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Tunnel excavation is difficult work, presenting inherent safety risks and other challenges, even for a relatively short tube. Schedules must be met and provisions must be made for the safety of underground workers, as well as protection of structures and facilities on the surface above the underground excavations.
There is always a risk of over-excavating. Getting the alignment just right and working with large tunneling machines requires the greatest precision and accuracy in order to avoid expensive rework. A pedestrian tunnel completed earlier this year as part of a development project in Grand Rapids, Mich., utilized a new measuring system to closely monitor construction.
Michigan Street TunnelCompleted in May 2006, the Michigan Street Tunnel involved mining a 103 linear foot pedestrian tunnel about 45 feet beneath the surface. The work was accomplished by Kiewit Construction of Omaha, Neb., one of the nation's leading tunnel construction specialist firms. The pedestrian tunnel under Michigan Street is part of a $120 million construction program--the largest development in the city of Grand Rapids in more than 10 years. The project will transform a six-acre brownfield site into a state-of-the-art medical and life sciences research complex. The complex will be anchored by the new Spectrum Health Lemmen-Holton Cancer Pavilion, scheduled to open in late 2008. By using the new pedestrian tunnel, patients located across the street in the Lettinga Inpatient Cancer Unit and DeVos Children's Hospital will be able to access services at the pavilion without crossing Michigan Street.
Tunnel Measuring SystemFor this specialized job, Kiewit's underground engineering and construction experts decided to use the Leica Geosystems (Norcross, Ga.) Tunnel Measuring System (TMS). The TMS combines both software and field hardware for the complete management of the tunnel building process, allowing the design model CAD file to be input into a Leica robotic automatic total station. The design can then be laid out to a high degree of accuracy and precision utilizing customized tunneling routines that run onboard the total station. It can be used for both setout and as-built applications. The TMS was under the control of Kiewit's surveying department, so professional surveyors were involved throughout the process of the tunnel's construction. "We found the straightforward user interface to be very logical and easy to use for both skilled surveyors and tunnel construction crews," said Paul Madsen, project manager for Kiewit Construction.
Project ProgressionAccording to Madsen, there were several procedures used for creating the pedestrian tunnel in Grand Rapids. "Before we started digging, it was necessary to stabilize the uncompacted sand base. This was accomplished by permeation grouting. A mixture of sodium silicate and hardener was used to chemically bond the sand particles into "sandstone' with strengths between 150 and 500 psi [pounds per square inch]."
After verifying the soil strength, excavation through the sandstone material began using a CAT 314 excavator with grinder head. The TMS SETout software was used to load the tunnel design onto a Leica TCRP1201 robotic total station, which was able to take continuous measurements from reflectors affixed inside the tunnel and provide precise guidance to the excavation crew as they advanced.
Surveyors placed the original control points outside the tunnel. As the tunnel excavation moved forward, new control points were established using the total station's onboard traversing and free-stationing routines. The total station has both an infrared EDM and reflectorless EDM mode, permitting measurements to be taken with and without reflectors. When reflectors were required, a specialized tunneling prism was used with an L-shaped bracket to anchor them to the sides or top of the tunnel. The TCRP1201 provided precision of one arc second in angular reading and two millimeters in distance measuring.
"The total station was used to paint an excavation line on the top heading one foot inside the target excavation," Masen said. "This allowed the grinder operator to remove a large portion of the profile with a reduced risk of over-excavating."
After excavating the five-foot top heading, lattice girder was set in place using a CAT 308 excavator with the Leica TCRP1201 providing precise placement. Shotcrete with steel fibers was used to secure the girder against the excavation and extend the initial liner. "The corresponding bottom bench profile was excavated afterwards, so the full face of the tunnel was never open at one time," Madsen noted.
A waterproofing system, consisting of a felt lining and Â¹/8 inch PVC membrane was installed after the initial shotcrete liner was completed. Welded wire fabric panels were hung inside the waterproofing membrane using BA anchors prior to the placement of the final shotcrete liner.
The TMS provided precise control over all facets of tunnel construction, reducing over-breaks and under-breaks. This translated into enhanced productivity and considerable cost savings due to less shotcrete and less rework. It also decreased the time it took to install tunneling support structures under the control of the automatic total station. In addition, the routines both within the total station and the office software helped to manage the ongoing tracking of quantities and as-built information. This provided real-time analysis of the project status in relation to the project design, and saved Kiewit money because engineers were alerted ahead of time of potential problems.
"There is no doubt that the TMS played a key role in completing the job safely, on time and within budget," Madsen said. "We were impressed with the system's accuracy and precision in guiding the tunnel excavation."