Laboratory Investigation on Advancing an Eco-Friendly Backfill Grout for Shield TBM Tunneling Using Biopolymer-Based Soil Treatment (BPST)

The mechanized shield Tunnel Boring Machine (TBM) method has been widely adopted in urban tunneling projects due to its high excavation efficiency and minimal ground disturbance. However, this method inevitably creates a tail void between the excavated ground and the segment lining, which must be properly filled to prevent potential issues such as ground settlement, segment displacement, or tunnel collapse. Although cement-based two-component grouts are commonly used for this purpose, they pose significant environmental concerns, including high carbon emissions, groundwater contamination, and soil alkalization. In response to these issues, this study proposes the development of a fully cement-free, eco-friendly backfill material composed of natural soil (sand and clay), biopolymers, and gypsum. A series of laboratory tests—including bleeding and flow table tests—were conducted to evaluate the mixture’s segregation resistance and flowability, while unconfined compressive strength (UCS) and volumetric strain measurements were performed to assess durability. Furthermore, a tail void simulation model was employed to investigate the injectability and optimal injection parameters of the proposed material under varying time conditions. The findings demonstrate the feasibility and environmental advantages of the developed grout, highlighting its potential for practical application as a sustainable alternative in mechanized tunneling operations.