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...)