Preliminary Research On Model Test Of Shield Tunneling Using Transparent Soil

In view of the fact that the transparent soil has great advantages in the research of soil internal deformation, using transparent soil as a model soil material is proposed. Through designing a model shield machine, constructing a model box and establishing a image acquisition system, a shield model test is carried out. Combining the image processing software—PIVview2C, information can be extracted from image data, the law of internal soil deformations induced by shield excavation is preliminarily discussed. In this paper, the main works and results are as follows:(1) Transparent soil is successfully made when fused silica is become saturated with the concentration of 59.8% calcium bromide solution, and its physical and mechanical parameters have been obtained by conventional soil tests. Testing results show that transparent soil has similar basic physical and mechanical parameters with natural soil.(2) The model test system is designed independently. The diameter of the model shield machine is 7 cm and the cutter can be removed. The advancing and retreating of the shield can be controled by manual rotating nut, which can simulate some engineering problems such as shield excavation and decrease of the support pressure. The planar laser is used to cut transparent soil and the digital camera will capture related images of the soil deformation continuously during shield excavation, PIV technology is applied to analyse the meso-mechanism of the soil deformation that induced by shield tunneling,which provide a new idea for us to improve the research of shield tunnel model test.(3) The deformation of soil around tunnel by shield excavation is analysed qualitatively by model test, the conclusions are as follows:①Cross-sectional laws:With the horizontal distance increasing from central line of shield, the lateral deformation increases to its peak first, and then gradually decreases to zero. With the increases of soil depth, the peak point gradually moves to the central line of the tunnel.With the increase of soil depth, the vertical displacement on central line of shield gradually increases, and then reaches a maximum when it is close to the vault crown, at last decreases to zero gradually. When the embedded depths of tunnel is one time the depth of the diameter, the focus is close to the bottom of the tunnel; When the depth is 1.7 times, the focus has a rising trend. This shows that the location of focus which is from vertical displacement is relative to the the embedded depth of tunnel. With the increases of embedded soil depth, the zero-point is closer to the central line of the tunnel.②Longitudinal laws:With the increases of the soil depth, the incidence of longitudinal displacement gradually decreases, and it reaches a maximum on the surface. With the horizontal distance increasing from the excavation face of the shield, the longitudinal displacement increases to its peak first, and then gradually decreases to zero when the range of height changes from the surface to the vault crown.The failure pattern of excavation face seems a shape with larger scope in upper regions and slower in lower, it also shows that there are certain cohesive betweem the transparent soil particles.③Testing results show that it is reliable that using the shield test system designed in this paper to study the mechanism of internal soil deformations induced by shield tunneling.