Changsha Metro Line 3, with Lingguandu Station as the center, has a 4.7-kilometer shield tunneling section that simultaneously presents two nationally rare construction challenges.

To the west, the section from Lingguandu Station to Fubuhe Station must cross the country's first long-distance underwater karst cave area, pioneering such construction in China;

To the east, the section from Lingguandu Station to Houjiatang Station to Dongtang Station has nearly 900 meters of overlapping construction with the already operational Metro Line 1, marking the first long-distance underpass of overlapping existing operational lines in China.

After overcoming numerous difficulties, these two major construction challenges have been successfully tackled, laying a solid foundation for the trial operation of Metro Line 3 next year.

A karst cave every 1 meter

The section from Fubuhe Station to Lingguandu Station has a total length of 2.65 kilometers, the longest shield tunneling section on the entire line. It must cross the Xiang River, passing through large karst development areas and fault zones, making it the country's first long-distance underwater karst cave shield tunneling section.

In the monitoring room of Lingguandu Station on Changsha Metro Line 3, staff monitor underground construction progress. Photo by Wang Zhiwei, Changsha Evening News

In the meeting room of Lingguandu Station on Metro Line 3, a cabinet in the corner displays rock samples selected from the shield tunneling pipeline. They vary in size and color, some extremely hard. The geological conditions in the shield tunneling area beneath the Xiang River are complex, truly a "geological museum."

Liu Bin, manager of the 5th bid section of Changsha Metro Line 3 for China Railway 14th Bureau, introduced:

The Xiang River karst development area crossed by Line 3's shield tunneling is located in Orange Isle, a national 5A scenic spot, within the secondary protection zone of the Xiang River water source. It has a total length of 323 meters, mainly consisting of limestone formed in the Devonian period 300 million years ago and glutenite formed in the Cretaceous period.

The karst area is like a giant honeycomb, with a cave encounter rate of 80.6%, several times that of ordinary karst areas. On average, there is one karst cave every meter, which is rare both domestically and internationally.

First use of slurry shield in Changsha Metro history

To adapt to high water pressure, long distance, large burial depth, and the complex underwater karst development area of the Xiang River, effectively control ground settlement, and protect the Xiang River water environment, a slurry shield is used for the first time in Changsha Metro history.

Zhou Xiaoming, general manager of the Municipal Rail Transit Group, introduced:

This is the section with the most concentrated difficulties and the most complex karst geology in Changsha Metro construction. The underlying strata vary greatly, with large burial depth and high water head pressure. It is essential to ensure that no roof collapse, main bearing seal failure, or tunnel water ingress occurs during the shield tunneling under the Xiang River.

At the same time, Changsha Metro established a scientific and technological research group, jointly with Central South University, to carry out design, construction research, and technological breakthroughs. They also hired renowned domestic experts to repeatedly demonstrate the risks of shield tunneling through karst development areas, ultimately determining the construction plan of grouting from a steel platform on the river surface.

By setting up a 6,000-square-meter construction platform on the river, converting karst cave treatment from water-based to "land-based" construction, and other series of plans, the technical challenge of underwater karst cave grouting was effectively solved. This not only shortened the construction period but also reduced the impact on flood discharge and environmental protection.

First long-distance underpass of overlapping existing operational lines in China

How many positional relationships can two metro lines have? The most common is when two lines intersect, forming a familiar transfer station.

For example, Changsha Metro Line 1 and Line 2 intersect in a "cross" at Wuyi Square Station. Besides intersection, another relationship is the overlap of Changsha Metro Line 3 and Line 1 at the transfer station Houjiatang Station.

The section from Lingguandu Station to Houjiatang Station to Dongtang Station on Changsha Metro Line 3 is about 2.04 kilometers long, with nearly 900 meters of overlapping construction with Line 1 in this shield tunneling section. The shield tunneling for Line 3 must pass beneath the operational Line 1.

Among them, the Lingguandu to Houjiatang section needs to underpass the overlapping section of Line 1 for about 721 meters, and the Houjiatang Station to Dongtang section has an overlapping section of about 114 meters.

The receiving and launching sections at Houjiatang Station are both overlapping sections. The minimum vertical clearance between the tunnel and Line 1 is only 5.5 meters, and above ground is the busy Laodong West Road. The shield tunneling difficulty is the first of its kind in Changsha Metro and even nationwide.

This is also the first long-distance underpass of overlapping existing operational lines in domestic shield tunneling construction.

Construction impact controlled to the thickness of an embroidery needle

Miao Xuan, project manager of the 6th bid section of Changsha Metro Line 3 for China Railway Fourth Bureau, told reporters that to reduce the impact on Metro Line 1 operations, during the entire shield tunneling process, they installed 9 intelligent robots within the 920-meter range of Line 1 to monitor displacement and settlement 24/7, collecting data every 15 minutes. If the warning threshold is exceeded, the system will automatically alarm.

Construction workers also manually compare data once a day and strictly control shield tunneling parameters through the data to ensure no mechanical failures during the crossing. At the same time, secondary grouting is performed multiple times in a timely manner, mainly at the crown, with strict control of grouting volume and pressure to minimize construction risks. Through various measures, the impact on Line 1 was ultimately controlled to within 1 millimeter, equivalent to the thickness of an embroidery needle.