Major water tunnels have the capacity to deliver several hundred million gallons of water each day. Tunnel inspection is an extremely complex and specialized process that requires forward thinking and advanced tools and technology. In an effort to minimize the time it takes to conduct inspections, and because of operational needs, tunnels are typically taken out of service for a limited window of time.
The Mott MacDonald team worked with Dibit Measuring Technique to capture the inside of a major freshwater tunnel. By building a detailed digital model of the interior surface, the engineering team could perform the needed assessments as a digital desktop study instead of conducting a lengthier traditional field investigation. The result was an inspection program that reduced the out-ofservice time needed by more than 80 percent while yielding a higher-quality, more complete and precise inventory of issues that will be repaired during construction.
Advantages of water tunnel assessments based on 3D data
Some water tunnels must be capable of withstanding high pressures and flow rates. This makes it mandatory to perform conditional assessments, ensuring safe and reliable water conveyance over dozens of miles.
Severe tunnel damage, like cracks or seeping construction joints, can cause a significant loss of water. This type of damage can also contaminate the water in tunnels from surrounding areas.
The complete drainage of a water tunnel is difficult to execute, and even more challenging if the tunnel needs to be drained for a long period of time. Traditional water tunnel assessment requires a tunnel shutdown of several weeks per mile to accomplish detailed manual and visual documentation. It also requires a large team of highly qualified personnel to accelerate the assessment, which can 360-degree, 3D digital assessment of subsurface water infrastructure result in additional costs.
Water tunnel assessment based on 3D data is a highly beneficial method to significantly reduce tunnel shutdown time. In this effort, the time spent in the tunnel is reduced to a minimum and the work is mostly concentrated on the tunnel scanning itself. The actual assessment is performed later, offsite.
This approach to tunnel scanning is faster than traditional assessments and requires less resources (Kontrus and Mett, 2019). It leads to a cost decrease, and the work can be performed within the safety and convenience of an office without the pressure of a limited time window to complete the work.
The approach involves scans of water tunnels to determine the condition of the tunnel surface (Fig. 1). It is important to capture significant damage and leaks to get a reliable state of the tunnel. Modern scanning systems offer the possibility to reproduce tunnel surfaces in the form of a 5 mm (0.196 in.) geometry accuracy combined with a highresolution photo texture of 1 × 1 mm (0.039 × 0.039 in.) per pixel. It ensures recognizable damages and cracks down to a size of 0.3 mm (0.011 in.) crack width (Mett et al., 2019b).
360-degree scanning system
The Dibit “Altira” (Greek for dumbbell) is a light, compact and variable measuring system for the 3D measurement of tunnels. As a hybrid measuring platform, it is equipped with a laser scanner and a photo unit and can be upgraded with a thermal unit. The laser scanner enables precise measurements of the tunnel geometry (Mett et al., 2019b). The photo unit delivers high-resolution photos for true-color texturing of the 3D data. With the help of thermal cameras (also known as infrared, or IR, cameras), additional information about the temperature behavior of the tunnels can be derived.
The system contains several high-resolution industrial cameras according to the latest standard, which are arranged on a horizontal axis in a helical structure. Due to sufficient overlap of the individual cameras, the entire interior of the tunnel is covered. Specially developed highperformance light-emitting diode (LED) flashes illuminate every measurement photo, even in tunnels without any interior lighting.
The manually pushed system reaches a speed of approximately 2 to 3 mph. With an attached control computer, the measurement can be continuously tracked, and the quality of the measurement data can be verified (Fig. 2).
The Altira measuring trolley can be used in water, road and rail tunnels. By manual rerailing, short mobilization times can also be achieved during its use. Narrow time windows can be used for the measurement: for example, in railway networks with continuous operation.
With high-resolution photos, cracks with opening widths down to 0.3 mm (0.011 in.) can be detected and measured in the 3D model. Geometric features can be detected with a 5 mm (0.196 in.) accuracy and temperature differences on the tunnel surface of 40 mK. The measurement unit is competing with other scanners like the Leica Pegasus Two and Riegl VMX (Leica-Geosystems, 2021, Riegl, 2021) and established tunnel measurement systems like the SPACETEC TS 4 (Spacetec, 2021).
Scan data and assessment of tunnel conditions in computer software
The scan data collected in water tunnels and the assessment of the tunnel conditions can be processed and performed in Dibit’s proprietary software Dibit-8. This software is specially, but not exclusively, designed for processing and analyzing 3D data captured in cylindrical structures like tunnels.
It supplies software standards to handle a huge amount of measurement data in the form of measurement photos (for example, RGB photos, infrared photos) and laser point clouds that are captured by the scanning device.
The viewer visualizes scan data in a fast and smooth manner, which allows for a professional analysis of tunnel conditions. It can be illustrated in a 3D viewer as well as a 2D viewer showing the unwrapped tunnel surface (Fig. 3).
A tunnel condition assessment includes a documentary of all significant damages and tunnel components like blocks (Kontrus et al., 2021).
All analyzed tunnel damages and components are stored in a database by a manual drawing of the objects, either in the 2D orthophoto or the 3D tunnel data (Fig. 4).
Furthermore, approaches based on artificial intelligence (AI) enable automated detection of tunnel objects and cracks. Each detected object has specific coordinates and can have a linkage to an inspection or measurement photo, or even a remediation protocol. The shapes of the objects can be selected in the form of a point, an open 3D polyline, or a closed polyline (planar annotation). The objects that have individually selectable names and attributes can be categorized in classes that allow a filtering and evaluation of the different classes. All important information like positions, lengths and attributes can be exported to a tabular format for further analyses.
A final step of finishing a tunnel assessment is the printout of a plot of the 2D orthophoto. This tunnel map includes the high-resolution photo-textured unwrapped tunnel surface (orthoimage), as well as all the drawn tunnel objects and classes.
3D point clouds and 3D-textured mesh models can be exported to other data formats like ASCII, E57, LAS, OBJ and many others. Ortho image data like TIFF and JPEG can be exported for further analyses in computeraided design or BIM software (Mett et al., 2019a).
Many tunnel objects and surface damages can be recorded in the 3D data (Fig. 4). This includes construction joints, cracks with several thicknesses, seeping holes or corrosions. Each object is cumulated in a specific class that has certain geometric information, like an open or closed colored polyline a point or a plane. Each class can have several objects indicated by numbering.
During the processing and analysis of measurement results, it turns out that 3D data is an excellent basis for detecting and measuring damages and cracks down to the submillimeter range.
Tunnel-specific conditions, such as the absolute darkness, are a challenge. The walls of water tunnels are usually covered with organic matter, which can lead to reflections on the measurement photos. A water-covered invert (up to 4 to 5 in.) can be fully processed and analyzed showing that hybrid LiDAR and photogrammetric Altira can scan areas where comparable scanners may not deliver reliable results.
The Dibit-8 software helps to perform the tunnel assessment with its unique combination of fast performance and display of high-resolution 3D data with a built-in database structure to classify the different tunnel objects and damages.
Each digital tunnel assessment delivers a snapshot of the conditions of a tunnel. By comparing the measured data with future scans, changes can be detected (for example, growth of cracks). It is a base for future maintenance that ensures safe water flow delivery to millions of people in the United States.
Kontrus, H., Mett, M. (2019). High-speed 3D tunnel inspection. 7 S. Proceedings of the Rapid Excavation and Tunneling Conference (RETC) 2019. 16.06.-19.06.2019, Chicago, IL. SOC FOR MINING METALLURGY. ISBN: 978-0-87335- 470-7.
Kontrus, H., Steinkühler, J., Peal, T. (2021). Highspeed 3D tunnel inspection in subway tunnels – Case study San Francisco BART. 6 S. Proceedings of the Rapid Excavation and Tunneling Conference (RETC) 2021. 13.06.-16.06.2021, Las Vegas, Nev. SOC FOR MINING METALLURGY. ISBN: 0873354923.
Leica-Geosystems (2021). https://leica-geosystems.com/de-at/products/mobile-sensorplatforms/capture-platforms/leica-pegasus_two. Website accessed on 10/6/2021.
Mett, M., Kontrus, H., and Holzer, S., (2019a). Dibit TIS – Das „Proto“- BIM für den Tunnelbau. Proceedings of the 20th international geodetical week Obergurgl. 10.02.-16.02.2019 Obergurgl, Austria. Edited by K. Hanke and T. Weinold. Arbeitsbereich für Vermessung und GEOinformation. Universität Innsbruck.
Mett, M., Kontrus, H., Eder, S. (2019b): 3D tunnel inspection with photogrammetric and hybrid systems. 10 S. Proceedings of the 14th International Conference on Shotcrete for Underground Support (ECI SUS XIV), Nong Nooch Gardens – Pattaya, November 17-20, 2019. Thailand.
Riegl (2021). http://www.riegl.com/nc/products/ mobile-scanning/. Website accessed on 10/6/2021. Spacetec (2021). https://www.spacetec.de/en/products/ts4/. Website accessed on 10/6/2021.