Yes, loose rock or a layer of sandstone are BOTH bad news!
One possibility is that they planned a core trench, but then hit a layer of rock and just did "the best they could" over the rock layer.
Another possibility, is that they encountered a lot of large chunks of rock and did not clear them from the pond bottom and dam area. You cannot compact material containing large rocks. When the rocks have enough point-to-point contacts, they begin to form a load-bearing framework. At that point, the clay material between the rocks can no longer be compacted any further.
Finally, as you suggested, the sandstone itself could be permeable (capable of allowing fluid flows). If there is not a well-compacted clay blanket over a ledge of permeable rock, then that can provide a pathway for a leak. Some sandstones and limestones in Missouri are impermeable rock that would hold water. However, if such a rock layer is jointed or fractured, it will still allow water to pass.
That is the bad news. The good news is that if your pond is only leaking a little, then your probably have some good clay material available for your use.
Generally, a foot of good clay-bearing material that was properly compacted in 6" lifts with the correct moisture content, will seal over any leaky rock layers for a pond up to 10' deep. (If you are going to build deeper, then you made need to thicken your clay blanket on the bottom in the deepest spots.)
My advice would be to keep working to get your pond drained and dry enough for you to walk around. I would then build a poor-boy probe with some re-bar. (You will need to weld a tee or a loop, to tap or pull out your probe when it gets stuck.) Then walk around your pond basin with your probe and a little 3 or 4# sledge hammer and try to find your rock.
If you see any exposed rock, that is a problem. If you hit rock 3-6" down, that is also a problem. If you find areas more than 12" to rock, good.
If you find areas several feet to rock, then maybe take some samples with your shovel. If it is at least 25% clay with no rock, perhaps you can deepen the pond in that spot and steal some of that material to fix your problem spots.
Does your pond only leak down to a certain level when full, and then quit leaking at that point?
If so, then still several possibilities.
- Could have a leak out of the side of the pond somewhere but high up in the bank.
- Could have a nearly impermeable leak in the bottom of the pond - that only allows water to pass when the pond is full and the "extra" hydrostatic pressure can push through some water. When the water level subsides enough to lower the hydrostatic pressure the leak ceases or is significantly reduced.
- Could be they just did a poor job on some aspect of the core trench. That could be poor compaction. However, perhaps they just ran out of clay-rich material and did not build the core trench up high enough and just added some crappy material with a high organic content to get the dam high enough.
(I think the third possibility is the most likely, based on your observation of moist ground on the back side of the dam.)
All of my writing above is some wild speculation. Keep probing and digging at your pond. If you can clearly define the problem, there are some people on Pond Boss that can offer some pretty good solutions.
Good luck on your pond improvement project!
