Digging a trench for a sewer line

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Cabin fever, Jun 10, 2012.

  1. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    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..
  2. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    why are you restricted to 1/8 inch per foot? The easiest way is to use a laser level with a grade stick.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,245
    Location:
    Cave Creek, Arizona
    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.
  4. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    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.
  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,245
    Location:
    Cave Creek, Arizona
    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.
  6. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    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.
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,245
    Location:
    Cave Creek, Arizona
    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.
  8. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    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?
  9. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    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.
  10. leejosepho

    leejosepho DIY scratch-pad engineer

    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: Jun 19, 2012
  11. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,245
    Location:
    Cave Creek, Arizona
    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.
  12. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    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.
  13. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    What is the reason for using larger pipes for lower slopes ( eg. 4" pipe for 1/8"/ft) ?
  14. leejosepho

    leejosepho DIY scratch-pad engineer

    Oops. You caught me thinking about a new installation where the drain field did not yet exist!

    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.

    If you cannot get one, just use 100' of clear 3/8" or 1/2" tubing, some washer fluid and a couple of yardsticks.

    Not sure about that one, but it likely has something to do with the physics of hydraulics.
    Last edited: Jun 20, 2012
  15. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    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 a moderator: Jun 22, 2012
  16. leejosepho

    leejosepho DIY scratch-pad engineer

    I have never tried to pour washer fluid into a tube quite that long, but I can imagine that would be tedious. Plain water will work, and you can fill the tubing with that by using a garden hose.
  17. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,245
    Location:
    Cave Creek, Arizona
    Force water through it with a hose or raise the hose next to the bubbles and keep them moving until they reach the end.
  18. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    Ok, I got the water level to work and got more accurate measurements. The distance from the cabin outlet to the septic tank inlet is 82 feet. The verticle drop is 11 inches. That gives me just about 1/8" per foot slope. The pipes will have a 90 degree bend starting at the cabin and two roughly 45 degree bends along the way to the septic tank.

    So. With the above information, do you think the sewer will work by gravity? Also can I use 3" pipes instead of 4" pipes? If not, why? Thank you for all the help.
  19. leejosepho

    leejosepho DIY scratch-pad engineer

    Be sure to use a long-turn or "sweep" ell there and not a regular 90.

    "Roughly?!" ;)

    45s are 45, so do some careful layout work ahead of time to be sure your parts will get you where you need to go.

    I used to work for a septic-tank manufacturer, and I was truly suprised by some of the things I learned there. A 24" storm drain there on the property ran for about 100 yards with barely any fall at all ... and yet never plugged up. There are likely some engineers here who can explain how all of that works, but no, the bigger the better in your case.
    Last edited: Jun 29, 2012
  20. Cabin fever

    Cabin fever New Member

    Messages:
    16
    Location:
    Texas
    Thank you very much for the great tips. The reason I said "roughly 45 degrees" is because I have to make a couple turns to get to the septic tank. I will do a dry fit before connecting everything permanently.

    Does it matter which end I start laying the pipes? In other words is it easier to run the pipes from high to low or low to high?

    Thanks.
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