Abstract: To realize efficient drainage of water-rich tunnels in high-altitude environments and ensure construction safety and progress, the extra-long Milashan tunnel of the Nyingchi-Lhasa highway in Tibet, China is investigated. The characteristics of water inflow in high-altitude water-rich tunnels, efficiency reduction characteristics of pumping and drainage machinery, and principle of equipment selection and configuration are analyzed through theoretical analysis and field tests. The pumping and drainage scheme of the main tunnel drainage assisted by an inclined shaft is designed, and the displacement and pump head are calculated. The results show the following: (1) the Milashan tunnel passes through a water-rich fault, and there is a large volume of water inflow, long water inflow time, and uneven water distribution; and (2) highaltitude environments affect pumping and drainage machinery and equipment. The test results show that the maximum output power of the generator set and pumping unit is only 66.2% and 90% of the rated power, respectively. Therefore, three levels of fixed pump stations are set in the construction section of the main tunnel assisted by the inclined shaft, and the pumping equipment of the pump station is composed of pumps and booster pumps of different power. Matching calculations show that the selection of water pipes and the pump head meet the requirements.

Key words: tunnel engineering, high-altitude water-rich tunnel, main tunnel drainage assisted by inclined shafts, efficiency reduction characteristic