Influence of Tunnel Construction Methods on Wastewater Quality and Inorganic Particle Characteristics

Ensuring the stable and compliant discharge of wastewater generated during high-altitude tunnel construction is critical for safeguarding regional aquatic ecosystems. However, how construction methods influence wastewater quality and particle behavior remains unclear. This study investigates two representative high-altitude granite tunnel projects to elucidate how drill-and-blast (DBM) and full-face tunnel boring machine (TBM) methods regulate inorganic particle characteristics and sedimentation behavior through multi-cycle water quality monitoring and multi-scale particle characterization. Wastewaters generated by both methods exhibited strong alkalinity (pH 11~12) and high suspended solids (SS) concentrations. However, continuous excavation under the TBM method markedly reduced SS variability and particle size dispersion. TBM-generated particles exhibited rougher surfaces, higher specific surface area, and greater porosity, promoting collision-induced aggregation and increasing the proportion of large particles (>100 μm). Consequently, the settling rate of particles produced via TBM was significantly better than that of DBM (with a 90% SS removal in 2 min for TBM compared to 30 min for DBM). Although DBM- and TBM-generated particles showed comparable mineral composition, wettability, and density, the rough surface structure of TBM particles under high pH and ionic strength conditions effectively compressed the electric double layer, disrupted hydration films, and enhanced mechanical interlocking. These effects collectively reduced interparticle repulsive barriers and promoted aggregation and settling. A natural sedimentation threshold of ~30 μm for granite particles was identified, with finer particles (<30 μm) dominating the residual fraction after treatment.