How would you design this storm Junction box

[DenverStruct]

Me and my colleague had an interesting conversation. We are designing the junction box highlighted in pink. All the way almost on the top of the photo is the FEMA 100 year flood elevation. My question is, how would you design the soil/water lateral force? How do you design the buoyancy (whether or not you need extended base). Where do you put the ground water elevation? I want to see what everyone's thought. The left pipe is existing, ther right pipe is new. The box inside dimension is roughly 8 ft x 12 ft x 21 ft tall.

 

[JoelTXCive]

The company I work for does a lot of submerged lift stations. For buoyancy water elevations, it varies by municipality. The City of Houston requires we run water all the way to the top to calculate buoyant forces. Other municipalities provide no guidance, so we will run water up to the 500yr flood elevation or we will have the hydrologist give us an elevation.

For the lateral forces we use PCA's Circular Concrete Tanks without Prestressing handbook. There is also a PCA rectangular tank manual, which would apply to you BUT, the rectangular manual has mostly tables in it. For a discussion of methodology, you want to refer to the Circular manual.

Both these manuals are getting a little old....I think the newest one is from the mid 1990s, but they are the only ones I am aware of.

Finally, we also design our reinforcement to ACI 350 standards, which makes the designs super-conservative.

 

[DenverStruct]

This is not a lift station. Here is where I am getting at. Wouldn't the junction box be filled up with water if it floods? Do you really need to design this as if the box has no water in it and you have all of this hydro static pressure all the way to top of rim elevation? I feel like we are being way too conservative?

 

[JoelTXCive]

I take your point.

I find that on these lift stations, and other civil structures people are very hesitant to change designs or methodologies. I think some things are just done because that's the way they have always been done.

In Houston and Southeast Texas, our clays are so wet, and the water tables are so shallow; that assuming the entire well/junction box is submerged is not an unreasonable assumption.

If you are in Denver, that might be an excessively conservative assumption though. Does the water table move up and down? If you do not have to worry about that, then you definitely have a valid point on the design being too conservative.

 

[DenverStruct]

I am in Houston now. Cant change handle unfortunately.

 

[JoelTXCive]

Ha! I'm siding with your colleague then......design for water to the top!

A lot of our roadway and land people do junction box designs. I'll ask them what elevations they run water to..

 

[DenverStruct]

I only ask because it is really close to the outflow. Almost impossible to have ground water all the way to the top with no water inside the box.

 

[cvg]

this is RCP which does not have gasketed joints. the joints are not watertight. the pipe to box connection might not have a waterstop either. We don't know what the hydraulic grade line is, but this pipe could be surcharged. Seepage / leakage from the pipe and structure could saturate the surrounding ground. Surface water could also saturate the soil from above. when the rain stops and the flow recedes you could have significant saturated soil above the ordinary ground water level. I would recommend the conservative approach and assume some level of groundwater and bouyancy.