Hello, I'm grateful for any advice that can be provided. I live on a storm water pond in Minnesota (man-made) - it was part of a master building development where ponds and a large lake were dug out, sand was mined to create building foundations for homes, and (I recently found out) the pond banks were lined with Peat around the perimeters.
We had record rainfalls last Spring, and a few of our pond shorelines collapsed where the sand met the peat (I am told the peat got saturated, dried out, and caused the collapse). We've had soil testing done to confirm this is the case.
My question is where do we go from here - I attached pictures of where the slough started - this is about 1/3 of the depth it is today (4 feet drop off in backyard, 6+ feet drop off/pond bank no longer exists). It is a cliff. I am told by the city that the proper "repair" is to dig out the entire peat area, replace with sand and restore the bank, then (possibly) dig and install a french drain across the property to help with drainage so the area can't get saturated and sink again.
I've had a dozen landscapers, county and city officials look at the problem, and offered solutions such as boulder walls, fill with light material (styrofoam or wood chips), one person said fill in with more peat, one said compacted rock in 4" lifts. The cheapest solution was $6200, the city's recommendation was $300,000, which obviously is out of the question.
This is the first time this collapse happened in my yard, but the second time in my neighbor's yard (first time was in 2011 when we had excessive rain as well). This time it sunk 3X deeper, however.
Any help is appreciated, my fear is that if left untouched the area will erode over time and eventually affect our deck foundations and then our home foundations.
What is the slope going to the pond? We have a lot of ponds dug in sand in this county. The banks are not stable (they slough off into the pond - sometimes like yours is doing) if the slope is steeper than 3:1. (1' drop vertically in 3' of run horizontally)
The slope of the pond *used* to be 3:1 I believe or whatever the city approved when the home was built for storm drainage standards - it was a gradual slope and looked fine. Now there is no slope. The land is flat, there is a cliff, then it's flat again, then where the bank used to be are cattails.
As far as the size of the pond, a rough estimate is 400' long by 150' wide. There are 26 homes around the perimeter of the pond, but only two have this severe erosion issue. There are 4 other homes with varying levels of other issues. The next most severe/costly is one homeowner who had a fence built close to the shoreline and the fence is collapsing.
Thank you to the person who posted the link to a similar issue. That link very accurately describes the same problem I am going through. I spent the entire summer contacting every entity known to man and nobody is taking responsibility for this. I have contacted an attorney and am told there also would be no guarantee to sue anyone, and the lawsuit could take 2 years.
I can't take any recent pics because we have snow right now but the sunk-in area is now quite large. I should add that I am on a corner lot, where there is a drainage culvert and our corner never freezes - and that the entire pond perimeter is completely covered by about a 10' ring of cattails.
Be thankful for the cattails, without their extensive root system the erosion probably would be much worse than it is.
If you can get some heavy equipment in there and make the slope 4:1 or even more gradual, your problems should stop. The slope should be carried out to about 1' water depth when the pond is at it's lowest water level. That slope will allow you to mow to the waters edge too.
I've seen retaining walls made from bags of concrete where the bags were just put in place and the concrete hardened in the bag. I don't know for sure, but I believe rebar could be driven thru the bags into the ground to help hold them in place before the concrete sets up.
Or, a steel interlock piling system could be pounded into the ground to create a seawall and stop the erosion.
Sorry if this is a dumb question but if we did a concrete bag wall (very interesting idea btw), could we then bury the wall and even out the grade/slope? Or would the wall only work if we had a 2-tiered yard?
My neighbor's (and part of my) yard is sunk down about 4 feet and I'd say 1/3 of his backyard is sunk in the shape of a rectangle. So if we left the sunken area as-is and did the wall, all that would grow there is weeds. Wondering if we could build the wall, and grade from the top of the wall into a slope down to the pond's shore like you mentioned, then sod the gradual slope without being able to visually see the wall. I would imagine our HOA is going to complain about the look of a visible wall of this nature. Just thinking ahead.
I am using the stacked bags of concrete and it works really good except it isn't pretty to look at. One could use the concrete then add limestone rocks to backfill and in front of concrete to "coverup" the wash should then fill in around the rocks and stop erosion then add grass and sand to fill in gaps . All the fill should be tapered to suit like Esshup said
Isn't it funny how nobody will take responsibility for anything anymore?
I have seen the concrete retaining walls many times here. And I have also seen metal and such walls. Some of the metals have turned out bad due to currents in high water, washing out behind the wall. Just bad over all. I think the cement bags look the best and if done correctly it can be nice. Some I have seen were laid in contour to the land to the water. I have seen some eye appealing walls. At 4 per bag and backfill it might also be the cheapest way.
Do not judge me by the politicians in my City, State or Federal Government.
The way I would approach it would to take a track hoe and pull the sand back up into position. Have rip-rap hauled in and with the same track hoe place it along the bank and down into the water as far as needed to hold the wall in place. Then get grass growing again on the newly excavated soil.
But I'm a farmer, not a contractor, so that is just an idea. No experience with sandy soils like you have.
Here are three links that do not address your problem directly, but they may give you ideas. You would probably need much larger rock than what is in some of the pictures to actually stabilize the bank.
Seems to me you have a shallower version of a sink hole going on. The soil under the top soil had to be washing away for it drop that far. I wonder next, where is all that soil going? Is there a cavern, or is it filling the retention pond? If going into the retention pond, flooding could become an issue with the reduced retention capacity.
I'd nix thinking about a lawsuit! You will pay a lawyer and even if you found someone to pay the repairs, the costs of the suit would probably be more....
According to a soil expert, the collapse was due to Peat. The Peat became water logged (multiple weeks of steady rains), upon drying, the peat disintegrated/collapsed, causing the sinkage. The peat was described to me like a sponge - when a sponge gets wet, you can put a lot of weight on top of it and it compresses as much as you can squeeze it. Nobody knows how deep the peat goes. One solution involved putting lightweight material (wood chips and styrofoam were mentioned) on top of the sinking peat, so that it doesn't compress further down. Styrofoam kind of makes sense because it won't degrade if you use the industrial blocks. Wood chips don't make sense to me, wood rots over time and will sink?
And I agree about the lawsuit. Our home is almost 10 years old and no landscaping is warranted. My sister in law is an attorney and she said we probably would be out tens of thousands even if we win the case. I even had the city council and Mayor out here for a meeting to look at it, they sympathize but won't put any money into it.
If the pond banks were lined with peat, then sand placed on top, I would have an excavator dig out the peat first, then backfill/taper with sand/clay. If the problem was caused by the peat, no matter what you do you will have the peat waiting to cause another problem if it stays there.
A building/wall/dam/etc. is only as strong as it's foundation.
I don't know...Your soil expert's explanation doesn't seem to hold water to me (pun intended). If the area was developed over 10 years ago and is just now collapsing, fill HAD to be at least gradually getting washed out. I would be pretty confident the peat had saturated and dried many times over the years. Granted, fill has to be brought in at this point, but I wouldn't buy into your soil expert's opinion...I'd look for, and correct the cause of soil loss.
In our subdivision, we have many steep hillsides built so yards and homesites could be leveled. A few years ago, our proactive city installed storm drains in some yards to divert runoff that was creating tunnels in the hills and causing collapses all over. During a heavy rain, my hill and back yard looked like it had fire hoses blasting water up from the ground with all the sub-surface water flow. I had to shovel mud out of my yard and re-fill the hill side many times.
The storm drains solved all the runoff issues till last year, when moles became a problem for us and neighbors. Now, we have tunnels all over the hillsides and more erosion, collapses than ever.
Did your retention pond ever have muskrat issues? What about moles or other burrowing critters? SOMETHING had to create a path for sub-soils to be removed....
Yes we have muskrats, voles, and gophers. Most of the voles and gophers are gone now because the area is more fully developed and they were scared away by people and more noise. This year we saw fewer muskrats as well. There is a clean line/break where the sand meets the peat soil - you could almost take a huge straightedge and place against it it's so straight - I don't see how an animal could have burrowed out that perfect of an area, but I am no expert on pond critters.
I should have clarified also that this is the second time this happened. The first time was in 2011 and was not in my yard, only my neighbor's yard. But when it happened the first time, it was in the exact same spot/square area as it happened this year - only this year it was 3 times worse/deeper.
I believe if we did a retaining wall we'd have to dig back 2-3 feet where the "good" soil remains, not place the wall foundation on top of where we believe the peat to be.
Jim T, I used to have a picture of a concrete bag retaining wall that was posted on the forum but I have since deleted it. It looked nice, and was maybe 6 feet high, much higher then you would need.
Could you take and post a few pictures from your yard out towards open water? I would like to see how the current water level comes up against this subsided ground along the lake shore line.
Submar Mats would probably work. You would have to contact the company for straight up prices. http://submar.com/
But I am going to have to go with Rainman on this one and say that there is something more going on here as in the bank is being eroded underwater somewhere and that is actually soil shifting towards the pond by the look of the picture. If it was just a soil supersaturation issue it would have more than likely seen an up-heave and then a return to normal saturation. I have seen similar things to your picture first hand when excavating underwater that if you undercut your ditch wall it will form a fracture and then eventually an all out collapse.
Anyways hope this helps I can not give you a cost estimate you would have to do the legwork on that yourself. I have used both these products and they work wonderfully for retention purposes underwater.
Yeah the burlap will decompose over time leaving just the blocks of concrete that will just look like stacked stone if you take the time to lay them out which actually doesn't take that long. I have placed thousands of these things underwater very easy to handle. The submar mats are the more expensive alternative but they have specially designed ones for erosion issues on residential properties.
The burlap sacks are nice to pre-mixed just add water or wait for rain and viola you are done type of thing. Hope this helps.
Yes we have muskrats, voles, and gophers. Most of the voles and gophers are gone now because the area is more fully developed and they were scared away by people and more noise. This year we saw fewer muskrats as well. There is a clean line/break where the sand meets the peat soil - you could almost take a huge straightedge and place against it it's so straight - I don't see how an animal could have burrowed out that perfect of an area, but I am no expert on pond critters.
I should have clarified also that this is the second time this happened. The first time was in 2011 and was not in my yard, only my neighbor's yard. But when it happened the first time, it was in the exact same spot/square area as it happened this year - only this year it was 3 times worse/deeper.
Jim, I didn't mean to imply an animal made the clean fracture lines. I was betting that underground critter tunnels created the original, hidden, sub-surface erosion that resulted in the fractures...most likely when soils saturated and could move more easily, and have more sheer weight. I would also bet the original tunnels were never filled in, and more critters expanding on those made it come into your property.
