7.1.2　Crossing Scheme

7.1.2　Crossing Scheme

At present，there are two main construction schemes for shield crossing the channel section in the middle of the river：the reinforcement scheme of overburden at the bottom of the river and the non-filling construction scheme at the bottom of the river.After the preliminary scheme is selected，the specific content of the scheme must also be determined by numerical simulation，theoretical calculation and field test，as shown in Figs.7.2 and 7.3.

1）Reinforcement scheme of river bottom soil

For the reinforcement scheme of covering soil，it is mainly by increasing the thickness of covering soil or using auxiliary work method to strengthen the covering soil，so that the covering soil can meet the requirements of shield construction.According to the shield construction practice at the bottom of the river at home and abroad，when the soil covering at the top of the shield cannot meet the safety requirements of the shield construction and the molding tunnel，the dumping clay is generally used in the deep water channel to increase the thickness of the overlying soil layer on the tunnel.In shallow water channel，due to the limitation of shipping and drainage，grouting reinforcement method is generally used to form an impervious hard shell layer，which is beneficial to the stability of soil layer and limit the floating of tunnel.When the overburden is too shallow，uplift piles and anti-floating slab structures are generally applied on the basis of grouting reinforcement to prevent roof fall caused by slurry pressure penetrating the overburden during shield tunneling.

2）Non-filling construction scheme of river bottom

When the buried depth of shallow overburden soil is greater than 0.7D and less than 1D，the dumping and filling measures are generally not adopted.Instead，the excavation surface is maintained by using high viscosity slurry，and the slurry pressure fluctuation and slurry flow of the excavation surface are controlled.The shield posture is strictly controlled in the tunneling process to ensure that the grouting is uniform and sufficient，and the safe and stable passage through the middle of the Yangtze River is achieved.When the shield passes through the shallow overburden section of the middle channel of the Yangtze River，due to the thin overburden thickness，high water pressure and high construction risk of the middle channel section of the Yangtze River，it is required to pass the shield at one time and avoid the long-term shutdown caused by equipment failure in the middle.Therefore，when the shield is close to the shallow soil section，the shield cutter should be stopped for inspection and replacement，a comprehensive maintenance of the shield equipment and the replacement of some parts，do a good job in inventory procurement of vulnerable spare parts，a comprehensive inspection and maintenance of ground slurry separation equipment，air compressor station，sewage pump and other equipment，and do a good job in equipment support for crossing the trough section.During the construction of the channel section in the middle of the river，high slurry pressure is prone to mud breakdown of shallow soil layer，and low pressure is easy to cause construction risks such as river bottom settlement and excavation face collapse.According to the characteristics of different strata，the tunneling and slurry parameters should be selected and adjusted timely and accurately so that the shield can pass through the middle channel section of the river smoothly.At the same time，the risks that may be faced during shield tunneling should be comprehensively evaluated and corresponding countermeasures should be formulated.

3）Construction scheme comparison

Because the flow velocity in the river is generally large and the clay particles are fine，the filling clay will cause local scour around the soil to disturb the stability of the original soil layer，so the bagged clay is needed to fill.However，this method is difficult to form a uniform overburden layer at the bottom of the river，and will lead to loose part of the soil filled with clay，which cannot become a whole with the soil under the original riverbed.The stability of soil pressure is small，the effect is negligible，and the economy is not suitable.In addition，the filling operation in the river，a huge amount of filling，not only time-consuming，but also affect the normal passage and safety of the waterway.

Therefore，compared with the dumping clay scheme with less adaptability and more disadvantages，the tunneling and slurry parameters should be adjusted in time，and the slurry quality should be optimized to stabilize the excavation surface so as to make the scheme of shield passing through the groove section more reasonable.At present，the Nanjing Yangtze River Tunnel project is a typical example of crossing the middle channel section of the Yangtze River in China.Through repeated analysis and demonstration，the project finally determines the construction scheme of“optimizing mud quality，fine control of pressure，strict control of posture，strengthening parameter matching，smooth control and propulsion，rapid segment assembly”，and achieves the purpose of safe and rapid crossing the middle channel section.