Metro tunnels require monitoring and maintenance even after completion, because tunnels are prone to degradation as a result of factors such as other construction projects along the metro line, pre-existing geological conditions, or simply material degradation. Common issues such as water seepage and cracks may occur, and can develop into falling concrete sections in extreme cases. To reduce the risk of a tunnel collapse, metro tunnels need to be regularly checked for problems, and defects must be treated promptly once they are discovered.
A company familiar with such metro tunnel assessments is Shanghai Geotechnical Investigations & Design Institute Co., Ltd. (SGIDI), a reputable Chinese geotechnical consultancy. In 2012, the company developed a set of proprietary procedures and data processing software to efficiently conduct high-precision, nondestructive testing on metro tunnels. By using FARO’s family of 3D Laser Scanners, SGIDI performed geometric measurements, laser reflectance imaging, and building information modeling (BIM) to assess the lining on metro tunnels.
Brief Account of Assessment Methodology
1. Scanning Method as Applied to Metro Tunnels
SGIDI’s method of acquiring data involves scanning metro tunnels in two states – static and mobile. Due to the FARO Laser Scanner Focus3D X 330’s ability for target-less scans and registrations, SGIDI can perform scans with ease, acquiring data a lot more efficiently than before. With FARO’s devices, the team manages a scanning speed of 500m/h in the static scanning state or 1,500m/h in the mobile scanning state, with just
two or three operators on the job. The Focus3D X 330’s portability, ease of use, and measurement speed of up to 976,000 points per second makes it the ideal tool for tunnel scanning, whether in meeting project time constraints or manpower limitations.
Static Scanning (left) vs. Mobile Scanning (right), where the FARO Laser Scanner Focus3D X 330 is mounted on a vehicular carrier.
2. High Accuracy Geometrical Measurements
With every scan it performs, the Focus3D X 330 captures a comprehensive set of geometric measurement results, including the tunnel structure’s horizontal diameter, tunnel convergence, segment displacement, and tunnel clearance. On one of their projects, SGIDI scanned 38km worth of tunnel data with the Focus3D X 330 and verified that their measurements of horizontal diameter were accurate to the tolerance of ±3mm. In addition, the scan data can at times be analyzed in sections, where a user can extract portions of axis and traverse sections from the main data file.
Axis and transverse sections of tunnels extracted from 3D scanning data.
3. High-resolution Laser Reflectance Imaging
The images of tunnel walls generated by the Focus3D X 330 are accurate to the millimeter-level. In fact, accuracy is at levels better than 5mm, and users can directly extract geometric measurements from the images. Oftentimes, it is through these images that tunnel defects are detected, or ancillary facilities are studied. Original point cloud data for a tunnel scan.
Tunnel 3D scanning orthophoto maps generated after projection and calculation
Laser reflectance images of tunnel defects
Laser reflectance images of tunnel facilities
4. Tunnel Building Information Modeling (BIM)
Furthermore, using the point cloud data and imaging results acquired by the Focus3D X 330, SGIDI was able to build digital representations of any tunnel, to reflect its actual size and texture. This allows metro operators to analyze the current state of their tunnels, and also serves as a powerful tool for ensuring quality control and operational safety. Tunnel BIM modeling & 3D scanned data.
Browser platform depicting the 3D laser scanning measurement results of a metro tunnel.
A Successful Application of 3D Laser Scanning Technology
SGIDI is dedicated in its quest to uncover more about 3D laser scanning, as applied to metro tunnel measurement. At present, the company has scanned nearly 100km of metro tunnels across cities like Shanghai, Nanjing, and Hangzhou. By providing metro operators with large volumes of rich data, SGIDI helps them to monitor and address issues related to tunnel degradation, facility investigation, and BIM modeling.
About Shanghai Geotechnical Investigations & Design Institute Co., Ltd. (SGIDI)
Established in 1958, Shanghai Geotechnical Investigations & Design Institute Co., Ltd. (SGIDI) is a reputable Chinese geotechnical consultancy company. With a long legacy in engineering applications, SGIDI boasts strong technical expertise and it invests in sophisticated equipment.
The company’s wide-range of services include geotechnical engineering design, consulting, engineering investigation, engineering measurement, engineering monitoring, engineering quality and housing quality testing, engineering geophysical prospecting, architectural design, engineering supervision, seismic safety evaluation, and geological assessment. The company also covers fields such as construction engineering management and engineering project management.
For more information, please visit SGIDI at www.sgidi.com.