The left line of the Haimen-Taicang Yangtze River Tunnel, which links Haimen district in Nantong with Taicang in Suzhou, East China's Jiangsu province, cleared the tunnel's deepest point 75 meters below the Yangtze River on Sept 21 after a shield machine bored 5,334 meters.

An interior view of the Haimen-Taicang Yangtze River Tunnel. [Photo/WeChat account: haimenfabu]

The crossing came four months after the right line cleared the same point, marking how both lines have passed the deepest point under the river.

This section is considered the most challenging and the highest-risk stretch of the project. The tunnel face passes through composite strata of mucky silty clay interbedded with silt and fine sand, with a maximum burial depth of 75 meters. Water pressure at the work face reaches 7.5 bar, equivalent to 7.5 kilograms pressing on an area the size of a fingernail.

The strata also contain large amounts of quartz sand, which wears down cutters quickly, and the soil is highly permeable. Even slight deviations can cause settlement, seal damage or pressure imbalance. Such conditions place high demands on shield posture, grouting precision and on-site control.

Zhao Xiaopeng, project manager of contract section A4 of the Haimen-Taicang Yangtze River Tunnel, said the shield machine on the left line will adjust its posture and bore upward. He said water and earth pressure will continue to decline, gradually reducing construction difficulty.