group effect in end bearing drilled shafts

[hebrone]

Hi guys,
I am conducting a geotechnical exploration for a site with
stiff soil profile for appriximatley 30 feet, then a layer of sandstone to 40 and limestone below that. axial loads in the order of 1300 ksf. I recommeded drilled shafts extending to 1 diameter into limestone bedrock (bearing capacity =60ksf). The building will have tilt up panels that are 11 feet wide. So the structural engineer placed small diamter piers (3 feet in Diamter) between the main drilled shafts (the main drilled shafts are 5 and 5.5 feet in diameter). The 3 feet diameter are placed to hold the panels only and he oversized the shafts to allow for panels to be placed at center. These piers have lateral loads in the order of 50 ksf.

I was asked if the proximety of these drilled piers, (almost 11 feet off center for all of them) will creat any group effect problems.

I don't think we should have any issues because 1)the piers are end bearing on good qualtiy rock, 2)realtivly low lateral loads (only 50 ksf).

I wanted to check if you guys have other thoughts and other recommendations?

P.S. I tried to get the structural engineer to place a grade beam between the main shafts, but for some reason he can't do that!!

 

[BigH]

How deep are the short shafts going? I tend to agree at first glance that founding in the limestone - if properly done, the small dias will not any effect at all. It doesn't sound like you have a problem with "normal" settlements - i.e., the shafts support is not settlement sensitive.

 

[hebrone]

everything is going to the same elevation, which is to lime stone. so really I have all piers going down to the same strata (good quality limestone) with low lateral loads (50 ksf).

 

[civilperson]

Group effect does not apply for end bearing shafts founded in rock.

 

[Teguci]

60 ksf end bearing sounds like moderate limestone. What do you mean by 50 ksf lateral? I've put 50 kips laterally into a 3 ft pier but certainly not much more than that.

Lateral shadowing may be an issue if the lateral loads are in line. I agree that axially there doesn't seem to be a problem.