If it's cranked down, it's probably a gate valve. There may be a piece of debris under it, and opening it a little bit may dislodge the debris, allowing the valve to close fully.
If it's 12 inch SDR 26, I have a few scrap pieces of that, and it's pretty thick and double gasketed at the bells. Rated for forced sewer mains at a maximum of 160 PSI. If it's SDR 35 or higher, it's not rated for pressure, but gravity feed only.
