Hi all, I have a pit dug that is about 12-13ft long and about 5ft wide. At it's deepest my best estimate is that I can make it about 21"-24" deep. I am considering a billboard tarp as a liner with underlayment and dirt or gravel over the top of it. My question to the forum is: Have any of you used a billboard tarp as a liner with lasting results? I have been researching it and have not found any examples. Thanks