Is my dam collapsing?

[Briarwood]

The footers were poured for our new house last week and the cement trucks had to travel over the 150 foot long earthen dam, not to mention all the other heavy equipment and gravel trucks, to get to our home site. Our gravel lane crosses the dam for our 3/4 acre pond. The pond was built in the 1960's and we have only owned the property for the last 4 years so I am not familiar with how the dam was built. There is a large diameter concrete tile that passes through the dam at one end for overflow. I just noticed this morning that there is a hole the size of a dinner plate in the tire rut on one side of the lane right over the location where the tile passes under it. Do you think the tile might have collapsed? What do I do next. Of course I'll talk to the contractor Monday because he is scheduled to pour the basement walls next week but I'll worry alot until then. What do you think? My wife says to just fill the hole with gravel and forget about it. I just envision the hole getting bigger and bigger and and deeper and deeper until we have a real problem if we don't already.

 

[DougL]

Briarwood,
Can you see inside the tile? How deep is the hole? Is there any dirt coming out of the overflow pipe? I would check these things first. Not knowing the construction of the dam it could be a point where there was some debris which rotted away and the weight and vibration of the trucks has caused it to settle. Also, what has your weather been like lately (rain or dry)? Do you see any water seepage from the backside of the dam? How thick and high is the dam. Do you have a picture? Sorry for all of the questions but it will give everyone a better overview of the situation.

 

[gerard]

One quick check would be to dump some food coloring down the hole and see if it comes out the discharge end. If so you have a clear path from the hole into the tile which is what you DON'T want. If the tile has been breeched it's probably only a matter of time until the whole thing collapses and I wouldn't want to be driving over it if it happened! /w3tcompact/icons/sad.gif

 

[JJT]

How big is the concrete tile? Is there water running through the overflow? Is it going in clear and coming out muddy? If the pipe's big and dry why not get a flashlight and do an inspection? If there water running and it's exiting as clear as it entered I doubt there's an issue.

I doubt the heavy equipment collapsed a concrete culvert.

How wide and tall is this dam?

 

[Briarwood]

In rereading my post, I realize I may have given the wrong impression referring to a hole. I guess "pothole" is more correct. Here is a picture. The surface dropped about six inches and I started to fill it with some mud clods left by the equipment the contractor carted out yesterday (note brown dirt in "hole"). I didn't intend to give the impresssion the hole might extend all the way to the tile. See pic attached. I will post more pics of the dam. We have had record rainfall here the last several months and it rained heavily yesterday but we were out of town. We realized only this morning that the contractor had been here yesterday and moved a very large piece of equipment then we discovered the pothole. I checked the discharge and it is clear (will try to give you another picture. The water level of the pond had been high consistently for the last two months and there is almost always water flowing through the tile. I cannot see through it at this water level. The tile is concete on the pond side but it is corregated black plastic on the other side. I have a pic of the outflow side and will try to post it. I can only guess at the size of the dam but I would say that it is about 20 feet high at the middle and maybe 150 feet long. Thanks for looking at this and helping a paranoid townie with his new country problems.

 

[Briarwood]

Here is a longer view of the drive over the dam and location of the pothole.

 

[Briarwood]

Here is a pic of the side of the dam. The blue circle is location of outflow pipe and red marks location of pothole.

 

[Briarwood]

Here is pic of the outflow tile. I guess the diameter is about 24 inches. Water is clear.

 

[JoeR]

Since this is a new house, has anyone trenched anything down the road? My wonderful phone company nailed two of my 18 inch culverts when they buried the phone line to my new house! I didn't know about it, until I had 4 inches of rain which caused the the dirt to become saturated and created pot holes!

 

[patrickg]

One good possibility is that the drain culvert is not installed in accordance with good engineering practice. Water can and often will seep/flow along the drain pipe/culvert and can, over time make a path/water course for itself that will allow water to flow along side the drain pipe/culvert and erode an ever increasing volume of dam material. This flow will erode the dam by doing exactly what the drain pipe was there to prevent, let soil be lost from the dam, typically lowering the water level and in extreme cases leading to a partial or worse dam failure.

The miniature "sink hole" you call "the pothole" is where a subsurface void had surface material collapse and subside into it. Good questions are, where did the void come from and when was it formed? If the original compacting was at fault,wouldn't you think this would have occurred and been noticed earlier? If, over time, seepage along the outside of the drain carried away dirt then there had to be a first time that the surface collapsed to fill the void and under a heavier than normal load would seem like a highly probably time for that occurence.

It is, of course, possible that the drain tile cracked under the strain of the HEAVY load and unless it is one of those subdivided into multiple parallel tubes, needs replacement, soon. Temporarily you could lay a HD steel sheet over it like you see used in road repair or some similar strategy to permit the construction to proceed.

I have attached a fine intricately detailed and self explanitory precision drawing depicting the anti-erosion "plates" (collars?) that should always be installed in drain lines of earthen dams. USDA recommendations via soil conservation districts detail how this is to be done under their auspices and they also give good advice regarding having an emergency spillway a bit higher than the "normal" drain as in flood conditions (heavy rain) the standard drain may be overwhelmed and yo don't want water to EVER go over the top of an unprotected earthen dam. Unprotected = no concrete spillway to prevent "backside turbulence" from eroding the dam proper.

Patrick (I guess now you know I didn't take mechanical drawing...)

 

[JohnS]

From the picture it looks like there is a tree located close to the inlet of the oveflow pipe. My understanding from reading a few books on the subject (not a pond owner yet) is that the tree roots can make another avenue for water to escape. i.e the water follows along the root path. I thought it was a problem anywhere on the earthen dam, not just near the overflow. But on a leaky overflow, the roots might have been drawn to the water. Also don't know, once a tree is established, is it worse to cut them down, and the roots will decay and create a void? Its been a while since I've read those books, and memory could be a little fuzzy, but it may pay to research that some more.

 

[Anonymous Poster]

Briarwood,

I'm surprised that no one has suggested that the two pipes may have a faulty connection. You say that the dam was originally built in the '60's, but corrugated plastic pipe wasn't widely used back then (if it was even available). I suspect that the plastic pipe was added much later. As an ex-highway man, I've seen lots of problems when trying to join two different kinds of pipe. I think there's a great possibility that the plastic pipe was just "jammed into" the concrete pipe and covered up. Water would eventually find its way around and into the pipe at this location carrying with it some of the fill material. It would eventually create a void. The void could exist for a very long time because the fill would "bridge over" it. I think it could have existed ever since the plastic pipe was installed or shortly thereafter, just lying dormant. The heavy load could compact the void and was most likely what caused it to become apparent.
I also think the reason the plastic pipe has been installed is because the last few sections of the original concrete pipe "fell off". Try taking one of those hand held, high powered 1,000,000 candlepower lights ( QBeam or some other brand) and see if you can tell how far the plastic pipe extends into the dam.
Uncontrolled water can be one of the most destructive forces in nature. I think you are very wise to try to find out what is causing this problem.

Highway Man

 

[beenthere]

Good thought, and fits with my suspicion that maybe the road across the dam was added later, requiring more width and thus requiring additional length to the culvert (plastic in this case). Your scenario of a problem connecting the two seems likely. That is a pretty good flow of water going coming out that culvert.
That concrete truck driver might like to know about the possible void under one side of his truck before he finds it when crossing over it.

 

[patrickg]

It seems you may be dam(ed) if you do and dam(ed) if you don't regarding trees in the dam. If the tree gets hit by lightening or dies for any reason the roots will rot away leaving hollow earthen "pipes" to drain the pond, especially if gophers use the hollows as a head start to tunnel. If you kill the tree you might start that problem. If you don't kill the tree the potential problem will grow and grow and grow. It is somewhat of a dilema.

I had two willow trees, both about 1 1/2 to 2 inches in diameter near a 6 inch pond drain pipe. I eventually had to remove their roots from the pipe. The root mass was 14 1/2 feet long and was 6 inches in diameter for the first 11-12 feet. I have pix somewhere, looks like a bloody anaconda or giant mutant eel.

I am pretty well convinced that I will take out all potentially dam ruining trees. I have 8 ponds and hope the majority will not suffer. If I don't act and depend on luck the odds are I would get away with it for a while but eventually, like walking across the street without looking both ways, something bad will happen.

Yes trees are a knotty problem. They are extremely unlikely to be the cause of this "pothole" but should be dealt with.

Patrick

 

[dmccarty]

Briarwood,

Highwayman has my two cents on what is most likely the problem. From
your pictures it looks like the pipe is covered by at least 12 inches of
material. There are some websites that talk about design issues with
culverts. There is a rule of thumb on how much to cover the culvert.
For what I intend to do with my driveway culvert 12 inches of covering
material will be all I need. If your culvert is larger than 24 inches you
might need more than 12 inches of material but I don't know.

I do know that 12 inches of material is over a plastic culvert should
hold 60,000+ pounds. How heavy are these trucks? 60,000+ pounds
is a loaded semi....

My neighbor has an improperly installed plastic culvert. Even with a
few inches of material over one end of the pipe he has had not had
deformation in the pipe from backhoes, dump trucks, and the septic
tank truck.

Thus my guess that if the pot hole is cause by the culvert its because
of a joining problem.....

Can you dig down a little bit and see if there is some sign of deeper
disturbance? It could be that when the dam was built wood got buried
and it finally compacted due to rot and load......

Good Luck...
Dan McCarty

 

[dmccarty]

Briarwood,

I just happened to run across this address. There is a section in the
document on culverts. They said, and it matches what I have read
from the makers of the plastic culverts, that there should be 12 inches
of material over the culvert if it is less than 25 inches in diameter.
For larger culverts the depth of material covering the pipe should be
1/2 its diameter..

http://www.state.me.us/dep/blwq/docwatershed/camproad.pdf

There is also a trouble shooting chapter that may give you some hints.

Hope this helps...
Dan

 

[DaRube]

Briarwood,

I'm another newbie to country life (moved in last August) who has inherited a dam that needs maintenance. No pothole problems yet for me, but I do have a virtual forest that has sprung up on the sides of my dam. I had invited the guy from soil and water conservation out, and that was the first thing he mentioned to me....that the trees HAD TO GO. This is my priority project for this summer.

Future years will see me repairing the drain inlet which has eroded some and lowered the water level 2 feet, and then possibly some dredging from the upper end where the creek feeds the pond. I still need to figure out what class of engineer does this type of work.

Good luck with yours!!

 

[Briarwood]

Thanks to all for your input. You have all provided some real good advice and I am inclined to believe the problem is most likely related to the joining of the two types of pipe. I cannot see through the pipe because the pipe is not a straight shot from the other side but has some curve in it.

Just to update you all. I told the contractor that's building my house about the small sink hole and he was not concerned. Just brought a load of crushed stone out and spread it over the drive and sink hole at the end of the dam. Well, last week we had eight huge concrete trucks deliver a total of 58 yards of concrete. (I have no idea what they weighed). Don't think I wasn't nervous. I told the contractor to warn the drivers about the sink hole. Well if he did warn them, they certainly weren't worried, since at one time I saw two sitting on the dam at the same time waiting their turn to unload. That must have been the ultimate test. I whish I had taken a picture of that. The sink hole is not evident yet under the new stone covering. The weight of the trucks and their wider wheels did make some minor depressions at two places near the outside of the lane that I filled with stone after they left.

It had been dry here and the flow has been very little to none through the culvert for the last week. It started raining again today, if we get alot it may be interesting to see if there is any change where the sink hole is covered.

The next step will be to have the trucks back to pour the basement floor, porch and garage floor. When the house is finished, I plan to rework my 1500 foot lane and make any necessary repairs to the dam. Hope it holds out until then.

Oh, BTW, did I mention that the house is a modular and they plan to bring the sections across the dam, too!!!! I will definately not be without my camera.

Thanx
Briarwood