How to prevent another leaning tower of pisa

[UtahWater]

Hello, all. I'm a water guy and our structural and foundation engineers aren't available...sorry if this is a rudimentary and painfully ignorant question.

I am designing a little pumpstation for pumping reuse water out of an old sewer pond. The client wants no, if possible, disturbance to the the existing clay liner of the pond. The pumpstation structure consists of a 20' tall tower of concrete manhole sections...so the "tower" would just sit on whatever pad is designed...on the bottom of the undisturbed pond. We don't know anything about the design and construction of the pond. Is this a shot in the dark as to making sure we don't build a mini leaning tower of pisa, or is there any type of rule of thumb?

The pumpstation will exert 1400 psf on whatever it bears on. I'm considering a 10" thick reinforced concrete mat on a 4" leveling course of clean 1" minus rock, with a separation fabric between the pond bottom and the rock, to get the psf down to below 1000 psf. Settlement up to 6" would be acceptable, as long as it's not differential This only considers bearing capacity, though, and I'm worried that there's some overturning factors that I should consider, too.

For y'all that know what you're about, I appreciate you sharing some guidance!

 

[dgillette]

The leaning tower has made Pisa a popular tourist destination, and has been a major portion of their economy for many years. Are you sure this is not an opportunity to profit?

Sounds like the client doesn't want you to drill for the investigation OR for a deep foundation, so yes, it is a shot in the dark, with potential repercussions for you and for anybody that relies on that water being available. You don't know what's below the clay liner. Your choices are to do a proper foundation investigation with drilling AT the site of the pump station, or to drill as close as you can, then design a foundation that allows for both thick and thin layers of extremely soft clay mine tailings overlying karst, peat, and swelling clay, with haz waste and a landslide. And maybe Jimmy Hoffa, too.

Yes, I'm being a smartass, but this really is a situation to stay out of.

Bon chance!
DRG

 

[UtahWater]

dgillette, thanks

I appreciate the feedback. It supports my theory that you can't really design something to succeed if you don't know what you're building on, you can only give an educated guess. Plus, it made me smile...man, somebody was teaching me about personality traits yesterday...I am so a C.

 

[civilperson]

Calculate the horizontal forces on the structure, (wind, waves, seismic), and apply to the proposed foundation. The 1000 pound per square foot design may not match a saturated subgrade, (a better than 90% chance that the soil is sufficient), but expansive or collapsing soils should be checked for and too big is better than too small. This is a twenty minute calculation after assuming a bearing capacity of the soil. (But this project will have too small of a fee to assume the design liability on a hunch).

 

[UtahWater]

Thanks, civilperson. You're right about the design fee being too small...$6,500 doesn't pay for very many tilted manhole sections after overhead and wages.