Now is the time to consider adding Tunneling to your survey services. Tunneling is a growth area that serves the construction industry for projects ranging from railroad tunnels and subways to critical utility infrastructure running beneath urban centers. While only 1% of the total construction market, tunneling is growing at 7% per year which is twice as fast as the global construction market (ITA, 2017). Tunnels are vital infrastructure investments for local communities requiring tendering and public bidding and approval stages. The growth of Building Information Modeling (BIM) is an additional driving factor for tunneling surveys to create as-built documentation that assists with future maintenance.
As a specialist in the challenges of operating in a GNSS-restricted environment and maintaining high accuracy over long, linear projects, a tunneling surveyor brings expertise and awareness that complement the construction process and aid the tunneling contractor. Tunnel surveys require unique techniques and skill sets that are not typically a focus of survey education programs. This presents an opportunity for professionals to distinguish themselves in the industry and provide services that few others can.
Tunnels require surveying for a variety of tasks, but most importantly, assisting the construction with navigating in the right direction.
Phases of Tunneling Projects
The pre-construction phase of a tunneling project is like any other infrastructure linear project. It starts with a project definition and initial topo surveys, as well as cadastral evidence to establish right-of-way and primary control. In this phase, the primary control network is established on the surface, spanning the length of the tunnel using often a combination of GNSS, optical (total station, leveling, laser scanning) and UAV surveying techniques. Once the above ground network is created, it is transferred into the tunnel using tedious measurements to reduce the errors. The survey control is crucial to ensuring the progression and final delivery of the tunnel position as all construction work is based on it. Due to the linear nature of the projects, any errors at this point are propagated into the tunnel.
There are two primary methods used to build a tunnel: 1. the traditional SEM (Sequential Excavation Method), often referred to as NATM (New Austrian Tunneling Method), which involves a combination of drilling, blasting, and excavation, and 2. the Tunnel Boring Machine (TBM) method. Having a good grasp of geology/geotechnology is important to select the best construction methodology for the conditions. With traditional SEM tunneling the surveyor guides the equipment to accurately blast and excavate the proper amount of material to ensure the tunnel conforms to the design. Measurements are only carried out when the area is safe to enter between blasting and excavation of material. Being efficient at collecting and analyzing the data while in the field is an important skill for tunnel construction surveyors.
With TBM, there is a total station system with prisms and a variety of sensors, some built into the machine, to guide the TBM machine along the correct alignment. Tunnel construction is often performed around-the-clock therefore it is important to ensure the total station is continuously collecting data and providing the TBM operator with real-time feedback. Rugged total stations, with effective driving mechanisms and communication technology is critical to avoid costly TBM downtime.
At the same time as the excavation or boring is taking place, there is also a deformation and tunnel convergence monitoring. All critical infrastructure above the tunnel must be monitored for movement, as well as inside of the tunnel to determine how the shape deforms after excavation.
On large tunnel projects there are dedicated survey crews who work in shifts to keep the construction progressing 24/7, whereas on a smaller project, like a short sewer line, the survey crew may be only required to intermittently check the alignment and as-built the finished tunnel.
Photograph taken in the Colorado rockies where Acklam, Inc. assisted in the survey and construction of a tunnel at high altitude using the Trimble S5 total station. (Photo credit Brenda Chinn)
Challenges of Tunneling
One of the primary challenges in tunneling projects is creating and managing the geodetic control network above and below ground. The answer often depends on regionally defined control network requirements, but the various options can be confusing, and often result in significant errors if not well coordinated between the survey and construction crews . Also, for longer projects (spanning several miles) the surveyor has to understand geodesy and how this will affect the vertical alignment of the tunnel over long distances.
The physical characteristics of tunnels make the surveying process somewhat different from conventional surveying. The underground environment prevents the use of GNSS, which forces surveyors to utilize optical instruments and scanning devices, and due to low light conditions, it can be difficult to acquire clean imagery for photogrammetry. One of the advantages of 3D laser scanning is the quality and density of data to calculate the volume of shotcrete needed and volume of material removed, as well as generating as-builts.
Downtime is costly for any construction project. Delays in the tunneling process can be substantial to the project since many trades rely on each other to progress. The construction crews work with the survey teams to provide guidance to the machines and quality check the excavation and reinforcement of the structure. This challenges the survey crew to be flexible and reactive to whatever each project can bring. These risks can be mitigated by having proper sensors and software to efficiently complete tasks.
After collecting data in the field, it must be managed effectively by the survey crew. It is common that the survey manager has multiple softwares for performing workflows such as data processing, quality assurance, reporting, and deliverable creation. Using a centralized software platform can reduce the need for multiple systems by having a single source for the data.
The Trimble S5 TiM holds a constant eye on the tunnel surface to understand movements and alarm stakeholders of deviations.
The process of tunnel surveying often involves unique instruments not used in typical above ground surveys. One of these is a gyroscope, a magnetic north-pointing instrument used in the underground environment to aid in setting the control networks for surveying. The surveyor also may use downward and zenith facing optical plummets to transfer control points from the tunnel shaft into the tunnel corridor.
As the world population grows and more cities move to underground infrastructure, the need for surveying expertise to build them increases. Surveyors are already at the forefront of measurement technology and can provide unique skills that are essential to build tunnels. The industry is ripe with opportunities for them to make their mark in tunneling.
Surveyors working on the Grand Paris Line 14 metro project need to be equipped with the right tools for any situation both inside and outside of the tunnel. (Photo credit Eiffage and Razel-bec)