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What is the most conservative way in calculation of the allowable bearing capacity using the unconfined compresive strength of the rock....example: UCS = 12.5MPa


I spent 6 years designing "Cut-'n'-Cover" tunnels and deep foundation support elements for the "Big Dig" Project in Boston, Ma. The design of "Rock Socket Drilled Shafts" required very specific knowledge of the type of rock, it's in-situ condition with respect to cracks and fissures, and in my case, a laboratory determined compressive strength values of core samples.

The conditions can vary greatly in a small area. I don't believe I would ever assume the risk of applying an arbitrary bearing capacity if the loads are of a significant magnitude to be of concern without some very specific information from a subsurface investigation.


we already conducted subsurface and the lowest UCS value is 12.5 MPa


Bearing in mind that the UCS value you have stated came from a controlled laboratory test, I wouldn't assume that it is "absolutely" representative of similar rock encountered over the entire site unless your boring logs show that level of uniformity, but that statement only applies if you are the geotechnical engineer responsible for calculating the bearing capacity. I am not familiar with your project.

If I was doing the calculation from the UCS for a report, I would use an equation that I have used, which I have in a deep foundation design text by Daas, Second Ed., which unfortunately is currently on loan to one of my associates. I will ask him to look it up and scan the calc for an email attachment. I will have to get back to you.

If you are calculating the bearing capacity for a Geotechnical Report to an engineering firm, then I'm sure you know that you must provide the factor of safety in your bearing capacity calculations. If you received a report from a geotechnical firm, and are using it to design a foundation (deep or shallow), then the factor of safety has already been applied.



definitely we should devide qu by 3 to get the allowable bearing capacity. We conducted rotary coring on the area because ground level is already a bedrock (3 boreholes at 10m). The only basis now of calculation of the allowable bearing capacity is the UCS from the lab thats why i'm asking what would be the conservative way of calculation of the allowable bearing capacity based on UCS. The bedrock is massive and moderately fractured. RQD ranging from 20-80%.  


I have given your question some thought in the last few days.

12.5 MPa is much greater than the bearing capacity of any granular soils (12.5 MPa= 260,000psf - approx., according to my Mathcad calc) I know of. This value is orders of magnitude greater than the highest values of soil bearing capacity I have ever seen, because it is rock, and not soil.

I am sure that the bearing capacity of the rock at your project site, which has a UCS of 12.5MPa, is NOT an issue for bearing capacity failure unless your loads on the footings are astronomical and / or concentrated in a very small area, which would likely require a deep foundation type design, which you indicated wasn't the case since the rock was close to the surface.

If I were designing the footings for your project, I would take the UCS, apply a safety factor of about 2.0, and call that qu or qall, and design my footings using the required ACI method, using the modified UCS as the allowable bearing capacity. I might even eliminate the footings, and anchor the column pedestals or foundation walls directly to the rock using drilled and grouted dowels.

I have used this approach in the past with great success, but I have no idea of the nature of your project, and cannot comment on it's applicability. That's an engineering decision you will have to make, and is your prerogative as the design engineer, Geotechnical or Structural, supported with calcs and assumptions of coarse.

I am still looking for an applicable equation to determine qu from the UCS of rock. I have found it to be a rarely, if ever used calculation in the design of shallow foundations since it isn't an issue for bearing capacity failure depending on the type and condition of the rock.



M.J. Tomlinson describes bearing capacity in rock in his Foundation and Construction book - in detail.


Please look at Bowles Chapter 4.16
   q allow rock = (qult(RQD)^2)/SF
   where qult = ult bearing pressure calculated by
                Terzaghi's  equation
Allowable bearing pressure mostly taken as 1/3 UCS for
    high RQD and 1/10 UCS for small RQD.

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