Rock Allowable Bearing Capacity 3

ets1,

AASHTO HB-17 (and I imagine LRFD 3rd Edition) presents guidance for computing allowable bearing capacity of rock with respect to Co (= UCS of intact rock) for different RQD or RMR values. These tend to be somewhat conservative, but I find that not to be a necessarily bad thing.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

I will check it out but I don't have any RQD values. I saw a report that used the UCS and split tensile value. We are doing some checks on an incomplete building that is for sale and made some cubes of the rock fragments from the footing excavation. The UCS values were from 170 psi to 278, one was 720psi. Any thoughts.

jdonville;

Would you be kind enough to research the formula for me because I don't have a copy and it looks like it would take days to get a $400 new copy.

ets1,

Firstly, I'm not sure how you're calculating UCS from cube samples. I believe that ASTM generally specifies cylindrical samples (from intact portion of rock cores) with L/D ~ 2, similar to concrete testing. I know that mortar or grout are often cast in cubes, but don't know how the UCS values compare to specimens with the cylindrical geometry.

Secondly, 170 to 720 psi are rather weak for limestone. Intact limestone in northern Ohio, in my experience, generally runs 10,000 to 20,000 psi. Based on your results, I would tend to use an average of about 200 psi for analysis.

If you want to see how close your cubes are, I would recommend that you hire a driller to do some bedrock cores and break the intact samples. They should report RQD values to help guide your analysis.

I imagine that the drilling costs should run you about $40/foot for rock coring and about $10/foot for blind drilling through the overburden soils. If you need a lot of drilling, it might be worth your while to hire the drill by the day. A rig and two-man crew should be available for $1K to $2K per 8-hour day, depending on the location. Preparation and testing of the rock samples should not be that expensive, as testing firms use the same equipment as for breaking concrete specimens.

You could probably have drilling and testing completed within a few days, depending on availability of drills and crews, if you are very hot for the results.

The formula in AASHTO HB-17 is qult(bearing on rock) = Nms * Co

where Co = rock UCS
Nms = f(RQD)

You could also request copies of the original geotechnical investigation report and ask if the original cores (if performed) are available. The original geotech report should include RQD values if cores were performed.

Hope this was helpful.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

Thanks, I found it in Caltrans manual on the web. We are going to get some rock cores done for $30/ft next week, because was you said the cubes are too low.

Thanks again

A really good reference for rock foundations can be downloaded at:


I really would not use some type of remolded rock fragment sample to get any strength of the rock, since that doesnt mean anything compared to the in-place strength of the rock.

What type of building is this (1-story, 10-story, masonry, ect?)

You may not need to know the ultimate strength of the rock, just if it will sustain the estimated loading (ie: the continuous footings will not go above 4 kips/lineal foot, isolated columns 4 ksf, so why test to prove that the rock is 15 ksf)

dmoler,
THanks..It is a 1-storey building with six power generators inside. Each generator is about 18kips. THe remolded strength does suck..I was thinking of getting a plate bearing test, but the soil is so uniform I thought cores would be okay.

If the client will pay for it, cores are the best. I would still check the rock thickness with auger borings due to some shear considerations.

Even very weak rock will support 2500 psf, which is probably all you need for that type of structure, since minimum footing dimensions might govern more than the loading in the interplay calculations with footing size/structural loading/ soil bearing capacity elements of design.

Also, if the footings are cast directly against excavated rock, the side shear of the footing would add extra capacity (sometimes substantial), which would be even more conservative.

One alternate approach is to do some SPT borings. If you get refusal, I would give them 3000 psf and so much the better, and if not, you could model the bearing capacity and settlement based on SPT data using sand models. That should be conservative compared to actual capacity and settlement as based on rock.

An old way we used to do for rock was simply drill auger borings, and categorize the drilling as easy, medium, medium hard, hard, and very hard. Medium hard to very hard was given 3000 psf without SPT for simple one and two story structures, but the questionable drilling was checked with SPT. You have to be experienced with drilling though.

dmoler,

Your 100% correct with the auger borings, we do the same proceedure but normally use around 5000psf. I have access to the SPT driller, but if I mobilize him I might as well do several bores at various depths. What Iam hoping to do is have a simple test I can carry out to do my preliminary designs without getting the drill on site until the preliminary design is done. I was wondering if there is a "miniture SPT" machine. I can't see the pocket pentrometer being very accurate and it seems to be more for clays than limestones. Is it? I was looking at getting one but not sure how well it works. Also, I see ELE has a "Clegg Impact Soil Tester"...
and a portable CBR machine and plater bearing machine.

Any suggestions...
This looks promising, any thoughts?

It seems like borings before preliminary design would be appropriate. That way your prelim design is more accurate.