Pond Boss Forums POND CONSTRUCTION Soil questions Trenching to disrupt water table / flow

Hi all - reporting back on the leaking pond and seeking advice of course!

I have a 1/4 pond with tiers at 3, 7 and 10ft...all tilled & packed with bentonite at this point.

Problem is I still have strong water flow below ground that blows out the bentonite during the rainy season, then leaks out when the water table / flow lowers. Last fall a french drain (drain rock, perf pipe feeding collected water into the pond) was put in to disrupt the flow, (see very artistic rendering) but it was not dug as deep as the pond bottom it is trying to protect. So it’s still blowing out the bottom few feet of the slope and leaking. A little hard to see in the pictures, but the water is flowing directly in from the slope…the slope above is dry. The bare dirt is where the sides sloughed off.




Here’s the dilemma – the original trench was done incorrectly and I want to go back to them with a little more knowledge this time. How do I create a trench that’s deep enough to disrupt the flow and still collect the water? Feed it somehow into the existing trench? The feed is gravity based so if I trench deeper than the 2nd tier of the pond…I need to pipe the feed into the 3rd tier?

Or trench the water around the pond completely and release it below the dam to get rid of the pressure altogether?



From a tactical standpoint what options are there for trenching 10+ ft down, as narrow as possible? Guessing this isn’t equipment I can pickup at Sunset Rentals and do myself.

Any insight is appreciated…

Brad in Oregon -

Brad:

How many lineal feet do you have to trench? The NCRS agent in the adjoining county recommends (for sandy soil, and we're talking about a dug pond - no dam) the following for ponds that are leaching out water due to porous ground:

Dig a trench around the pond down to or below the water table, drop a sheet of visqueen vertically into the trench, then backfill being careful not to tear the visqueen. Pond liner is tougher than visqueen (but significantly more expensive).

I take it the water is leaking thru the dam? Hopefully some of the dirt experts will chime in shortly.

From digging French Drains around a house (to protect the basement), here's what we'd do. Dig a trench down below the basement floor, line said trench with weedblock fabric/landscape fabric. Place perforated tile, holes down, covered with a silt sock on the bottom of the trench. Fill in trench to 12" of surface with 1" gravel. Fold remaining fabric over trench and gravel, and fill the rest of the trench up with topsoil. We'd slope the bottom of the trench 1" for every 10' of run. Depending on your pond and soil type this could get to run into some $$! We were doing most of the drains in heavy soil, so it would keep it's shape and not collapse. All of the work was done from the top, you wouldn't want to be in a trench 10' deep and have it collapse on you!

Good question on the leak location, forgot to cover that. It appears that the water is going straight back down into the water table through areas that slough off. We packed bentonite in the deepest section 2 years ago and that held fine.

now we've packed it everywhere and the leaking stops right at the bottom of the slough areas...won't really know until drier weather.

the original trench (much like your description) is almost 200 feet long along the uphill edge of the pond. I think the next round could be maybe 1/2 that distance. The first 100ft of the original trench seems to be working because that area of the pond is only 3ft deep.

For the visqueen idea...does the water then go downward? towards the edges? wonder if I could visqueen / line the bottom 4-5ft and then channel into the existing trench to give the water an outlet.

I've seen a similar idea packing the trench with bentonite to create a water barrier. forces the water to go elsewhere, I'm just not sure where that would be given the slope I'm on.

Ideally I need the water to fill the pond...but I also need to control it. Pipe dreams !

Have thought about the visgueen before . The trench caved in before the the visqueen was in place completely.

Let us know how it turns out this could be a good deal.

If the soil lets water in and out on the sides then it could also do it on the bottom.

Hey T: I just want to echo what esshup said about safety. (Sorry, it is what I do for a living). Trench excavations are very dangerous, especially when working in non-virgin soil. If you go to ten feet, do everything from the sides of the trench and make sure that the spoil pile is at least two to four feet away from the sides to give you room to work safely from those sides. There are lots of other considerations and options, but the safest is to check with a reputable contractor and make sure that safety is a priority.

No need to apologize Victor and good input. People get killed every year because of trench cave in.

Thanks guys - I do appreciate the safety notes too. One of the main reasons behind my DIY question. I'm okay admitting if a project is over my head...

How heavy is the pond liner / visqueen stuff? Trying to work out how you drape 100ft of it in the trench, hold it firm up top and backfill.

This will not be a one man job.
A lot of hand work with a shovel to get started.
Backfilling the trench without letting the plastic fall in with the dirt will take some time and patience.

Hey all - time for an update and any helpful thoughts from you cuz I'm really confused !

Just in time for the rainy season, put in 18" wide trench around the top 190' of the pond. Runs from 8-10ft deep in order to get below the 1st two levels of the pond. Deepest level held water all summer so I wasn't worried about that one. Lined with trench fabric and backed with 1.5" drain rock.

we've had close to 8" of rain since then and the pond level is up over 18"...but I can also see that the trench did nothing to help. Water is still flowing out of the slope as in the picture above...maybe 5ft below the full pool level.

Feel like I'm watching a magic act. Trench is deeper than this level of the pond so water flow down the slope has to be intercepted. (sloped and piped into the deepest section of the pond).

Could be coming up from the bottom, but 10 ft to the right or left has nothing. This is also the highest part of the natural slope (pre dig) so I would expect to see water coming into the lower levels 1st.

I dug a couple test holes 12-18" deep in the area of the top picture where I could see water. Coming in from seams, bubbling up from the bottom...

Plan on draining the top few feet this weekend to see what's really happening now that we're into the wet season. Could be stuff going on that's usually hidden underwater.


Any thoughts? Tough to visualize without more pictures...but I'm really mystified at this point...