The under-construction northwest loop of Phase II of Harbin Metro Line 3 in Heilongjiang Province is the city's largest underground rail transit project to date, crossing four core urban districts, serving more than 100 residential communities and connecting nearly 180 bus routes. Construction of the northwest loop of Harbin Metro Line 3 is currently in full swing; track laying across the entire line will be completed by the end of this year, and trial operation is expected by the end of 2024. Unlike conventional side-by-side tunnels, the tunnel sections from Heshan Street to Hesong Street and from Hesong Street to Highway Bridge adopt an underground 'stacked' design, making it Northeast China's first underground stacked tunnel.

What is an underground stacked tunnel? According to the site manager of the Harbin Metro Line 3 project at the CCCC Tianhe Harbin project department, in the sections from Heshan Street to Hesong Street and from Hesong Street to Highway Bridge, the two underground tunnels for the two directions would normally be arranged side by side, but due to constraints of the shield tunneling environment, the two tunnels were built one above the other. The stacked section totals 600 meters in length, and the minimum clearance between the upper and lower tunnels is only 2.1 meters—less than the height of one story—making tunneling extremely difficult. This is rare not only in Northeast China but also in tunneling construction worldwide.

The stacked tunnel design effectively addresses problems such as interference from external factors like elevated bridge piles and high-rise building pile foundations, as well as narrow construction sites in underground spaces. It reduces the underground space footprint and provides a better option for urban rail transit route planning. The difficulty in shield construction lies in the fact that the shield machine must first tunnel through the lower section, but when tunneling through the upper section, because the clearance is so small, every effort must be made to minimize 'disturbance' to the completed section, with extremely stringent requirements for segment circular convergence, segment grouting, and soil disturbance settlement control.

As an urban rail transit project, the northwest loop tunnel of Harbin Metro Line 3 has been successfully completed, setting multiple national records during construction. For example, the project is China's largest metro loop line in a cold region; it built the country's first metro stacked section in water-rich sand strata—the section from Heshan Street Station to Hesong Street Station; and the country's most difficult metro stacked section in water-rich sand strata—the section from Highway Bridge Station to Hesong Street Station. CCCC Tianhe, which undertook the tunnel boring task, specially developed 18 'Ice City' series shield machines for the project.

While setting multiple records for cold-region metro construction, the northwest loop of Harbin Metro Line 3 also passes underground beneath China's largest protected architectural complex—the Chinese Baroque Historical and Cultural Block. The Chinese Baroque architectural complex in the block is currently the largest and best-preserved Chinese Baroque complex in China. Its unique architectural culture, folk culture, and culinary culture, among other tourism resources, attract local residents and tourists from all over. Since September 29 this year, multiple stations on the northwest loop of Harbin Metro Line 3 have officially opened and begun operation. Local residents and tourists from all over can enjoy a 'historical and cultural tour' while traveling by metro.