Different bearing capacities point of view 3

[pelelo]

Hello Engineers,

Few years ago I was involved in a project which consisted in the construction of a 12-story building + 3 levels of basement (27 ft deep excavation).

The geology of the area is weathered coralline limestone (see picture attached).

Consultant A (the original geotech consultant) recommended an allowable bearing capacity (Qall) of 5000 psf, using the typical bearing capacity equations by Terzagui. This BC was recommended at the bottom of the excavation (27 ft below original surface).

Somehow someone on the client' side decided to get a second opinion with consultant B.

Consultant B recommended for the same building an allowable bearing capacity (Qall) of 8300 psf. He computed the bearing capacity using the typical BC equations like Consultant A however Consultant B added the overburden pressure at the bottom of the excavation. 5000 psf + 120 pcf x 27 ft ~ 8300 psf.

I understand that you may add the OB pressure (120 x 27 ~ 3300 psf) because there is a compensation at the time the excavation is performed, however, do you think it is correct to add up the full OB pressure?. I ask that because due to the weathered limestone material, cohesion plays a factor, so i don't think the full height of the OB (27 ft) would exert at the bottom of the excavation. Due to this, i can not tell what percentage of the OB would it be proper to consider for BC recommendations.

If the OB was clean sand, I would agree with Consultant B.

On other note, I found interesting that consultant B computed settlements considering a net foundation pressure (qn) of 5000 psf (local shear, Qall - OB). I understand that since he recommended a Qall of 8300 psf (considering OB) this should be the stress to be considered for settlement computations.

Please let me know your thoughts.

 

[1503-44]

I would not usually expect limestone to have a high cohesive value.

Other than that, it seems that one consultant may have calculated net bearing pressure and the other gross soil bearing and neither were specific as to which allowable they recommended.

I've never had any problem adding full weight of the depth of overburden removed to a net soil bearing capacity.

https://civilengineeringx.com/foundation/terzaghis-bearing-capacity-theory/amp/

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[HTURKAK]

It is hard to comment without knowing that the Consultant A allowable bearing capacity (Qall) of 5000 psf is if Gross or Net bearing capacity..

The following is excerpt for the bearing capacity FS. (from Principles of Foundation Engineering by BRAJA DAS )

My suggestion would be try to see the bearing capacity is gross or net and adjust it .







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[EireChch]

1503-44 is right, its hard to know what they are referring to as they have not be specific.

The Terzaghi general bearing capacity (with Meyerhoff factors, but this is irrelevant) method give us ultimate bearing capacity (qu). To get allowable you divide it by a SF usually 3.

Nowhere in the Terzaghi method does it say to add the overburden stress on to make it gross. Consultant B is wrong IMO.

Regarding settlement calculations Consultant B is correct in only using the q[sub]allowable net[/sub].

If removed you removed 150kPa of overburden in an excavation and then constructed a foundation that applied 150kPa of pressure then the foundation will not settle settlement (this is only applicable to Mohr Coulomb soils which assume a linearly elastic soi).