- SPECIAL REPORTS
- THE MAGAZINE
Since opening in 1962, Washington Dulles International Airport has become the largest and busiest airport in the Washington/Baltimore area. In 2010, it handled more than 23.7 million passengers and 330,000 takeoffs and landings, including commercial and private air traffic. Dulles also handles the bulk of the region’s international flights. But for many passengers, traveling between Washington and Dulles can be a challenge. For all its jet-age capabilities, the airport lacks a rail connection to and from Washington.
The Dulles International Airport Access Highway (DIAAH) provides toll-free vehicle access to the airport. It runs parallel to the Dulles Toll Road (DTR), a local expressway serving the communities of Northern Virginia. The 30-mile (48-kilometer) trip requires a taxi, bus or car and can take an hour or more depending on traffic. By contrast, passengers arriving at Ronald Reagan Washington National Airport, less than 5 miles (8 kilometers) south of Washington, can go from the airport to the city center in about 10 minutes and for less than five dollars.
The difference--aside from the distance--is Metrorail. Since the Washington Metropolitan Area Transit Authority (WMATA) opened its Metrorail rapid transit rail service in 1976 with five stations in the city, the system has grown to 86 stations connected by 106 miles (170 kilometers) of track. The trains operate on five routes connecting the Washington city center with surrounding cities and suburban areas in Maryland and Northern Virginia as well as Reagan National--but not with Dulles.
Adding Metrorail service along the Dulles access corridor would make life easier for airport travelers and local residents. Plans to extend Metrorail to serve Dulles have been on the WMATA drawing board for years. Now the ideas are turning into concrete and steel.
When construction is complete, the Dulles Corridor Metrorail Project will extend the Metrorail system by 23 miles (37 kilometers). In addition to the airport, it will serve Tysons Corner and the Reston-Herndon areas, the two largest employment centers in Virginia. The project’s first phase, which began construction in March 2009, consists of 11.5 miles (18.5 kilometers) of track that will create a new line branching from the existing line just west of East Falls Church Station. There will be five new stations, including three elevated platforms. The Phase 1 project includes 2,400 feet (730 meters) of twin-tube tunnel beneath the Tysons Corner area. With a projected cost of $2.7 billion, Phase 1 is planned for completion in 2013. The lead contractor on Phase 1 is Dulles Transit Partners LLC (DTP), a team led by Bechtel.
The DTP surveyors draw from a variety of positioning tools. Optical instruments include rotating laser levels, optical and digital levels, total stations and 3D scanners. Surveyors and contractors are using GNSS equipment for site positioning and Trimble GCS900 Grade Control Systems for GNSS-based machine control on the project’s excavators and earthmoving machines. 3D laser scanners are used in planning and quality control. Behind it all is a common database for project information that helps prevent systematic positioning errors. Each technology offers unique strengths, and Betit’s team is skilled at selecting the best approach for each task on the project.
Primary geodetic control comes from NAD83 points established by WMATA prior to construction. That’s not the only control on the project, though, and the DTP survey teams worked to bring the various coordinate frameworks together into a consistent system for the engineers and construction groups. Recognizing that any control point in the packed construction zone would soon be destroyed, DTP decided to install a dense network of 3D control points along the project; all of the points would be visible from within the construction corridor. The resulting network of roughly 2,000 points provides the control for all of the work done using total stations. Reflectors were attached to fixed objects along the corridor, and each reflector’s position was measured and computed in a least squares adjustment. Before any control point could be added to the control file, it had to be reviewed by at least two surveyors. To provide control for real-time kinematic (RTK) GNSS, DTP installed a Trimble SPS851 Site Positioning System GNSS Receiver as a reference station in a construction yard near the midpoint of the project corridor.
In the construction area, the position and orientation of the total stations is determined exclusively using resection (sometimes referred to as free stationing). “Traverse is not part of our methodology,” Betit says. “Nothing survives for long in the construction zone, so we use resection for everything. High-precision instruments and EDMs are really critical to our mode of surveying.” For much of the optical work, the crews use Trimble S6 or S8 Total Stations or a Trimble VX Spatial Station. DTP has done analyses to determine optimum patterns and distances for their resection work, and most resection measurements are held to 200 feet (60 meters) or less. At that distance, uncertainty in the angular component of the measurements is consistent with the precision of the EDMs.
Surveys for construction include stakeout, quality checking and as-built measurements. Setups for construction are approached in the same way as monitoring, with resection and extensive checking. “There is nothing that gets a carpenter more upset than seeing a second mark 1/4 inch (6 millimeters) from the one you had before,” Betit says. “After a crew has done a resection, they check into a point they staked previously, just to make sure things fit well.”
Like many companies, DTP uses two-person crews with their robotic instruments. It is normal for the instrument to operate unattended, with both people “out front” at the prism target. One surveyor operates the controller and calls instructions to the second person, who is often kneeling to hold the prism and mark points on the ground. The person standing is also on the lookout for hazards and moving machinery on the congested worksite. Because of the required precision, the DTP surveyors avoid using a standard prism pole, preferring to hold the prism close to the ground on very short rods or using targets on small angle iron mounts. The crews work to establish points to a precision of 1/8 to 3/16 inch (3 to 4 millimeters). The DTP teams use the robotic total stations for both stakeout and checking. It’s common to see a robot set up on a pier, with a team of two surveyors working on the next pier 100 feet (33 meters) away. Sometimes a total station may be on top of a pier for several hours, so the surveyors periodically check against random control stations. If they start to see any drift, then they will redo the resection and check into existing points.
Creating the Trimble Connected Site for the Dulles project required strong collaboration between Trimble and DTP at both the technical and policy levels. The two companies worked together to ensure secure, reliable operation, and to develop detailed plans for the layout, implementation and operation of the system. Additional network equipment, firewall rules and VLAN configurations were put in place in order to allow Trimble traffic to ride along with the DTP secure network traffic. “The key to success was open communication of the goals and system requirements between DTP and Trimble,” says DTP Senior Project IT Manager Misha Nikulin. “The collaboration addressed substantial technical issues as well as strategic business IT concerns.”
The survey team’s monitoring activities provide an example of how data is shared across the network. When monitoring measurements are completed, the data is immediately transmitted to network adjustment software. The office team looks at the resection data and measurements to the monitored points. If it’s acceptable, the data is extracted into a specialized spreadsheet that makes automatic computations. The spreadsheet is available on an internal website accessible by project geotechnical and structural engineers, WMATA and the Virginia Department of Transportation (VDOT). If something is moving (not uncommon with flexible supportive excavation), then an instrumentation and monitoring engineer sends out an email alert. The alert includes information on what commodity moved, by how much and in what direction.
The two-way communications network is especially important in troubleshooting. If there is a suspected problem, office technicians can look at the design models in a particular machine and can push a new model onto a machine when needed. The system makes it possible to observe machine operations and even capture as-built information in real time. For example, when installing final sub-ballast for a section of track, a grader makes the final passes over the surface. The machine’s GNSS or optical sensors record position and height data, and send the information over the two-way network.
Like many construction projects, survey crews are frequently pulled from one task to another with little notice. The DTP instrumentation and sophisticated IT and communications systems contribute to the flexibility. The control points and alignment data are in standard files, and crews can quickly download the data needed for a specific location. “We’ve learned how to disperse the surveyors so they are working in operations areas and still maintain very tight communication of common procedures,” Betit says. “The crews have learned how to communicate with each other to transfer crew resources. Surveyors on construction sites often like to have a single location where they can meet and deploy. But if you want really scalable solutions, then you have to learn to work remotely and still maintain very close relationships with your procedures and your ability to share resources.”