3.4.1　Summary of Engineering Problems

There is a section about 150 m long in the middle section of Nanjing Yangtze River Tunnel，and the thickness of the overburden is about 11m.The main overburden in this section is permeable silt，which is about 20 m underwater.The shield begins to be excavated from a certain distance from the shallower overlying soil layer，and gradually passes through the shallow overburden.When the shield passes through this section of overburden，it will cause great disturbance to the strata.At this time，whether the shallow overburden can remain stable，restrain grouting and settle too much when the shield passes through has an important influence on the safety of tunnel construction.Therefore，it is necessary to simulate and predict the settlement law，shield attitude change law and excavation surface stability in the process of shield crossing in shallow overburden section.

Fig.3.15　Distribution of the thickness of the shallower overlying soil

According to the thickness of tunnel overlying soil，the shallow overburden section is divided into four working conditions for comparative analysis：

①KLK5+900～LK5+975　section；②KLK5+975-LK6+025　section；③KLK6+025-LK6+075 section；④KLK6+075-LK6+150 section.

The main contents are divided into three types，namely，surface subsidence，excavation surface stability and shield attitude.The total length of the selected section is 250 m，and the distribution of overlying soil thickness is shown in Fig.3.15.