2.1.1　Main Construction Methods of Shield Initial ...

2.1.1　Main Construction Methods of Shield Initial Stage

Shield initiation refers to the use of temporary assembly segments and other reactive equipment to push the shield onto the launching platform，from the starting point into the stratum，starting a series of operations along the given line.The shield launching stage mainly includes two parts：the soil reinforcement at the end of the tunnel and the initiation construction technology.According to the different methods of removing temporary retaining wall and preventing stratum collapse of excavation face，the main construction methods in the initial stage are the following types（Fig.2.1）.

（1）The end soil reinforcement method is to use reinforcement measures to stabilize the excavation stratum，and then shield tunneling into the reinforced stable stratum.Reinforcement methods used in the past are more precipitation，air compression and chemical grouting reinforcement method，now gradually using high pressure injection method and freezing method more safe construction method.

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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_2

（2）Pile pulling method is a process to overcome the pile side friction（the soil around the pile acts on the surface of the pile）and the self-quality of the pile depth，and pull the pile out of the surface，including the double steel sheet plate pile method，excavation backfill method and SMW core pulling method.Double steel sheet pile method is to start the shaft steel sheet pile retaining wall as two layers.The shield tunneling after pulling out the inner steel sheet pile can ensure that the outer soil will not collapse due to the retaining effect of the outer steel sheet pile，which ensures the stable tunneling of the shield.When the shield is pushed to the front of the lateral steel sheet pile，the lateral steel sheet pile is pulled out.Due to the stability of the soil between the internal and external steel sheet piles，the shield can be maintained to continue to advance after the lateral steel sheet pile is pulled out.Excavation backfill method is to use the starting shaft as a rectangle（longer than 2 times the length of the shield），set a partition in the middle of the shaft（or build two parallel shafts），half as the starting shield assembly，and backfill when the shield machine is pushed into the other half of the shaft.Due to the isolation supporting effect of backfill soil，the stratum can not collapse when the steel sheet pile of the end shaft wall is removed，which provides a reliable guarantee for the safe penetration of shield into the stratum.The SMW core pulling method is to use the SMW retaining wall as the lining of the starting wall of the shaft.Before the shield starts，the I-steel core material is pulled out，and then the shield starts to excavate and cut the shaft wall without core material.

（3）Direct excavation method includes NOMST method and EW method.The characteristics of NOMST method is that the wall material of the starting port is special，and the tool can be directly excavated without damaging the tool.The starting operation of NOMST method is simple，without auxiliary method，and the safety and reliability are good.The principle of EW method is that I-steel，the core material of retaining wall，is corroded by means of electric erosion before shield launching，which brings convenience to shield excavation and cutting directly.The advantages of EW method are the same as those of NOMST method.

（4）Large-depth initiation protection method includes special mortar replacement method and cutting concrete method.The special mortar replacement method is to use the excavator to dig out the soil of the original protection part directly from the surface，and then pour the low strength（compressive strength at month age＞10 MPa）without separating the mortar in water.After the completion of all the mortar conversion soil，the in-situ curing method of wall protection slurry is used above the mortar（until the ground）.The cutting concrete method is to use PC steel wire to reinforce the starting wall constructed by limestone concrete.When the shield starts，the PC steel wire is pulled out to make the starting wall in a state of no reinforcement，and then the shield cutterhead is used to directly cut the starting wall.

For the starting construction of a specific shield tunnel，one of the methods in Fig.2.1 can be used alone，and the combination method can be used.Details on the characteristics，application scope and construction points of various reinforcement methods are provided in Sect.2.2.The specific selection of construction methods depends on factors such as geology，groundwater，overburden，shield diameter and construction environment，and the requirements of safety，construction，cost and progress should also be considered.

Taking the launching construction of large-diameter slurry shield in Nanjing Yangtze River Tunnel as an example，the main construction methods of soil reinforcement at the end of the launching stage are introduced.The soil layer of Nanjing Yangtze River Tunnel is silty clay，and the stratum is reinforced by high pressure jet grouting pile in the 0-18 m section ahead of the tunnel portal，with the reinforcement depth of 23 m and the width of 23 m.Due to the connection between groundwater and Yangtze River water，the high-pressure rotary jet grouting slurry is easy to be taken away by flowing water.The reinforcement effect of high-pressure rotary jet grouting pile by drilling core detection is not ideal，and it does not condense into a whole.There is a local weak zone that cannot seal sand water，which is prone to sand gushing and water gushing，affecting the normal exit of the broken portal and shield.In order to better enhance the strength and sealing of soil and ensure reliable water sealing of reinforced soil，a 1.6 m wide frozen soil wall is designed between the soil reinforced by high pressure jet grouting pile and the underground diaphragm wall at the end.The frozen soil wall formed by artificial refrigeration process cements the reinforced area of high pressure jet grouting pile and the underground diaphragm wall，so that the soil reinforced by jet grouting and the frozen wall jointly resist the water and soil pressure to ensure the safety of the portal breaking and shield launching.Fig.2.2 is the schematic diagram of the reinforcement area at the starting end of the shield.