Steep RCP Pipe

[civilman72]

Preliminary drainage design for storm drainage (in a parking lot) behind retaining wall (approx. 8'-12' in height) includes curb and gutter with a storm inlet and a 12" RCP outlet culvert that discharges on the downhill side of the wall. Existing slope downhill of wall is approx. 2:1 and in order to meet this slope the culvert has been designed to match the existing slope, so culvert grade = 50%. The only way (that I can see) to flatten the grade of the pipe would be to significantly increase the inlet height. In order to get the culvert safely below the RW foundation this would require about a 16'-18' storm inlet height; the current design requires only a 7' inlet. This is a small parking lot, with very smalll flows, so building such a large inlet seems like overkill.

So, my question is: Is there any special design considerations for design of such a steep pipe. My biggest concern is the potential for this pipe to slide away from the inlet. I've designed "steep" sanitary sewer lines and there are numerous design considerations to avoid manholes/pipes on steep grades from pulling away from each other. Does anyone know of any similiar design consideration for storm sewer? I would be more concerned if the pipe was much larger/heavier.

BTW, we have plenty of energy dissapation designed at the outlet, so I'm not concerned with velocities and erosion, more just about the structural integrity of the pipe and inlet connnection.

 

[civilperson]

Build a steel reinforced pipe cradle with occasional turndowns for the steep length of RCP with 2-#5 bars continuous just below the spring line. This will stay in place and hold the segments of RCP.

 

[TerryScan]

While it sounds unusual, the 18' deep inlet seems like the most economical and constructable.

 

[gbam]

I agree with Terryscan. Can you use a manhole with a grated lid? I have found that a manhole configuration is typically more cost effective than extreamly deep catchbasins.

Also, I typically do not exceed a 10% slope for pipes. On steep slopes, I would anchor the pipe into the slope.

 

[civilman72]

Thanks for the ideas...

I'm leaning towards using a CMP or HDPE pipe (lighter) and do like the idea of using a manhole, instead of a full-blown inlet, and agree that this will save costs.