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Situation: Guy’s got an owner-built septic that ain’t no system. Evidently, whoever built the “system” came from the moreisbetter school of waste pipe pitching. A peek under the house reveals a pipe pitched like the rake of a chalet and grimly, it’s steepening by an eigth as it hits the dirt. Happily, the municipality is extending their 8 inch line to the property line, hence the phonecall to me. Guy wants to know cuanto cuesta to get down the slope to tie-in. Oh yeah, forgot to mention the slope, about 10%. Approach I’m taking thus far is: compute proper fall for 3″ ABS over 230′ run – 230/4/12=4.8ft. Total fall is approx. 23′, this leaves me with an extra 18 or so feet to shed. Dividing 230 by four yields four properly pitched runs of 57.5 ft. I’m planning on having a 6′ vertical drop ending in a sweep at the ends of the upper three runs. This “waterfall” approach means more complicated, more expensive trenching. The agency with jurisdiction says it sounds good- that is after I collapse and backfill the old tank. Reason I’m figuring ABS is it’s going under his gravel drive. Am I missing anything?
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Freelance, give me 'till tomorrow. I read your problem to my husband (Civil Engineer, waste water management specialist), and he said if I print out your posting and bring it home with me he'll work it out and have an answer for you.
*Freelance, DH looked at your problem, and this is what he said:A fall at 24 feet over 230 foot run yields a 5.96% slope. ABS has a low friction factor, but even so, this slope on a 6" pipe flowing full yields a velocity of 8.5 feet per second. This is high, but below the 10 fps maximum. Downside is this pipe may wear out and need to be replaced in 20 years instead of 25 years. If you exit the house beneath the foundation 3-4 feet below the surface you may be able to reduce the slope to 4% or 5%, drop your velocity accordingly and gain a few years on the laterals lifespan.
*Dear CLS- Thanks for handing my post to DH. Seems to be some confusion however. DH figures 5.9% for 23' fall over 230' run. This is obviously discrepant (if that's a word) to my approximate slope calculation of 10%. This sends me to the books to make sense... In "Pipe And Excavation Contracting" by Dave Roberts, Craftsman Book Company, 1987; to calculate slope as a percent, divide the vertical rise by the horizontal distance, then multiply the answer by 100. So 23(vertical rise) divided by 230(horizontal distance)=.1x100=10%. If DH is interested in clarifying this point and the downstream considerations thus affected, please tell him I'm interested, for as of now I must disregard his information as erroneous until his method of calculation is explained and shown to be numerically correct. Perhaps there is some multiplier or factor he's using of which I'm ignorant..?Thanks for any time or interest you invest.Freelance
*Freelance, my fault. Engineers handwriting sucks! I misinterpreted his scribble. Sub in "5.96 degrees" or 10% slope. The rest was correct. Sorry about that!
*If you are concerned with the lifespan of the pipe, simply switch to schedule 80.