Sustainable solutions: Transforming waste shield tunnelling soil into geopolymer-based underwater backfills

Abstract

Waste shield tunnelling soil is often utilized as backfill materials in engineering practice. However, traditional backfill materials exhibit poor backfilling performance in underwater condition, which is even more commonly encountered in backfilling projects. In response to this issue, this study introduces a geopolymer-based underwater controlled low-strength material (UWCLSM) that utilizes recycled waste shield tunnelling soil and granulated blast furnace slag (GBFS). The UWCLSM demonstrates promising underwater performance, making it a viable candidate as a new underwater construction material. Key characteristics include a flowability exceeding 280 mm, unconfined compressive strength ranging from 1 to 2 MPa, high resistance to water erosion with an anti-washout strength ratio consistently above 80%, and a permeability coefficient lower than 10−7 cm/s. The addition of anti-washout admixtures (AWA) plays a pivotal role in enhancing the UWCLSM's underwater performance. Polyacrylamide (PAM) effectively attracts soil particles, depolymerized GBFS monomers, and cement particles, reducing dispersion in water. An optimal amount of PAM forms cross-linked composites with GBFS and soil particles, resulting in slight improvements in UCS, shear strength, and impermeability. The combination of PAM and carboxymethyl cellulose enhances UWCLSM's cohesion and viscosity, optimizing its suitability for underwater applications. Moreover, the UWCLSM's cost, CO2 emissions, and energy consumption are only 55.6%, 16.7%, and 16.1% of those associated with traditional materials, contributing to sustainability and offering significant environmental benefits, respectively.