Abstract: Research on the excavation and ventilation technologies for hydraulic tunnels with ultra-small cross-sections is of great significance for developing hydropower resources in high-altitude regions. Based on the case of the Sanjen Khola hydropower station located in an uninhabited area on the southern slopes of the Himalayas in Nepal, this paper reveals the mechanism of gradient blasting for creating free surfaces in ultra-small cross-section tunnels, which involves one-time drilling, pre-blasting of the central pilot tunnel, and subsequent blasting of the side and crown arches. We compare air pressures and airflow losses along the length of iron ventilation pipes versus HDPE pipes, and develop a theoretical model and a full-process construction methodology for accurate deviation control in highly inclined ultra-small cross-section shafts, covering the induction, generation, formation, correction, and prevention of excavating deviations. Practical applications on the construction site shows that for excavating such tunnels, the single-cycle advance is increased from 1.5 - 1.8 m to 2.8 - 3.1 m. Under a ventilation distance of 3000m, the airflow and air pressure losses along the HDPE pipes are lowered by 83.6% and 45.5% respectively, compared to the iron pipes; The maximum deviation rate of the inclined shaft was about 0.18%. The findings are important and useful theoretically and technically for safe excavation and ventilation of ultra-small cross-section hydraulic tunnels in high-altitude regions.

Key words: hydraulic tunnel, high-altitude, gradient blasting, small cross-section ventilation, high-inclination shaft