Optimizing Tunnel Shaft Design for Performance and Constructability--The Singapore Experience

This paper summarizes design enhancements to improve operational performance and constructability of wastewater tunnel drop shafts. The enhancements were developed from focused computational fluid dynamics (CFD) modeling and a review of recent physical modeling studies. The enhancements include: reducing the vortex drop shaft diameter to ensure consistent hydraulic performance in dry-weather conditions. CFD confirmed the adequate performance for peak wet-weather conditions; incorporating fillets to the approach channel for stable hydraulic performance across the range of flow conditions; modifying the deaearation chamber cross-section to ensure adequate self-cleansing flow in dry-weather conditions; increasing the diameter of curved adit connections for adequate performance during peak wet-weather conditions; replacing a helicoidal ramp diversion shaft with a baffle plunge alternative to reduce bypass pumping during construction. A superhighway for used water management, the Deep Tunnel Sewerage System (DTSS) is a core water infrastructure project which provides a cost-effective and sustainable solution to support Singapore's continued growth and meet its long-term needs for used water collection, treatment, reclamation and disposal.