ShapeArray reduces uncertainty in soft ground tunnelling

ShapeArray provided more precise ground movement data above the tunnel shield during the progression of an Earth Pressure Balance Tunnel Boring Machine

The Regional Connector Transit Project will connect two existing underground rail lines in Los Angeles. Completion of the Los Angeles County Metropolitan Transportation Authority’s (Metro) design-build project is expected in 2021. The result will be a new underground light rail system, approximately 3 kilometres long, to connect the Metro Gold Line to the 7th Street/Metro Center Station.

The $1.756 [1] billion project includes the construction of three underground stations, approximately 1.8 km of twin tube bored tunnel, and approximately 1.5 km of cut and cover tunnel, amid Los Angeles’ busy city streets. A tunnel boring machine (TBM) created the twin tube portion, using the technique of pressure balanced mining. Construction officially broke ground in 2014.

During project planning, Metro performed a building and utility protection assessment, which determined several buildings were at risk from the planned tunnelling activity. Engineers determined a limit of 13 mm of maximum movement for structures and utilities. In response, Metro developed a comprehensive monitoring strategy to allow tunnelling under existing buildings and utilities with very low ground cover.

Metro installed ShapeArray 1 m over each of the twin tunnels to monitor the path of the TBM. With ShapeArray’s sensorized segments running along the centreline of the TBM’s path, the operation parameters of the TBM could be confirmed or modified in real-time. A multipoint borehole extensometer (MPBX) was also installed 400 mm vertically and 300 mm laterally from the SAAX model ShapeArray. Data from the two measurements agreed to better than 2 mm as the TBM passed underneath [2].

Photo credit: Photos courtesy of Joe Linton/Streetsblog LA

Reference:
[1] Los Angeles County Metropolitan Transportation Authority, “Regional Connector Transit Project Overview,” 2017. [Online]. Available: https://media.metro.net/projects_studies/connector/images/factsheet_connector_2017_overview.pdf

[2] Hansmire et al., “Challenges for Tunneling in Downtown Los Angeles for the Regional Connector Project,” Proceedings of the World Tunnel Congress 2017. Bergen, Norway, 2017.