Sanitary line slopes – pitches that are outside of the norm
Working on the sanitary design for our vacation home up north. The site is perched on a bluff about 70′ above the road. The local planning department would not permit a driveway be put into the home site. Top soil ranges from just inches to 3′ in the deep pockets. The Soil & Water department is requiring us to us a water based sanitary system and after extensive work we have conceptual approval. The system will be based on a holding tank located next to the road. The tank will be about 300′ from the house and there is a vertical 50′ high cliff the pipe will need to go over. S&W is aware of this and we anticipate them approving a variance to not have 18″ of soil cover. As homeowner, one of my jobs is to build a “flume” of sorts to support the waste pipe, a water pipe and some electric wires as they tip over the cliff face. Although I can eventually go vertical, it won’t be right away. Because of boulders, we need to kick out from the face of the cliff a good 50′. Rather than running out horizontal, I was hoping I could just point the pipe at my desired target and pitch it down immediately. What I’m wondering is what pipe pitches I should be targeting. I mean, I don’t know how pipes are sloped when they break out of the standard 1/8-1/4″ per foot range. I’m guessing that we’ll be in the 2-4″ per foot range.
Replies
Well
not scientific but.
Our home is located 38'-45' above the sewer line. We ran 6" plastic down the hill (couple hundred ft) which was approved and has worked for 20 yrs.
One would venture that in case of a blockage from sediment not being carried down that it would be a mean snake job to clear it out.
We however have the benefit of domestic use wastewater going down also, not just a flush.
Your example confirms that "excessive" slope is no problem, because if it was, the 6" line would clog more readily, due to reduced scouring action that large pipes have compared to smaller ones. I'm assuming that 4" would have met the sizing needed for your house, but that you decided to go bigger.
The sewer excavator chose the size..........
And did a very nice job.
Instead of laying the dig out back high on the trench, I had him pack down level. Gave us a nice slight hollow for coaster riding through the woods in the winter.
This is in the woods.............
you win IF you make it to the bottom.
Man what a blast!
This may not be relevant to your situation, but I can think of one other pitfall. And this is from designing big gravity sewers.
If you have a line that changes from high-slope to low slope, and it's running near capacity, you can have problems. Basically, the water goes from supercritical (rushing) flow to subcritical (slower, deeper) flow. If the velocity downstream is in the right range, it will entrain air but not push it on through the pipe. You get a big air bubble that accumulates until it eventually forces its way back upstream to the nearest vent/manhole. There you get a geyser we refer to as a "hydraulic cannon." Can pop off manhole lids and generate all kinds of drama.
Probably not relevant but that's what I thought of when reading the OP.
We had a sewer cannon go off here in town a few years ago--a guy dumped his lawnmower gas down the storm sewer, then realizing that was a bad idea, tried destroy the evidence by dropping a match down. Blew manhole covers skyhigh and cracked the concrete storm sewer up over several sections. Cost the city over $30k to repair--I don't know if they ever got any $ out of the guy.
I don't know if this applies, but water/wastewater systems that flow steeply and change directions need a thrust block designed to prevent the elbow from blowing out. Don't recall under what circumstances this might be required. Water flowing down hill steeply has a lot of force when it changes direction. Thrust blocks are common in plumbing designs to prevent the blowout. Basically it is a block of concrete cast (I think) into the elbow of the piping. This provides adequate support for the elbow (even one that is buried).
I believe that is more of a problem in cases where the pipe runs full, as in fire hydrant lines and water mains, or, in the case of sewer lines, it could be a problem in lines that flow full because of being pumped uphill (as in the lines from a lift station that are "forced mains).
You are probably right. Domestic waste is fairly intermittant and generally pretty small in terms of bulk volume. It's not like the sewer line that may fill up w/ 20 houses all at one time for short periods.
Actually, intermittent is worse -- you get water hammer. Water flowing at a steady rate won't try to move the pipe (though it may try to bend it).
Water hammer in a sewer line? That's a new one on me.
Why?
Other than a forced main being pumped up from a lift station, I can't imagine a sewer line running full enough and fast enough to cause water hammer.
I have never known of it or heard of it myself, and have never seen a design for a sewer line that included thrust blocks. Have you?
50' vertical drop warrents "thrust block" type support
nuff said.
Consider that the water free-falling 50 feet could achieve speeds in excess of 30mph.
Thrust blocks
I have never known of it or heard of it myself, and have never seen a design for a sewer line that included thrust blocks. Have you?
Yes, but not for a residence.