Rock bearing capacity. Which method?

[Silty]

Hi Everyone,
There are various tables and formulas for bearing capacity of rocks. However they give very different numbers ranging from 3 MPa to 35 Mpa for the site that I am working on. 35 MPs is not a reasonable number to me but want to hear other opinions.
The methods used are as simple of correlating with RQD, Rock type, UCS and also usace method considering joint spaces. The rock is granite but joints are as close as 5 inches and in some cores RQD is as low as 60 percent.
UCS is more than 150 MPa as you can imagine. But I think joints are controlling factor not strength of a piece of intact rock.
Please let me know your thoughts and your estimated guess based on above.

Thank you for your contribution
Appreciate this forum

 

[EireChch]

You will have a huge bearing capacity with UCS of 150MPa. Probably more than the structural capacity of your concrete. That is likely the limiting factor.

Sorry I can respond with recommended method later. Shooting out now.

 

[Silty]

Thank you ErieChch.
Can anyone else share their thoughts. The question is basically about effect of closely spaced joints on bearing capacity. Thanks

 

[MTNClimber]

We typically recommend 20,000 psf (~1 MPa) for footings bearing directly on sound unweathered hard bedrock, such as granite. I've never had a client be upset at that value but I haven't been in a situation where that is a limiting value. We also provide them minimum footing dimensions for continuous and isolated footings which probably govern more than the allowable bearing capacity.

An RQD of 60 isn't bad so for shallow foundations so I wouldn't be concerned.

I would start being concerned with really low RQD's, low recoveries, sloping bedrock, active faults, soft rock, weathered zones, ect. I can't tell you specific values for each item as I feel like it's more experience and local knowledge that governs when dealing with shallow foundations on rock rather than a manual.

 

[EireChch]

I would recommend that you determine mohr coulomb parameters using Hoek Brown criteria. This will give you c' and phi'.

You can plug these into equation 2.26 of Tomlinson Foundation Design and Construction 7th. This will most certainly give you an ultimate well over 1MPa.

And yes you are correct, any jointed rock strength should be assessed to consider the discontinuities. This is what Hoek Brown criteria considers.

 

[Settingsun]

1 MPa allowable is what we start calling rock, and it's sandstone.

 

[MTNClimber]

I have some questions for you.

What are you building? Will the client benefit from a less conservative allowable bearing pressure?

How thorough was your subsurface investigation? Did you actually core below bottom of footing elevation?

Do you know who will be doing the rock blasting/excavation? Are you confident they won’t over-drill and over-blast?

These are the things that would go through my mind before recommending a non-conservative value. Bedrock quality will vary with depth and location across the site. Settlement will typically control. So you have to be confident that there won’t be highly weathered zones in your rock below the footing. I’ve seen enough variation in my regions to warrant being conservative.

 

[dik]

See... any comments from real geotekkies? Earlier I posted a link in the electrical forum that I thought was great... and wasn't so great.

http://home.iitk.ac.in/~sarv/New Folder/Presentation-14.pdf

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Mccoy]

What I usually do is employ Hoek & Brown method to assess the phi-c' parameters of the rock mass and then treat the rock mass as an extremely very stiff-dense soil. This has already been described by EireChCh previously.
However, this method is correct only as far as the rock mass is isotropic at the scale of the potential failure surfaces involved. If the rock mass is anisotropic, that is discontinuities govern, then there are more elaborate methods and the phi-c' parameters at the joints, families of joints must be considered. I confess I rarely used the more elaborate methods in the small or medium sized projects I'm usually involved with.
The above being said if the foundation is on rock mass and if it's a rigid, not weak rock, even with pervasive joint networks (low GSI number) the bearing capacity will invariably be huge.
Bottom line, the calculations have a practical sense only if the loadings on the rock are very substantial.

 

[Mad Mike]

I follow the same reasoning as McCoy - there are "rules of thumb" if I recall, that in a very well-jointed rock mass with poor joint quality, you would divide the rock UCS by 10 to obtain a safe allowable bearing pressure; in the instance of a tight rock mass you would divide the UCS by 3 for same.

As already noted, the calculation becomes academic since prescribing a "bearing capacity" in excess of 1 to 2Mpa for a shallow footing doesn't produce any noticeable reduction in the foundation costs, to the best of my knowledge. I rarely prescribe an allowable bearing pressure in excess of 1MPa and no bridge engineer has ever complained or asked for anything higher...

Best,
Mike