The Swiss Alps have for centuries presented a major barrier to transport in Europe. Even today, modern highways and railroads struggle to negotiate the steep, unforgiving terrain. To ease the bottleneck, Switzerland is making major investments that will integrate it into the growing network of high-speed railways in Europe. A key component is construction of the New Rail Link through the Alps (NRLA). When completed, the NRLA and its high-speed AlpTransit trains will reduce the 3-hour, 40-minute travel time between Zurich, Switzerland, and Milan, Italy, by a full hour. In addition to numerous new bridges and facilities, the NRLA will include approximately 120 kilometers (75 miles) of new railway tunnels. The centerpiece of the tunnel system is the Gotthard Base Tunnel between Erstfeld and Bodio in southern Switzerland (see Figure 1). When completed in 2017, the Gotthard will be 57 kilometers (35 miles) long — the longest tunnel in the world.
Working a 35-kilometer-long (22mile-long) section in the northern part of the Gotthard tunnel, Swiss company terra vermessungen ag is providing surveying services for STRABAG, the tunnel’s construction contractor. The variety of the work, demanding requirements for precision, and difficult working environment present the surveyors and their equipment with unique challenges. Surveyors contend daily with high humidity, noise, poor lighting, and schedule pressures.
One of the more difficult hurdles comes from the narrow sight lines in the tunnels. When sighting in a tunnel, it is common to have multiple prisms visible in a total station’s field of view. The prisms appear to be very close to each other. While manual observers can distinguish the desired target from the others, robotic total stations with automatic pointing systems can struggle to select a single, correct target. To solve the problem, terra vermessungen selected the Trimble S8 Total Station. The 1-second (angular accuracy) robotic instrument combines precise angle and distance measurement with Trimble FineLock pointing technology to provide fast, high-accuracy measurements in the confined spaces.
Precise underground control
Working at the core of the project are two giant tunnel boring machines (TBMs), operating in parallel tunnels approximately 60 meters (200 feet) apart. Each TBM is capable of excavating as much as 40 meters (130 feet) per day, and it is crucial to keep them operating and moving in the correct direction. To do so, surveyors regularly maintain and extend the network of control points used by the TBMs, as well as densify the tunnel control network behind the machines.
Surveyors extend the control networks by about 200 meters (660 feet) at a time by placing new control points on the tunnel walls or roadway slab. For each point, crews install rigid threaded bolts equipped with an adaptor that holds the survey prism. The intervisible control points are set at sight distances ranging from 20 to 300 meters (66 to 980 feet).
Each day’s surveying is completed in shifts of approximately eight hours. During this time, the surveyors share the tunnel with crews performing maintenance work on the TBM as well as work on logistical items such as power, water, ventilation, and conveyor belts. The surveyors must coordinate with — and stay clear of — the other activities. Extending the control network usually requires three instrument setups over two hours, in which the team completes multiple measurements to approximately 25 targets. The measurement data is collected using Trimble Survey Controller software running on a Trimble TSC2 Controller.
Each day’s data is downloaded and analyzed in the local jobsite office. To verify the internal accuracy of the measurements, the surveyors first compute a strict adjustment using an unconstrained network. Next, they use a Helmert transformation to transform the new, unconstrained network onto the existing tunnel control. The maximum residual errors must not exceed one millimeter. The surveyors attach prisms to the new points, which will serve as control points for the TBM until the tunnel moves ahead. Then new points are installed and measured as the cycle repeats.
Keeping the tunnel in shape
Tunnel construction often alters the natural forces and loads spread through the rock in which the tunnel is bored. The resulting deformations occupy relatively small areas called convergence zones, and typically occur in known fault zones or in areas with visible cracks or falling rock fragments.
The surveyors monitor the convergence zones by collecting periodic cross-section measurements. Depending on the level of risk and movement, three to seven prisms are mounted per cross-section. The intervals between cross-sections vary from a few meters to as much as several hundred meters.
With the convergence zones defined and marked, project geologists and local staff determine how frequently measurements should be taken. Survey crews must decide whether to measure the prisms conventionally, or to install an automated monitoring system. It’s not an easy process, as the null (or baseline) measurements must be taken as soon as possible after the convergence is discovered. Because no information on possible subsequent deformation is yet available, the surveyors need a flexible and expandable approach to monitoring the convergence. They can start with a conventional measurement configuration using the Trimble S8 controlled by a surveyor on site.
If necessary, the team can quickly convert to an automated system. In this approach, the Trimble S8 operates as a remotely controlled automated monitoring system to take continuous observations without the need for a surveyor at the instrument. Remote software controls the instrument and analyzes the measurement data to detect movement of the prisms. To guarantee a complete series of measurements, the prisms installed for the null measurement are used for the ongoing observations. Once the rate of deformation subsides, the observation intervals may be changed or crews can resume conventional, on-site monitoring.
Blast, muck, and measure
While the TBMs do most of the digging, tunnel sections that are short or have irregular cross-sections are often excavated using the blast and muck method. In this approach, a jackhammer drill bores holes, approximately 80 millimeters (3 inches) in diameter, several meters into the heading face. The holes are filled with explosives and are fired simultaneously.
Immediately after the shot, the tunnel face is littered with rock and debris (sometimes several hundred cubic meters). Access to the face is prohibited until the newly excavated tunnel is stabilized. However, the tunnel crew is in a hurry to remove the rock material, and the heading foreman wants to know immediately if his shot has been successful or if further blasting will be needed to achieve the required profile. Each blast creates a new cavern, 2 to 5 meters (6 to 16 feet) deep, and accurate surveying is needed to compare the cavern with the required profile. When working in a tunnel, even this relatively simple surveying task becomes complicated.
To handle the work, terra vermessungen has developed a “motorized laser” approach using a Trimble VX Spatial Station. A surveyor mounts the Trimble VX on a console table attached to the tunnel wall approximately 100 meters (330 feet) behind the heading face. A massive steel cage protects the instrument from blast debris. Using the free stationing routine in Trimble Survey Controller software, the total station precisely determines its three-dimensional position in the tunnel coordinate system. Design information on the tunnel’s alignment and profile are stored in the Trimble TSC2.
At this point, the surveyor turns the instrument over to the heading foreman. Immediately following each blast, the foreman — running a specialized routine in Trimble Survey Controller — uses the robotic function and red laser of the Trimble VX to measure several locations on the face and tunnel walls. The measured points are compared with the design stored in the Trimble TSC2, and any deviation from the required profile is displayed on the control unit. Any points or areas showing insufficient rock excavation (underbreak) are displayed in red. Urs Müller, survey engineer for terra vermessungen, said that the video capability of the Trimble VX is especially useful. “In the poor lighting conditions, the foremen really appreciate the video display,” he said. “It helps the foreman know he is pointing to the correct spot.”
Every few days, surveyors check the measurement configuration and the quality of the free stationing solutions. When the tunnel advances by 50 meters (160 feet), the console table is moved ahead to a new position and the instrument position is remeasured. The approach carries several benefits for the tunnel-driving crew. The system can be operated by the heading foreman, and is available at all times for measuring the results of each blast. It’s a big advantage when working in a long tunnel with access routes that can be covered only by train.
According to Müller, the Trimble technology has lowered the cost of surveying on the blast and clear tunnels. It has reduced the number of surveyors needed at each heading face and eliminated delays in waiting for qualified staff to arrive.
Scanning for quality assurance
Shortly after they are excavated, the tunnel walls are sprayed with shotcrete. After the first application of shotcrete, the surface of the tunnel is measured for quality assurance. Laser scanning is a good solution because it can quickly capture an accurate depiction of the tunnel walls several hundred meters at a time.
In practice, however, the tunnel is filled with machinery and clutter. Often just the first few meters behind the heading face can be measured. And during the construction phase, the quality assurance teams don’t need the millions of points that laser scanning provides. Their goal is simply to check that the tunnel has been driven in the right position and that the structure gauge has been met. To do this, they set up the Trimble VX mid-tunnel and orient to the tunnel system using the free station method. The instrument is set to automatically measure a series of profiles at intervals of 1 meter (3.3 feet) over sections of tunnel 60 meters (197 feet) long, collecting points every 50 centimeters (20 inches) along each section. The resulting profiles are stored in the TSC2.
The entire process takes less than one hour to capture and store 61 profiles. Back in the jobsite office, the profiles are printed and delivered to local site supervision staff. This data enables the construction teams to optimize the inner concrete formwork. Müller said that optimizing the concrete thickness to 3 centimeters (1 inch) leads to savings of 1 cubic meter (1.3 cubic yards) of concrete for every 1 meter (3.3 feet) of tunnel.
Surveying for the Gotthard Tunnel includes a variety of stakeout work. One of the most demanding is the alignment of the TBMs. Each TBM is more than 400 meters (1,300 feet) long, and the concrete blocks that support it must be placed properly to ensure accuracy of the machine’s travel. Working beneath the TBM, surveyors install profile points at intervals of 10 meters (33 feet) on the tunnel walls to the left and right sides of the machine. On a typical day, the TBM advances approximately 30 meters (98 feet), so crews need to set out six points at a time.
For this work, the surveyors choose a setup location for the Trimble S8 total station where it will not conflict with maintenance and other activities. They use free-stationing to tie into tunnel control. To locate the pre-computed points along the fluctuating tunnel walls, the crews use specialized routines for tunnel setting-out in Trimble Survey Controller. The total station automatically turns to the correct direction, activates the red laser, and makes iterative measurements until the appropriate point is located on the tunnel wall. At this point, the surveyor uses a rock drill to install a small rock anchor. He confirms the correct position with the laser dot — visible on the rock anchor — and stores the point for analysis and quality assurance. Typically, it takes less than one hour to set up the instrument and install six points into the tunnel walls.
For the TBMs alone, terra vermessungen will set approximately 7,000 points. Using similar methods, the survey crews set out points for roadway alignment, concrete formwork, and other applications. As the construction progresses, terra vermessungen will continue to keep its Trimble systems busy. They will conduct profile checks, locate and document any cracks in the new concrete surfaces, and scan the entire tunnel over the 35-kilometer (22-mile) length of their contract.
Müller is pleased that the new technology and specialized field software has repeatedly proven its value. “After a few hours, the skeptical attitude of the heading foremen changes,” he explained. “And after a few days, they do not want to continue without it.”
John Stenmark, L.S., is a writer and consultant working in the AEC and technical industries. He has more than 20 years experience in applying advanced technology to surveying and related disciplines.