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Tunnel face stability is critical for tunneling safety, especially in constructing a tunnel adjacent to another. In tunnel construction, if there is an adjacent existing tunnel and the active failure of the tunnel face happens, there will be severe consequences. Therefore, the active stability of shield tunnel face should be studied considering existing tunnels nearby. In this work, a series of transparent clay model tests were conducted to investigate the active stability of a shield tunnel face adjacent to an existing tunnel based on the development process, failure mode, influence range and support force. In addition, discrete element method was used to study the shield tunnel face stability under various conditions. The role of existing tunnels on the shield tunnel face stability was discussed. The failure mode of the shield tunnel face is an overall failure mode and similar to a wedge shape and ladder shape along the longitudinal and transverse section, respectively. The critical support force ratio is approximately 4.5%. The relative vertical and horizontal distance have significant influences on failure mode, while the influence of burial depth and overlapping angles are not significant. Key words: active stability, tunnel face, transparent clay model test, numerical simulation, existing tunnel, 1. Introduction 1.1. Background With the arrival of the climax of the metro construction in China, the urban rail transit system has developed from a single line to a complex [...]

The deformation of soft clays under cyclic loading is controlled by microfabric changes. Cyclic triaxial tests, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were conducted to investigate the deformation and the corresponding micromechanism under cyclic loading. The correlations between the microparameters and deformation of Tianjin soft clay are discussed. The deformation increases linearly in the initial compression stage (number of cycles, N < 500), at a decreasing rate in the later shearing stage (N < 5000), and eventually stabilizes when the cyclic stress ratio (CSR) is 0.30. The probability entropy of both the soil particles and pores have a weak correlation with the deformation on the whole, reflecting a fluctuation within a small range in the initial compression stage. The correlations between the median pore diameter, median particle diameter, and specific surface area and the deformation are validated by using the gray correlation method. More specifically, the specific surface area and the deformation are highly correlated in the initial compression deformation stage. The median pore diameter and median particle diameter are highly correlated with deformation in the later shearing deformation stage. Key words: soft clays, deformation, micromechanism, gray correlation method. La deformation des argiles tendres sous charge cyclique est controlee par des changements de la microstructure. Des essais triaxiaux cycliques, la microscopie electronique a balayage (MEB) et la porosimetrie au mercure (PM) ont ete effectues pour etudier la deformation et le micromecanisme correspondant sous une charge cyclique. Les correlations entre les microparametres et la deformation de l'argile tendre de Tianjin sont discutees. La deformation augmente lineairement dans la phase initiale de compression (nombre de cycles, N < 500), a un taux decroissant dans la phase ulterieure de cisaillement (N < 5000) et se stabilise finalement lorsque le rapport de contrainte cyclique (<< CSR >>) est de 0,30. L'entropie de probabilite des particules du sol et des pores a une faible correlation avec la deformation dans son ensemble, refletant une fluctuation dans une petite fourchette dans la phase initiale de compression. Les correlations entre le diametre median des pores, le diametre median des particules et la surface specifique et la deformation sont validees en utilisant la methode de correlation des gris. Plus precisement, la surface specifique et la deformation sont fortement correlees dans la phase initiale de deformation par compression. Le diametre median des pores et le diametre median des particules sont fortement correles avec la deformation au stade ulterieur de la deformation par cisaillement. Mots-cles : argiles tendres, deformation, micromecanisme, methode de correlation des gris., 1. Introduction The dynamic responses of soft clays under cyclic loading are affected by the confining pressure, number of cycles (N), cyclic stress ratio (CSR), and vibrational frequency (f). The [...]