Damage characteristics and micro-mechanism of solidified clay under dry–wet and freeze–thaw cycles in shield tunnels

The freezing and grouting methods are among the main construction techniques for the lateral connection passages of shield tunnels in soft soil areas. Therefore, the surrounding rock undergoes freeze–thaw (FT) and dry–wet (DW) cycles caused by water level changes during operation, leading to the deterioration of mechanical properties and instability. However, this research achievement is very limited. In this study, the macro and micro damage mechanisms of the surrounding rock in lateral connection tunnels under FT and DW cycles were systematically investigated. Initially, clay was sampled from a cross-tunnel of Hangzhou Metro Line 4 in Zhejiang Province. Cement (NXI), ground granulated blast furnace slag (GGBS), and fly ash (FA) (NXII) were used to solidify the clay subjected to DW and FT cycles. Finally, the uniaxial compressive strength and microstructure were examined using scanning electron microscopy and X-ray diffraction (XRD) to obtain 15 DW cycles (0, 5, 10, 15) and 12 FT cycles (0, 4, 8, 12) after 7 and 28 d curing periods. The results indicated that the compressive strength decreased after the DW-FT cycles, with rod-like hydration products (macropores) transitioning to needle-like ettringite (AFt) in the micropore-dominated structures. Simultaneously, the GGBS-FA mixture (NXII) promoted tight microstructures via hydration-induced bridging and pore filling, enhancing the water stability by 23% and DW-FT resistance by 18% compared with cement-only formulations. The NXII composite demonstrated superior long-term strength retention (89% at 180 d) and formed distinctive hydration phases, including calcium silicate hydrate and hydrotalcite-like compounds. Subsequently, the increasing pressure on the surrounding rock was calculated to degrade its mechanical properties (20% and 24.4%, respectively). Finally, a life-cycle assessment confirmed that the GGBS-FA system reduced material costs by 35% and carbon emissions by 42% compared with conventional cement-lime stabilization. These findings elucidated the microscale hydration damage mechanisms of GGBS-FA systems for soft soil solidification to advance sustainable tunnel engineering.