Gravity San. Sewer in Poor Soils and Groundwater
Gravity San. Sewer in Poor Soils and Groundwater
(OP)
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?





RE: Gravity San. Sewer in Poor Soils and Groundwater
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Gravity San. Sewer in Poor Soils and Groundwater
RE: Gravity San. Sewer in Poor Soils and Groundwater
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).
RE: Gravity San. Sewer in Poor Soils and Groundwater
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Gravity San. Sewer in Poor Soils and Groundwater