Watertight tunnel jacking

This article describes the watertight concrete design for the jacked tunnels in Boston's Central Artery. At the southern end of the gigantic Central Artery project, an interchange between two main roads will be completely rebuilt, with six levels of elevated roads and underground tunnels linking 28 routes. At the heart of this complex interchange, some roads must pass underneath eight main line railway tracks. Priority has been given to ensuring safety and minimising disruption to traffic during the construction. As a result, full-size tunnel jacking has been applied so that the tunnels under the railway can be constructed and installed without needing repeated relocations of track, signalling, and safety control equipment. This tunnel jacking scheme evolved along with strategic modifications to the detailed design of the interchange, which now has three jacked tunnels of lengths between 48m and 107m and cross sections up to 24m wide and 11.5m high. The use of jacking required a critical review of the project's concrete specification, especially its approach to watertightness based on durability, so that it could be designed without problematic external, impervious waterproof membranes. Waterproofing will be ensured by monitoring the self-sealing process, then if necessary grouting the few remaining areas of seepage.