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I need to extend an underground drain pipe.

The current pipe is underground a few inches and ends at the top of a hill that's a few feet tall. The hill is angled at (perhaps) 35 degrees.

The current pipe is 4" PVC "teleduct".

My original plan was to take an adapter I'd already purchased to connect the teleduct to a more standard form of PVC, then use an angled coupling to route it down the hill, underground.

My neighbor (who is helping with the project; it is close to his property) suggested (1) reducing the diameter, as a small diameter is easier to bury, (2) using some kind of rubber or elastomeric connector with hose clamps (the kind with a threaded worm screw). The pipe he bought is flexible PVC.

My question: is his approach reasonable?

My concerns over his approach are (1) a compression connection won't last as long as a cemented PVC-to-PVC connection, (2) I see allusions online to "don't reduce the diameter in the direction of flow".

The advantage of his approach is the flexible pipe is easier to deal with (don't have to get angles exactly right).

Regarding my concerns, I'm not sure (1) is well founded, since online there are many reducing connections that are "underground rated". Re (2), intuitively I'm not sure it really matters, as long as the smaller diameter is enough to handle the rainwater flow. That said, it seems like he's thinking of making the connection to the existing pipe without angling down at first, which I think is a mistake because reducing the diameter on a near-horizontal pipe will mean water will get caught (and pool for some length) at the reduction.

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    Some thoughts: if it's the type of flexible that is ridged, it can collect debris and plug easier. Reducing the diameter is generally bad unless you know 100% that it's massively oversized as it is. The rubber connectors are nice and snug, so the concern is the band rusting away. Sep 9 '20 at 15:07
  • Agree re ridged; I use those for above-ground drainage but wouldn't for below. Sizing: going to consult some charts. My concern there is that the reduction might lead to trapping debris. Band: I'd probably try to ensure it's stainless steel. Sep 9 '20 at 16:49
  • I don’t worry much about things plugging up with a slope like that I have made adapters out of a piece of plastic slip the pipe inside or outside depending if you stay large or go smaller wrap with plastic and if you want it bulletproof proof pack some mortar around it. That’s how we used to do concrete pipe that was damaged it has worked for pvc and steel over the years when I did not have the proper adapter.
    – Ed Beal
    Sep 10 '20 at 21:54

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