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Increasing the water–cement ratio and water-reducer dosage of cement slurry enhances its fluidity. However, a high water–cement ratio diminishes the beneficial effects of water reducers on fluidity. The stone content of the slurry decreases as the water-reducer dosage increases. Additionally, the water–cement ratio significantly affects stone content. However, when the water–cement ratio exceeds a threshold value, stone content decreases. Furthermore, the threshold value of the water–cement ratio decreases with increasing water-reducer dosage. Without the addition of the water reducer, as the water–cement ratio increases the overall integrity of the grout stone decreases. The addition of the water reducer alters the surface pore distribution, wherein "uniform small pores" change to "localized large pores." Based on the multi-objective optimization of Matlab, the recommended optimal mix composition for a slow-setting cement slurry is a water–cement ratio of 0.25 and water-reducer dosage of 1.5%. With the use of this optimized mix composition, the stone content and compressive strength increase by 7.8% and 145.6%, respectively, compared to those obtained using the recommended mix ratio in the specifications. Additionally, all relevant performance parameters meet the requirements specified by previous standards. [ABSTRACT FROM AUTHOR]

With the development of urban metro systems, shield tunnels that pass through existing bridge pile foundations have become an inevitable engineering problem. Therefore, ensuring the stability of the strata and existing bridge piles during tunnel construction is a common goal in engineering practice. Currently, research on the mechanical responses of strata and existing piles under different conditions of upper-soft and lower-hard composite strata during shield tunneling has not been conducted extensively. This paper presents a numerical simulation of a shield tunnel passing through an existing bridge pile foundation in upper-soft and lower-hard composite strata. Subsequently, the surface subsidence and mechanical responses of a single pile were analyzed and evaluated. Additional stresses generated in the soil by existing bridge piles and the selection of grouting pressure were considered to optimize the driving pressure of the slurry shield. Allowable values were proposed to evaluate the construction disturbances caused by the tunnel excavation. The results show that the disturbance to the soil and existing piles is significantly influenced by the hard-rock height ratio, and the surface subsidence increases when the hard-rock height ratio decreases. The displacement and internal force of a single pile are significantly influenced by the load applied to the pile. This study demonstrates the changes in the mechanical responses of a single pile during shield tunnel boring, and provides in-depth insights into the deformation control caused by shield underpassing structures in upper-soft and lower-hard composite strata. [ABSTRACT FROM AUTHOR]

• Grouting Failure Investigation: High w/c ratios caused excessive shrinkage, impacting anchor body filling. • Failure Mechanisms Identified: Inadequate grout filling led to weak bolt-to-rock connections. • Optimization Solutions: Optimizing w/c ratio and using water-reducing agents curbed volume shrinkage. • High Ground Stress Impact : Maoxian Tunnel experienced >1.0m deformation, affecting rock bolt efficacy beyond 8.0m. Maoxian Tunnel is located in a high-stress region and has undergone over 1.0 m of horizontal deformation, which has caused concrete cracking in the initial support and the failure of full-length grouted rock bolts owing to extrusion. Investigations revealed that higher water-cement (w / c) ratios lead to higher grout volume shrinkage (V s), consistency, and inadequate borehole filling. Combined with the lower compressive strength of grouting, this results in weak rock-bolt bonding. The wet contact interface further reduces the ultimate bearing capacity of the interface. Therefore, the rock bolts are squeezed out and fail. Reducing V s by adopting a smaller w / c ratio and optimizing the w / c agents can enhance the grout's compressive strength. Additionally, the strength of the rock–bolt interface is improved in soft rock. The varying bolt length significantly influences the rock deformation control, but the tunnel stress relaxation in areas with high geo-stress extends beyond 8.0 m. Strengthening support structures and expediting installation are both crucial measures for resisting structural forces in such environments. [ABSTRACT FROM AUTHOR]