My last trip to Roundrock involved two (yes, two!) major projects. I already told you about my battle with the tree that I was sure was going to fall across the road (and apparently wasn’t). The other major project was rebuilding the retaining wall above the overflow drain in the dam. Over the years the ground beneath/behind this wall has eroded from high water events (in the dim past) and critter incursions.
You see above the wall as I had originally built it. You’ll also see that I have put the blocks in backward. The idea, I think, was that making a concave curve meant I had to put the blocks beside each other such that they were wider on the “outside” than the “inside.” I suppose that seemed to make sense at the time. I don’t really recall. Regardless, there is a design flaw in the overflow drain that — literally — undermines my wall. You can see the black circle in the concrete drum under the wife mesh. When the drum fills with water, it will eventually spill into that black circle, which is actually a one-foot diameter pipe that opens at the bottom of the dry side of the dam. Thus the excess water is bled off before it would go into the spillway.
The problem is that there is no seal between the black pipe and the concrete drum. Thus any water that might pour into the pipe can also leak into the space around the pipe. And if the water goes into there, it will leak into the dam itself, eroding the dirt there and compromising the seal. Granted, such high-water events are not common, but the erosion had washed away enuf of the dirt behind the drum to cause the wall I had built to fall backward. Further, critters seemed to find the space behind my wall a safe haven. There was an obvious path from the edge of the wall to the the water (far) below.
The point of the wall was to keep the dam above the overflow drum from eroding into the drum. Instead, the wall was eroding into the dam. So it was time to take it apart, address the erosion as much as possible, and rebuild it.
You see the cavern that the critters were exploiting behind the wall. I dreaded what I would find in there when I took the blocks apart. In fact, I fully expected some critter to come forth and object to my efforts. Had that happened, I might have tumbled back into the lake.
This is what I found when I took the blocks of the wall apart. No critters. No den. But still a mess. Lots of rock. You can even see the cedar tree trunk (bottom center above the drum) that comprises the “dirt” of my dam.
My plan was to backfill this cavity with Bentonite and “good” dirt then build the wall again in front of it. I pulled out as much of the cedar tree as I could, launched it over the dam, and rearranged the rocks a little. Then I poured my ($9) bag of Bentonite into the space. My hope was what it would filter down into the dam, along the routes any errant water had found its way. I had done this when I built the wall, and the very same thing happened this time. I poured the Bentonite into the space, and it quickly seeped through the gap between the black pipe and the concrete drum, collecting in the bottom of the drum and in the black pipe. It does me no good there, of course, but not much of the ($9) bag of Bentonite went there. Most of it stuck around at the top of the cavity. Well, that’s better than nothing. I then poured my three bags of good topsoil into the rest of the cavity, packing it with my gloved hands.
It turned out that three bags of good topsoil (on top of one big bag of Bentonite) was not enuf to fill the cavity. So Libby and I drove up to the pine plantation where I happen to have some good soil, and dug enuf of it to refill the three empty soil bags and the one empty Bentonite bag. Then we returned and added that to the cavity. We managed to have a little dirt left over (and we used it elsewhere). Then I rebuilt the wall.
You can see several things from this photo. One is that I placed the stones with their “outside” facing out. It certainly looks better this way. The second thing you can see is that the dam above the wall is sort of hanging there. I expect this to settle into place, but if I were a rich man, I’d place another series of blocks in my wall to bring it up higher to make a clear demarcation between dam and drain. You can also see where the Bentonite has collected in the black pipe. That will wash away in the next high water event.
So, better than before, but still not perfect. This will bear watching over the coming months. I may have to do more work on it. But I don’t mind.