Chinese engineers have tackled another world-level challenge: Holing through a subway tunnel under a busy railway hub in Xi’an, northwestern China’s Shaanxi province.

The tunnel was completed on Jan. 9, according to Chinese newspaper Science and Technology Daily.

Xi’an Metro Line 4 is a newly planned project that crosses the city’s busy railway station, one that was built on wet loess. An average of 240 trains pass by the station each day.

With 15 tracks and eight turnouts, the slightest deformation of the rail might lead to a derail -- not to mention the tracks are paved on wet loess that is susceptible to collapse.

“It’s like having cardiac surgery without the anesthesia,” said chief engineer of the project Liang Xijun.

Normally, girders are used to prop up the tracks above tunnels in case of ground subsidence. However, such a method is not applicable for turnouts.

In addition, excavators could not be used either, since the settlement must be kept within five millimeters.

Thanks to the innovative application of robot technology, however, the engineers found the most appropriate solution. They hardened the ground to minimize possible settlement, and divided the section of the tunnel into four parts to enhance stability. Through a remote controller, the excavating robot successfully holed through the tunnel.

“It was the first time a robot was used this way,” said a constructor, adding that the machine is usually applied in pipeline projects.