Bearing capacity for Calcarenite rock for mat foundation

[MeladHaweyou]

Hello,

I have a geotechnical report that estimates the allowable bearing capacity for isolated footing and raft foundation on a site predominantly composed of Calcarenite sedimentary rock.

The geotechnical engineer estimated the bearing capacity for a raft foundation to be 2600 kPa, which, in my opinion, seems too high. The building is designed to be 8 stories high above ground with 2 stories underground.

I have attached a screenshot of the geotechnical report section addressing this.

I would appreciate any opinions on this matter. I am a structural engineer.

 

[geotechguy1]

Did they do any supporting laboratory or field testing (UCS / Triaxial / Point Load)?

Is it unusual because you have never seen 2600 kPa allowable bearing capacity for this type of rock in your area? What parameters are usually given? Or have you not worked with this rock before? Are there similar buildings nearby with similar heights and loads?

What are you using the bearing capacity for? Bearing capacity, or are you doing the cheeky structural engineering thing and taking that allowable bearing and converting it into soil springs for slab design?

 

[EireChch]

No way anyone can comment without knowing the UCS of the rock. It could easily be 2600kPa or 26000kPa depending on the rock

 

[MeladHaweyou]

Thanks EireChch for yours reply. I am attaching here what he provided from the UCS test. It was estimated to be 2.64 MPa as a mean for a frequency distribution with the number of tests they did.

 

[MeladHaweyou]

And here is the distribution they provided. Thank you very much for your time.

 

[MeladHaweyou]

Thanks Geotechguy1 for your reply. Yes, they have done UCS test. I Uploaded in the previous reply. To me 2.64 MPa seems low. Typically we get 400/500 MPa for the area of the project.

“cheeky structural engineering thing and taking that allowable bearing and converting it into soil springs for slab design l This is what I wanted to do, I asked the geotechnical engineer for a better estimate with a plate test and his response was to use that correlation\equation!!)

 

[geotechguy1]

I don't agree with that approach but in any case, it's preferable to at least design for a range of spring parameters (i.e., a soft spring, a 'likely / characteristic spring' and a much stiffer spring) to consider a range of cases; and also because 'conservative' in the manner that geotechnical engineers think when coming up with these parameters is liable to be the opposite of what is conservative for the structural engineer and thus potentially dangerous if not properly managed.

 

[EireChch]

With a UCS of 2.4MPa, 2 storey basement and raft foundation (assumming its 10x10 or that), your allowable bearing capacity is easily 2.4MPa.

Also you approach is flawed, work out what settlement you have and then work out a spring stiffness. Even that approach is somewhat flawed but more accurate than yours i think