Predicted and Documented Construction-Stage Impacts of Urban Mega Infrastructure Projects: Environmental Follow-Up, Material Circularity and Social Acceptance in Athens Metro Line 4

Urban mega infrastructure projects are justified by long-term mobility, and environmental and social benefits, yet their construction imposes pressures that are concentrated in particular neighborhoods and periods and whose magnitude is rarely quantified at the assessment stage. This study examines the relationship between anticipated and documented construction-stage impacts in Section A of Athens Metro Line 4, from Alsos Veikou to Goudi, and extends the comparison to excavation-related material flows and to the conditions of social acceptance. A qualitative case-study design analyses the environmental permitting framework, the annual environmental reports of Elliniko Metro S.A. 2021–2025, the applicable waste legislation and the relevant literature, applying an analytical sequence that links predicted pressure, documented condition, management response, residual effect, resource-recovery pathway and social relevance. The documentation confirms that management mechanisms corresponding to the anticipated pressures were in place for noise, vibration, dust, traffic and public-space occupation, although it does not allow their effectiveness to be verified quantitatively. The clearest documented outcome concerns materials, since excavation products were directed to quarry restoration, fill and cover applications and cement production, while construction and demolition waste (C&DW) arising from archaeological works and utility relocations was transferred to licensed facilities. The findings indicate that separating excavation products from C&DW is necessary for an accurate reading of project circularity and that social acceptance can be addressed only conceptually in the absence of representative survey data. The framework offers a transferable basis for construction-stage follow-up of mega infrastructure projects and waste-management facilities.