Air-entrained concrete has been extensively used in bridges and water conservancy projects in cold regions. The objective of this study was to investigate the effect of air entrainment on the mechanical properties, chloride migration, and microstructure of ordinary concrete and fly ash concrete. Twelve concrete mixtures were prepared with two water-to-binder ratios (0.53 and 0.35), three levels of air content (no air entrainment; moderate air entrainment, with 4–5% air content; and high air entrainment, with 7–10% air content), and two levels of fly ash (no fly ash and 30% replacement of cement). The results indicated that, compared with ordinary concrete, the addition of an air-entraining agent (AEA) to fly ash concrete resulted in a greater decrease in compressive strength. Care must be taken when introducing artificial air bubbles into fly ash concrete because the bubbles will affect the mechanical properties. With proper air entrainment (air content of 4–5%), stable, closed, and well-distributed air bubbles are introduced into concrete so that the connectivity of the pore system is interrupted and chloride migration is consequently minimized. However, if the concrete contains a large amount of entrained air, a substantial amount of large and detrimental pores is introduced, causing air bubbles to overlap and merge, as well as causing additional chloride migration. This effect is more prominent in fly ash concrete.

In view of the shortcomings in the risk assessment of deep-buried tunnels, a dynamic risk assessment method based on a Bayesian network is proposed. According to case statistics, a total of 12 specific risk rating factors are obtained and divided into three types: objective factors, subjective factors, and monitoring factors. The grading criteria of the risk rating factors are determined, and a dynamic risk rating system is established. A Bayesian network based on this system is constructed by expert knowledge and historical data. The nodes in the Bayesian network are in one-to-one correspondence with the three types of influencing factors, and the probability distribution is determined. Posterior probabilistic and sensitivity analyses are carried out, and the results show that the main influencing factors obtained by the two methods are basically the same. The constructed dynamic risk assessment model is most affected by the objective factor rating and monitoring factor rating, followed by the subjective factor rating. The dynamic risk rating is mainly affected by the surrounding rock level among the objective factors, construction management among the subjective factors, and arch crown convergence and side wall displacement among the monitoring factors. The dynamic risk assessment method based on the Bayesian network is applied to the No. 3 inclined shaft of the Humaling tunnel. According to the adjustment of the monitoring data and geological conditions, the dynamic risk rating probability of level I greatly decreased from 81.7% to 33.8%, the probability of level II significantly increased from 12.3% to 34.0%, and the probability of level III increased from 5.95% to 32.2%, which indicates that the risk level has risen sharply. The results show that this method can effectively predict the risk level during tunnel construction.

The concrete lining in subway tunnels often undergoes cracking damage in coastal cities. The combination of cracked tunnel lining structures and high concentrations of corrosive ions in the groundwater (e.g., chlorine) can accelerate concrete erosion, reduce the mechanical performance of the lining structures and shorten the tunnel service life. This paper investigates the chloride ion concentration in the groundwater of several subway tunnels in the coastal city of Qingdao, China. Indoor experiments and numerical simulations are conducted to investigate the chloride ion transport behaviour and service performance of cracked concrete linings. The results are applied to predict the service life of lining structures. The crack depth in concrete linings is found to have the most significant effect on the transport rate of chloride ions, followed by the crack width. The numerical simulations are carried out using COMSOL software to study the chloride transport behaviour in cracked specimens and predict the service lifetimes of lining structures of different thicknesses, and the results correspond well with the experimental data. The durability of a concrete lining can be enhanced by increasing the thickness of the protective concrete layer. Additional measures are proposed for treating cracked concrete linings to resist chloride ion attack in subway tunnels.

In order to investigate the development process of crack formation in shallow-buried sandstone tunnel, biaxial compression tests were conducted on a similar model of the real straight-walled arched sandstone tunnel. The results indicate that the initial crack appeared at the arch line on both sides of the tunnel and propagated downwards, eventually leading to spalling of the rock mass on the surface of the tunnel, forming a V-shaped groove. Additionally, slab cracks were observed in the straight wall on the right side of the tunnel, which were approximately parallel to the vertical load. The failure characteristics of the tunnel were closely related to the fractal dimension of the crack geometry distribution. During the tunnel compaction and elastic deformation stage, the fractal dimension of the cracks in the tunnel surface increased linearly, while during the crack propagation stage, the fractal dimension increased gradually, with a sudden increase occurring just before the rock mass reached its peak load. The acoustic emission results revealed that AE ringing counts and amplitude were inactive during the first 4239 seconds of the test. And they only increased during the crack propagation stage. The continuous decrease of the b-value and the sudden increase of the fractal dimension of cracks can serve as a reliable precursor of tunnel failure.