A New Alignment and Breakthrough Accuracy Optimization Strategy in Long Immersed Tunnel Surveys

For ultra-long tunnels, the effective means of alignment and breakthrough accuracy optimization is to measure the gyro azimuths or transfer the coordinates of surface known points or the directions to the inside of the tunnel through vertical shafts or inclined shafts. Compared with bored tunnels, immersed tunnels have different engineering characteristics and construction environments, and have higher alignment control accuracy requirements. However, the immersed tunnel has no vertical shafts or inclined shafts that can be used for coordinate transfer to improve the measurement accuracy. In order to provide high-accuracy alignment control and breakthrough for long immersed tunnels, we put forward a new strategy after network stability analysis, and consistency analysis between the results of the element positioning system and the results of the control network. The results of the element positioning system are taken as constraints to improve the accuracy of the control network. The analysis on 100,000 simulations of a 20 km-long immersed tunnel shows that the accuracy of the last point pair of the traverse network with angle and distance observations is 60.8 mm, while the accuracy of the same point pair after adopting the new strategy in this paper is 11.8 mm. The results of a 7 km-long experimental traverse network show that the lateral breakthrough accuracy is improved by 73%. All of these indicate that the new strategy can effectively improve the measurement accuracy and the lateral errors can be kept at a small level. Following the new strategy, the high-accuracy alignment and breakthrough can be achieved for long immersed tunnels.