Abstract: High-water-content slurry containing residual surfactants produced during shield tunneling construction poses environmental pollution and safety risks when disposed of in traditional landfills. Furthermore, the diminishing capacity of disposal sites leads to continuously rising treatment costs. To address these challenges, this study proposes a synergistic solidification approach combining enzyme-induced carbonate precipitation (EICP) with the industrial byproduct phosphogypsum to explore a new pathway for the resource utilization of this type of slurry. By varying the mass fraction of phosphogypsum and the number of EICP treatments, and considering the influence of residual foaming agents, macromechanical tests such as unconfined compressive strength and ultrasonic wave velocity are conducted. Based on scanning electron microscopy observations, the mechanical properties and structural evolution of the solidified specimens are comprehensively evaluated. The results demonstrate the following: (1) Surfactants present in the residual foaming agent adsorb onto the surface of clay particles. This process enhances the electrostatic repulsion between particles, thereby improving the penetration uniformity of the EICP reactive solution and promoting the formation of calcium carbonate. Consequently, specimens prepared in a foaming agent environment exhibit superior unconfined compressive strength and ultrasonic wave velocity, compared to those prepared in a deionized water environment. (2) The addition of phosphogypsum provides sufficient additional Ca²⁺ for the EICP reaction, promoting additional calcium carbonate precipitation and enhancing the cementation effect. However, excessive phosphogypsum incorporation leads to pore clogging by fine particles and increased pore fluid viscosity, which reduces slurry permeability and consequently reduce solidification performance.

Key words: earth pressure balance shield tunneling, soil solidification, residual foaming agent, enzyme-induced carbonate precipitation, unconfined compressive strength, ultrasonic wave velocity