The pilot of the glider was being towed to altitude near mountainous terrain when the glider encountered turbulence and rotor wave activity. The pilot reported that, after releasing from tow, he noted that the glider was in “some bad sink.” The pilot initially turned back to the airport but continued to lose altitude quickly. He then noticed that the spoilers were extended, retracted them, and attempted to reach a diversionary airport; however, the glider continued to descend rapidly. The pilot said he entered the airport traffic pattern rather than making a straight-in landing approach, but realized he had insufficient altitude to clear trees, and landed in a nearby field. Upon landing, the glider ground looped when it contacted tall grass resulting in substantial damage to the tail boom. The pilot said there were no mechanical malfunctions or failures of the glider that would have precluded normal operation.