Chongqing, the largest city in southwest China, is burrowing forward with what is believed to be the country's largest urban tunnel project.

On completion, the Daping urban tunnel project will boast two 1,435-meter-long tunnels connecting to a 4.35-km bridge that spans the Jialing, a tributary of the Yangtze River.

It will be the widest tunnel in Asia.

Tunnelers struck through on the left part of the tunnel on Saturday after more than a year of effort. The right tunnel, just 30 meters from completion, is expected to be finished on Christmas Day, according to Fan Jianguo, deputy secretary of the Party committee of the No.1 company with China Railways No.8 Bureau, the builder.

As for the bridge, construction workers have so far completed four fifths of the work, and will now spend more time on tasks such as road surface improvement, illumination and erection of road signs and fire control devices, according to Fan.

The Daping urban project starts from a mountainside near the Daping water tower, traverses Daping Mountain, connects to the Jiahua Bridge and ends at Xindongfu Garden in Huangshaxi. The dual tunnels are sandwiched in between existing tunnel tracks for urban light track rail and a separate railway tunnel connecting Chongqing and Xiangfan, a city in western Hubei Province.

The three-lane tunnels will restrict cars to speeds of 80 km per hour. They will connect Jiangbei District with downtown Yuzhong and Yangjiaping districts, cutting travel time by half an hour.

The 2.2 billion yuan (about 275 million U.S. dollars) tunnel project will go into service next June.

Chongqing, whose urban districts are divided into three major parts by the mighty Yangtze and its tributary Jialing, is well-known for its mountainous terrain. Travel around the mountainous city used to be very difficult but improved transport infrastructure has made commuting much easier.