Above ground lies the Shantang Ancient Street, a 4A scenic area in Suzhou, home to famous ancient buildings such as the Wanshun Tower and Bao'en Temple. Parallel to and 350 meters ahead of the ancient street is one of China's busiest high-speed rail lines, the Shanghai-Nanjing Intercity High-Speed Railway, with trains speeding past every 3 minutes. Below ground, a 13.25-meter large-section slurry shield is tunneling through the underground tunnel of the Tongjing Road North Extension Project in Suzhou.

"If there were even the slightest deviation, the consequences would be unimaginable." The crossing project began on January 1. During this period, Liu Yanwen, project manager of the Tongjing Road North Extension Project of China Railway Fifth Survey and Design Institute (hereinafter referred to as "CRFSDI"), barely slept.

On January 11, the large-diameter shield successfully crossed the 350 km/h Shanghai-Nanjing Intercity High-Speed Railway. Process monitoring data showed that the cumulative settlement did not exceed 1 millimeter.

Professor Liu Songyu, Executive Vice President of the Institute of Future Underground Space at Southeast University, who witnessed the successful crossing on the 11th, believes that this is the first successful case in the world's construction history of a large-diameter shield crossing an operational high-speed railway, and also a classic case of a "package" solution covering design, construction, and equipment manufacturing inspired by the need to address urban development demands.

Liu Yanwen told the Science and Technology Daily reporter that to complete this "package" solution, CRFSDI spent a full four years on design and research.

The Suzhou Tongjing Road North Extension Project is located about 2.1 kilometers west of Suzhou Railway Station, with a total length of 2.49 kilometers. The tunnel is designed as a double-tube six-lane structure and is an important part of the "seven vertical" main roads in Suzhou's overall urban planning of "seven vertical and five horizontal" roads. "The ancient buildings above ground and the operating Shanghai-Nanjing Intercity High-Speed Railway, Beijing-Shanghai Railway, and North Ring Expressway all present the highest risk level difficulties for underground crossing," said Liu Yanwen. This longest north-south main road crossing the city has therefore become a "pain point" in Suzhou's decade-long difficulty in north-south connectivity.

To accomplish such a crossing is a comprehensive test of China's infrastructure technical strength from design to construction to equipment manufacturing.

Since February 2017, CRFSDI formed a project general team and joined forces with multiple units to carry out technical research.

The Tongjing Road North Extension Project passes under the Shanghai-Nanjing High-Speed Railway bridge using a 13.25-meter diameter large-section slurry shield. "Previously, domestic crossings under high-speed railway bridges all used shields with a diameter of about 6 meters," said Liu Yanwen. The shield diameter has more than doubled.

To ensure design quality, CRFSDI designers conducted multiple site surveys and adopted targeted different solutions for different crossing points.

Shantang Ancient Street is a national AAAA scenic area and a Chinese historical and cultural street. When crossing under this section, designers specifically used steel floral pipes as isolation measures to further reduce the impact on ancient buildings. After more than ten rounds of coordination with railway, planning, road, environmental protection, cultural relics, power, and river management authorities, and after optimizing and comparing multiple schemes from various angles, the shallow-buried shield tunnel scheme was successfully finalized, opening a "crossing path" with short distance and shallow depth.

When crossing under the Shanghai-Nanjing Intercity Railway bridge, not only is the settlement control requirement limited to no more than 2 millimeters, but the working space under the bridge is only 4 meters. "To install 30-meter-long isolation piles within a 4-meter clear height to ensure the passage of the large shield, during the crossing, any settlement exceeding even a tiny amount would cause abnormal stoppage of the entire high-speed rail national network, with incalculable losses. There are no similar successful cases at home or abroad," said Mao Zhongliang, head of equipment research and development for the "low-clearance full-casing cast-in-place pile machine" at CRFSDI.

To address this technical difficulty, CRFSDI innovatively developed a "low-clearance full-casing cast-in-place pile machine" that can meet low-clearance construction conditions. "It mainly consists of a frame, a rotary drive system, a soil excavation system inside the casing, a lifting and traveling control system, and an electrical control system," explained Mao Zhongliang. Through these systems, the casing is sunk section by section, and soil is removed in sections inside the casing to the design elevation of the pile bottom, thereby greatly reducing the equipment height and construction clearance height.

The project also proposed new processes such as using MJS piles to reinforce soft ground, making it "possible" for large shields to cross operational high-speed railways and breaking through the technical barrier that had blocked Tongjing Road for ten years.

The Suzhou Tongjing Road North Extension Project is scheduled to be completed and open to traffic in 2022, which will greatly improve regional traffic conditions in the thousand-year-old city of Suzhou.