Raft foundation placed on hard rock layer

[Olaa]

Dear Gents:

I have raft foundation of 600/1200 mm thickness will be placed on Limestone Rock of high value of soil vertical subgrade modulus equal 600000 KN/m3 and the NABC is 1000 Kpa. My question that is there are any recommendation to put structural fill or bedding layer above the limestone to enhance the stresses distribution below the raft foundation since I found one paragraph on CODUTO-Foundation_Design_-_Principles_and_Practices that in the case of the strong rock, the allowable bearing capacity shall be limited to 0.33Fc, moreover the IBC put some limitations to the allowable stresses for the materials used in deep foundation, refer to the screenshots below.


Regards.

 

[Olaa]

Please find the attached screenshot cropped from IBC


Regards.

 

[dik]

I'm not a geotekkie... Why a raft foundation on bedrock... are your loads that high or is the bedrock that poor? With raft foundations you want as uniform a bearing material as you can find. Part on rock and part on other soil is not likely a good idea.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Olaa]

Dear Dik,

Since you are not Geotechnical engineer so, please let me clarify some points to you in this regard:

There are 4 basements, the soil report recommended raft foundation.
The Ground floor slab will be transfer slab supporting heavy loads that will be transferred to the foundation system.
The Ground water table is at the mid level of the 4 basement so, the raft foundation looks reasonable.

Regards.


dik (Structural)24 Jul 23 14:28
I'm not a geotekkie... Why a raft foundation on bedrock... are your loads that high or is the bedrock that poor? With raft foundations you want as uniform a bearing material as you can find. Part on rock and part on other soil is not likely a good idea.
-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SlideRuleEra]

I have raft foundation of 600/1200 mm thickness will be placed on Limestone Rock...

...any recommendation to put structural fill or bedding layer above the limestone...

...in the case of the strong rock, the allowable bearing capacity shall be limited to 0.33Fc

The short answer to your question is "No Recommendation". The long answer: There are occasions when the Engineer may choose to put compacted structural fill or bedding on rock, then construct foundation on that bedding.

The references quoted are not restrictions. They give the Engineer clear guidance: Foundation's allowable bearing is the lower of the rock's allowable bearing or 0.33 Fc of foundation concrete.