On February 7, a reporter learned from the Chengdu-Kunming Railway Company that at 3 p.m. that day, the parallel heading of the Xiaoxiangling Tunnel in the Emei-Mianning section, the longest tunnel on the Chengdu-Kunming Railway double-track line, was successfully holed through. The parallel heading is more than 21 kilometers long; its breakthrough has verified the geological conditions for construction of the main tunnel bore, solved the problems of ventilation and traffic organization in the long tunnel, and accelerated the construction progress of the Xiaoxiangling Tunnel.

The Xiaoxiangling Tunnel is located in the territory from Yuexi County to Xide County in Liangshan Prefecture, with a total length of 21.775 kilometers, and is the longest tunnel on the Chengdu-Kunming Railway double-track line. The tunnel lies in the middle and high mountain areas of the Hengduan Mountains, passes through 10 faults and 2 folds, and has a maximum buried depth of 1,350 meters. Its geological conditions are extremely complex and active, making it a Class I high-risk tunnel.

Liu Qian, secretary of the Chengdu-Kunming project department of China Railway Tunnel Group, introduced that since construction began in June 2016, tunnel construction has encountered nearly 90 incidents of various scales, including rockbursts, collapses, mud bursts, water inrushes, and deformation of initial support. In October 2018, the tunnel revealed the Xide underground river system, with a maximum water inflow of 520,000 cubic meters per day. The cumulative water inflow has reached more than 200 million cubic meters, about the volume of 15 West Lakes, which is rare in tunnel construction nationwide. To deal with the above problems, the project department spent more than 500 days.

It is understood that during this year's Spring Festival holiday, all tunnel construction personnel of China Railway Tunnel Group stayed on duty without taking leave. A total of 428 people worked in rotating shifts and remained at their posts, with 8 working faces in full operation 24 hours a day without interruption. The main tunnel advanced on average 32 meters per day, and 2,305 meters remain. Construction personnel are racing against time for the final sprint toward tunnel breakthrough.

The Chengdu-Kunming Railway double-track line has a total length of 865 kilometers, starting in Chengdu and ending in Kunming. It is a key project in the 13th Five-Year Plan and an important corridor in the national "Belt and Road" development strategy connecting to international trade ports in Southeast Asia.

At the end of December 2017, the approximately 129-kilometer Chengdu-Emei section within Sichuan was opened; at the end of May 2020, the approximately 108-kilometer Mianning-Panzhihua section was opened; on January 10, 2022, the Mianning-Miyi section was completed and opened, and Fuxing EMU trains entered the Daliang Mountains for the first time. At present, only the Emei-Mianning section of the entire line remains unopened. This section is under accelerated construction and is expected to open to traffic in 2023.

After the entire line opens to traffic, the travel time of conventional railway trains from Chengdu to Kunming will be shortened from the current 19 hours to about 6 hours. This will build a major railway transportation artery for personnel and logistics exchanges in the Southwest region and is of great significance to promoting economic and social development in the Southwest region.

Did You Know: The Parallel Heading of a Tunnel

A parallel heading is a heading built on one side of a tunnel, parallel to the tunnel alignment, with its excavation face always ahead of the tunnel excavation working face. Parallel headings are mostly used in long mountain-crossing tunnels exceeding 3,000 meters in length when topography or other conditions make cross adits unsuitable or impossible. The parallel heading is connected to the main tunnel bore at multiple points through cross passages, and each cross passage, after entering the main tunnel, can add two new working faces.

The roles of the parallel heading in tunnel construction include: creating additional working faces; serving as advanced geological exploration; facilitating construction ventilation (the parallel heading can be used to form a gallery ventilation system); drainage, dewatering, and surveying (using the parallel heading to set up an in-tunnel traverse network to improve survey accuracy); resolving transportation interference; and accelerating construction progress.

(Photos provided by Chengdu-Kunming Railway Company)