110' interceptor drain to stop perched water from interfering with my septic.

[mikefromgeorgia]

Gentlemen,
I am near Atlanta on a slightly sloping lot. A soil scientist reports that the area where my septic field is located is Alcovy with indications of a perched water table at 30-50". The septic field has an average depth of 70", which would make the deepest point around 80". The soil expert stated that this septic plan would no longer be approved, however since the septic functions well other than when we have heavy rain I may simply intercept the water before it flows into the septic.

I have found numerous references stating that to ensure I intercept the water, I should be below the depth of the field. The soil scientist agreed, much to my dismay as I would much rather only have to go down to 50". I have researched online for a few months and have read multiple threads on this forum discussing materials and techniques, but would greatly appreciate guidance to solidify my plan.

The plan is to use an excavator with a 12" bucket, starting at 90". Georgia requires a 15' offset from the septic field, so I'll comply with that. At this point I am planning on using 4" corrugated with slits, without a filter fabric, and granite sand up to about a foot from the surface. The total distance of drain will be 110 feet, and then the pipe will eventually daylight downhill.

Thoughts?

 

[oldestguy]

While a soil scientist may have enough background and "smarts" at draining ground water, I'd at least get an experienced geotechnical engineer's opinion. Their expertise (as far as I know) is in growing crops. Have you taken a series of hand auger borings, with slotted of otherwise suitable pipes in them to have a good grasp at the ground water table levels through the whole area? Is there a topographic type plan drawn up with ground and septic field elevations? Do you have a plan (on paper) showing the profile of your proposed work and a positive discharge location and showing proposed new ground water levels? Remember you cannot expect the water table down hill from your drain to be any lower elevation than the pipe? Have you tested the granite sand to know it is pervious? Some granite materials weather to a clay situation and may not drain well. I'd opt for ASTM C-33 fine aggregate (concrete sand) if there is any question and have it at least up a foot or two in trench and then your granite sand.

Will your project fix one area and then cause a problem elsewhere? Will a perforated pipe lose some if its water before it is supposed to and cause problems? Sorry, but I've seen too many jobs like this not work as first thought out, some built by engineers also.

Another thought. Is there any possibility of the up-hill interceptor also taking on any "gray" water from down hill where it may easily flow by gravity to that deeper drain? Is it suitably filtered so as to not cause a problem at the daylight discharge? Draw a cross section of the drain with the u-hill known water levels as well as the down hill water levels. Remember once under ground that water doesn't follow the slope of the surface necessarily.

 

[mikefromgeorgia]

Thanks for replying Oldest Guy! I've learned a ton from your posts and am thrilled that you monitor this forum!
Topographically, I have a continual slope that my septic field is in. All water in my 2 acre area moves from up the hill, toward the creek in my front yard a few hundred feet away. Am I correct to assume that the water within 10 feet of the surface likely tracks the topography considering that the restrictive layer from 30-50" was consistent?

I did consider measuring the ground water level, however from my research I am under the assumption that so long as the restrictive layer is reached and the trench depth is lower than the septic field depth the effect will be the same. Additionally, the ground water level is not an issue more than 2-3x per year. In the last week of December we got about a foot of rain in a few days. I dug a hole in another area away from the septic field and the next day the hole was filled with water. In the case of ground simply being totally saturated, I am attempting to simply remove the negative effect of the restrictive layer applying water from uphill into my septic. At this point I believe the percentage of water that seeps from landing directly on my septic field will likely not overwhelm the system an if it does I will follow up with mitigating surface drainage further.

My theory is that water lands on the area and moves downhill toward the creek. A percentage of it seeps into the ground, which then flows laterally at 30-50" until it hits my septic field which acts like a french drain. Is this sound logic? Also, with regard to the gray-water I assume that the 15' offset is to prevent this and although it would be easy to place the drain further away from the septic field, I see that many states only require 10' offset so I'm inclined to attempt placing it as closely as possible. Either way, the only way that I can think of completely preventing gray water that may seep the 15' and end up in daylight is to not go below the septic field level, which is counter intuitive to all the technical drawings I've found.

 

[oldestguy]

You are making progress. It would help with an attachment of a profile from up hill to down hill. Placing these things on paper will help you to plan. A simple hand level (or even use f a carpenter's level) set of elevation shots will help you a lot. It also will help you to see where your septic field flows to. It doesn't just disappear in the ground unless that ground is somewhat previous. Many tight ground disposal fields depend on a lot on evaporation from the ground surface. A post hole digger or hand auger set of borings showing the soil profile should help you. Perhaps that soil scientist can help on that score, since to size the field there probably was some sort of investigation possibly with test pits..

Am I correct to assume that the water within 10 feet of the surface likely tracks the topography considering that the restrictive layer from 30-50" was consistent?

That may be correct, assuming that "hardpan" is sloped also. That series of borings will show that. Once in the ground, only ground conditions control. What is the geology of the area? What is the pedologic soil series name? That may be in maps the soil scientist has.

You mention French Drain at the tile field. That "drain" is for sdding water to the ground, not draining it.

This forum is not usually to be used for designing anything, but to give ideas as to how to solve problems. That's why I'd suggest using a local geotechnical engineer at least for a general review of what you have and propose. Knowing your general location may help us.

What you have seems something like a situation with a moat around a castle, but with that castle also is dumping water into the ground. I'd not depend on what is termed "drawdown" to a drain. Only in very pervious soil does that come about effectively. Also, it would not be nice to see you cited for draining sewage water to a creek, in case something was missed in planning. Any such help should have had some experience with drainage of ground water, but very few civil or geotechncial engineers get any training in that regard. So far you seem to be on the right track, but more than one or two borings and that surface topo survey are needed I think.

 

[oldestguy]

You should have something like this to help in deciding elevations, etc. Is there really a barrier hardpan there?

You should have some reasonable assurance that you know the way water will move.

For instance setting the drain at location B may bring gray water into the system.

Quite likely there is at least some downward drainage there or the drain field probably would never be permitted.

 

[mikefromgeorgia]

I see your point. What you drew up perfectly depicts my issue.

Considering that addressing this problem would be really affordable if the trouble is from the restrictive layer at 30", what do you think about simply trenching at 3 feet and leaving it open for a rainfall? In this scenario my farmer logic says that during/after the next rainfall I should be able to see good outflow from the trench and also have no issue with field saturation if 3 feet is adequate. Assuming the trench does not collapse much I'll complete the sand and pipe if it was successful.

 

[oldestguy]

Your idea of different depths of observation holes is OK, except that to do that properly you need much more in the way of doing it. We use porous tips on plastic pipes, surrounded with sand at the level we want to measure and backfill the rest of the hole with a mix of sand and bentonite clay to seal that. Otherwise, with time each hole will fill to the same level. If I were doing the checking, assuming I am correct as to yur site, the observation holes would all be into water, set a piece of pipe (downspout pipe or similar), backfill and leave it. Record depth to water every so often, maybe once a week. Holes would be 20 to 30 feet apart over a broad area. I'm thinking you want to cover an area maybe 200 to 300 feet square, at least. You could drill some holes in the pipe to be a little more confident that you are not plugged down there. A couple in your drain field might help you see what is going on there. A hand level or a cheap transit would be ideal for summarizing exact elevations. Once you know ground elevation at each pipe, developing the contours of water elevations goes easily.

An open trench to get an idea of what is going on is fine, if it doesn't cave in. Saturated soil more likely will cave in. When you dig a trench that then will have a drain pipe, start at the low end and roll in the pipe early on behind the digging, backfill at least the pipe zone with clean sand immediately. That way you can be pretty sure it will go in right. Cave-in on that sand is then no big deal. You may have to hand clean out cave in that gets there too early on before the pipe is laid. Some up and down of that pipe may result, but it will work if the higher parts are still lower than your planned new water elevation. If you start at the top end first, you may find water coming at you that has no place to go.

 

[msquared48]

My brief comment is to reinforce the point that any french drain installed should be well above in elevation and above the slope to the drain field so that the French drain NEVER picks up any discharge from the drainfield. Otherwise you are defeating the purpose of the drainfield.

Mike McCann, PE, SE (WA)

 

[mikefromgeorgia]

Sorry for the delay. Last week we were about 5 days after a drenching rain fall. The septic had appeared to work fine since the day of the heavy rains. I decided to attempt to see if I could test the theory that the restrictive layer, which according to the soil scientist, starts at about 30". I bore holes down to 3 feet at each of the lettered bore holes depicted below.

The result was that A B C D all had water at 2 feet below the surface, which filled within 10 minutes of my drilling.
This common depth was despite the significant difference in slope over the area.

Predictably, G which was over the septic field, had no water even down at 5 feet.

E F H I had no water at 3 feet. I then dug down to my max of 5 feet and water slowly wet the bottom over about an hour.

So, here are my thoughts.
1. I believe that because ABCD, directly north of the septic field, all had water at a consistent depth of 2 feet, confirms that a restrictive layer is moving water laterally due to the fact that water depth mirrored the natural grade.

2. G being empty makes me believe that my septic is simply intercepting the ABCD water, and 5 days after this heavy rain, was intercepting the water and processing it in the septic field despite the pressure not being sufficient to cause a backup.

3. I could still have an additional restrictive layer moving water deeper than 5 feet that my testing would not show, however due to the fact that I found no equal water level at EFH my problem is likely from the ABCD water which obviously is problematic at 30".

Does this sound logical? If this is the case that my problem is this shallow, and as you both suggested it's not desirable to go beneath the septic field if I don't have to I should just go with a 4 foot trencher to minimize my cost.

Thanks again for your advice.

 

[oldestguy]

I'm a little confused. Wasn't the original post discussing possible up-hill drainage due to the excess ground water getting to the septic field? Then now the water table there is way down, not saturating the field? Have you installed pipes in those test holes to see what happens with time? Also, what is the story about the field not being approved? Isn't it in and doing its job? As to depth of the subdrain, the deeper the better, especially if it can extend to the barrier layer.

 

[mikefromgeorgia]

The original theory proposed by the soil scientist seems to be what the test holes validated. The soil has a restrictive layer starting at 30" and is moving water laterally at a consistent depth of 2'.

Your original posts did address the possibility of it being a rising water table, however due to the fact that the problem only comes about within hours of extremely heavy rains, I continued to think it was likely the restrictive layer issue. Thankfully my restrictive layer is close to the surface I guess.

I'm not sure where I said anything about approval. This septic has been in for 14 years and performs fine other than when we get hammered with rain, and even then it starts working within hours or a day of the heavy rain.

This would be a cross-section.

 

[mikefromgeorgia]

Oldest guy, I see the initial confusion. I said it was perched water. I may have misused that term as my belief is that the water descends and then moves laterally once it hits the restrictive layer. Like I mentioned with the test holes, I saw the test holes stabilize 2 feet below the surface despite the grade change in ABCD.