Abstract
Shield tunneling under rivers often requires monitoring riverbed deformations in near real-time. However, it is challenging to measure riverbed deformation with conventional survey techniques. This study introduces a comprehensive method that uses the Global Positioning System (GPS) of the USA and the BeiDou navigation satellite system (BeiDou) of China to monitor riverbed deformation during the construction of twin tunnels beneath the Hutuo River in Shijiazhuang, China. A semi-permanent GPS network with one base station outside the river and six rover stations within the river was established for conducting near real-time and long-term monitoring. The distances between the base and the rover antennas are within two kilometers. The network was continuously operating for eight months from April to December 2018. The method is comprised of three components: (1) Monitoring the stability of the base station using precise point positioning (PPP) method, a stable regional reference frame, and a seasonal ground deformation model; (2) monitoring the relative positions of rover stations using the carrier-phase double-difference (DD) positioning method in near real-time; and (3) detecting abrupt and gradual displacements at both base and rover stations using an automated change point detection algorithm. The method is able to detect abrupt positional-changes as minor as five millimeters in near real-time and gradual positional-changes at a couple of millimeters per day within a week. The method has the flexibility of concurrent processing different GPS and BeiDou data sessions (e.g., every 15 minutes, 30 minutes, one hour, one day) for diffident monitoring purposes. This study indicates that BeiDou observations can also achieve few-millimeter-accuracy for measuring displacements. Parallel processing GPS and BeiDou observations can improve the reliability of near real-time structural deformation monitoring and minimize false alerts. The method introduced in this article can be applied to other urban areas for near real-time and long-term structural health monitoring.
Highlights
Ground surface deformation monitoring during urban tunnel excavations is a common means for ensuring construction quality and safety, including the safety of people and equipment inside the tunnel and structures located on and near the ground surface
Itresulting is beyond the constructions scope of this paper to tunnels speculate how excavation tunnel investigations. It is beyond the scope of this paper to speculate how excavation of the tunnel induced riverbed deformation, and how excavation of the second tunnel interacted with the first induced riverbed adeformation, excavation ofseries the second tunnel interacted with the first tunnel
A quick look at the displacement time series does give some impressive results that may help to understand the complexity of the riverbed deformation during shallow tunneling
Summary
Ground surface deformation monitoring during urban tunnel excavations is a common means for ensuring construction quality and safety, including the safety of people and equipment inside the tunnel and structures located on and near the ground surface. Conventional ground surface deformation monitoring involves the use of total stations, levels, inclinometers, and other devices that need to be deployed on the ground surface. It is inconvenient to utilize these conventional surveying techniques in open water areas, such as rivers and lakes. Surrounding soil disturbances and tunnel deformations may lead to cracks and water infiltration, threatening the safety of tunnel construction and operation. Continuous, long-term, and near real-time riverbed deformation monitoring has become a challenging topic for both researchers and engineers in urban tunneling. There is an urgent need for developing new techniques and methods for riverbed deformation monitoring
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