Instrumentation controls for chemical grouting during tunneling operations. Boston, Massachusetts

摘要

The Central Artery Tunnel Project located in Boston Massachusetts is the largest and most complex highway project in American history. One of the Projects main elements is the replacement of the elevated Interstate 93 through downtown Boston with an underground highway. At its deepest point the underground highway is 120 feet (36 meters) below grade and passes directly beneath an existing subway tunnel/station. To tunnel beneath the existing subway a significant underpinning structure was required consisting of stacked drift tunnels and post tensioned roof girders or cross adits. To facilitate mining two shafts were dropped approximately 120 feet (36 meters), the water table was temporarily lowered and two access tunnels/grouting galleries were mined in a cohesive granular till. The underlying stacked tunnel drives were driven in pervious gravel deposits overlaying Argillite bedrock. The stacked tunnel drives and cross adits were mined open face with a combination of ribs and liner plates. Permanent dewatering could not be performed due to settlement concerns, since, the nearby adjacent high rises bear on mat foundations and the nearby historical structures are supported on timber pile foundations bearing in clay. To facilitate the open face mining, of the lower drifts in the pervious gravel soils, ground stabilization was required. Prior to the ground modification being performed, an instrumentation system was installed and initialized in the active transit tunnel located approximately 1 meter (3 feet) above the crown of the access tunnels. The instrumentation system included 3 separate "strings" of vibrating wire liquid level displacement transducers. The strings, each approximately 50 meters (150 feet) long, were installed along the inbound and outbound track platforms and along the cen-terline columns between the tracks. All of the strings were connected to an Automated Data Acquisition System (ADAS) which was monitored remotely via modem. Ground modification was performed by injecting sodium silicate grout through tube-a-manchette or TAM pipes creating a mass of chemically stabilized soil under each access tunnel. During the early grouting program vertical upward deformations were measured by the liquid level system and confirmed by optical survey. Multiple modifications were made in an attempt to limit the heave while still creating the required grout curtains. Such modifications included: additional pressure relief via dewatering wells, reductions in injection pressures, reductions in flow rates, reductions in the grout viscosity, revisions to the grouting sequence, reductions in the number of active packers, a replacement of sodium silicate with acrylate grout. All of the contract modifications were performed under a heave based limiting criteria. In summary, the monitoring system provided a degree of confidence during the grouting operations allowing the maximum amount of grout to be injected while maintaining heave within acceptable limits to the Owner. Over 950,000 liters (250,000 gallons) of grout were injected and the drifts and cross adits have been successfully mined without incident.