Is my catch basin too close to my sewer lateral line?
I just dug a drainage ditch and catch basin, but hit my PVC sewer lateral (at about 20” depth (in the Seattle area, Zone C). So I piled an inch or two of dirt on top and placed my 18” deep plastic catch basin over it. By happenstance, the 4” drainage pipe is directly 8-12” over the sewer line for a couple of feet. In the attached pic, you can see the sewer lateral’s clean out directly next to the drainage pipe (so maybe it’s not completely parallel, but offset a bit?).
The latest cold snap got me thinking that while my sewer lateral is below frost line, it now has only 1-2” of insulating soil over it at the catch basin (which has a 14” x 14” footprint) and 8-12” of soil over for the couple of feet of length where it runs in parallel with the drainage line.
Is this a danger for future freezing? If so, what’s the best fix here? Insulation between the two systems, or just straight up relocate drainage?
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A rule of thumb is that an inch of foam equals a foot of dirt.
An inch and a half of foam and you wouldn't have to worry about freezing. It would also provide physical protection. Things that are buried can press on each other and cause damage.
This is a great point. I have a 14” catch basin that I may swap in, which would give me 5-6” of separation total between catch basin and lateral. That’d give me ample space for a 2” EPS board. I suppose I’d want to angle that against the house to let seeping water drain away from the house. Or maybe just glue some to the bottom of the basin. That probably makes more sense. Then line the drainage trench with it.
That may be my best play here. Thanks!
Or maybe just lay some foam at the bottom of the inside of the basin and put a brick on top, then undermine the drainage pipe with a layer. That would be the easiest solution.