Particle morphology and electrostatic screening shape salinity-dependent flocculation and settling of fine blasted tunnel sediments

Franco Tapia, Franziska Lorz, Gerit Harmut Orzechowski, Oyvind Thiem, Andrew Gravelle, Bernhard Vowinckel

Rock-blasted fine-grained sediments generated during tunnel excavation may enter receiving waters that range from freshwater to marine conditions. Yet the combined roles of primary-particle morphology and salinity-dependent electrostatic interactions in their settling remain poorly understood. In this work, fine sediments from five Norwegian tunnel construction sites were investigated using bulk-settling experiments over 0--35 PSU, microscopy-based particle characterization, and $\zeta$-potential measurements. Bulk particle settling exhibited two distinct regimes. At low salinity, settling was weakly dependent on salt concentration but differed substantially between sites. These differences correlated with a morphology--size index $\mathcal{M}$ combining particle size and aspect ratio, indicating that primary-particle geometry conditions the baseline clarification response. At intermediate salt concentration, the suspensions underwent a marked transition toward enhanced settling, with operational critical coagulation concentrations between approximately 0.36 and 1.60 PSU. The magnitude of this salinity-induced clarification correlated with the change in $\zeta$-potential, linking the macroscopic settling transition to the electrokinetic response of the suspended fraction. At higher salinity, clarification approached a common high-clarification regime. The results indicate that morphology and electrostatic interactions govern complementary stages of the settling process: primary-particle geometry conditions the low-salinity clarification state, whereas increasing salt concentration activates additional flocculation and clarification of the remaining sediments in suspension. This coupled framework provides a basis for predicting the fate of tunnel-derived fine sediments discharged into freshwater, brackish, and marine receiving environments.