Abstract:

Foam concrete severs as a deformation reserve layer between the initial support and the secondary lining， effectively resisting the rheological deformation of the surrounding rock in deeply buried tunnels with high-ground stress. The incorporation of fibers enhances its compressive performance and ductility， addressing the issue of low compressive strength under specific fiber characteristics. Orthogonal test and Box-Behnken design method of response surface were used to systematically investigate the effects of various characteristics such as fiber mass fraction， length， and types， on the compressive strength and elastic modulus of foam concrete. A characteristic regression model was developed to optimize the mix ratio. The results show that the regression model established by the response surface method demonstrates high accuracy and reliability. Among the various fiber characteristics， the fiber mass fraction has the greatest impact on both the compressive strength and elastic modulus of foam concrete. Meanwhile， the interaction among multiple characteristics significantly influences compressive strength while slightly impacts elastic modulus. Particularly， the interaction between fiber length and fiber type has the most obvious impact on compressive strength. Furthermore， by maximizing the compressive strength and minimizing the elastic modulus， the optimized mix ratio result derived from the model shows that the absolute values of the relative errors are less than 5%. The small relative errors indicate that the proposed model can provide support for the multi-objective optimization of foam concrete with different fiber characteristics in the project application.

Key words: tunnel reserve layer, foam concrete, Box-Behnken design（BBD）, response surface method, multi-objective optimization