REBUILDING THE RED LINE AT ST PETERSBURG

St. Petersburg, Russia, is site of one of the most difficult sections of metro tunnel ever encountered. The project is the rebuilding of 800 meters of the city's metro line, which was closed in 1995 after chronic problems with water and soil intrusions. The most problematic section crosses a deep glacial channel filled with fine sands and loam suspended in groundwater. The twin tunnels running through the channel were originally built by freezing the soil, but bores some seven years after they were completed showed the soil had thawed. Eventually, the section was sealed and filled with water to prevent further damage. Unanticipated side effects from the ground freezing, faulty materials and shoddy construction are among the suspected contributing factors. The Ministry of Construction of Russia chose to excavate two new tunnels, though their alignments had to match up with existing underground stations. Even with adjustments, the hydrostatic head is potentially as high as 5.5bar, which is the most challenging conditions encountered for soft-ground pressurized tunneling techniques. Customized tunnel segments are being built in Russian factories to the contractor's specifications. Despite delays caused by fiscal crises and technical challenges, the governor of St. Petersburg promises the tunnel will be completed by 2003 in time for the city's 300th birthday celebration.