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Thread: Digging a trench for a sewer line

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    DIY Junior Member Cabin fever's Avatar
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    Default Digging a trench for a sewer line

    Hi all,

    I am planning to run a sewer line from the cabin to the septic tank. I will be using 4" PVC pipe and the pitch will be 1/8" per foot. My question is how do I dig the trench to get the proper pitch? The ground is not level and so I cannot just measure from the top of the ground. Any suggestions will be greatly appreciated. Thank you..

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    Plumber Winslow's Avatar
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    why are you restricted to 1/8 inch per foot? The easiest way is to use a laser level with a grade stick.

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    Moderator & Master Plumber hj's Avatar
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    Dig the trench, then put an 8' piece of 2x4 on the bottom. Put the level on the board, and then dig under the board until the level shows the correct pitch and there is support all along the board. Or dig the ditch, install the pipe while checking the pitch as you go. You probably only need 3" pipe, if you can get 1/4" per foot grade.
    Licensed residential and commercial plumber

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    DIY Junior Member Cabin fever's Avatar
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    thanks for the suggestions. the reason i'm going with 1/8 inch/foot pitch is to minimize the depth of the a new intake hole in the septic tank. my problem is the original intake is too high to give me enough slope for gravity to drain the sewage. therefore, my plan is to drill a lower intake hole that will give me the minimal require pitch but is still higher than the overflow hole into the second septic tank.

    my other question is are there certain connectors that i will need to acheive this slope, or do i just use couplers and have the the pipes slope down by weight of the backfill dirt? thanks.

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    Moderator & Master Plumber hj's Avatar
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    Your entire plan is faulty. The inlet hole is at the OPTIMUM point and should be about 4" above the OUTLET hole, not the pass through to the second chamber. You cannot just arbitrarily drill a new opening in the end of the tank. Someone "screwed up". Either the tank was not installed deep enough, or the plumbing was not installed so the slope would be adequate, depending on who was there last, usually the septic guy.
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    DIY Junior Member Cabin fever's Avatar
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    I think the septic tank was not buried deep enough. The inlet is only 3-4 inches below ground. When I purchase the property the septic tank, foundation and rough plumbing was already there. However, the plumbing has not been tied to the tank. I had the plumbing redone in accordance with the new floor plan. The main drain coming out of the cabin is as shallow as the plumber could make it. But it is still not high enough to achieve the 1/4 in per foot slope. I guess I will have to install a sewage pump system.

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    Moderator & Master Plumber hj's Avatar
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    quote; The main drain coming out of the cabin is as shallow as the plumber could make it

    There is sometimes a big difference between as "shallow as possible" and "as shallow as YOUR plumber could make it". But at this point it is no longer negotiable. The inlet is usually about 12" below the top of the tank, which would usually put the lid above the ground at your dimension. To go with 1/8" slope, you will have to use 4" in most areas.
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    Plumber Winslow's Avatar
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    You said the ground is not level. Is it sloping toward the septic tank. How did you measure the difference in elevation between the invert at the house and the invert of the inlet on the tank?

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    DIY Junior Member Cabin fever's Avatar
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    I strung a level line between the two ends and measure the verticle distance from the top of the pipes to the string. The difference between the two readings tells me how low the septic tank inlet is relative to the cabin outlet.

    If the septic inlet is 12" below the top of the tank then I need to excavate further to investigate the septic end. The pipe going into the tank is only about 4" below the top of the tank.

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by Cabin fever View Post
    ... not high enough to achieve the 1/4 in per foot slope. I guess I will have to install a sewage pump system.
    Not if the far end of the pipe is lower than where it exits the house and you can get 1/8" slope with 4" pipe (like hj has mentioned). I once had only 3.5" fall available for a 35' run (giving me 1/10th" per foot) and that line worked just fine. However, a line with low slope must be straight -- no dips or high spots in the run -- from one end to the other. And, that can easily be checked with a string before the pipe is covered. Also, a water level (a length of clear vinyl hose and some windshield-washer fluid) is a great and relative inexpensive way to check your actual grade beforehand if you do not have access to a transit or laser level.

    Keep the tank inlet right where the manufacturer placed it ... and if you absolutely cannot get enough fall, have the tank lowered before moving away from a gravity system. Sewage pumps are for elevating sh-t, and sooner or later they get tired of doing that and must be repaired or replaced (where nobody will ever figure out a way to charge a power, maintenance or replacement fee for gravity).
    Last edited by leejosepho; 06-19-2012 at 08:13 PM.
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    Moderator & Master Plumber hj's Avatar
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    You CANNOT just "lower" the tank, because you would also have to lower all the effluent piping. A string is a poor method of checking elevations. A "string level" is so short that it is not accurate for the entire length of the string, only the section it is on.
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    DIY Junior Member Cabin fever's Avatar
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    You're right. You cannot just lower the septic tank without messing up the drain field. I will try to get a laser leveler to get a more accurate measure. The distance between the cabin and septic tank is 80'. That means I will need a drop of at least 10" to achieve the 1/8" per foot slope.

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    DIY Junior Member Cabin fever's Avatar
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    What is the reason for using larger pipes for lower slopes ( eg. 4" pipe for 1/8"/ft) ?

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    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by hj View Post
    You CANNOT just "lower" the tank, because you would also have to lower all the effluent piping.
    Oops. You caught me thinking about a new installation where the drain field did not yet exist!

    Quote Originally Posted by hj View Post
    A string is a poor method of checking elevations.
    I was suggesting a string only to be sure the line has no high or low spots along its grade ... kind of like looking at a guitar string to see whether the neck is straight. Personally, I would never use a string-level for anything. And for an 80' run, I would check the two ends and the middle with a water level for grade and then run string between those points to check for straight.

    Quote Originally Posted by Cabin fever View Post
    I will try to get a laser leveler to get a more accurate measure.
    If you cannot get one, just use 100' of clear 3/8" or 1/2" tubing, some washer fluid and a couple of yardsticks.

    Quote Originally Posted by Cabin fever View Post
    What is the reason for using larger pipes for lower slopes ( eg. 4" pipe for 1/8"/ft) ?
    Not sure about that one, but it likely has something to do with the physics of hydraulics.
    Last edited by leejosepho; 06-20-2012 at 10:02 AM.
    "Great minds discuss ideas, average minds discuss events and small minds discuss people." --Eleanor Roosevelt

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    DIY Junior Member Cabin fever's Avatar
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    I couldn't find a laser level so I tried using a 1/2" clear tube as a leveler. However, I spent over an hour trying to get all the air bubble out of the 100' long tube. Was not successful. How do you fill the tube without getting air bubbles? The reduced slope as the pipe size increases is counterintuitive, but the assumption is that the larger pipe will usually have a greater volume of water flowing through it. This is also the reason that 6" and larger main line sewers have 1/16 or 1/32 slopes.
    Last edited by hj; 06-22-2012 at 07:01 AM.

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