The metro tunnel construction mainly passes through the downtown area, including ground buildings, infrastructure, underground pipelines, existing subway lines, and station floors. However, it is very difficult to control the ground deformation, especially in the water-rich soft ground or in the tunnel engineering construction with strict requirements for surface settlement control. The traditional stratum reinforcement method may not meet the engineering needs. In this case, the artificial freezing method (AFM) provides a solution for such engineering problems. This paper summarizes the basic principle and characteristics and application scope of AFM, and compared with the other three stratum reinforcement methods, AFM has better effect in controlling surface displacement and vault settlement, while grouting method, pipe shed method, and pipe curtain method have absolute advantages in controlling surface uplift. Then, through engineering examples, the reinforcement effect of AFM in the four aspects of subway connecting passage, shield inlet and outlet, interval tunnel, and damage repair is introduced. Through analysis, it is found that the reinforcement effect of AFM in water-rich weak stratum is very good. Finally, the future development of AFM is prospected in order to provide some experience and guidance for the follow-up research.

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Water inrush hazard seriously threatens construction safety of subsea tunnels in unfavorable geological areas. In recent years, a large number of subsea tunnels have been built worldwide, some of which have experienced many water inrush disasters, especially in Japan and Norway. In this paper, a systematic methodology is proposed to rigorously review the current literature about water inrush in subsea tunnels. Emphasis is placed on recorded causes and evolution processes of water inrush, as well as relevant mitigation measures. In particular, the geological conditions that generate such water inrush hazards are initially discussed by counting cases of tunnel water inrush in the past decades (43 cases of water inrush hazards in tunnels (including mountain tunnels)). The process of formation of failure modes of water inrush, and the corresponding research methods (including theoretical, numerical and experimental) are reviewed, and can be used to pave the ways for hazard prevention and future research. This is followed by a summary of the prevention methods and mitigation measures used in practice, and a short discussion of the achievements and limitations of each method. Then combined with the evolution characteristics of the failure area, the water inrush process of different modes is divided into three stages, with a proposed a grouting scheme for each stage. Finally, concluding remarks, current research gaps and future research directions on subsea tunnel water inrush are provided and discussed.

With the acceleration of tunnel construction, it is inevitable to cross the collapsible loess region. The engineering properties of loess are very poor in the water-rich environment. Additionally, if the buried depth is shallow, the load bearing capacity is further weakened, which can increase the difficulty of tunnel construction. Therefore, this paper provides a case study on the Pangwan Tunnel based on a geological survey, numerical simulation, and on-site monitoring. The failure characteristics of the studied area under the original support scheme are analyzed in detail, and the extreme deformation mechanism is discussed combining the geological conditions and surrounding rock properties. The results show that there are five main reasons resulting in the extreme deformation of the Pangwan Tunnel, which are: abundant groundwater, low strength of surrounding rock, disintegration of loess after water immersion, depth of tunnel is shallow, and unreasonable original support scheme with low bearing capacity. Then, the corresponding countermeasures are proposed, which are advance drainage, large arch foot and foot-lock bolt, sealing tunnel face and grouting timely, as well as advance pipe shed and small pipe grouting. Finally, from the feedbacks of numerical analysis and field application results, extreme deformation of the Pangwan Tunnel is effectively controlled. This work can provide some helpful guidance for similar projects.