Safe passage

February 2011 » Web Exclusive » PROJECT CASE STUDY
As construction progresses, advanced technologies protect buildings above a high-speed railway tunnel.
Carl Morton

High-speed railways are enjoying strong growth across Western Europe. Capable of traveling at speeds in excess of 190 mph, high-speed trains require specialized tracks and alignments. Working with a set of international standards for constructing and operating high-speed rail networks, Western European countries are creating cross-border links and encouraging high-speed rail travel. Such projects can deliver an economic boost and provide faster, more efficient transportation across the continent.

In Spain’s northeast corner, construction is under way on the section of the high-speed rail link between Barcelona and Figueres. Scheduled for completion in 2012, it is the final link in the line that connects Spain’s existing high-speed train system (Alta Velocidad Española, or AVE) to the French border and the French high-speed railway (Lignes à Grande Vitesse, orLGV). The connection will enable high-speed travel from Madrid to Paris and beyond.

The last big project of the Barcelona-Figueres line is the La Sagrera station in Barcelona. Construction on the underground station began in June 2010. Covering more than 3.2 millionsquare feet, La Sagrera will become the city’s largest building, incorporating an intermodal transit hub with commercial and residential space. The station is expected to handle 100 million travelers per year and will connect high-speed trains with local and regional railways as well as buses and taxis.

High-speed trains coming from the south and west will reach La Sagrera by traveling through a new tunnel from Sants, Barcelona’s main train station. Construction of the tunnel and station represent one of the most ambitious engineering projects ever undertaken in Spain. The project is supervised by Adif, the Administrator of Railway Structures, which is part of the Spanish Ministry of Public Works. The prime contractor for the work is U.T.E. La Sagrera, which is a combination of three companies: Sacyr, Scrinser, and Cavosa. The tunnel, approximately 3.5 miles long, has been carefully routed to avoid going beneath residential and historic structures.

Construction monitoring
Most of the excavation on the Sants-La Sagrera tunnel will be done by a tunnel boring machine (TBM). The TBM chosen for the work is an Earth Pressure Balance machine, which maintains a balance between the earth and the machine’s shield pressure. The machine’s rate of advance matches the rate of soil removal, reducing disturbance and potential displacement around the tunnel. Nicknamed “Barcino,” the TBM can bore a hole 34 feet in diameter and advance roughly 59 to 82 feet per day. As the Barcino advances, it sets in place the reinforced concrete blocks that make up the tunnel lining. Although the tunnel is roughly 92 feet below ground, Adif is taking action to ensure there are no adverse effects on the surface.

Workers pour concrete on one of several access shafts that support construction as the TBM bores the tunnel. When the tunnel is operational, the shafts will provide emergency exits and ventilation.

Prior to construction, engineers conducted detailed studies of the terrain, soil characteristics and location, and condition of surrounding buildings. To guard against disturbances on the surface, the Sacyr and its subcontractor, Soldata Iberia, have installed a geotechnical monitoring system. The system includes ultrasonic sensors installed along the tunnel alignment at 490-foot intervals; the intervals are shorter in sensitive areas and subsidence zones. The sensors connect to the Internet using WiFi and cellular communications.

A second monitoring system uses optical surveying technology and an extensive above-ground network. This system uses precise observations of more than 2,800 prism targets attached to buildings and structures along the tunnel’s route. The prisms are monitored by a network of 31 Trimble S8 Total Stations located on buildings and pillars in or near the tunnel construction zone. The instruments are installed in custom mounting cages and connected via wireless communications to a central monitoring control center. Each instrument is remotely controlled to make multiple measurements to dozens of prisms, using Trimble’s FineLock technology. At 30-minute intervals, the instruments automatically locate and point to the prism targets and then take precise angular and distance measurements; the measured data are sent directly to the control center for compilation and input to data management and analysis software.

To reliably detect tiny movements of the buildings, the monitoring system must measure the prisms’ positions to 1 millimeter (0.04 inches) or better. In addition to high-precision angle and distance measurements provided by the Trimble S8, the project uses atmospheric sensors to apply corrections to the optical measurements. A single software system controls the atmospheric sensors, total stations, and ultrasonic sensors.

The measurement and alert system is customized based on the structure to be monitored. Project operators define threshold levels of motion for different areas along the route. For example, modern buildings can tolerate more motion than older structures. On newer buildings, the system looks for changes exceeding 10 millimeters (0.4 inches). For older buildings, the tolerances are tighter. When a sensor detects any change that exceeds these levels, the system sends alerts to key project personnel via e-mail and SMS (text messages).

As the TBM advances underground, the total stations and prisms can be moved to keep pace. During the course of the project, each instrument will occupy three different monitoring locations along the tunnel’s route. At each location, the total stations are placed to provide line-of-sight to monitoring targets on buildings and structures of concern. Before the construction reaches the new monitoring area, the system completes pre-construction measurements and creates a baseline database of the positions of all of the prism targets. During construction, statistical analyses compare the baseline positions against measured results; any motion is confirmed with a high level of confidence.

A series of prism targets also are placed outside of the construction zone to provide known reference points for the instruments. By regularly calibrating to these fixed targets using the Trimble S8, the monitoring system ensures highest accuracy and precision for detecting motion.

Throughout the project route, the Trimble S8 total stations installed on the roofs of residential and commercial buildings operate silently, reducing disruption to nearby residents. The project technicians noted that the instruments require little servicing beyond annual maintenance and calibration.

Typical installation for a Trimble S8 total station. The instruments are installed on the roofs of residential and commercial buildings. In the background: Sagrada Familia.

One of the key challenges in the monitoring efforts was the communications system for the instruments. Antonio Alvarez, chief of topography for the Railway Head Office, described the system. “Communication takes place with a WiFi and 3G network installed expressly for controlling the stations and data that the network sends out,” he said. “Along the route, repeaters are mounted on top of the highest buildings to ensure good communication along the entire path.”

Protecting a landmark church
One of several critical buildings on the tunnel’s route is the church of Sagrada Familia, a unique structure designated as a World Heritage Site by UNESCO (United Nations Educational, Scientific, and Cultural Organization).

The cathedral Sagrada Familia was sanctified in 2010. The building is still under construction and will expand above the train tunnel.

Because of its height and loads onto the ground, Sagrada Familia called for additional precautions. To protect the church, engineers installed a safety system consisting of a barrier of 104 piles with a diameter of 4.9 feet and a depth of 135 feet. The barrier is made up of a longitudinal concrete die and consolidation injections to improve the terrain stability.

Outside of Sagrada Familia, engineers installed 104 barrier piles to protect the building.

Inside the church, teams installed four Trimble S8 Total Stations and 149 prism targets. The prisms are placed in strategic locations among the interior facades, walls, and pillars where motion might occur.

Monitoring targets on a column inside Sagrada Familia. The prisms are placed in strategic locations throughout the cathedral.

The total stations make continuous measurements to the prisms, and provide uninterrupted data on the behavior of the priceless structure.

A Trimble total station monitors the interior Sagrada Familia. Four instruments were installed in the famous cathedral.

Outside the church, additional total stations and sensors keep close watch for settlement or other motion.

A Trimble S8 monitors the area near Sagrada Familia. The wireless antenna connects the instrument to the monitoring control system.

In the past, buildings would be monitored using conventional leveling, according to Alvarez. “The advantages of the robotic total stations are the large number of readings and the automatic alerts,” he said. “And we can still use conventional leveling to verify the ultrasonic and total station observations.”

Because of the exceptional value of buildings along the project, a team of 20 technicians from the Instituto Técnico de Materiales y Construcciones (Technical Institute of Materials and Construction, or Intemac) reviewed the documentation and procedures for monitoring the church and project areas. The review included surveys of the terrain and hydro-geological conditions as well as a special ultrasonic study to analyze how the rock and soil will respond to the tunnel’s construction.

Using information obtained from the ultrasound and geotechnical explorations done by Sacyr and Soldata, the Intemac study issued a positive report on the TBM passing beneath the church of the Sagrada Familia. The project demonstrated that sophisticated monitoring technology, combined with WiFi and 3G communication technologies, provides project technicians with a view of the entire construction zone. If an incident should occur, they will be ready to react quickly using accurate, up-to-the-minute information.

Carl Morton is a writer specializing in the geomatics, civil engineering, and construction industries. Drawing on his extensive training and experience, Morton focuses on applications and innovations in equipment, software, and techniques.


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