Grand Paris Requires Grand Surveying
Advanced surveying solutions for tunneling and monitoring help speed construction of the largest regional transportation expansion in Europe.
One of the most visited and beloved cities in the world, “The City of Light” began the Grand Paris initiative in 2007 with the goal of keeping Paris competitive with the world’s greatest cities into the 21st century. The Grand Paris project aims to improve quality of life, social and economic sustainability, strategic development, and environmental protections, all while preserving the city’s great history and heritage.
Part of the initiative added three neighboring districts—Hauts-de-Seine, Seine-Saint-Denis and Val-de-Marne—to the metropolis. The combined population grew from 2.2 million to more than 12 million inhabitants, rivaling London and other major European cities.
To support the expanded metropolis, a huge and aggressive transportation plan, the Grand Paris Express, is producing the largest subway and urban rail expansion in the western hemisphere. Already known for its exceptional public-transportation system, Paris will gain increased transportation coverage and capacity while reducing auto traffic and its accompanying congestion and pollution. In addition to streamlining access to the three Paris airports, the transit system will tie suburbs into the region’s business and arts districts, recreation and sports facilities, and schools and universities.
With a projected cost of more than $35 billion, Grand Paris Express seeks to enlarge the rail capacity of the century-old Paris Métro and the regional express (RER) networks by adding 200 kilometers (124 miles) of new lines along with upgrades to existing lines. The rail expansion will deliver a long-desired ring route, creating 68 new stations encircling Paris. Transit times to the Paris airports will be reduced, much of the rolling stock will be entirely new, and the time between trains at most stations will be considerably reduced. The lofty goal is to serve 2 million additional riders per day by 2030, an increase of roughly 40 percent.
Some critics deem the expansion project’s timeline unrealistic. But while large transportation projects often come with delays and cost overruns, Grand Paris teams are aggressively working to contain costs and time. And despite ongoing concerns, funding issues and early construction delays, the project is essentially on track. Much of the credit for this success stems from the use of new technologies and automation by the AEC industry.
Using Technology to Manage Risk and Reduce Surprises
The high-visibility, high-pressure project is not a place for beginners. Grand Paris Express has involved many of the world’s most-prominent and innovative design and construction firms. With 90 percent of the new rail lines to be underground, two major engineering and construction companies, Eiffage Génie Civil and Razel-Bec, joined forces to construct tunnels and related structures and facilities.
Both firms have extensive experience in the types of construction undertaken at the new ring route (Line 15) and the expansion of Line 14 through downtown Paris. Eiffage GC is one of the top three construction firms in France, top 10 in Europe and top 30 worldwide. The firm’s expertise is in heavy civil, vertical and energy construction as well as infrastructure operation. Razel-Bec, the civil division of the Fayat Group, is also a global company and one of France’s largest independent construction groups.
About 90 percent of the project’s lines will be underground, and tunneling is particularly susceptible to schedule and budget-busting hazards—a problem in one area can shut down work over an entire line. Eiffage GC and Razel-Bec turned to their hardware and software providers to seek out new solutions to reduce risk and speed construction. The decision to involve their technology suppliers is paying dividends. Two of Eiffage GC and Razel-Bec’s Grand Paris joint ventures implemented several advanced surveying applications for tunneling, including automated real-time monitoring (see “Monitoring: The Key to Safe and Successful Projects,” towards the end).
Automated Monitoring Protects Buildings and Businesses
On the southeastern portion of the new ring-route near Champigny Centre, the new L15B tunnel is being built using a mixture of tunneling techniques. There are many houses, businesses and utilities nearby that could be affected by the massive excavation, including subsidence or tilting resulting from the tunneling activities. Eiffage GC and Razel-Bec established a comprehensive monitoring system to check for any motion related to the trenches during construction and for a period after completion.
At the L15B tunnel site, Eiffage GC and Razel-Bec installed a Trimble 4D Control monitoring system (T4D), including 25 Trimble S7 total stations that feed measurements to a T4D server for data storage, processing and network adjustment. To ensure continuous measurement data for the analysis software, teams installed Settop M1 wireless communications hubs at each total station. In the event of internet disruption, the hubs automatically continue the measurements, buffering the data for transfer to the T4D server when communications are restored.
Through these hubs, the monitoring team can remotely operate the total stations and access onboard cameras, a vital feature for the team. For example, if the monitoring system can’t measure to one or more of the target prisms, remote operators can use the instrument’s camera system to see what the telescope is seeing in real-time. This helps determine if a prism has been lost, the view has become obscured or if the instrument crosshairs aren’t pointing close enough to the target. The live video reduces trips to the site to check on targets.
The L15B worksite and surrounding structures to be monitored are in a densely developed area, and it was often difficult to establish or find project control in view of some of the total stations. So the teams formed a network of stations, another option provided by the T4D solution. With multiple total stations sighting common control points, the software uses least-squares network computations to leverage the multiple instrument locations, and optimize performance and accuracy.
Surveying in the Tunnels
Eiffage GC and Razel-Bec also partnered on construction of the L14 South line—an upgrade to rail access between the city and Orly Airport. The L14 South tunnel is being constructed using a tunnel-boring machine (TBM). The underground work included as-built surveys by total station and scanner on the access trench and associated facilities used by the TBM as well as measuring each tunnel ring—the segmented concrete rings that line and support the tunnel walls.
The as-built surveys serve multiple purposes. They check fidelity of the bored and lined tunnel rings to the design plan, providing construction crews with timely and precise measurements for the finishing phase. The surveys also provide feedback to the TBM operators on how well they’re controlling the machine to conform to the design. With precise measurements, adjustments can be made to improve quality as the TBM progresses along the route. Again, Eiffage GC and Razel-Bec needed to automate surveying processes as much as possible if they hoped to keep up with the tight schedule.
Eiffage GC’s Olivier Gauche is a survey engineer with 30 years of experience. For this project, he used the Tunnel Module for Trimble Access software, which includes a solution specifically designed for automatically measuring and analyzing arrays of points on the inside of the tunnel rings. The Trimble Access software allows the total station to profile the tunnel rings at specified intervals. For example, it can shoot 20 to 30 profiles at 1-meter stations along the tunnel alignment, and every 20 centimeters around the ring—all this at a rate of 10 to 15 points per second.
In the L14 South tunnel, Gauche can confirm the integrity and proper placement of the arched tunnel lining by using underbreak and overbreak analysis routines in Trimble Access. The software compares measured data to the designs and provides the results immediately onsite. Gauche uses the tools to perform the field surveys, providing data for reports generated by external software packages.
“In the office, we use Trimble Business Center (TBC) to compute the traverse, check and average measurements,” adds Gauche. “And we use the TBC Tunneling Module to create alignments and templates and transfer them to Access.”
Gauche also uses a Trimble SX10 scanning total station for monitoring in the tunnel.
“We measure control points to check the convergence of the ring and determine if the tunnel is moving,” notes Gauche. “We use it to survey the vertical wall, ceilings and floors in the subway station. We have used it to perform tunnel profiles as well.”
The SX10 also is used to scan sections of the tunnel and associated structures; and the resulting point cloud is compared to the design model in TBC.
Handling the Pressure
Project owners worldwide are pressuring construction and engineering teams to use modern, digital approaches to engineering design and construction. This includes Building Information Management (BIM), 4D BIM to manage schedules, and connected construction practices to increase collaboration and shared data throughout projects. Stakeholders recognize the value of these tools and solutions to avoid rework, change orders and related delays. The high-tech approaches used by Eiffage GC and Razel-Bec are coming into mainstream use, and projects such as the Grand Paris Express and HS2 (the second phase of high-speed rail in the UK) are proving grounds for new solutions, processes and workflow-enhancing technologies that are delivering savings in time and costs.
The monitoring and as-built surveying automation adopted for Grand Paris Express tunnel sites by Eiffage GC and Razel-Bec have already yielded improved timelines and richer data than legacy tools and methods used on previous projects. Based on this success, they’re looking to deploy these solutions at other Grand Paris sites and beyond.
Eiffage GC and Razel-Bec are confident automated surveying and monitoring for tunneling has reduced total time spent surveying, and it also can rapidly identify potential issues and provide measurements to refine and streamline other construction activities. Their experience demonstrates that progress can be made—one site and one workflow at a time. There’s no panacea (yet) for all cost and scheduling woes for large projects, but as construction firms succeed in raising efficiency in individual project elements, the
tide can begin to turn.
Monitoring: The Key to Safe and Successful Projects
Construction-related damage to adjacent structures can dampen progress and, if substantial, halt all work at a site for indefinite periods. As a result, monitoring has become almost mandatory for modern tunneling work. It protects existing natural and built features on the surface as well as underground assets and personnel. Early detection of deflection and subsidence is essential for construction teams. Timely feedback helps them adjust operations or seek design changes to mitigate issues.
Monitoring provides critical feedback to operators of the giant tunnel boring machines (TBMs). Precise, real-time measurements on the surface and in the tunnel are essential for the TBM operators to balance the machine’s speed and head pressure to avoid an excessive “bow wave,” which is the deflection and perturbation of the ground above the moving TBM head. Bow waves can damage structures on the surface and compromise adjacent underground utilities. Monitoring systems can also track subsidence related to pre-excavation dewatering for the construction of tunnel access structures.
For all its benefits, manual monitoring surveys can be costly and time-consuming. In many instances, they can’t practically be performed frequently enough to provide the essential data needed to meet tight project timelines. The solution lies in automated systems that use fleets of sensors that produce continuous measurements to detect unwanted surface motion.
One of the most-common approaches to real-time tunnel monitoring is the use of Trimble S Series total stations and automation software. Prism targets are placed on buildings, utilities and structures. Regularly scheduled, pre-programmed rounds of total station observations are fed into the T4D software. Data are processed by motion engines in the software to produce high-precision time series. Long- and short-term trends in movement are tracked, and systems may be configured to trigger alarms if rapid or pronounced movement is detected.
Author’s Note: All photos were taken prior to the required implementation of COVID-19 safety measures.