LAUSR

The immersed tunnel is composed of elements which are prefabricated elsewhere and floated to the tunnel site to be sunk into the prepared trench. Each element and the closure joint must line up exactly for the watertight gaskets to seal properly. For the construction of extra-long immersed tunnel, particular care should be taken in the perspective of geodetic control. Therefore, the design and implementation of the geodetic basis and hierarchical surface control networks are described. To decrease the influence of lateral refraction and to improve the configuration strength and increase the number of redundant observation compared with traverse, the design of underground surveying network named duo-linear joint chain is presented. The data-processing methodology, including outlier detection for excluding outliers from observation and variance component estimation for realistic precisions of observation, is described. Hong Kong–Zhuhai–Macau immersed tunnel surveying data are analysed and the performance of the geodetic networks and data processing methodology is demonstrated.