Objectives  In recent years, the convergence of shield tunnel using the terrestrial laser scanning is more and more widely, but stations changed frequently have an extreme influence on monitoring efficiency. The position of endpoints will change with the tunnel deformation when the laser point clouds is applied to fit curve in order to calculate the coordinates of endpoints. Therefore, it is difficult to ensure that the same convergent endpoints are obtained at different times using fitting curve based on point clouds which have an influence on the precision of convergence.

  Methods  Aiming at the problem of low efficiency of station-based 3D laser scanning, this paper introduces an economical, efficient and reliable mobile laser scanning monitoring method. For the calculation of convergence value, this paper introduces a new method based on tunnel expanded image in detail. Firstly, the expanded images of tunnel were obtained by the cylindrical projection of point clouds, the expansion of standard cylinder and gray rasterization, interpolation and gray enhancement of image. Secondly, it can accurately locate the section of point clouds of each ring in shield tunnel through the binaryzation of image, connected analysis, automatic identification and manual correction of ring cracks. Then, the positions between segments of each ring were manually drawn in the expanded image and the orientations of the fixed endpoints were deduced. Finally, the coordinates of the convergent endpoints were calculated and the convergences were calculated by fitting the segments.

  Results  The results show that the difference between the repetitive results of the convergence value using mobile laser scanning is less than 1.5 mm.

  Conclusions  The proposed calculation method can efficiently monitor the convergence of the single-circle shield tunnel with reliable accuracy.