Project Case Study: Tracking structural stability

August 2005 » Feature Articles
When completed this fall, the Reno Transportation Rail Access Corridor (ReTRAC) project will ease traffic congestion and beautify downtown Reno, Nev., by sinking 2 miles of on-grade rail lines into a 33-foot-deep trench. Among the project's challenges is how to avoid undermining the foundations of existing structures-some of them historic buildings.

Wi-Fi technology streamlines tilt monitoring for Reno ReTRAC trench construction.

Project
Reno ReTRAC

Design/build contractor
Granite Construction Co.

Project application
Wireless tiltmeters provide continuous monitoring of buildings adjacent to and spanning a 33-foot-deep trench.

BY ETIENNE CONSTABLE, DOUG BLEAKLY, R.G., AND CLAUS LUDWIG, P.ENG.

When completed this fall, the Reno Transportation Rail Access Corridor (ReTRAC) project will ease traffic congestion and beautify downtown Reno, Nev., by sinking 2 miles of on-grade rail lines into a 33-foot-deep trench. Among the project's challenges is how to avoid undermining the foundations of existing structures-some of them historic buildings. As part of its project plan, design/build contractor Granite Construction Co., of Watsonville, Calif., called for installation of tiltmeters on sensitive buildings, which are located close enough to the trench alignment that construction activities inadvertently could cause structural deformation or foundation settling. Granite determined that stability could best be assured by using digital tiltmeters to document the effects of trench excavation on the buildings; provide a record of the structures' performance as a defense against damage claims; and protect workers, the public, and building owners from injury or property damage.

Additionally, Granite specified that the tiltmeters and the associated data acquisition system be configured to integrate seamlessly into, and communicate with, the project's planned Ethernet (Wi-Fi) network.

ReTRAC overview

A combination of open concrete Uchannel and standard retaining walls, ReTRAC is 2.2 miles long, 54 feet wide, and an average of 33 feet deep. After years of discussion and delay, the city of Reno finally took the project to bid and awarded the $171 million contract to jointventure partners Granite and Parsons Transportation Group (Pasadena, Calif.) in summer 2002.

Crews spent the first 20 months building the shoofly-temporary track lines that Union Pacific Railroad trains are using while the trench is built in the old railroad right-of-way. Union Pacific activated the shoofly in late April 2004, and excavation for the trench began in mid-May. Since May 2004, Granite has removed more than 700,000 cubic yards of material to make way for construction of the 2.5-footthick U-channel. As of July 2005, about 80 percent of the 100,000 cubic yards of concrete had been placed.

The proximity of the new trench walls to eight sensitive structures along the alignment necessitated the use of extensive underpinning to allow safe excavation of the trench and construction of the U-channel.

Schnabel Foundation Co., of Walnut Creek, Calif., designed and constructed the underpinning for the structures with handdug piers and micropiles.

Trench walls were built directly below four of the sensitive structures. Here, continuous hand-dug piers extended to a depth of approximately 10 feet below the groundwater table. The continuous underpinning piers also serve as the new trench walls. The largest building underpinned in this manner was the Fitzgerald's Garage, a seven-level concrete parking structure that spans the railroad trench. This structure required continuous underpinning along the two column lines, each 300 feet long, directly above the new trench walls. The remaining four sensitive structures along the alignment had enough horizontal clearance from the new trench walls to permit micropiles for underpinning.

In all, the various forms of excavation support included soil nail wall shoring (200,000 square feet), vertical piling with tiebacks, micropile underpinning, and conventional underpinning. The choice of support technology depended on right-ofway constraints, the proximity of existing buildings and the shoofly, and continuance of surface-street traffic parallel to and across the project during construction.

Additionally, the excavation extended below the groundwater table in a significant portion of the trench. Jet grouting of the soil between the vertical piles created a vertical impermeable cutoff wall in wet areas. Where conventional underpinning was performed, permeation grouting from the original ground surface produced an impermeable soil mass below the groundwater table. After excavation of the underpinning pits, an unreinforced seal slab at the bottom of the excavation was poured, completing the nearly watertight “bathtub.” This allowed Granite to build the conventional reinforced-concrete structure in the dry.

Precast concrete tension bridges now in place provide 11 at-grade crossings over the trench. When the U-channel is finished, Union Pacific will lay the 60- mph permanent rail line and begin testing the two mainline tracks. The Granite/ ReTRAC team is aiming to have trains in the trench by Thanksgiving of this year.

Granite then will remove the shoofly, regrade the crossings, and finish site work and landscaping. Final completion is slated for late spring of 2006.

Stability monitoring of sensitive buildings used Wi-Fi tiltmeter installations on Reno's famous National Bowling Stadium, the Fitzgerald Garage, and Fitzgerald's Hotel Casino Rainbow Bridge spanning the trench. The instruments also recorded the excavation's effects on three historic buildings that abut ReTRAC: the Amtrak station, where thousands of tourists arrive in Reno each year; the Men's Club, a historic natural-stone structure; and the two-story Freight House and associated warehouse.

“It was originally contemplated that two of these three historic buildings would have to be relocated,” said Ron Dukeshier, Granite's project manager on the ReTRAC Project. “But immediately prior to bid time it was determined they would just clear the trench. The contractor was given the option to underpin or relocate, and it was a lot more economical to underpin.

Wireless tilt monitoring

In April 2004, Granite awarded to Applied Geomechanics Inc. (AGI) of Santa Cruz, Calif., the contract to install 36, Wi- Fi-equipped tiltmeters on the eight buildings identified as sensitive. In order to establish response baselines for each structure, the contract called for the instruments to be on-line some weeks prior to the start of excavation. The 36 digital tiltmeters have remained fully functional since construction began in May 2004, providing a record of building performance during the life of the project.

In accordance with Granite's specifications, AGI modified its off-the-shelf MD900 digital tiltmeter by adding a realtime clock for synchronization and timestamping of each data point, and mating each tiltmeter to an outdoor-rated 2.4- GHz, 802.11b Ethernet Wi-Fi radio transmitter.

Originally, it was intended that each tiltmeter would communicate directly with the project's Wi-Fi system. However, when Granite encountered technical difficulties that prevented completion of the Wi-Fi network, AGI was asked to develop an alternative communication/data collection method. AGI's solution was to reconfigure the tiltmeters and their Wi-Fi transmitters to permit “drive-by” data downloads. To improve line-of-sight access to tiltmeters mounted in remote locations, two allweather wireless routers with high-gain omnidirectional antennas were installed in strategic positions where each could serve as a hub for nearby clusters of tiltmeters.

In the drive-by arrangement, a field technician with a Wi-Fi-equipped laptop computer can connect to every tiltmeter (or instrument cluster) from any line-ofsight location, within several hundred feet of a tiltmeter or router, and initiate queries, download data, and confirm operation.

Technical challenges

Tiltmeter monitoring is a well-established technology for tracking structural stability, but each project presents technical challenges to identify (preferably in advance) and to surmount. Monitoring system costs, ease of operation and maintenance, and data quality all depend on how competently the technical issues are addressed. In the case of Reno ReTRAC, most of the challenges proved to be those typical of large, complex projects.

Documenting the stability of varying building types adjacent to the trench-from historic structures with native stone walls on unreinforced stone foundations, to concrete block on-slab, to pillar-andshear- wall construction-demanded instrumentation that would acquire reliable data and communicate flawlessly using the project's Wi-Fi network.

The most critical issue that arose proved to be the unanticipated absence of the project-wide Wi-Fi network. When instead, drive-by downloading was implemented, a consistent, once-every-six-days schedule for data downloads had to be set up and followed to avoid filling each tiltmeter's flash memory.

Wi-Fi benefits

Wi-Fi monitoring proved to have the following important benefits beyond its originally intended ease of integration with other ReTRAC data communication systems: Reduced costs-A typical Wi-Fi system involves less cabling and no data loggers or multiplexers, requires less labor to install, and generally uses less power.

Easier equipment moves-Wireless systems have fewer components to remove and reinstall.

Limited need for surge protection-A wireless system needs only a single surge protector at each antenna.

Faster, more flexible data download options-Options include drive-by access from line-of-sight locations, or direct wireless communication to a central project computer.

Data digitized at the measurement site-Digitization improves data reliability, lessening the chance of information loss during transfer of analog signals.

The applications engineering team at AGI envisions further improvements to the Wi-Fi monitoring system as it was deployed for Reno ReTRAC. Among several enhancements currently in development are adding on-board, integrated Wi- Fi transmitters to shrink the combined tiltmeter/transmitter's footprint and reduce power consumption, and increasing onboard memory to allow maximum time between downloads.

Although it can be seen as innovative, the choice of Wi-Fi tilt technology for Reno ReTRAC is clearly “on track” with the industry trend toward wireless data collection and communications.

Etienne Constable is technical services manager and Doug Bleakly is a registered geologist with Applied Geomechanics, Inc. Claus Ludwig, P.Eng., is with Schnabel Foundation Company. For more information about tiltmeters, visit AGI's website at www.geomechanics.com, or telephone 831- 462-2801.


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