This paper aims to investigate the tunnelling stability of underwater slurry pressure balance (SPB) shields and the formation and evolution mechanisms of ground collapse following face instability. A laboratory SPB shield machine was employed to simulate the entire tunnelling process. Multi-faceted monitoring revealed the responses of soil pressure, pore water pressure, and surface subsidence during both stable and unstable phases. The morphological evolution characteristics of surface collapse pits were analyzed using three-dimensional scanning technology. The experimental results indicate that: (1) The key to stable tunnelling is balancing the pressure in the slurry chamber with the tunnelling speed, which ensures the formation of a filter cake in front of the cutterhead. (2) The torque of the cutterhead, soil pressure, and surface subsidence respond significantly and synchronously when the tunnel face becomes unstable, while the soil and water pressures are relatively less noticeable. (3) Excavation disturbance results in a gentler angle of repose and a wider range of collapse in the longitudinal direction of the collapsed pit. (4) A formula for predicting the duration of collapse is proposed, which effectively integrates the evolution patterns of the collapse pit and has been well-validated through comparison with the experimental results. This study provides a reference for the safe construction of tunnel engineering in saturated sand.

Damage to a masonry building induced by tunneling greatly depends on the settlement of its foundation. Compared with pile foundations, group cemented soil column (GCSC) foundations have lower bearing capacity and stiffness. Tunneling through a GCSC foundation may have a significant influence on the settlement of the above masonry building. Field tests and numerical simulations were performed to investigate the settlement behavior of a single cemented soil column (CSC) and GCSC foundation during tunneling. Moreover, the stiffness of the GCSC foundation was investigated by using the concept of area replacement ratio. The reinforcement effect of the GCSC foundation was much greater than that of a single cemented soil column (CSC). From the test result, the settlement of a single CSC was four times that of GCSC. The GCSC foundation could be considered a large reinforcement area that could reduce settlement. The volume loss decreased from 0.2 to 0.02% as the tunnel passed through the reinforcement area, and the relationship describing the transition between the reinforcement area and the green field was linear. Compared with the in situ test results, the building stiffness yielded reasonable results, particularly for the interaction between the building and GCSC foundation at the final stage of tunneling. The results of this study could be used to evaluate the settlement of a building with a GCSC foundation during tunnel construction.

The stress state of soil may affect the building settlements induced by tunnelling, which, however, has not been well understood. In this study, three dimensional numerical analyses combined with in situ measurements were performed to investigate the geostress-associated settlements of a raft-foundation building due to tunnelling in soft ground. Basically, two types of geostress fields were investigated: the first type considered the effect of additional stress generated in the foundation soil (FAS) due to building weight, while in the second type, a sequential twin tunnelling was presumed, and the effect of additional soil stress induced by the first tunnel (TAS) on the building response to the second tunnel was considered. The results indicated that FAS may aggravate the stress release of the foundation soil, and thus gave rise to a larger building settlement or inclination. In the sequential tunnelling process, the effect of TAS can be more complex: when the first tunnel lowered the stress of foundation soil, TAS effect of the first tunnel may help reduce the building settlements induced by the second tunnel; otherwise, it may aggravate building settlements. In addition to TAS effect, the sheltering effect was also found to play an important part in twin tunnelling.

Excessive ground deformation caused by shield tunnelling is prone to irregular settlement and deformation cracking of the overlying building. Hence, accurately assessing the extent of damage to the building is crucial for the effective strengthening and repair of the structure. This paper presents a comprehensive case study of a metro shield tunnel conducted beneath a masonry building. We systematically monitored and investigated the settlement and crack development of the masonry building and discovered that the cracks in the masonry building were mainly situated at the maximum slope of the building settlement curve, rather than at the peak. After completion of the tunnel construction, the maximum settlement of the masonry building was 37 mm and the cracks were predominantly oblique cracks with a length of 0.6-7.6 m and a width of 0.5-5.0 mm. The maximum principal tensile strain in the walls of the masonry building was 0.153%, and the masonry building was evaluated to be moderately damaged according to the assessment criteria considering the extent of damage to the building surface. Then, we proposed a building damage assessment method that considers soil-structure interaction and subsequently verified it through finite-element results and field monitoring results. Finally, the effects of key parameters on the stiffness of the building were analyzed. The stiffness of the building was mainly affected by the opening ratio and the effective coefficient of the building cross section. These research results have significant guiding and reference values for safeguarding buildings during metro tunnel construction.

Due to its inherent advantages, shield tunnelling has become the primary construction method for urban tunnels, such as high-speed railway and metro tunnels. However, there are numerous technical challenges to shield tunnelling in complex geological conditions. Under the disturbance induced by shield tunnelling, sandy pebble soil is highly susceptible to ground loss and disturbance, which may subsequently lead to the risk of surface collapse. In this paper, large-diameter slurry shield tunnelling in sandy pebble soil is the engineering background. A combination of field monitoring and numerical simulation is employed to analyze tunnelling parameters, surface settlement, and deep soil horizontal displacement. The patterns of ground disturbance induced by shield tunnelling in sandy pebble soil are explored. The findings reveal that slurry pressure, shield thrust, and cutterhead torque exhibit a strong correlation during shield tunnelling. In silty clay sections, surface settlement values fluctuate significantly, while in sandy pebble soil, the settlement remains relatively stable. The longitudinal horizontal displacement of deep soil is significantly greater than the transverse horizontal displacement. In order to improve the surface settlement troughs obtained by numerical simulation, a cross-anisotropic constitutive model is used to account for the anisotropy of the soil. A sensitivity analysis of the cross-anisotropy parameter alpha was performed, revealing that as alpha increases, the maximum vertical displacement of the ground surface gradually decreases, but the rate of decrease slows down and tends to level off. Conversely, as the cross-anisotropy parameter alpha decreases, the width of the settlement trough narrows, improving the settlement trough profile.

With the widespread application of Earth Pressure Balance (EPB) shield technology, the generation of shield muck has been increasing yearly. This paper aims to investigate the effectiveness of bentonite-silty clay modified slurry (BSC) as a soil conditioner for enhancing the workability of sands during EPB shield tunneling, thus enabling the recycling and reuse of the discarded muck (waste silty clay). Standard slump tests were conducted on three typical sand specimens from Shenyang Metro Line 6. The influence of the types of conditioners and slurry injection ratio (SIR) on slump values were examined to determine an optimal conditioning scheme tailored to the specific formation conditions. Furthermore, the study explored the combined use of BSC and foam to improve workability, employing a three-factor four-level orthogonal experiment. Finally, the rheological parameters (yield stress) derived from the slump tests provide valuable insights for assessing material flow within the tunneling system. The results show that comparative analyses with pure bentonite slurries reveal that BSC is a suitable, economical, and effective alternative for soil conditioning. The particle size distribution of sand specimens significantly influences the conditioning process, necessitating adjustments to SIR and slurry viscosity for optimal results. When the slump value of slurry-conditioned soil falls within the range of 150-250 mm, the slump test can be effectively used to estimate its yield stress under atmospheric conditions. This study contributes to the development of sustainable and economical solutions for soil conditioning in urban tunnel projects, particularly by utilizing excavated materials effectively.

The clay shock method is used to fill the excavation gap caused by shield taper and deviation during shield tunnelling. The rheological properties and diffusion-related performance of clay shock (CS) slurry have an important influence on its ability. In this study, the authors tested the rheological properties of CS slurry by using a rotational viscometer with a stepless regulation in speed. The influence of the parameters of preparation of CS slurry on its rheological properties was analyzed. The results showed that the Bingham plastic model is more suitable for characterizing the shear thinning behaviors of CS slurry. The apparent viscosity and yield stress of the CS slurry first increased and then decreased with increasing duration and speed of stirring. They increased with the duration of standing and decreased with increasing the water-powder ratio of component A. The numerical diffusion simulations and model tests with constant pressure were carried out to investigate the influence of the water-powder ratio of component A, the permeability of the stratum, and the net injection pressure on its diffusion-related performance. The process of diffusion of CS slurry mainly involved filling and permeability diffusion. The filling diffusion stage mainly occurred in the first 30 s of injection and can reach 70-80 % of the final injection mass. The effect degree of the aforementioned factors on the diffusion distance of CS slurry was in the following order: permeability coefficient of stratum > water-powder ratio of component A > net injection pressure. CS slurry diffused over a distance of less than 10 cm in sandy soil, where this was negatively correlated with the viscosity of the slurry and positively correlated with the coefficient of permeability and porosity of the stratum. The filling time of the excavation gap was positively correlated with the viscosity of the slurry, negatively correlated with the net injection pressure, and was not significantly correlated with the coefficient of permeability of the stratum. The diffusion termination time was negatively correlated with the viscosity of the slurry, and positively correlated with the coefficient of permeability of the stratum and the net injection pressure.