The application of polymer flocculants plays a pivotal role in the slurry separation process of shields, and the dispersant used for treating cutter mud cakes can significantly impact the effectiveness of polymer flocculants, potentially leading to reduced efficiency in slurry separation. Experiments were conducted to select appropriate flocculants and investigate the influence of dispersants on flocculant effectiveness, aiming to assess the effect of flocculants and explore the relationships and mechanisms governing their influence. Changes in the patterns of slurry flocculation were revealed in terms of flocculation-driven precipitation and vacuum-filtration effects. The purpose of this article is to provide a reference for the field application of polymer flocculants in the shield field. The conclusions are as follows. Inorganic flocculants containing 0.5% polyaluminum chloride (PAC) exhibit the most effective flocculation, demonstrating strong charge neutralization action. Organic flocculants containing 0.1% cationic polyacrylamides (CPAM) exhibit the most effective flocculation, demonstrating strong bridging and net capture effects. The dispersant sodium hexametaphosphate (SHMP) can significantly weaken the charge-neutralizing action of flocculants and slightly enhance bridging and net capture effects. SHMP can impede the flocculation of slurry with PAC. For CPAM, SHMP can enhance the flocculation of slurry at a low mass fraction (0.1% and 0.3%), while SHMP can significantly hinder flocculation at a high mass fraction (0.5% and 1%). A low mass fraction of SHMP reduced slurry viscosity to 246.3 mPa.s and enhanced vacuum filtration, while a high mass fraction of SHMP increased slurry viscosity to 667.2 mPa.s and hindered vacuum filtration. In conclusion, while dispersants reduce the effectiveness of inorganic flocculants at any mass fraction, a small number of dispersants enhances the performance of organic flocculants; thus, the organic flocculant CPAM is recommended for slurry separation.

In this paper, the initiation of the fracture of a segment caused by the pressure of the jack and other factors during shield construction is discussed. Based on the Rots model in the finite element software Diana 10.4, the fracture width is solved. Combined with in situ measurements, the mechanisms of concrete fracturing of a segment under external loads, such as the jack thrust deflection angle and uneven jack thrust caused by the changes in the segment due to the upward buoyancy and shield attitude, are studied; additionally, the occurrence conditions and engineering control measures for segment fracture are summarized. The results show that when the attitudes of the shield and segment are identical, the total thrust of the shield is recommended not to exceed 21,000 kN, and is strictly limited to 24,000 kN. When the attitude inclination angle between the shield machine and the segment is less than 1°, the impact on the segment quality is small. When the inclination angle reaches 2°, the total thrust of the shield is recommended not to exceed 16,000 kN, and is strictly limited to 18,000 kN. When the inclination reaches 3°, a fracture is easily produced. When the total thrust is 19,000 kN, it is recommended that the loading increase or decrease in the left and right four grippers should not exceed 20%, and they are prohibited to exceed 30%. The fracture width increases exponentially with the increase in misalignment between adjacent segment rings. These research results provide a theoretical basis for jack pressure control during shield construction.

A novel C-type flocculation reagent was developed to address the high water content of sludge and the environmental pollution caused by wastewater discharge during the mechanical dewatering of the Zhanjiang Bay undersea tunnel. Microscopic parameters, such as granularity and zeta potential, were analyzed to elucidate the mechanism of flocculation and dewatering in C-type reagents. The applicability of the C-type reagent was further validated in a field test and compared to that of the conventional reagent. The results showed that the C-type reagent reduced the water content of the sludge by approximately 20% and decreased the supernatant turbidity by approximately 79%. The C-type reagent demonstrated enhanced bridging and charge neutralization activities, thereby facilitating the aggregation of fine particles (50 μm), ultimately enhancing the efficiency of flocculation. The C-type reagent can also facilitate the formation of a calcium drainage channel skeleton with enhanced water permeability and reduce slurry viscosity to improve the efficiency of mechanical dewatering. These effects enhanced the pore size distribution of the slurry, resulting in a range of 346 to 8656 nm, which was approximately 15% to 41% greater than the overall pore size of the original slurry. The on-site utilization of 0.5% C-type reagent, compared to the original reagent, can enhance the solid content of the sludge by 35.7%, resulting in a 20% reduction in cost. Moreover, it meets the required sewage indicators, such as pH, and thus qualifies for direct discharge. The findings of this study offer valuable insights for the selection of environmentally friendly and efficient reagents in the context of the combined flocculation-mechanical dewatering method.