Visualization of tunnelling-induced ground movement in transparent sand

This paper presents an experimental investigation of internal soil deformation ahead of a tunnel boring machine by using transparent sand and digital image correlation (DIC) techniques. Soil deformation and its control are often very critical issues for protecting adjacent properties and services during tunnel construction. Currently, most of soil deformation measurements are limited to ground surface settlement since natural soil is not transparent. The visualization of spatial deformation inside soil masses will improve the understanding on the influence in tunnelling. Transparent sand is used in this study, which is made of fused silica and a calcium bromide solution with a matching refractive index. An optical set-up is developed that consists of a laser, camera, and computer. The laser is used to illuminate the targeted traverse section ahead of a scaled shield machine. The images of laser speckles generated through interaction between the laser light and silica are captured by the camera and then transferred onto a computer. DIC is used to calculate the soil displacement between two images obtained before and after the machine movement. Two model tests are performed with an overburden cover that varies by one to two times the tunnel diameter. The results show that soil deformation changes with increases in tunnel depth. The settlement troughs at various soil depths are similar to Gaussian curves. As expected, the trough becomes narrower as the soil depth increases. The influence zone changes from a rectangle over a reversed trapezoid shape in the shallower tunnel to a bell over a trapezoid shape restrained within the soil mass in the deeper case. The limitations of this study are also discussed in the paper. (C) 2013 Elsevier Ltd. All rights reserved.