Retrofit underground storage tank

The present invention is directed to a retrofit underground storage tank (30) and method for making the same. A portion of an underground primary tank (32) its uncovered and an access opening (42) in formed therein. A plurality of trifolds (50, 120, 122, and 124) comprising three folded up and banded together panels (52, 54, 56) which are jointed together by hinges are inserted into the primary tank. A monorail or conveyor (90) is inserted into and suspended from above the primary tank (32). Trolleys (102, 104) on the monorail (90) assist in transporting the trifolds (50, 120, 122 and 124) within the primary tank (32) and then the monorail (90) is removed. Trifolds (50, 120, 122 and 124) are unbanded and unfolded and are arranged circumferentially about the tank. Jack stands are used to hold panels of the trifolds in place. Circumferential and longitudinal joints are formed between abutting edges on the panels using fiberglass reinforced plastic mats, woven rovings and resins layups thus creating an inner cylindrical wall (230) within primary tank (32). Further panels are joined together to form a pair of end caps (260, 300) which are jointed to inner cylindrical wall (330) thus creating an inner tank (312). The access opening (42) in primary tank (32) is then covered with a panel which is sealed to primary tank (32). The inner tank and outer primary tank (32) cooperate to form retrofit double walled storage tank (30). Brine, pressure, vacuum, or a dry monitor are installed between the inner and outer tanks to allow monitoring of the tank. In a second embodiment, double wall and or single wall panels are joined together to construct a multi-walled inner tank within an outer primary tank to create a retrofit tank (30').