At just two miles, the State Route 99 Tunnel in Seattle will not be an especially long tunnel boring project; however, it will be a record breaker in terms of diameter. That's partly because it's not a rail tunnel. Instead, the tunnel will function more like an underground bridge for cars, with two generous two-lane freeways stacked on top of each other. That takes a lot of volume, and a very large-diameter tunnel – 57.5 feet, to be exact. For comparison, the tunnels being bored in New York's East Side Access project (currently in the running for world's biggest ongoing tunnel project) never exceed 22 feet in diameter.
The tunnel is so big, in fact, that a brand new, $80 million tunnel boring machine (TBM) was built especially for the project by Japan's Hitachi Zosen Corporation and shipped to Seattle in 41 pieces. When assembled, "Bertha" – the name was picked by Seattle elementary school students – is 326 feet long and weighs 6,100 tons.
This giant machine is already at work, digging the world's widest tunnel, at a rate of 6.5 feet per day, directly underneath Seattle's historic Pioneer Square and the downtown business district – an area with high groundwater and notoriously unstable soils. If all goes well, work will be completed in late 2015. You can see why monitoring the boring process and the precise positions of the buildings and other infrastructure directly overhead might be important to Seattle-area businesses and residents.
A model of precise monitoring
It's not as if tunnel collapses are unheard of. As recently as 2009, a rock fall idled a mega-TBM on a Niagara River water diversion tunnel for more than two months. And even in Seattle, tunnel projects have run into trouble. A July 27, 2013, article in the Puget Sound Business Journal noted: "It will be up to—¦ Seattle Tunnel Partners to make sure things go smoother (on the viaduct) than for King County Metro's Brightwater sewage tunnel near Bothell, or for Seattle's Beacon Hill Sound Transit light rail tunnel. Work on Brightwater stopped for months in 2009 when a tunnel-boring machine was damaged. At Beacon Hill, construction worker Michael Merryman died in a 2007 accident, and the digging opened —voids' in the earth and caused house foundations to crack."
"There have been a lot of projects where there were huge damages," conceded Loic Galisson, a project manager for SOLDATA, a Seattle-based firm that specializes in risk mitigation on large construction sites. "And in many cases, the damage is increased because there is little or no monitoring. It's not that good monitoring can prevent damage, or collapses – there are a lot of risks and threats that simply can't be predicted. But, monitoring can help tunnel boring projects to react well to changing conditions, and to react sooner."
SOLDATA is the subcontractor charged with monitoring aboveground changes during the tunnel boring. The firm understands that they're dealing with a special challenge. "In some ways, this is very similar to projects we've completed in Europe and Asia," Galisson said. "But on the other hand, this is really a very big tunnel, one of the biggest ever drilled, so everyone is wondering how it will turn out."
To cope with the unknowns, SOLDATA has assembled one of the most comprehensive and sensitive monitoring networks ever attempted – a network that has deployed about 4,000 sensors that rely on dozens of separate technologies, including extensometers (measure vertical deformation of the ground), inclinometers (measure horizontal movement of the ground), piezometers (measure static pressures in fluids, usually groundwater), crack and strain gauges (applied to beams and other building supports), several types of levels and tiltmeters, and even some radar interferometry. All of these are in addition to thousands of sensors on the TBM itself.
The most visible sensors – and the ones that have most captured the interest of Seattleites – are TM30 total stations, made by Leica Geosystems. These are specialized instruments, built for real-time monitoring. SOLDATA has set 37 of them in permanent locations on (and, in two cases, under) buildings and roadways covering the projected tunnel path. SOLDATA refers to the TM30s affectionately as "Cyclops" or "Centaur," and it is fair to say that they're the heart of the network.
All 37 total stations are connected 24/7 to a web-based GIS and each of them automatically measures about 50 points in cycles lasting 20 to 30 minutes. Very small positional changes – as small as a quarter inch and up to an inch, depending on ground conditions and the type of structure being measured – immediately sound alerts that are conveyed to TBM crews and, in some cases, can shut down Big Bertha more or less automatically.
It's a lot of responsibility, and Galisson and his staff take their work seriously. A team of 18 was onsite for several months prior to boring commencement, designing and installing the network and establishing baselines for building movement. A team of six will remain onsite for the entire dig. What will they be doing? "Watching screens!" Galisson said cheerfully. Completely automatic reporting isn't feasible with a sensor network this size, and SOLDATA staff will spend a lot of time "monitoring the monitoring," with someone keeping an eye on onscreen reports continuously. And obviously, a lot of time will be spent checking, maintaining, calibrating, repairing, and replacing all the many sensors.
Designing the TM30 network alone took many weeks. Engineers used existing surveys and aerials to designate the most important and desirable locations for total stations. The actual spot was important, as project specifications called for the TM30s to remain in place throughout the 14-month construction phase.
"We discussed —leapfrogging' the stations along the tunnel path, as the TBM progressed," Galisson said. "But ultimately, having consistent readings of one big set of positions is more important than minimizing the number of Cyclops used." So SOLDATA wanted, basically, secure locations with good views of critical points, and they wanted these to be distributed evenly along the bore path.
Having established ideal locations, they then compromised as needed. "We had a theoretical optimum, of course," Galisson explained, "but in some cases we couldn't get access to buildings, and we also tried to minimize tree trimming."
Vandalism or theft, of course, had to be considered. Most of the aboveground TM30s are set on rooftops with key-controlled access, or on secure public structures, and most are also set inside covered steel cages. And every TM30 is equipped with an anti-theft system that covers the tribrach and prevents unauthorized adjustments.
Leica Geosystems, of course, is keenly interested in the monitoring project, and in the highly visible use of one of its premier instruments. "We worked closely with the SOLDATA project management team through all phases of this project," said Gerard Manley, Leica Geosystems' vice president of engineered solutions. "To begin, our team had to prove to SOLDATA that our TM30s would perform throughout the demanding weather seasons expected in Seattle. So we tested the instruments on another SOLDATA project that had similar precipitation levels. Once approved, we had to meet the demanding installation schedule and provide them with guarantees of available backup instruments if needed. I am pleased to report that all instruments are performing to their expected standard."
Working with the data
Analyzing the data from 4,000 sensors, in real time, and then reacting – immediately if necessary – is a world-class feat of analysis. The day-to-day operation of the TM30 network is controlled by proprietary software that SOLDATA calls TACT, for Target Acquisition Controlled by Theodolite. Similar to Leica Geosystems' GeoMoS automatic deformation monitoring system, TACT directs each TM30 to cycle through its assigned series of measurements, compiles the readings, and uploads to other systems as needed.
To manage the data, SOLDATA is using a proprietary web-based GIS called GEOSCOPE. "Its strength is to be able to collect data from nearly any kind of sensor and to combine various types of data sources to analyze ground behavior, or any another environmental phenomenon," explained Galisson. "We've been building GEOSCOPE for the last 15 years, taking advantage of real experience and feedback from all our business units on the major urban construction projects in the world (Europe, Asia, Russia, North/South America). So it is a really flexible tool, adapted to construction, environmental monitoring, mine operations, etc. Addressing the specific requests we've had from our various clients have made this software really powerful, and very well suited to user needs."
Seattle is the site of one of America's most ambitious landscape transformations. The Denny Regrade neighborhood, now a flat triangular area just north of the central business district, used to be a very large hill. In a series of massive projects extending from 1897 to 1930, Denny Hill was first dug up and leveled in spots via informal projects, then sluiced into Elliott Bay with hydraulic mining techniques, and then, finally, regraded completely with steam shovels. It goes without saying that none of this work was "monitored" in the modern sense. And, in fact, there were many disastrous landslides and unplanned home removals in Seattle's regrade era.
So it's a good thing that construction project monitoring has emerged as a surveying specialty in our time. Addressing the world's massive infrastructure shortfalls will absolutely require that large, ambitious projects be undertaken in densely populated urban areas. As SOLDATA's work on the SR 99 Tunnel Project is proving, monitoring is now a mature construction technology that is helping cities to remake themselves, safely and efficiently, without doing damage to existing buildings and landmarks.
Angus W. Stocking, L.S., has been writing about construction and infrastructure since 2002. More information about monitoring solutions is available at www.leica-geosystems.us