Overview
In the last twenty years, more than fifty miles of tunnels have been constructed in the Los Angeles area. Recent projects include The East Central Interceptor Sewer (ECIS) completed in 2004, which spans over eleven miles. The Northeast Interceptor Sewer (NEIS) completed in 2005, spans 7 miles. The most recent project, the Metropolitan Transportation Authority’s Gold Line Eastside Extension (MCGLEE) completed in 2006, spans 1.3 miles.
All of the tunnel routes lay beneath heavily populated urban environments so minimizing ground surface settlement was critical to the success of the projects. Previous tunneling efforts in the Los Angeles area using open shield methods resulted in significant settlement which damaged roads, utilities, buildings and other structures. Earth Pressure Balance (EPB) or Slurry Machines, were specified for these projects to reduce subsidence risk. The contractors chose EPB Tunnel Boring Machines (TBM) for the work.
The closed and pressured face of the EPB TBM reduces loss of ground at the tunnel face and, when the machines are used in combination with gasketed pre-cast tunnel liners and backfill grout behind the installed segments, losses from over-cut of the excavated surface are also reduced.
Evaluation
To monitor the specified maximum allowable settlements for all three contracts the projects included comprehensive settlement monitoring along the paths of the TBM’s. This was accomplished using Multiple-Position Borehole Extensometers (MPBX’s) with 3 anchors. The lower anchors were installed five feet above the tunnel crown; surface anchors at five feet below ground surfaces; and middle anchors installed between upper and lower anchors. The MPBX’s were installed at 15m (50’) intervals in high-risk areas and at 75m (250’) intervals in lower-risk areas. Conventional survey of settlement points was also undertaken in conjunction with the automated monitoring to verify the data.
The three tunneling projects combined utilized approximately 360 MPBX’s to monitor thousands of surface settlement points along the various routes. Most of the extensometers and survey points were installed/located at traffic lanes and streets. This allowed a definitive analysis of tunneling performance during the excavation process and provided for development of action plans in the event of subsidence exceeding approved limits.
What We Did
The MPBX’s were installed above ground to follow the path of the TBM’s underground. This required drilling and installation of manholes in high traffic urban areas to accommodate the extensometers, the head assembly with transducers and the data collection package. A view of a typical MPBX installation in the urban environment is shown at left. The MPBX’s were equipped with vibrating wire transducers to provide stable and highly reliable displacement measurements.
We were tasked with design and supply of the automated data acquisition platforms. Over the course of working on all 3 projects we provided several versions of the automated data collection packages. The improvements were a reflection of advances in wireless communications. All three versions used a Campbell Scientific CR510 with power supply (to provide for up to 3 months operation without charging) and sensor interface packaged to fit beside the extensometer head in the manholes. Field personnel would periodically visit the manholes and collect data by connecting a portable data storage module which automatically collected data. The second version included a spreadspectrum radio which allowed the field personnel to drive to the vicinity of the manhole and, with a base station radio attached to a PC, data were collected wirelessly. This provided a much safer method of collecting data as most manholes were located in high traffic areas. The last version included a CDMA cellular modem which provided for data collection from the contractors office. A view of this package is shown at right.
We also supplied our MultiLoggerDB software package with Insite client data access software for managing the systems and the data collection from all locations. With the CDMA equipped units, data collection and reporting was completely automated thereby saving a great deal of time and money and providing near real-time monitoring of the TBM progress.
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