7.1 Technical Difficulties and Schemes for Shield ...
At present，the slurry pressurized shield method is almost the required method for river-crossing and sea-crossing tunnels.When the large diameter slurry pressurized shield machine is used to cross the river，it will inevitably cross some shallow soil sections（the thickness of the soil is less than twice the diameter of the shield）.In the shallow overburden section of the middle Yangtze River，due to the high water pressure，part of the overburden is scoured by high water pressure for years，thus forming a flushing groove.Therefore，this section is called the middle channel section of the Yangtze River，which is often U-shaped landform and the terrain fluctuates greatly，as shown in Fig.7.1.
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J.Chen et al.，Construction Technology of Large Diameter Underwater Shield Tunnel，Key Technologies for Tunnel Construction under Complex Geological and Environmental Conditions，https://doi.org/10.1007/978-981-16-5896-9_7
Fig.7.1 Schematic diagram of the middle channel section