In Zhengzhou, China, soft ground tunneling has made a speedy entrance. The metro’s first TBM, one of two 6.3-m (20.7-ft) diameter Robbins EPBs for Zhengzhou’s Line 1, achieved a project record of 720 m (2,362 ft) in one month.

Daily rates have been as high as 22 rings (33 m/108 ft) in two 10-hour shifts. The rates are not only a project record amongst nine other machines, but also rank as some of the highest rates ever recorded for Chinese EPB TBMs in the 6 to 7 m (20 to 23 ft) diameter range.

The Robbins machines were launched in November and December 2010, and have both achieved intermediate breakthroughs into a cut and cover station site, most recently on March 29, 2011. The TBMs emerged into the station at the 1,300-m (4,250-ft) mark after excavating in conditions including soft, powdery soils — a benefit for swift tunnel boring.

“The breakthroughs went perfectly. Machine downtime during tunneling has been very minimal, and the organization of this project has been very good,” said Zhou Shuqing, General Mechanical and Electrical Engineer for contractor CRCC, 11th bureau. Ground in later sections of the tunnels is expected to consist of clay, fine sand, loess and some pebbles, with little groundwater.

Both EPBs, for the 11th Bureau of the China Railway Construction Corp. (CRCC), are excavating under downtown areas with cover as low as 8 m (26 ft) for nearly half of the tunnel length. The parallel 3.6 km (2.2 mi) long tunnels are passing through four intermediate stations between Kaixuan and Tongbo areas of the city. All sections are expected to be complete by September 2011.

The high advance rates are being achieved despite challenges including a section of tunnel just 7.0 m (23 ft) below Xi Liu Lake, and nearby structures such as building foundations and a highway interchange bridge. In order to reduce settlement, foam and bentonite are being injected for soil conditioning. The advance rates and muck removal volume are being closely monitored to prevent subsidence.

Zhengzhou, a city of 7 million people, is planned to become a center for rail commerce in China. Up to four metro lines will be built in Zhengzhou, while freight lines traveling between Beijing and Guangzhou (North-South), and between Xuzhou and Lanzhou (East-West), will eventually intersect in the city.  Once complete in 2013, Line 1 of Zhengzhou Metro will include 26 km (16 mi) of tunnel and 22 stations. The Zhengzhou Metro company has invested Yuan 10.2 billion (USD 1.5 billion / EUR 1.1 billion) in the new metro lines, which will total 188 km (117 mi) by their completion between 2015 and 2030.

This article was supplied by The Robbins Co.

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