The Kuthiran Tunnel, a critical infrastructure component in the challenging terrain of Kerala, India, requires a systematic and proactive maintenance strategy to ensure its structural integrity, operational safety, and long-term functionality. Traditional reactive maintenance approaches are often inadequate for such complex, high-traffic, and environmentally sensitive structures, leading to unexpected failures, prolonged closures, and increased life-cycle costs. This paper proposes a Proactive Maintenance Plan (PMP) developed and scheduled using Primavera P6, a leading enterprise project management software. The plan integrates condition-based monitoring, predictive maintenance triggers, resource-level scheduling, and risk-aware activity sequencing to transition from corrective to preventive and predictive maintenance paradigms. Key components include structural health monitoring (SHM) integration, environmental impact considerations, safety compliance milestones, and optimized resource allocation for labour, materials, and equipment. The study demonstrates how Primavera P6 can be utilized to create a dynamic, adaptable, and visually coherent maintenance schedule that accounts for real-time data inputs, seasonal constraints, traffic management requirements, and regulatory inspections. The proposed framework aims to reduce unplanned downtime, extend tunnel lifespan, enhance safety, and optimize maintenance budgets. This research contributes to the field of infrastructure asset management by providing a replicable digital model for large-scale tunnel maintenance in hilly and eco-sensitive regions.