing capacity values in the region of a few megapascals, depending on the input parameters. Again, in isotropic or quasi-isotropic condition (well fractured rocks) Roclab is darn good.

I would experiment with various input conditions to get a reasonable range and a reasonable lower bound for bearing capacity.

Yet, I would try to detect potential weaker sliding surfaces, clay fillings, weak areas and so on, just to sleep well at night.