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The 23.1-mile Dulles Corridor Metrorail Project will expand the reach of the existing regional rail system, connect the existing system to Washington Dulles International Airport and points west, and reduce travel time into and out of downtown Washington, D.C. The project owner is the Metropolitan Washington Airports Authority (MWAA), and the end user will be the Washington Metropolitan Area Transit Authority (WMATA). It is up to the Dulles Transit Partners, LLC (DTP) design-build team, lead by Bechtel, to complete the 11.5-mile alignment of Phase 1 of this project by August 2013.
While much of the Metrorail project construction is above grade, there is one segment in Tysons Corner that requires the construction of two new railway tunnels to facilitate the rail line. By comparing mobile laser scan data with a 3D model of the tunnel, which included pipe arch canopies, engineers determined that there was a minimal amount of cover from the top of the tunnel to the road surface. Throughout tunnel construction, DTP needed to monitor the active roadway intersection for settlement-an area termed an Intensified Monitoring Zone (IMZ)-to ensure safety of construction workers and the public.
Bechtel teamed with the Engineered Solutions group of Leica Geosystems, Inc. to implement one of the first reflectorless measurement road surface monitoring projects in the U.S. that utilizes automated software and a total station to minimize highway safety hazards and provide unprecedented, continuous monitoring throughout the project.
Optical Solutions
Phase 1 of the Dulles Corridor Metrorail Project, which runs from East Falls Church to Reston, Virginia, includes five new stations. Crews must also construct two 2,400-foot underground tunnels that will connect two of the four Metro stations in Tysons Corner. Tunnel construction began in October 2009 with the New Austrian Tunnel Method (NATM) a method that uses the surrounding rock mass to stabilize the tunnel. A key component of this mining technique is to have survey geomonitoring equipment measure the deformation of the tunnel rock after excavation. Similar instruments would be used to monitor the roadway above.
Bechtel and Leica Geosystems considered traditional survey methods such as ground-based total stations and prisms to monitor the roadway. However, these methods would have required survey crews to work within an active roadway 24 hours, seven days a week—an ineffective and unsafe approach. Also, the maintenance of traffic plans would have been difficult to arrange and very costly, requiring extensive crash barrier trucks, cones, and labor crews to install, as well as the presence of local police officers for safety. Instead, the team opted to put in place an optical automated surface monitoring system using direct reflect (DR) on road surfaces with no targets.
Direct Reflect Response
The monitoring system included GeoMoS software in conjunction with a Model TS30 total station. The Leica Model TS30 total station is a 0.5" angular accuracy instrument, which includes a powerful reflectorless reading EDM capability. Along with the road-based DR measurements, the system was also set up to monitor surface movement along the tunnel centerline alignments using a select number of high precision glass prisms mounted on concrete cylinders set in the surface. These cylinders and their installation requirements were designed and installed in accordance with the project Instrumentation and Monitoring procedures.
The Leica GeoMoS software suite includes three separate applications: Monitor, Analyzer and Adjustment. GeoMoS Web web-based interface is available as a service for data presentation and reporting.
The GeoMoS Monitor application controls the interface to the sensors, which may include geomatic or geotechnical sensors. The Monitor application allows the communication and parameter settings to be configured for the sensors. Additionally, the application manages the point coordinates for specific groups, primarily control points or monitored points. It handles all of the displacement calculations, alert level settings and system messaging, such as who receives an email for a certain alert. Monitor performs all of these actions, schedules the sensors to take readings as required, and writes the data to an open Microsoft SQL database.
The Analyzer application is used to query the database for analysis and preparation of graphing and reports. Bechtel developed a script within Analyzer to automate data extraction from the instruments in the tunnel and on the roadway. The Analyzer application runs independently of the Monitor application, making it possible to query the active database for up-to-the-minute data review. The GeoMoS Adjustment application was not used on this project.
The next challenge was to figure out a way to monitor the active roadway without prisms and still keep traffic flowing. Reflectorless measurements taken from high above the roadway seemed to be the answer. These measurements were carried out with the TS30 Electronic Distance Meter (EDM) using the Direct Reflection capability. DR enables surveyors to obtain distance measurements to remote objects or surfaces without the use of special reflective glass prisms or sheet reflectors.
Towering Techniques
Positioning the total station above the active roadway would make it possible for the equipment to take reflectorless measurements. Naturally, the accuracy of the reflectorless monitoring system would rely on a very stable elevated instrument location. The team opted to construct a tower using a 50-foot long reinforced H-pile with a 36-inch flange positioned outside the expected area of impact from the tunneling operation. The pile was embedded in concrete standing on end 25 feet into the ground adjacent to the roadway.
The column had no movement in the vertical axis. The horizontal axis was reinforced with additional bracing and by the addition of a concrete collar around the H-Pile at the ground surface to minimize potential movement.
The team also constructed an engineered multi-story scaffold system containing a staircase and a platform at the instrument level. The scaffold system surrounded the column and provided access to the instrument for operation and maintenance. The system was designed so that there was no contact between the scaffold system and the column. This isolation between the scaffolding (including the platform) and the instrument pillar prevented any motion on the scaffold from being transmitted to the instrument (similar to the old National Geodetic Survey Bilby Tower system).
It was expected that there would be some thermal impact on the instrument’s position due to solar heating or weather. To mitigate thermal issues, the scaffold system also included a roof system covering the entire extent of the instrument pillar and platform. In the shelter of this protective roof, the TS30 operated without further environmental protection 24/7 for nine months, continuously, until it was time to demobilize the system (the tunnel had successfully passed the required distance beyond the IMZ) and send the instrument to Leica for its annual calibration certification.
A stainless steel mount for the instrument was welded to the top of the column. To compensate for any minor movement induced by weather or solar activity, surveyors implemented an automated resection routine in the GeoMoS software. On the hour, as a part of each automated measuring sequence, the instrument would carry out a new resection to position the instrument for its automat
ed measurement data capture session. Permanent AC power, a digital thermometer connected to the instrument, and a wired (Cat 5) local network access were installed in a weather-tight enclosure at the top of the column.
The survey team found that the TS30 EDM would gather DR readings from wet road surfaces, but heavy rain degraded the road surface readings a small but noticeable amount. Snow caused road surface heights to change and triggered the automated alert system. Snowfall and dense fog were the only environmental conditions that interfered with the DR road surface readings. These were rare environmental conditions and did not affect the overall automated monitoring program significantly. Snow was the only condition that required shutdown of the system due to inability to obtain useful road surface readings.
The system was so accurate that the Bechtel team could track the movement of the sun during the daytime operating periods to within 3 mm. Rain did not hinder the DR readings of the road surface enough to reduce the effectiveness of the system in meeting surface deformation monitoring requirements of the Virginia Department of Transportation. The instrument sometimes moved outside the level compensation system limits, and on these occasions manual attention by a surveyor was required to re-level and reset the system. All other operations were carried out remotely using the GeoMoS server network connection to the TS30.
Access and Accuracy
The reflectorless instrument on the tower was equipped with an Ethernet device and connected to a dedicated GeoMoS DMZ server isolated within the Bechtel internal network. The DMZ server allowed two-way communication with the Leica GeoMoS website in Heerbrugg, Switzerland. At the end of every measurement system, the new data was pushed to the Leica GeoMoS Web server and also stored on the local GeoMoS DMZ server. The monitoring database measurements always occupied two completely independent hardware platforms, guaranteeing no loss of data due to hardware failure.
By remotely logging into the GeoMoS server from their office workstations, the Bechtel surveying team remotely operated the TS30 instrument at the Dulles project site. An automated GeoMoS multi-level alert system notified project team members via email if surface movements occurred outside the defined limits. As well, in the case of an alert, the approved NATM Tunnel engineers and surveyors were able to access the project measurement data over the Internet at any time, day or night, to use the GeoMoS Analyzer automated analysis and charting system housed on the Leica GeoMoS web server in Switzerland to assess current surface movement data.
In addition, Bechtel blended the GeoMoS data from the roadway measurements and the data from the prisms located on the interior walls of the tunnel using Geodata’s EUPALINOS tunnel software for tracking convergence in the tunnel walls. The combined data was used in a daily meeting to help NATM Tunnel mining engineers compare actual surface deformation conditions and NATM Tunnel convergence movement with predicted mining design scope and safety parameters.
The 2,400-foot underground tunnels that connect the two Metro stations in Tysons Corner are complete. The monitoring system worked beyond the Bechtel team’s expectations, providing confidence in the construction methodologies and subsurface conditions with 24/7 monitoring, while keeping surveyors and the public safe.
Joe Betit is Survey Manager with Bechtel and lead surveyor on the Metrorail project. He undertook the installation and management of the system with Leica support for the duration of the monitoring period.
Craig Hewes is a registered professional land surveyor in Pennsylvania and New York. He currently manages the installation and support of monitoring projects for Leica Geosystems Engineered Solutions Group.
A 4.557Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE