Cast In Place 37 ft deep Storm Water Wet Well Means and Methods 3

[structuralsteelhead]

Tasked with designing a cast in place wet well structure. 37 feet deep, 12' by 17' Not sure the reasoning this was not sent out to pre-caster, but that ship has sailed and I was not part of the reasoning why I'm asked to design a cast in place option. Typically the pre-caster would form sections in his plant and ship. I'm considering doing the same with a cast in place option having the contractor pour two or three separate sections and form the top and bottom joint similar to how a pre-cast company might, maybe it would require more sections. Not sure on the means and methods side here but I do need to address it. I don't want to just show a monolithic detail of the wall structures and call it a day. Or should I?

Not sure how thick the walls will be, assuming 10"...waiting for geotech report design parameters. I'm sure forming blowout will be an issue, so even if formed full height, I'm sure there would be a limit in height and thus a construction joint necessary. The wet well/lift structure is for storm water, not sewer by the way.

Penny for your thoughts for those designing these on a day to day basis. (Although I know and respect these thoughts worth a little more than a penny!) Thanks in advance nonetheless for your grossly underpaid thoughts.

 

[DaveAtkins]

I have designed several of these. I have not worried about means and methods (construction joints, etc.).

The wall thickness will be controlled by the shear capacity of the concrete, since you typically don't provide stirrups in a wall. My guess is it will be a lot thicker than 10".

Flotation is an issue, since presumably much of the structure will be below the water table, and when the structure is empty, it will want to float.

I recommend using at rest earth pressure, since the structure is so stiff. The at rest earth pressure must be combined with the hydrostatic pressure.

I recommend creating a finite element model of the entire structure for analysis.

DaveAtkins

 

[JoelTXCive]

I have designed several of these also. Any chance of switching to a circular section versus your 12x17 rectangular?

Shear at the bottom is going to be large. The 90-degree corners are points of fixity so you'll develop large horizontal shear along with the normal vertical shear. I wouldn't be surprised if your wall thickness exceeds 24" at the bottom. A circular well gets rid of the horizontal shear and horizontal moments.

I'm with Dave.....I don't worry about the construction joints either. At 37' foot depth, I would probably show 2 C-Joints on my plans and label them optional. I would also have bar splicing at any joint clearly shown so that you communicate to the builder that you want to maintain development lengths regardless of the joint locations.

Another consideration... are they going to open cut the well, or use the caisson method to sink the well? If they are going to sink the well, the circular wet well is much more stable.

 

[JedClampett]

First of all, 37 ft is not that deep. And as the above commentors mentioned, get your mind away from 10-inch-thick walls. You can't install waterstops in a 10-inch wall. Plus, if there's groundwater, you'll need the mass to avoid buoyancy. I'm guessing 18 inches thick, but maybe with your short wall lengths, you might be able to reduce it.
You concerns are not important. Forms are designed by a form supplier, and I've never once had a problem in 27 years. You need to design for soil pressure, water pressure (separately), surcharge and buoyancy. Joel has the right idea with a sinking caisson, but with a rectangular pump station, that might be more difficult.
If you're not familiar with waterstops (and their details), surcharge and buoyancy, you need to get help. From the nature of your questions, I'm worried.

 

[structuralsteelhead]

Much appreciated, to all of the above. Again,...the contractor has asked for a cast in place design,...the water table below the bottom of the elevation of the well by about 10 feet. I suppose I could insist the shape be changed to a circular design and could design the well as a circular tank then and let the contractor worry about forming. However I suspect if I do that, they will wonder why they are not waiting for the pre-caster to be available as forms for such diameters would be hard to come I would imagine unless you are a manufacturer. I will produce an FEM of the structure and let the numbers do the talking with regards to stresses and reinforcing design. I will inquire about the excavation method and placement. Still waiting on Geotech design parameters. Thank you gentlemen! I may circle back looking for a sub-consultant.

 

[Rabbit12]

You should probably be using ACI 350 too.

 

[structuralsteelhead]

yes thank you I've order it! @ Rabbit12, and ASTM C 433 as well and ASTM C 478