no bottom for sanitary sewer

[jmrosell]

I am in the middle of a sanitary sewer upgrade. The line we are replacing runs along a lake so needless to say the soil isn't the greatest. There are a couple of spots in particular that don't have good soil for 5 to 10 feet below the invert of the pipe. Does anyone have any suggestions for supporting an 18" RCP sanitary sewer other than driving piling?

 

[dicksewerrat]

Why do you need an RCP pipe? Or is that is what is there and it is sinking?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[dicksewerrat]

I just thought of some more questions. Gravity or force main? Will you be excavting and replacing? What about rehabilitaion of existing?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[jmrosell]

This project is an upgrade. We are going from 12" vcp gravity sewer to 18" rcp gravity. The existing pipe is broken due to sinking.

 

[semo]

There are probably options such as casting reinforced concrete beneath the pipe to act as a beam; but, it will probably cost as much or more than just digging the poor soils out and replacing them with good soils or rock.

Some of this depends on the length of run that is over poor soils. If it is a long run, you might have to drive piles and combine it with other supports.

 

[Maury]

If you can't install piling or excavate out the poor soil, then you'll have to be prepared to deal with some settlement or displacement. Be prepared to do cleaning on a regular basis. Also, I would use ductile iron pipe rather than RCP. It's stronger, has better and fewer joints.

 

[rconner]

I would agree that the only way to guarantee continued constant elevation here for any wastewater structures (reportedly without dependable foundation for several feet under the pipe) including piping would be with piles or piers and the pipe etc.strapped down to same, and ductile iron pipe has great beam and shear strength when properly designed for such installations. If on the other hand the line must be installed without pier support and without mucking out the reportedly poor foundation soils beneath the pipe, the "bulk density" of this size say a 16" (flow I.D. ~16.72") or 18" (flow I.D. ~18.80") minimum Pressure Class ductile iron pipe partially full of wastewater would likely be slightly less than the density of water (I guess one could run the numbers for with actual required thickness of pipe and anticipated actual loading conditions). If such installation is below the water table (as it appears it likely is) it would appear there would be little tendency for ductile iron gravity sewer piping to "sink", unless of course the entire soil mass around and below the pipe were to settle/consolidate downward and would drag the line down with it. Of course also and on the other hand, ductile iron piping would have lesser of a tendency to float than any lighter pipes that might be considered for similar applications (by virtue of its weight and perhaps also outside surface texture.)

 

[jmrosell]

Thank you all for your input. It appears as though we are going to have to excavate to good soil and use pilings and do our best to keep the water out.

 

[dicksewerrat]

Look at pipe bursting with HDPE. this line is lighter than the DIP or RCP. Easier to clean and excavation will be much less. Horzontal directional drilling may be able to lay the line also.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[rconner]

May need to be careful with a perception that "lighter" is better -- see thread at

http://www.eng-tips.com/viewthread.cfm?qid=130290&page=5

and also unfortunate findings at

http://news.yahoo.com/s/nm/20051112/ts_nm/hurricanes_dead_dc).