A Solid Link
November 1, 2009
In an ambitious undertaking of civil and railway engineering, the Chicago Transit Authority (CTA) in 2006 embarked on a subway tunnel project to allow access between Midway and O’Hare International airports. The plan was to link the CTA Blue Line to the CTA Red Line in Block 37 situated in the heart of downtown Chicago. The Mills Corp.*, the developer of Block 37, would construct the part of the subway link that fell within its project limits, while the City of Chicago and the CTA would construct the station and the interface to the two existing subway lines.
The CTA contracted Kiewit-Reyes AJV, a joint venture of Kiewit Western Co. and Reyes Group Ltd., to undertake the construction at the two interface sites. The work entailed installing slurry walls and soldier pile and lagging walls, constructing deck beams to support roadway plates, and excavating to a depth of approximately 50 feet below street level to allow for the proposed subway connections. This construction was performed in two very busy street intersections in downtown Chicago.
Kiewit-Reyes AJV contracted GZA GeoEnvironmental Inc. to provide, install and manage geotechnical and structural instrumentation to monitor the impact of the construction and excavation on the installed slurry and soldier pile and lagging walls and the two subway tunnels as well as two nearby historic buildings and a section of the city’s historic freight tunnel network. The two historic buildings--the Reliance Building, built in the 1890s, and the Delaware Building, built in 1870s--both are designated as Chicago Landmarks. “Unlike most of the other buildings near the proposed construction, these historic buildings are not founded on caissons or piles and thus were more likely to experience settlements due to adjacent construction activities,” says Niels Jensen, PhD, PE, project manager for GZA. “We had to ensure that the entire construction process provided minimal impact to all the surrounding structures.”
Automated MeasurementThe GZA team installed inclinometers and extensometers to monitor the slurry and soldier pile and lagging walls; robotic total stations and strain gauges to monitor the subway tunnels; robotic total stations, tiltmeters and liquid level sensors to monitor the historic buildings; and tilt-beam sensors and tunnel profile sensors to monitor the freight tunnel.
While all of the equipment served a useful purpose, the robotic total stations (also called automated motorized total stations, or AMTS) were key to acquiring the necessary data in the tunnels and on the buildings. “We opted to use AMTS systems instead of traditional tunnel profile monitoring systems within the subway tunnel on this project for several reasons,” Jensen says. “One advantage is that the zero or datum reference for comparison of the measurements is not within the area that may be influenced by the construction. Additionally, data processing is, arguably, much easier.”
A traditional tunnel profile system uses a network of electronic tiltmeters and extensometers affixed to the walls of the tunnel that can measure deformation of the tunnel section. Where robotic total station technology cannot be considered equal to the tunnel profile is in the accuracy of the measurements. A tunnel profile system can provide submillimeter results. In comparison, the resolved coordinate total station accuracy was generally in the range of 1.5 millimeters in 100 meters inside the subway tunnels and about 3 millimeters in 100 meters for the exterior installations. However, the robotic total stations reduced transit operation downtime, labor costs and wiring requirements, compared to tunnel profile systems, for an overall cost savings. The reduced labor on the tracks also increased project safety.
Four separate robotic total stations--Leica TCA1800 models with automatic target recognition--were installed in the CTA subway tunnels with one in each of the four tubes making up the Blue Line and the Red Line. Each total station system consisted of one total station and 18 Leica GMP104 mini prisms, including three mini prisms that served as backsights for the total station. The mini prisms were installed in three cross sections to provide a reasonable amount of data for engineering analysis. The four total stations were controlled automatically by two PCs with I-RIS software installed in electrical control rooms located on the platforms of the Blue Line and the Red Line. The communication between the total stations and the PCs was accomplished using radio frequency modems running the serial RS-232 protocol. Each PC had a dedicated DSL line that allowed remote access to the PCs as well as automatic upload of data to a remote database server.
The historic building monitoring system consisted of two total stations--one mounted on the Daley Center building, which monitored prisms on the Delaware Building, and the other mounted on what was then the Marshall Field and Co. Building (now Macy’s), which monitored prisms on the Reliance building. The building total station setups were equipped with Raven cellular IP modems, which allowed for remote control of the total stations. The two building total stations were controlled from a PC with I-RIS software located in GZA’s New York City office. “The real-time communication capability of both the tunnel and building total station systems allowed for full automation and near-real-time monitoring with associated alarms and deformation data,” Jensen explains. “If any movement of the structures was noted, rapid changes could be made in the construction sequence or methodology.”
Installation IssuesAs with any project of this magnitude, GZA had to overcome various challenges with the installation of the robotic total station systems. The project was further complicated by the fact that the instrumentation system installation locations within the freight tunnels, subways and private structures were not accesssible prior to the start of the project.
The team had originally decided to install the total stations on the tunnel walls and had ordered wall-mounted brackets. However, once in the subway, the team realized that two of the four total stations could not be mounted as planned. “Obstructions along the walls would have prevented sighting the mini prisms,” Jensen says. “We had two days to procure brackets that could be installed in the tunnel ceilings.”
Another bracket issue surfaced when trying to mount the total station on the Daley Center building. The bracket, manufactured by GEO-Instruments, was so heavy that it required two people to lift it. Additionally, drilling through the Daley Center’s ¾-inch steel plate with a hand-held drill proved impossible. “We had to rent a magnetic drill press and mount it upside-down in order to drill the holes to install the bracket,” Jensen says.
Mini prisms on the historic buildings were attached using epoxy to avoid damaging the exterior surfaces of the buildings. However, finding an epoxy that would allow for vertical attachment proved difficult, and in the end, most of the prisms were attached on horizontal surfaces.
Remote Data Access
The data acquired from the total stations were sent to a Web server in GZA’s New York City office. The server runs Interfels’ ARGUS Monitoring Software, a Web-based data management tool that handles all data processing, including storing the data into a MySQL database, performing calculations on the data, presenting the results in graphical and numerical format, generating alarm messages and creating automated PDF reports. Team members were thus easily able to access all of the project data remotely through a Web browser throughout the two-year duration of the project.
Budget constraints and development issues have prevented the CTA tunnels from being completed. However, the interface sections were left in a state where additional excavation can take place in the existing tunnels without any further street-level disturbances.
Thanks to the efforts of the entire project team, the construction did not cause any movement in the adjacent structures. For GZA, the project was a successful endeavor. “The technology was smoothly implemented, and we established positive relationships with Kiewit-Reyes and the CTA,” Jensen says. “What’s more, based on the success of this installation, AMTS systems are now being used in other monitoring projects around the country.”
* In April 2007, The Mills Corp. was acquired by an investment group comprising Simon Property Group and Farallon Capital Management. Simon Property Group assumed management of the former Mills properties after the acquisition, including Block 37. Retail development is being handled by Joseph Freed and Associates LLC.
Sidebar 1: Equipment and Software SuppliersGEO-Instruments LLC (www.geo-instruments.com) – Communication interface equipment, RS-232 radios, Raven IP modems and total station power supply setups
Geokon Inc. (www.geokon.com) – geotechnical instrumentation
Interfels GmbH (www.argusmonitoringsoftware.com) – ARGUS Monitoring Software
Leica Geosystems (www.leica-geosystems.us) – robotic total stations and mini prisms
RST Instruments Ltd. (www.rstinstruments.com) – geotechnical instrumentation
Soil Instruments Ltd./ITM (www.itm-ltd.com) – I-RIS software
Sidebar 2: Recent Developments in Total Station MonitoringThe deployment of the robotic total stations for the Block 37 project was close to state-of-the-art at the time these systems were installed. However, current monitoring specifications now usually require that systems be able to continue to operate and collect data even if data communication is interrupted. The Block 37 building total station setups relied on cell service and did not satisfy this requirement. The PC-controlled tunnel total station systems did satisfy this requirement, but having a PC on site to control the total station is not always practical due to the PC’s power and space constraints.
In recent installations, GZA has deployed total stations controlled by dataloggers manufactured by Campbell Scientific Inc. These dataloggers are typically used to monitor geotechnical and structural instrumentation and are designed to operate with very low power consumption. Using this setup, it is possible to control a total station locally and power it by solar panels. Additionally, the datalogger can be used to simultaneously monitor other instruments. For example, the datalogger controlling the total station shown above also collects data from a number of wireless tiltmeters and multiple-point borehole extensometers. This capability simplifies data collection and post processing.