Model tests and theoretical analyses of influences of surface surcharge on confining pressure of shield tunnels

When sudden surface surcharge occurs around the tunnel, the surface additional stress on the tunnel segments will be generated. When the surface surcharge is too large, it will cause the opening bolts of the longitudinal joints of the tunnel to be exposed, and the structural edges and corners of the segments at the top of the arch will be damaged. In response to the hazards of sudden surface surcharge on the tunnel, the indoor reduced size model tests with geometric similarity ratio C_L=15.5 are carried out to study the change of confining pressure of the tunnel comprehensively considering the depth of the tunnel, the size and location of the surcharge. The method of theoretical analysis is used to study the change of confining pressure and total confining pressure of the tunnel under the action of surface surcharge, and finally the theoretical analysis and experimental results are compared under the same working conditions. The results show that when the surchage is accumulated step by step, the change of the confining pressure of the tunnel basically shows an equivalent increase, and it exhibits an overall downward trend with the increase of the eccentric distance within a certain range. When the surcharge position is 0.5D₀ and 1D₀, the decrease of the confining pressure of the tunnel at the eccentric side is obviously much smaller than that at the non-eccentric side. However, when the surcharge position is 1.5D₀, the changes of the confining pressure at both sides of the tunnel are basically the same (D₀ is the outer diameter of the tunnel segment). As the buried depth increases, due to the increase in the thickness of the soil at tunnel crown, the influences of the surface surcharge on the confining pressure of the tunnel are relatively reduced. The theoretical results are in good agreement with those of the indoor model tests, and thus the accuracy of the experimental and theoretical analyses is demonstrated.