2.1.2 Construction Method of Shield Arrival
The arrival construction of shield machine refers to the whole construction process from 50 m before shield tunneling reaches the receiving well to the tunnel entering the receiving well into the receiving well and being pushed onto the shield receiving pedestal（usually the initial base of shield machine）.The construction of shield arrival includes the reinforcement of the reinforcement of shield to front end，and the shield arrival method and the shield receiving method.The specific scheme of each part is described as follows.
1）Soil reinforcement of shield to front end
With the change of stratum conditions，the soil at the end of the shield arrival section also has the soil stability problem，and the reinforcement method is referred to the reinforcement method of the shield arrival section.
2）Shield arrival method
Shield arrival construction refers to the whole construction process from 50 m before the shield tunneling to the receiving well to the shield tunneling through section tunnel into the receiving well is pushed to the shield receiving base（usually the shield arrival base）.
According to the different time of removing the retaining wall，it is divided into two kinds：one is to remove the retaining wall reaching the shaft after the shield arrives，and then push it forward，the other is to remove the retaining wall in advance，and then push it to the designated position.
（1）The construction method of removing the retaining wall after the shield arrives.
The method is to push the shield outside the retaining wall reaching the shaft，stabilize the stratum by stratum reinforcement，remove the retaining wall，and then push the shield to the specified position.When the retaining wall is removed by this method，the shield is stopped in front of the open surrounding rock，and the gap between the cutter head and the shaft is small，so the stability is strong.However，it is widely used process and good construction，it is widely used.Because the stratum is prone to collapse when the shield is re-propelled，it is mostly used for small and medium section shield projects with good stratum stability.
（2）The method of removing the retaining wall before the shield arrives.
This construction method needs to remove the retaining wall in advance，so highstrength strata reinforcement should be carried out before removal，and steel partition wall with easy removal and good stress should be built in the well.Then，the retaining wall is removed from the bottom to the top，and the gap between the stratum and the reinforcement and the partition is filled sequentially with cement or poor ratio mortar.After completely replacing it with cement soil or poor ratio mortar，the shield is pushed to the front of the partition，and the partition is removed to complete the arrival process.Because the shield is not allowed to advance again，the construction method has the advantages of preventing stratum collapse and strong anti-seepage at the entrance of the tunnel，but the scale of stratum reinforcement increases，and the partition wall must be set up，so the scale of arrival preparation operation is expanded.This method is mostly used in large-section shield engineering.
3）Shield receiving method
In order to solve the engineering problem of poor reinforcement effect in some strata encountered in the field construction，the construction technology of water arrival and the receiving technology of steel sleeve have been developed in recent years，as shown in Figs.2.3 and 2.4.
The construction technology of shield arriving in water refers to the construction technology that，in order to prevent or control the engineering danger caused by the large amount of groundwater and soil gushing out of the open tunnel circle during the shield arriving，the water and soil of the shield arriving well is backfilled actively by using the mechanism that the water and soil pressure balance inside and outside the arriving well can be controlled and the shield is pushed into the arriving well safely under the condition of water and soil pressure balance.
Fig.2.3 Construction technology of arrival in water
Fig.2.4 Steel sleeve receiving technology
The steel sleeve receiving technology adopts the main design principle of the steel sleeve receiving shield，which is a steel sleeve structure that can completely cover the shield machine body in the shield well.The soil pressure condition of the normal excavation of the tunnel is simulated in the steel sleeve，and the normal tunneling pressure is established in the process of shield breaking the door，and the door ring is closed to the steel sleeve to prevent water and soil from pouring into the shield well during the hole exit，so as to ensure the safety of shield receiving.The construction method has the advantages of safe construction，short construction period and reusable，which has become an effective measure for shield safety reception.