On January 20, the first phase of Changsha Metro Line 3 successfully crossed a 323-meter honeycomb-like karst cave area under the Xiangjiang River, a first in history, pioneering similar projects in China and marking a new major breakthrough in China's underwater shield tunneling technology. What does this underwater karst cave section look like? What will be encountered during shield tunneling? What is most feared? How to solve it? Karst caves like a honeycomb, cave encounter rate up to 80.6%
"This underwater karst cave area is located in the Meiyuan Garden of Orange Isle and the western branch of the Xiangjiang River, belonging to the Xiangjiang karst development area, with 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," said Yang Junsheng, professor at Central South University and vice chairman of the Hunan Provincial Society for Rock Mechanics and Engineering. The reporter learned on site that the first phase of Changsha Metro Line 3 is 2.75 kilometers long, with 1.4 kilometers crossing the Xiangjiang River. Among them, the 323-meter lava area is like a honeycomb, with a cave encounter rate as high as 80.6%, averaging one karst cave per meter, just like a huge karst cave museum, rare at home and abroad.
This 323-meter karst cave area is not only dense but also has irregular distribution and depth. The shallowest burial depth is 2.5 meters, the deepest is 22.46 meters, and the widest is nearly 50 meters. "Karst caves are underground spaces formed by karst processes in soluble rocks, belonging to limestone areas. Due to long-term dissolution by groundwater, insoluble calcium carbonate in limestone is converted into soluble calcium bicarbonate under the action of water and carbon dioxide," Yang Junsheng told the Science and Technology Daily reporter. This characteristic forms the unique landscape of karst caves that are independent, diverse, steep and beautiful.
Water steel platform construction, grouting into karst caves from the river surface The bizarre and beautiful karst cave landscape is, underwater, the most feared devil's zone for tunnel construction.
"The karst cave area has large stratigraphic changes, large burial depth, and high water head pressure. Once problems such as shield machine roof collapse under the Xiangjiang River, main bearing seal breakdown, and tunnel water ingress occur, they will connect with the ground or river water, leading to serious accidents such as ground subsidence and river water inrush," said Shi Qingbin, project manager of the China Railway 14th Bureau Group. The project department established a scientific and technological research group, jointly with Central South University and Shandong University, to carry out construction research and scientific and technological breakthroughs. They hired well-known domestic experts to fully demonstrate the risks of shield tunneling through karst development sections, and finally determined the water steel platform construction plan. By building a construction platform of nearly 6,000 square meters, the project department transformed karst cave treatment from water construction to "land" construction and other series of plans, effectively solving the technical problems of steel pipe pile driving in shallow karst cave areas. "This method has a short construction period for karst cave treatment, small impact from flood season, small impact on flood discharge, and low construction risk," Yang Junsheng believed. It is reported that in the karst cave area, more than 3,000 grouting holes were drilled, and about 80,000 cubic meters of grouting was injected. During the entire process of crossing the karst cave area, there was no water leakage, and the most feared situations such as shield "nodding" and "sinking" did not occur. What ingenious methods were used?
"The project department, together with Central South University, conducted detection at least 15 meters ahead of the excavation, scientifically formulated various parameters for tunneling and subsequent strata, and effectively avoided construction risks," Shi Qingbin told the Science and Technology Daily reporter. During construction, the project department also used various technical means to conduct tests, continuously summarized the parameters of slag samples and tunneling, and strictly controlled the tunneling level. Orange Isle is a national 5A-level scenic spot and is located in the secondary protection zone of the Xiangjiang River water source. How to ensure that the water source is not polluted during construction through the karst caves?
"We developed a new technology for hole formation in complex karst strata. Through testing, analysis, and experiments on various grouting materials, we innovated an environmentally friendly modified paste slurry as grouting material, which not only achieves good reinforcement effects but also avoids pollution of the river water," Shi Qingbin said. The reporter also learned that in order to reduce the impact on water quality, construction personnel used steel cofferdams with a diameter of more than 1 meter as "protective walls" to isolate the grouting construction from the river water, effectively protecting the Xiangjiang environment. "In the autumn of flowering reeds, on a night of the Xiaoxiang, the beautiful scenery of Orange Isle is like a screen painting. In the blue mist, under the bright moon, a small boat has just finished fishing." 1,200 years ago, the late Tang Dynasty poet Li Xun wrote such beautiful words while passing the Xiangjiang River in a small boat. If he knew that today one could cross the Xiangjiang River through the karst cave area from the riverbed, how would Li Xun write about Orange Isle?