Study on early microscopic characteristics and splitting ring-opening behavior of casing material for tunnel sleeve valve pipe grouting in water-rich environments

Sleeve valve pipe grouting technology is widely applied in water-rich stratum engineering, but C-B casing material often suffer from poor grouting quality due to improper ring-opening pressure selection under water-rich conditions. This study systematically investigated the early performance evolution and splitting ring-opening behavior of C-B composite materials under water-rich conditions using multi-scale experimental methods combining macro- and microscopic approaches. Through comparison of five mix proportion schemes, the optimal mass ratio of cement:bentonite:water was determined as 1:1.5:2. A self-designed simulation apparatus revealed the ring-opening pressure evolution patterns under 0–8 m water head conditions, showing that compared to 322 kPa under dry conditions, water-rich environments significantly reduced ring-opening pressure, and with increasing groundwater pressure, the ring-opening pressure increased from 155 kPa to 280 kPa, while material penetration into soil expanded significantly with more uniform distribution. XRD and SEM-EDS microscopic analyses revealed that water-rich environments hindered hydration product formation by diluting Ca 2 + ion concentration while promoting ion exchange reactions between bentonite and cement, resulting in persistently low Ca/Si molar ratios and loose porous microstructures, thus explaining the macroscopic mechanical property change mechanisms. This research provides scientific basis for material optimization, curing time determination, and pressure parameter selection in water-rich grouting engineering.