Precision Alignment Aids Expanded Transit System

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 More than one million athletes, judges, volunteers and visitors are expected to converge on Vancouver, Canada, and surrounding areas during the 2010 Olympics. Yet many of these visitors won't realize the Olympic-sized engineering and surveying feats accomplished to prepare this beautiful coastal and mountain region for so many guests.

Of particular medal-worthy vision is an innovative span-to-span elevated guideway for a new transit system, pieced together like a beaded necklace with more than 2,600 precast concrete segments­ each segment positioned with millimeter accuracy high above the city streets.

The project is a key part of the nearly $2 billion, 19-kilometer Canada Line metro railway expansion project, currently under construction, that will ultimately connect the nearby city of Richmond and Vancouver Airport to downtown Vancouver. This metro railway, designed to serve the region for the next 50 to 100 years, will provide additional transportation capacity equivalent to 10 major road lanes.

Archie Gal, Survey Manager on the elevated guideway portion of the project for RSL Joint Venture, says, "The high precision requirements defined on the guideway project, coupled with the innovative construction techniques used to piece each element together, challenged our expert survey team and our equipment. It's not every day that we are asked to achieve two-millimeter accuracy while straddling a scaffolding support beam eight meters above an active highway."

Hanging Segments
Construction of the Canada Line expansion project (formerly called the "RAV Line") began in September 2005, and includes a bored tunnel section, a cut-and-cover tunnel section, two bridges across the Fraser River, as well as the elevated guideway to Richmond and the elevated and at-grade guideway to Vancouver Airport. The overall project, managed by SNC Lavalin, a Canadian engineering and construction firm, is broken into a number of separate joint ventures to handle individual sections.

The RSL Joint Venture (a joint venture between SNC-Lavalin and Rizzani de Eccher of Italy) is responsible for designing and constructing the high profile elevated guideway in Richmond and the elevated and at-grade guideway to Vancouver Airport, as well as the two bridges crossing the Fraser River.

The approximately 7.3 kilometers of elevated guideway are constructed using an enormous horizontal crane, called a launching girder or truss. This massive truss structure is suspended high above the busy roadways and is used to lift, support and align precast concrete segments from span to span.

To begin the project, crews erected 230 substructure columns and foundations along the proposed path approximately 36 meters apart. Once a number of the columns are complete, the launching girder is lifted into position and assembled at the tops of the columns across the first span.

Following completion of the truss assembly, superstructure work begins with 3.0m segments of precast concrete blocks (called "rocks") delivered from the precast yard, located in south Vancouver, to the work site via trucks.

Precision Alignment
The average segment weighs approximately 30 tons, though some are as much as 90 tons or as little as 20 tons. Each segment is uniquely constructed to fit in a given location along the defined route. Each span is typically composed of 12 segments that extend approximately 36 meters.

Before span assembly can begin, all the segments in a span must be hoisted and held suspended in the air by the truss. Construction crews assemble the first two segments using torqued threaded bars to hold the span until final post-tensioning is stressed. Once all concrete segments across a span are lifted and elevated on the truss structure, the segments are connected with post-tensioning cables. The RSL Joint Venture surveyors are then called on to provide final alignment.

Well before project go-ahead, the RSL Joint Venture survey team selected survey equipment able to achieve the tight tolerances required. This included the Leica TCA2003 automated target recognition total station from Leica Geosystems, one of only a few high-precision half-second instruments in the industry with a one millimeter EDM. Other survey solutions included the DNA03 precision digital level and TCR1201 & TCRP1201 total stations.

Precarious Balance
By the time the last segments are suspended from the truss, the RSL surveyors are set up with a total station and level, ready to begin span assembly. They initially establish a resection point from ground-based control, and reference elevations at the guideway level.

During span assembly, elevations are checked at each stage and small adjustments are made. The survey checks are done to prepared data that takes into account curvatures, camber and information from the precast yard defining the as-cast geometry of the segments.

Archie explains, "The purpose of the survey at this stage is to land the span within ten millimeters in alignment and ten millimeters above the final elevation­it cannot be low, to ensure the span is above the bearing plinths at each column."

Once fully assembled, the truss lowers the span approximately ten millimeters onto the columns and spacer blocks that are later replaced with bearings.

The unique launching truss presents many obstacles for survey crews, not the least are the lifting frames that are used to lift the segments into place while hydraulic actuators tilt and slant the segment as required. Sometimes, the level can only be set up to shoot under or below the frames, or the grade is restrictive. Then the level must be set up using a 60-centimeter short tripod.

"Other times, to aid visibility of ground control over the parapet wall, or over a crest, we use targets on three meter `Super-Legs' accessed by stepladder," explains Archie. "Our safety regulations require fall arrest protection above the level of the guideway, or on the truss, so both surveyor and assistants wear harnesses with double lanyards. When accessing the end of the span, the assistants climb over each lifting frame with the harness lanyard clipped to an overhead lifeline."

Once the entire span is aligned, the truss self-launches forward horizontally to the next set of columns and prepares to hoist and assemble the next span.

From 10 to 2
While the truss crews are preparing supports and anchoring the truss, the survey crews return to the erected span completed at least three launch cycles prior (to minimize vibrations and movements from the truss) and determine the final in-place shape of the assembled span.

Archie explains, "We conduct a `best fit' survey to assess where the greatest number of track pad anchor bolts are closest to alignment, and the elevation of the track pad so as to require the fewest track shims and no grinding. This is a high-precision survey. Once started, the best fit survey must be completed from the same setup."

The resulting data is sent to the survey office where the shape of the span is modeled against the design alignment where it is rotated and translated to achieve the best fit position of the span.

When the best position of the span is determined, the span is jacked into position using high pressure hydraulic jacks capable of lifting 300 tons. For this purpose, a resection point is established at least one span away, using ground control.

"Span-jacking is done by an expert crew of surveyors, experienced in the precise movements and reactions of a large concrete structure. Setups can only be above a column for stability. The ground control network must be set at fifty to sixty-meter intervals to tie within tolerances of less than two millimeters between monuments at two-hundredfifty-meter intervals, to enable setting the span into its final position within two-millimeter precision," Archie says.

There are some spans that must be aligned perfectly within two-millimeters on first erection, without subsequent final span-positioning. These spans are suspended in place, and a closure pour is made to bridge approximately 30-centimeter gaps between span-ends. This situation occurs at the passenger stations, where the spans become part of a monolithic station structure. These spans are checked and double-checked for movement before and after closure forming is built.

The End of the Line
Construction on the entire Canada Line expansion project is on schedule for completion by 2009, in time for the 2010 Winter Olympics. The elevated guideway in Richmond and Vancouver is now complete.

The RSL Joint Venture is also constructing a cable stay suspension bridge over the North Arm of the Fraser River which incorporates two important features that reflect the bridge's location over a busy marine route and its proximity to Vancouver Airport. A longer distance between the foundation piers will accommodate the significant marine traffic that will pass under the bridge. A cable system supporting the bridge will be held by lower towers, avoiding potential interference with overhead flight traffic to and from Vancouver Airport.

RSL Joint Venture's Archie concludes, "This has been a truly exciting project that ties innovative engineering with the highest precision surveying skills. When we're done, we'll leave behind a transportation infrastructure that benefits the region well beyond the Olympic events."

Vicki Speed is a freelance writer based in Dove Canyon, California.

A 2.666Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE