Herrenknecht has shipped the first of two 15.4m diameter slurry machines, claimed to be the world’s biggest of their type, to Shanghai for the Shanghai Changjiang Under River Tunnel Project in China.
The project will see the construction of twin road tunnels, each carrying three lanes of traffic on one level and a safety passage and rescue lane in the bottom of the tunnel cross section. Crossing the Yangtze, China’s biggest river, the scheme aims to open up the river’s delta. To link the riverbanks between mainland Pudong and the island of Chongming, the twin tunnels will stretch 8.95km from the mainland to Changxing island, a bridge will connect Changxing and Chongming.
Shanghai Changjiang Tunnel & Bridge Construction Development Co will use the TBMs to bore 7.17km at 2.9% grade through ground conditions broadly characterised as sand, clay and rubble. The first TBM is planned to start boring this year, with the second following on a three-month lag. The planned construction period for the project is four years.
Herrenknecht told T&TI that one of the challenges for its designers was the large shield diameter; the other was the predicted geological and hydrological conditions. The entire tunnel alignment is through very weak clayey soils below the groundwater table. Local incalations of silty and sandy layers as well as shell residues are expected. At their deepest, the tunnels run 65m below the surface, a maximum groundwater level above the tunnels is expected to be 47m above tunnel axis, so the TBMs are designed for a maximum working pressure of 6.5bar.
The cutting wheels are designed with six accessible main spokes that are sealed against the water pressure. To avoid adhesion of sticky clay at the cutting wheel, the centre area is equipped with its own slurry circuit, large openings in the wheel also help mitigate banking of material in the centre.
Ten of the cutting tools, (two buckets and eight soft ground tools), have an electronic wear detection system. This system generates realtime data on the state of wear allowing better planning of maintenance works and hopefully minimising costly compressed air interventions. The cutting wheel was designed to allow access under atmospheric pressure with tool change devices integrated in the cutting wheel to allow the replacement of tools.
The TBM backup allows the installation of the road elements concurrent with advance. This means the TBM supply can be by trucks that drive along the road slabs laid by the backup crews.
