A Rapid Shield Tunnel Profile Measuring Method Based on Close-range Photogrammetry

With the rapid development of tunnel construction in China and growing demand for tunnel inspection, the tunnel measurement task becomes increasingly imperative. Currently, as traditional methods cannot meet the increasing inspection requirements of a large number of tunnels anymore, a new efficient and precise measuring method is urgently needed. A novel method of measuring tunnel profile based on the close-range photogrammetry was proposed in this article, where a linear laser source was used to set massive target points on the lining section, forming the profile to be solved. Laser range-finders were installed to project several control points, whose corresponding distance and angle were recorded. On the basis of 2D-DLT (Direct Linear Translation) equations, the tunnel profile was fitted. Based on the principles above, a dedicated inspection system MTPM-1 was designed and manufactured. Relevant algorithms were programmed. Field tests were carried out in a tunnel. The impact of image resolution as well as the number and distribution of control points on the fitting precision was analyzed thoroughly. A standard operating procedure for determining the optimal image resolution, the number of optimal control points, and the distribution of optimal control points was proposed. For different structural sections or different measurement requirements, the optimal image resolution, the optimal number of control points and their distribution form can be determined. For the tunnel with an inner diameter of 5.5 m, the maximum error was below 5 mm when eight uniformly-distributed control points were used with image resolution of 2 957×1 958 pixels. It proves the strong feasibility and high accuracy of the method. The further optimization design scheme of this method was proposed, including shooting distance optimization design, fish eye lens optimization design and dual laser optimization design. This method realized rapid and economical inspection of shield tunnel profile. © 2021, Department of Journal of the China Railway Society. All right reserved.