Tunnel Deformation Monitoring
Monitoring of existing tunnel to measure potential deformations and vibrations during construction
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The Challenge
- Ensure existing tunnel unaffected by construction of new one
- Existing tunnel remain operational during construction
The 50-year-old 3,500-ft-long two-lane Midtown Tunnel provides a vital link between Norfolk and Portsmouth, VA. It is the most heavily traveled two lane road east of the Mississippi and its usage has increased 600 percent since its construction. The new tunnel consists of a two-lane tolled tunnel under the Elizabeth River, parallel to the existing Midtown Tunnel.
This second tunnel will provide redundancy, additional capacity, and reduce congestion to nearby communities. The challenge on this project was to ensure that construction of the new tunnel did not have any adverse effects on the existing tunnel, which remained operational during all construction activities.
The Solution
Geocomp was engaged in two major roles. The first was to provide monitoring of the existing tunnel before and during construction. This baseline monitoring was established more than one year before construction of the new tunnel began. It consisted of ten Robotic Total Stations (RTS) installed within the tunnels alignment, thirty tiltmeters, and a series of crack meters. The data collected from this early monitoring was used to model and help engineers understand the existing tunnel’s seasonal movement before construction of the new tunnel began. During construction, Geocomp’s role included the installation and maintenance of over forty-five ShapeAccelArray inclinometer strings, forty-five load cells, and over one hundred strain gages to monitor the approach structures for the new tunnel construction.
These instruments, along with additional manual survey instruments, were used to confirm design criteria and provide performance monitoring of the support of excavation system during the dredging and tunnel installation phases of the project. The instrumentation and monitoring system was vital to ensure the stability of the existing tunnel during construction, minimizing the risk of project delay from unexpected performance.
