 Complete the following phrase:  To reduce construction costs, the transport authority decided to use an abandoned canal bed to form part of the system, and did not dig deeply. By using the canal bed, the underground section is only 23 meters under the street surface, and was the worlds shallowest metro. There were only 1,000 full-time construction workers at that time; the others were volunteers from factories, schools and institutions all over the city. The average number of volunteer workers exceeded 2,000 a day, which rendered the project unique in the world then. Construction was delayed in 1976 when an 