Stability Analysis Of Shield Tunnel Excavation Face Using Transparent Soil

The shield tunneling method is an efficient, economical and safe construction methods and is widely used in urban subway construction. As the complex environment of urban construction, shield construction safety accidents often occur. The accidents caused by the excavation of the surface instability are the most common. So the study of the excavation face stability is significant. At present, researches on excavation of the surface instability of ground movement are more concentrated on the stratum surface. In this article, synthetic transparent soil is used as a shield model soil, combined with digital image processing technology. Using PIVview2C software to process and analyze photos of shield excavation surface instability of the internal ground movement, including the following:(1) design a fused silica sand and a concentration of59.8%calcium bromide synthetic transparent soil, the soil samples were examined in indoor routine laboratory and direct shear test, proved similarity with properties of the natural soil.(2) analysis of the principle of image digitization, combined Laser Speckle and PIV technique, get the target surface by the CCD camera, the images will be digitized and imported into software for the process of cross-correlation calculation.(3) The test results were obtained in three different buried diameter ratios (C/D=1, C/D=1.5,C/D=2), analyze the overall cross-sectional and longitudinal disturbances form soil surface and within the level of displacement, vertical displacement law, the soil arching effect and the loose earth pressure theory.The analysis shows, in the experiment of transparent land Shield, due to the sand soil arching effect, the depth is greater than1.25D, the soil deformation will not develop to the surface, shallow buried (C/D=1)correspond to Terzaghi loose earth pressure theory, the overall disturbance shape cross-section similar to the pants-shaped and the vertical section is similar to the triangle, when deeply buried (C/D>1.25) in line with Protodyakonov’s theory, the cross-sectional disturbance morphology is similar to egg-shaped, longitudinal disturbed morphological development chimney-shaped, deep soil settlement trough is similar to a normal distribution. Longitudinal vertical displacement and horizontal displacement are little deffrent, the maximum vertical displacement in the shield dome is between0.3-0.7D, with the shield of the increase of depth, the largest area of the vertical displacement gradually away from the shield excavation surface. Cross-section is mainly the vertical displacement. The maximum vertical displacement is about0.5D. The horizontal displacement maximum point is near the inflection in the sedimentation. The distribution pattern is similar to the Poisson distribution.