Abstract: When adopting a construction scheme consisting of excavation followed by laying of tracks prior to tunnel breakthrough, there is a technical problem with accurate docking of ballastless tracks through parallel adit without an azimuthalclosure check condition. Consequently, a method ensuring the directional accuracy of the CPⅡ traverse by measuring the azimuth with a highprecision gyroscope is proposed. By discussing the accuracy matching and poor tolerance standards of the two azimuthal coordinates as well as confirming the matching of tunneltracklaying construction clearance, checking the closure error of the highprecision gyroscope, and optimizing the matching between the lateral deviation and the construction clearance, the following constraints for using a CPⅡ gyroscope in an extralong tunnel are obtained.(1) First, the accuracy of the CPⅡ traversecoordinate azimuth and gyroscopecoordinate azimuth is almost the same; (2) the closure error of the coordinate azimuth and relativeclosure errors of the extremelength traverse meet secondclass tolerance requirements for the tunnel; (3) matching of gyroscope with the tunnelconstruction clearance should be optimized; and (4) the lateral deviation of the constructioncontrol traverse points in CPⅡ should be within 5 m of the tunnelconstructionclearance distance. According to the constraints for application of gyroscope azimuth side, a method for the calculation of constrained gyroscope azimuth is determined for CPⅡ traverse in the tunnel. The results of this method show that every indicator complies with the requirements of the inspection standards of construction. This indicates the feasibility of the CPⅡ traversebased method for the constrained gyroscope azimuth in the tunnel.

Key words: extralong tunnel, track laying survey, CPⅡ traverse, gyroscope azimuth, constrained adjustment, construction clearance