The pilot of the amphibian airplane reported that while on approach to the destination airport, he attempted to extend the landing gear. The nose landing gear and right main landing gear extended and locked; however, the left main landing gear only partially extended. The pilot could not correct the anomaly by troubleshooting and using the emergency checklist. He subsequently elected to return to his home airport and perform a gear-up landing on a grass area adjacent to the runway. During the landing, the airplane sustained substantial damage to the lower fuselage bulkheads and right wing. The pilot and a mechanic recovered the airplane to a hangar. Several days later, under the supervision of a Federal Aviation Administration inspector, they were able to cycle the landing gear five times with no anomalies noted. The mechanic added that perhaps ice interfered with the left main landing gear, but he could not be sure. .