septic system search

Our septic system is being replaced and today the backhoe ripped through the concrete foundation where a chicken house once stood. It burned down many years before our time and I thought I would check out what was underneath it. First find was a cartridge case marked super win 30-30. Then I got the circular iron thing I don't recognize and the iron hasp. After that it was a beer can and a huge number of nails which I got sick of digging and gave it up. I plan to check out the trench dirt when they finish. The second photo is the area I tried detecting.