Level trench digging

doane001 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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 · May 11, 2005

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!

EddieWalker · May 11, 2005

When I do it, and I'll be doing one in a few months. I check with my laser level about every 20 feet, give or take a few. It's real easy to drift up, or down, when looking at the trench from the height of the seat.

Since you've never run a backhoe before, I'd also suggest leaving the engine idle while operating until you get comfortable with the controls. I've seen people pick it up in a few hours, and for some it takes days.

dummy · May 12, 2005

for some it takes days Or longer. Oh wait, that was me! But if you stop learning things and trying new stuff, wouldn't your "quality of life" factor degrade quickly?

MadDog · May 12, 2005

Amen to that!! Stop learning and you'll probably stop enjoying your life and when THAT happens it's not just "quality of life" that suffers; your health is apt to decline, too.

I first ran an FEL and BH last spring at the age of 64. This summer, I am going to learn to weld. /forums/images/graemlins/grin.gif Unfortunately, my hair is getting thinner anyway. /forums/images/graemlins/blush.gif

RoMo · May 12, 2005

Whatever You do, Please do not get in the ditch (Yourself or anyone else} and level, as there are people that die from cave-ins.

Highbeam · May 13, 2005

That's a whole different discussion. Depends on the trench. Standard drainfield trenches are not too deep, less than 4 feet which is the cutoff for trench boxes. You need to get in the trench to do the work. I suppose you could dig a huge trench and buy trench boxes if it bothers you.

John_Mc · May 23, 2005

( 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.)

Are you sure you didn't get an extra "0" in there on those measurements? .005" is about the thickness of a couple of human hairs or a sheet of paper. A grain of sand is bigger than that. .05" is about double the thickness of the wire in a typical paper clip... tough to hold that over 10 feet.

John Mc

HomeBrew2 · May 24, 2005

I know this is an international site and, even in the US, local ordinances vary but, I thought I'd chime in to relate how real life is here in the foothills of central Kalifoenea. My experience, only dates to '79 and is rural residential specific. For the record, I have read all the theoritical septic design lit. In practice, around these parts, with some of the most restrictive env regs in the country: 1) as another poster pointed out, all leach lines are dug as deep as the contractor's hoe will dig ~ trench side-wall "counts" as leaching surface area 2) All attempts are made to keep the trenches mostly flat/level 3) Nobody lasers or sprirt-levels trench bottoms and no inspector does more than a cursory check regarding levelness 4) Nobody, not even the grungiest of contractors, puts anybody in a trench deeper that waist-deep without a shield ~ THAT worries everybody 6) Nobody pulls a shield for a leach line 7) Under "standard" circumstances, no leaching failures occur under these circumstances. My experiences are however limited to: fully developing 2 properties myself, and those experiences over 2 decades of friends and family in the same area. For reference, around here, the top 2-3 feet is bullet-proof clay, under that is usually decomposed granite.
Cheers!

LBrown59 · Jun 9, 2005

( 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. )
**********
Uniform sand is not much different than gravel with a pretty high void ratio
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What does VOID RATIO Mean???

Highbeam · Jun 9, 2005

Most all uniform material, uniform in grain size, has a void ratio of about 30%. That means for every 100 cubic feet of material, there is 30 cubic feet of air or water. The void space is what allows your effluent to pass through and soak in. No voids and you have a waterproof material.

Well graded material with a broad mix of big and little stuff will have fewer voids since the small material fills in the spaces between the big stuff pretty well.