Steep (~50%) RCP Storm Drain Installation

[Hoagie]

I'm about to specify a short (~75 l.f.) section of 24" RCP storm drain at a 50% slope. I have unrippable rock, so I've minimized the cover (2.5-ft) and have specified cut-off walls to keep the bedding in place. The storm drain terminates at a manhole with grate (for emergency release)and interior baffle (for energy dissipation) then the water discharges out a drain at 1.0% to a plunge pool and rock-lined channel to its final destination.

Any concerns with this?

 

[WL123]

Is this for emergency overflow only? If not, you want to check you flow velocity for scour, if it's over 10 fps, it will wear down your pipe in no time. You might want to consider a drop manhole in between with flatter slopes.

 

[LCruiser]

What do you mean "for emergency release"?

How are you going to put the interior baffle in it?

How long is the 1% grade?

 

[Hoagie]

LCrusier (answer 1) the grate will allow a release if in the event the mh is inundated. (ans 2) mechanically. (ans 3) 20 ft, say.

im not too concerned with scour. can always reline later.

thanks guys.

 

[Dozerman56]

I'd be kind of concerned about the box at the lower end staying in place and the pipe staying jointed.

 

[Drumchaser]

WOW.....if you are locked in on placing RCP this steep I would think a cement stabilized backfill (sand) would be in order. Also I think you will discover that placing at this slope will be next to impossible...you may as well place 75 V.F. and change your downstream appurtances.

Not sure what "unrippable rock" (existing soil conditions?) or "cut-off walls are.

 

[cvg]

I think by cutoff walls, he really means anchors. You will probably need anchors for each joint of pipe. It won't be so much to keep the bedding in place as it is to keep the pipe from sliding down the hill. at this steep of slope, in un-rippable rock, how will you dig the trench, place the bedding, pipe and construct the anchors? With RCP, I believe this is not very constructable. You might be better off with a fully welded steel pipe or even with welded PE pipe. Also consider a desilting structure upstream of the downdrain to remove any abrasive sand in the water. You may want to place baffles along the entire reach of pipe.

 

[rconner]

While I guess one could argue the relevance to stormwater or culvert applications, you may be interested in the information contained in The Ten States Standards document at

http://10statesstandards.com/wastewaterstandards.html

including,

“33.45 High Velocity Protection

Where velocities greater than 15 feet per second (4.6 m/s) are attained, special provision shall be made to protect against displacement by erosion and impact.

33.46 Steep Slope Protection

Sewers on 20 percent slopes or greater shall be anchored securely with concrete, or equal, anchors spaced as follows:

a. Not over 36 feet (11 m) center to center on grades 20 percent and up to 35 percent;

b. Not over 24 feet (7.3 m) center to center on grades 35 percent and up to 50 percent; and

c. Not over 16 feet (4.9 m) center to center on grades 50 percent and over.”

See also with regard to cvg’s guidance maybe away from RCP for such situations what appears to be (for whatever reasons) a concurring statement on page 372 of the reference at

http://books.google.com/books?id=tL...X&oi=book_result&resnum=2&ct=result#PPA999,M1

.

I would also note ductile iron pipes are also sometimes used on steep slopes as they have rather good constructability and toughness e.g. to resist concentrated support reactions etc. (e.g. see

http://www.acipco.com/international/engineeringlibrary/engineering-data/poss.cfm

and

http://www.acipco.com/pipeprogress/indexss99.html

etc.), though on very steep slopes I would think restrained joint pipes at least would most often be specified, and particularly if there is less than one support/anchorage per stick of pipe. While certainly not professing to be any sort of expert in all aspects of very steep sewer/pipeline constructions, I would think if it is possible (and it would appear maybe it would in this case) to somehow key the “cut-off walls”, anchors/supports, or whatever one wants to call them into solid rock, that would perhaps also be helpful to keep “the whole shooting match” (pipeline and bedding) from sliding downhill, under the rather inexorable forces of gravity and water movement etc.