Pretty good idea, King County. But why skimp on conrete and use wood? PT or not, it will be rotted to next to nothing within 20 years. Maybe sooner, with the near-constant dampness. I'm going to look into a variation on a job I am currently doing, but it won't be PT lumber.
And why so deep for the influent pipe, unless you are trying to pickup some infiltration, which is actually a good use of land.
20% inflow gradient? Man, that water will be moving. I think I'm going to back that down a lot.
25 feet on level 4" pipe, with one cleanout? Probably OK, w/ 6" pipe, or more frequent CO's.
Needs note spec'ing 25' from bldg foundation.
Why solid cover on inlet? I'm using a grate, for better dispersal and larger inflows.
I'm allowing square or v-notch overflows. Constructability will be a nightmare w/ concrete v-notches, if cast in place.
I would actually call out AASHTO #3 stone (or whatever you prefer). I've been bitten too often by the Contractor saying "that IS clean stone", while pointing at a pile of Suitable Backfill.
Call out AASHTO Class 2 geotextile. I've been bitten too often by the Contractor saying "that IS filter fabric", while pointing at a roll of some flimsy stuff of indeterminate tensile stress.
Engineering is the practice of the art of science - Steve
