Level trench digging

[doane001]

I will be renting a BH to install a conventional septic system on my property and need some guidance on how best to dig a level trench. According to Environmental Health, the bottom of the trench should not deviate more than a quarter of an inch in ten feet.

Can anyone suggest methods they use to ensure a level trench. I have operated lots of tractors in my time, but never a backhoe. I have read a considerable amount about backhoe operation, unfortunately, none of it refers to specifics about digging a level trench.

Any help would be greatly appreciated.

Thanks,
Ed Doane

 

[Bird]

Ed, I don't think the "trench" has to be exact. In fact, what I've seen is for the trench to be a few inches deeper than the perforated pipe for the leach field. Then you put gravel (crushed rock) in the trench and spread it in such a manner that when you lay the pipe in you have about a quarter inch per 10 foot drop, cover that with additional crushed rock, then a barrier fabric, and cover with dirt.

 

[Highbeam]

It needs to be flat. Otherwise the effluent will travel to the lowpoint and saturate that area. Once the effluent leaves your perf pipe it travels down through the drainrock. Once at the bottom it hits soil and gets absorbed at some rate, often this rate is slower than what you are sending down the line so it puddles and then flows along the trench bottom.

The uniform drain rock provides about 30% void space to act as a conveyance and also for a storage space.

You will overload the lowpoint of the system leading to failure of that area. Which will overload the rest of the lateral until you experience system failure.

Best to use a laser level or even an optical level to check grade as you go. I can't imagine that you need to be within 1/4" but the flatter the better.

 

[Bird]

That makes sense to me, Joe. I just didn't think it had to be as exact as that quarter inch per 10' measurement; maybe it really does.

 

[JerryG]

Ed,
It takes a lot of practice to be able to dig a level trench. There is no way that anyone (that includes me when I first owned a hoe) can dig a level ditch without lots and lots of experience. If you want to dig it yourself and have it level, I would suggest that you dig it the best that you can and then pour a thin layer of sand in the ditch. That way you can take a rake and level it out the required amount. Also, when you are digging, take it slow, that way you can master the controls and have less work with the rake.

 

[dummy]

Level comes in more than one direction. From the seat, level left to right and front to back. An easy way for left to right is to lay a "bubble type" level across the control area when you adjust the outriggers. That will help ensure a flatter bottom. Front to back is either laser or transit time. You might want to double check the 1/4 in 10 parameter. Are they asking for a true flat grade or do they want 1/4 in 10 slope away from the distribution box? What depths do they have you setting the trench and pipe?

 

[dummy]

Maybe, depends on what their local engineers are looking for I guess. We tend to look more at the overall project and determine from there how to set it up. The maximum depth for pipe is 5 feet (here, according to the last time we checked), but the trench can go deeper depending on the site. Last summer, had to redo a failed field. New trenches were 18 feet deep. Yep, that meant 13 feet of gravel before we laid pipes. Needless to say, wasn't too awful concerned about the grade on bottom /forums/images/graemlins/wink.gif

 

[doane001]

Thanks to all of you for replying. It answered the questions I had. I live in a county where Environmental Health and Building Inspections are very, very, very strict!

Thanks again,
Ed

 

[Highbeam]

In that case, get it as close as you can with a machine and then get in there with a rake and carpenters level to clean up. I recommend using the native dirt in the trench to fill the low spots so that the absorbtion rate is pretty close to the rest of the trench bottom. Uniform sand is not much different than gravel with a pretty high void ratio.

We are pretty much splitting hairs as I'm sure there have been many systems installed by eye that function just fine. If the soil is pretty tight like silts or clays then I would be especially careful to get it right. Sand and gravel, well, it's hard to mess it up to the point that your system fails.

 

[Ronzonie]

The trench does not have to be within 1/4". There is useually 6" to 1 foot of stone in the trench. The pipe sits ontop of the stone,that should be .005 per footor .05 for evey ten feet of perf pipe.You should ues a laser, with a receiver. The best way to set it is to put some stone to set the pipe on,leave it a little low,put smoe stone ontop of the pipe set the pole on the pipe with the receiver and ift the pipe until you get a solid tone on the laser. Then fill around the pipe. It's not as hard as it sounds. Good luck!