Feature: Surveying 24 Bridges in 40 Days: The T-REX Project
Professional Surveyor Magazine - January 2004
Tom Service, PLS
Using Cyrax scanning technology, David Evans and Associates, Inc. (DEA), surveyed 24 bridges in 40 days without disrupting traffic on the heavily traveled Interstate 25 through the Denver, Colorado metropolitan area. T-REX, the Transportation Expansion Project launched by the Colorado Department of Transportation (CDOT) includes highway expansions, improvements, and the addition of light rail along 18 miles of Interstate Highway 25 and I-225 in the metropolitan Denver area. The $1.67 billion design/build project includes reconstruction of interchanges and bridges, a new drainage system, and improved pedestrian and bicycle access.
CDOT selected Southeast Corridor Constructors (SECC) for the project, based in part on SECC's aggressive work plan for finishing T-REX 22 months ahead of CDOT's schedule. SECC is a joint venture between Kiewit Construction Company and Parsons Transportation Group (PTG). Turner Colley Braden (TCB), Sverdrup, and DMJM are assisting PTG in the project design. Kiewit is responsible for gathering the necessary design survey information as well as construction staking and final right-of-way surveys.
After reviewing the tight schedule and design requirements, Jim Bodi, the Survey Manager for SECC, decided to combine conventional terrestrial surveys with aerial mapping and to use a Cyrax scanner to gather the bridge and mainline freeway as-built data. Jim hired my company, David Evans and Associates, Inc. of Denver to provide terrestrial surveys and Cyrax scanning services.
My task as the Project Principle was not made easier by the fact that DEA crews had 40 days to complete field surveys for basemaps of 24 bridge sites over or under I-25. Right at the start I decided that we needed two Cyrax crews for the Cyrax scanning and three field crews. DEA performed mapping under and over many structures while six to eight lanes of traffic buzzed by uninterrupted. In the office DEA employed six survey technicians to manage the Cyrax data: three to process the data, one to process the field survey data, and two drafters to produce the base maps.
We learned a lot during the project. One of the more amazing facts relating to this project is that even with five field crews working 12 to 16 hour days, we did not require traffic control or lane closures for any of our work. We worked through morning and evening rush hour while listening to the helicopter traffic reporters on the radio talk about the work we were doing, but at no time did we impede the flow of traffic.
The Cyrax scanner has been used for the past three years; however, it has never been put to work on so many structures in such a short time frame. DEA had extensive experience using the Cyrax equipment and processing the data for bridges, intersections, roadways, parking lots, and buildings. Our survey technicians are trained and experienced in merging the scan data with the field crew's conventionally gathered topographical mapping to prepare base maps for design.
The bottom line is that we knew it would work for T-REX and believed we had the right combination of field and office procedures to complete this project in the time allotted. The further we got into the work, the more we found that our normal survey procedures also applied to the new technology. Even though the scanner collects data differently (quicker and more of it), the same type of control is necessary. Each scan has to have at least three control points or registration points in it. If the control is poor the scan data is useless. Proper planning and attention to detail is essential for a successful project.
We used one scanner mounted on a tripod and the other scanner mounted on a boom truck, dubbed the "Scan-Van." The tripod scanner performed the under-bridge scanning and tight area work, while the Scan-Van provided proximity for the railroad scanning and mainline work. The Scan-Van has a boom arm that raises the scanner up and increases the range and visibility, thus increasing the amount of data that is captured. We could park off the shoulder with the van and scan the freeway while the cars drove by. The cars would show up in the scan as a single line. The scanning technicians could then delete the cars from the scan before sending the data to the mapping technicians. If the traffic was moving very slowly or standing still we did not attempt to scan.
We also learned that by setting up a conventional instrument, such as total stations with each scanner, we were able to increase our productivity. We could tie in the control points while the scanner was operating and pick up the obscure area topo that the scanner could not observe. The scanner can only pick up what it can see, therefore the top of a curb may be mapped but the flow line would not be measured. Likewise we found that areas around parked cars, guardrails, or brush could be mapped using the conventional instruments.
DEA recognized the Cyrax Scanner as another valuable instrument that we could add to our inventory. We have been using GPS since 1988 so adding the 3D laser scanning equipment and technology was seen as a natural extension of the equipment already in use.
This is so far our largest deployment of Cyrax technology, that we used in conjunction with other methods as well. In fact the main challenge was to coordinate the use of a number of different hardware and software platforms. These included the Cyrax systems (used mainly on bridges and overpasses) and total stations, aerial survey, and GPS equipment for the main roadway and control. The Scan-Van mounted Cyrax achieved about 240 feet distance and the tripod mounted Cyrax about 180 feet, not a dramatic difference.
DEA used the following workflow:
-- Collect control survey data and Cyrax scan dataÑ2 person field crew
-- Process survey data in Liscad (create control point coordinates)
-- Load survey control data into Cyclone (Cyra's own software) and register scan data (includes quality control check of registration)
-- Reduce Cyrax Cyclone point cloud data into conventional AutoCad data
-- Export from Cyclone into AutoCad
-- Review line work in AutoCAD and correct any layer errors
-- Translate from AutoCAD to Microstation (InRoads) for final file/drawing preparation
Final deliverables to the client were Microstation drawing files and InRoads DTM (digital terrain model) files. The client can use planimetric drawings for their horizontal designs and the DTM for all of their vertical calculations.
DEA delivered 24 bridge sites within the 40 days. We continue to provide survey services on the project on an as needed basis.
Construction on the project is expected to extend from 2001 to 2008.
TOM SERVICE is Senior Vice President of David Evans and Associates, Inc.
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