I live in the Nashville basin probably not far from where you are. I recently finished my pond and here's what I'll say about core trenches. I really don't think it's necessary to core your dam for the following reason. I built my own and when I was ready to start building the dam I thought long and hard about whether or not to core. I had argued in the past on this forum that coring wasn't necessary on a good clay base and not even practical in our part of the country with such thin soils. Here you have to keep the water from going into the basin soil or it hits the rock and will bypass any core unless the core goes halfway to China. That being said when it was time to build my dam since I have my own machine I decided I would cut a core trench since it was only more spare time and diesel and the core couldn't hurt anything whether it helped or not. I cut about 4 ft into the subsoil when I started hitting limestone bedrock and had to stop. I then compacted the core with my pull behind roller as it was filled with my best susoil (48-51% clay, Mimosa silt loam). I then constructed the dam in 8" lifts with my best soils in the middle (all of my subsoil has adequate clay). I finished up the dam last fall but before I could compact the basin I got very busy with work and had to postpone pond work until later in the fall. Then in late Oct I had to go out to CA for a week and when I came back the pond drain had gotten clogged and the spring and a rain event had put about 5' of water in the pond. I drained it out but that late in the year it was impossible to get it dry in the bottom. Being anxious, I packed the upper part of the basin and plugged the drain with my fingers crossed. The pond picked up another 6 ft of water pretty rapidly, but was leaking about 1/2 inch/day, which bothered me because I wanted it to hold like a bucket from the get go. So, with no rain in sight due to El Nino, I opened the drain back up in mid Feb and determined that I would wait until it dried enough to pack it proper in the basin. I had to go out to CA again in early March and when I got back home the pond was fairly dry except for a 30' x 30' patch in the very bottom. I rented an Ingersoll Rand self propelled vibratory padfoot compactor over a weekend and pounded the hell out of the pond basin with 56,000 lb force on the pad feet. I then closed the valve back up and watched the spring start fillin it back up. I've had 2 moderate rain events since and the pond is now showing 6.5 ft on the stick and appears to be holding like the plastic bucket I was pining for. Research your soil type and I will bet that one of the limitations listed for it by the soil service is 'difficult to pack' like my Mimosa silt loam, but it is still perfectly capable of holding water as long as it is packed properly and a dozer simply can't do it, not enough ground pressure. Unless you have the $50,000 it would take to core down 50 ft into the rock and fill it with concrete I'd save my money on the core trench (might work) and spend $500 on a padfoot compactor for a weekend (will work). Core trenches are only practical in certain situations where you have impervious soils overlain with pervious soils. If your subsoils are thin and prone to seepage because of the soil structure (not lack of clay), then you are wasting your time and money on a core. Compacting the subsoil in the basin works on the same principle as a plastic liner. If the water can't seep into the soil, then it doesn't make it down to rock, and if it can't seep into the rock, it can't get past the dam. My common sense told me this was true before I started and my experience thus far has borne it out. I'm going to post some pics one of these days.
Hillbilly
