Model Design and Application for Excavation Face Stability in Upward Shield Tunneling

The emerging upward shield method (USM) for constructing vertical shafts has been used in various projects, including the Midosuji utility tunnel in Japan. A scaled-down model testing system, featuring a geometric similarity ratio of 1:30, was developed specifically for studying the USM. This system incorporates sand inflow control, propulsion control, data acquisition, and water level control. It facilitates detailed observation and recording of parameters such as vertical displacement of surface soil layers, support force at the excavation face, and earth pressure within the model box. Consequently, it enables investigation into the excavation face instability process, modes, and the formation and evolution of the soil arch zone above the excavation face during upward shield tunneling. Additionally, through the application of particle image velocimetry (PIV) technology and GeoPIV-RG software v1.1, quantitative analysis of soil displacement fields during excavation face instability is conducted, capturing microscopic displacements and deformations of soil planes. This approach more accurately elucidates the accuracy of understanding the dynamic response of soil. Pre-test research using the model testing system explores the variation patterns of excavation face load displacement, vertical earth pressure within the failure zone, surface displacement, and internal soil displacement during the instability process. Analysis reveals that excavation face load variation typically progresses through three stages: rapid growth, slow growth, and descent. Moreover, vertical earth pressure shifts upward in tandem with excavation face displacement, while overall surface displacement initially shows slight settlement followed by accelerated uplift.