Foundation design for an arena

[oengineer]

I am working on a project for a rodeo arena. There are existing drilled pier foundation on site. My company is gong to demolish the old bleachers, concrete barrier wall, and concession stands and come in with put new ones. There are existing drilled pier foundations in the soil right now, but we are going to design new drilled piers to support the bleachers that we are adding. The old concrete barrier wall was supported on drilled piers, but I am told that the plan is to come in and support our new barrier wall on a continuous wall spread footing. The new soil report for this project calls for drilled and underreamed footing bearing at a depth of 14 ft below existing grade in the fat clay soils. The footing may be sized for a net allowable bearing pressure of 3000 psf for dead plus sustained live load. The soil report also states that " Allowable bearing capacity at a depth of 24" below existing grade: Dead Load: 1,000 psf and Total Load: 1,500 psf.". I am concerned about using a continuous wall spread footing in lieu of drilled piers when the soil report does not explicitly say to. I have been told to just have the continuous wall spread footing go 3 ft below the finished grade in order to use the 3000 psf allowable bearing. Should I be concerned? Also, we are building concessions stands that will just be rectangles. The walls of the concessions stands will be composed of CMU walls. I have been told to make the foundation a slab-on-grade supported by only grade beams. Based on the soil report information I just provided, is this possible? Any comments are appreciated.

 

[dhengr]

Oengineer:
We can’t see the site, soils samples or soils report from here. I wonder what would happen if you asked these very questions of the soils engineer who you trusted enough to write the soils report and reviewed the various soils testing in that process. He probably even knows the local soils conditions better than any of us from thousands of miles away. He can also do testing and inspection during the construction process. I thought that was what you hired soils guys to do.

 

[oldestguy]

One of the important items from the soil report should be what differential settlements will occur for various steps in pressure for footings or for that slab possibility. I'd suspect commercial bleachers could withstand a lot of differential settlement, thus possibly saving much on required foundations.

 

[Ron]

If the allowable bearing pressure is 1500 psf at 24 inches, it won't be 3000 psf at 36 inches. According to your post, the 3000 psf is at 14 feet below grad. If the old wall was supported on drilled piers, and if it was performing well, then you could do the same or even provide a grade beam between the piers to support the wall; however, the wall itself will likely serve as a deep beam between the drilled piers.

Agree with OG....maybe your differential settlement criteria are not as stringent as you might think. Check with the geotech.

 

[oengineer]

Regarding the settlement, the soil report says:"A detailed settlement analysis was beyond the scope of this study. However, previous studies by the author indicate that total settlement should be less than one inch. Differential settlemet is typically assumed as about one-half the total settlement." I was told that there is a provision for an increase in soil as you go lower in the IBC code.

 

[oengineer]

My boss wants to avoid using the old drilled piers. The issue is that the original existing concrete wall was a skewed and we are now making the wall run straight. So many of the existing drilled piers do not sit directly underneath our concrete wall.