The condition of Boston Harbor has greatly improved as a result of key structural elements made of concrete, including a tunnel for carrying primary-treated sewage water from one island to a new secondary treatment plant on another and a tunnel for carrying secondary-treated water into the vast Atlantic. Boston Sand & Gravel Co. supplied the concrete required for 44 of the 61 projects on one of the islands and produced all of it onsite because a mitigation agreement with a town next to the new secondary-treatment plant limited truck traffic to eight per day. The island has received barged materials up to six times a week. The contractor for the five-mile-long inter-island tunnel opted for cast-in-place tunneling despite problematic concrete delivery. Drivers pulled up to a 300-foot-deep shaft above the tunnel, and concrete was chuted or pumped into a 12-inch-wide tremie pipe. Segregated concrete was intercepted below in a re-mix pot, dispensed into a hopper that filled four air-powered agitator cars, railed to work areas and poured into plywood/metal forms.The contractor consortium for the outfall tunnel decided precast better suited the world's longest such tunnel. Six segments were cast for each ring of the 24-foot-diameter tunnel.