The Hongdoushan Tunnel, a key tunnel project of the Dali-Lincang Railway, was successfully drilled through on Wednesday in southwest China's Yunnan Province after the four-year construction work.

Construction on the 10.6 km Hongdoushan Tunnel started in May 2016. However, toxic gases were detected in the tunnel in May 2017, which, in combination with extremely high temperatures and the gushing water, increased the construction difficulties.

The complex geological environmental conditions of the Hongdoushan Tunnel that allow various toxic gases to gather are very rare, according to experts.

Factors such as high geothermal, ground stress and seismic intensity, active neotectonic movement, geothermal environment, external dynamic geological function and superficial geological transformation have all contributed to the large number of types of toxic gases in high concentrations.

Under such circumstances, the project's construction contractor worked with universities and institutes to solve the problem, and the construction had proceeded steadily.

"After conquering four major difficulties in four years, 1,200 of our staff members finally gained the hard-won victory on construction of the Hongdoushan Tunnel. At present, all the over-10-kilometer tunnels of the Dali-Lincang Railway have been drilled through. And we are confident that the remaining works can be completed on schedule in mid-October," said Zhao Yu, manager of the Dali-Lincang railway project department of the Railway No. 10 Engineering Group Co., Ltd.

The Dali-Lincang Railway, with a length of 202 km and a designed speed of 160 km per hour, is a key section of the China-Myanmar international railway. It is scheduled to be completed and open to traffic by the end of 2020.