Monitoring of Movements above Large Shallow Jacked Tunnels

The construction of CA/T Contract C09A4 entails the jacking of three (reduced from four) tunnels beneath eight sets of Amtrak and MBTA commuter rail tracks at South Station in downtown Boston. The tunnels range up to 38 feet (11.5m) in height and 79 feet (24m) in width, and crowns are separated from the tracks by only 8 to 23 feet (2.4 to 7m) of manmade fill. The original design called for the tunneled ground to be extensively treated with a combination of dewatering, jet grouting, penetration grouting and soil nailing, but surface settlements could still have reached as much as 13.5 inches (34cm). There was need to monitor both subsurface movements and rail settlements to protect the tracks, so designers provided for 200 subsurface instruments of eight different types, upwards of 1200 optical survey points on tops of rails, 1200 railroad hubs for determining dynamic rail movements, and special software developed to quickly sort and report hundreds of rail elevation measurements made every day. Studied and rejected were a computerized hand propelled Portable Geometry Trolley for measuring static rail elevations and an extensive array of remote reading Electrolevels and Beams for determining both static and dynamic rail elevations. The post-bid deletion of one tunnel and replacement of design ground treatment with ground freezing forced construction managers to continually adjust the mix of instrument types, numbers, locations and installation procedures to stay abreast of developments.