Abstract:

Completely decomposed granite exhibits strong cohesiveness because of its high fine-particle content, posing a major challenge to earth pressure balance shield tunneling in metro projects in the southeastern coastal region of China. This material tends to adhere to the cutterhead and form mud cakes, which can sharply increase cutterhead torque, hinder muck discharge, and reduce excavation efficiency. In severe cases, frictional heating may solidify the mud cake, forcing shutdowns for chamber opening and creating substantial economic and safety risks. Foam conditioning is an effective approach for treating such cohesive strata. However, existing studies lack a robust strategy for systematically optimizing foam components for completely decomposed granite strata. As a result, it is difficult to balance multiple objectives, including foaming performance, conditioning effectiveness, and cost-effectiveness. In addition, the complex interaction effects within multicomponent foam systems have often been overlooked. To address this issue, this study employed response surface methodology (RSM) combined with central composite design (CCD) to develop a predictive model for foam performance. The independent variables were the anionic surfactant sodium alpha-olefin sulfonate (AOS), the nonionic surfactant alkyl polyglucoside (APG), the foam stabilizer xanthan gum (XHG), and the inorganic dispersant sodium hexametaphosphate (SHMP). In terms of performance evaluation, foaming ability was characterized by the foam volume measured with a modified Ross–Miles foam tester. Foam stability was assessed using both foam half-life and the 5-min defoaming rate. The ability to reduce soil adhesion was quantified by the interfacial adhesion force measured with a modified Yida conical bao pull-off tester. Multi-objective decision-making was performed using the desirability function approach, and the optimal compound formulation was identified as the Fuyan-1S foaming agent (FY-1S), containing mass percentage contents of AOS, APG, XHG and SHMP are 4.18%, 1.59%, 0.16% and 0.95%, respectively. Laboratory tests showed that the FY-1S foaming agent outperformed three selected commercial foaming agents in key properties, particularly stability and reduction of interfacial adhesion. Field engineering validation was conducted in an EPB shield tunneling section of Xiamen Metro Line 3. By comparing the conditioning effects of FY-1S with those of commercial foaming agents, this study systematically evaluated both soil properties, including interfacial adhesion force, undrained shear strength, and vertical slump, and shield tunneling parameters, including cutterhead torque, total thrust, advance rate, and chamber pressure. The results demonstrated that FY-1S effectively improved soil workability and significantly reduced soil mechanical strength, cutterhead torque, and excavation resistance, thereby enhancing shield tunneling efficiency and operational adaptability.

Key words: completely weathered granite, earth pressure balance shield, foam conditioning, response surface methodology, central composite design, desirability function approach