Gravity San. Sewer in Poor Soils and Groundwater

[jmrosell]

I am installing 18" Gravity sewer in poor soil conditions. The soils consist of mostly marl and loose sand. The water table for the most part is 2 feet below ground level. I have looked at using RCP since it is the spec'd material per local agency requirements, but this would require pilings. We have been allowed the possibility of using PVC pipe instead. I am required to do sink/buoyancy calculations for both concrete and pvc. I am worried about the pvc possibly floating and understand that this can be remedied by anchoring. My question is what calculations i should use for the sink/buoyancy caculations?

 

[dicksewerrat]

Wieght of pipe full of water and weight of pipe empty, the two worst cases. What is holding up the buildings using the sewer?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[jmrosell]

thank you, there are no buildings in this area. it is about a 400' stretch that is by a lake.

 

[rconner]

Interesting – it appears there may be a perception that very heavy concrete pipes might sink, and/or a worry that very lightweight plastic pipes and slippery on the outside might float. Following this up with sort of "Three Bears” reasoning, perhaps ductile iron pipes, with weight in-between and at least a little (soil keying?) texture on the outside, might be “juusst right”?? Seriously, it is possible to make assumptions of a buoyant condition for any piping material, thereafter calculate the bulk density/and thereafter buoyancy of any piping material with whatever weight empty or with variously flowing contents conditions, and then determine opposing downward vertical earth loading due to overburden with appropriate reduction for water level/buoyancy effect on soil particles etc. One such paper that talks about a procedure I have seen is “Technical Report – Buoyancy Effects on Ductile Iron Pipe” available from DIPRA.
ASCE MOP #60 (WPCFMOP FD-5),"Gravity Sanitary Sewer Design and Construction", also contains some potentially helpful general guidance with regard to dewatering, bedding and support etc. of gravity sewer pipelines. I noticed it contains the quotes, "In those instances where the trench bottom cannot be stabilized satisfactorily with a crushed rock or gravel bed, and where limited and intermittent areas of unequal settlement are anticipated, a timber cribbing, piling, or reinforced concrete cradle may be necessary." It goes on to talk in generalities about such subjects as supporting pipelines in even large extents underground etc. on pilings etc., and eventually also taking particular care to prevent and recognize uplift in construction (of course when the soil is disturbed and all over-burden may not be present) in wet areas.

In other words with regard to the former, I read into this that even without concerns of buoyancy pipelines in sort of unstable soils may be a threat to settle along with the soil mass (due to natural consolidation of underlying soil strata , movement of ground water, voids etc.), and the only dependable means to assure vertical alignment in some cases might be with piles.

I would only add further, that should you decide to use underground external anchoring/supports for the piping, it is of course advisable to choose a pipe material that is capable of handling the potentially great concentrated beam/loadings that can eventually come to bear (due to such).

 

[dicksewerrat]

I guess people still use concrete pipe so we can all stay in business. that is the last pipe i would use. Sewage eats concrete. Maybe some HDPE?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[jwhite1133]

I would not use RCP. I recommend using a lined DIP or PVC. Pour concrete ancors or caps to prevent floating. Also are you installing any manholes in this 400'?