Summary:When the subway shield section passes through the deep buried sewage pipeline, due to the loss of soil layer, it will cause a certain degree of pipeline sinking and deformation, which will affect the safety of pipeline operation, and then affect the implementation and operation safety of the subway section tunnel. Through calculation and analysis, the impact of shield tunneling on the pipeline is quantified, so as to determine the control indicators that need to be taken for subway construction and the necessary relocation and modification scope of sewage pipelines. For large-diameter sewage pipelines, strict calculations and special designs must be carried out as a risk source in the early design of the subway to minimize the interaction between the subway and the sewage pipeline. 1.OverviewWuxi Metro Line 4 Phase I Project Shield Tunnel along the side and under the deep buried sewage main and the current shield construction sewage pipe caisson, and the shield section jacking process will lead to soil settlement, thereby causing the sewage pipeline and caisson settlement, and then lead to pipeline deformation. If the deformation exceeds the bearing capacity of the flexible interface of the pipeline, it will lead to the misalignment and leakage of the sewage pipe connection. Therefore, in the early design of Wuxi Metro Line 4, special calculations and studies were carried out on the deep-buried sewage main and caisson along the line, and from the perspective of ensuring the safe implementation of construction, the rationality of the subway scheme and the necessity and rationality of pipeline relocation and modification were studied, the impact of shield tunnel construction on the deep-buried sewage main and sewage caisson was analyzed, and the reasonable and feasible construction and pipeline relocation scheme were studied to ensure the feasibility and safety of the subway implementation. 1.1 Overview of sewage pipelines along the line There are 18 stations in the first phase of Metro Line 4, and the stations are basically shield sections between them. The construction of the station and the section will affect the sewage pipe network of Wuxi Chengbei Sewage Treatment Plant, Lucun Sewage Treatment Plant, Taihu New Town Sewage Treatment Plant and other service areas from north to south. Guangshi Road, Fengxiang Road, Jianshe Road, Lixi Road, Yinxiu Road, Jincheng Road, Jinshi Road, Zhenze Road, Gonghu Avenue and other trunk roads through the subway line are all laid under the trunk pipe network of each sewage plant, with pipe diameters ranging from D800 D1650 and buried depths of 5 8 m. Among them, Liutan Station, Guangshi Road Station, Qingshi Road Station, Sheng'an Station and Huishan Guzhen Station mainly affect the main pipe network of Chengbei Sewage Treatment Plant; Siyuan Station, Helikou Station, Jianshe Road Station, Sports Center Station, Yinxiu Road Station and Lihu Park Station mainly affect the main pipe network of Lucun Sewage Treatment Works; Grand Theatre Station, Wuhu Avenue Station, Datong Road Station, Civic Center Station, Wudu Road Station, Business Center Station and Gonghu Avenue Station mainly affect the main pipe network of Taihu New Town Sewage Treatment Plant. The above main pipe network is basically a reinforced concrete pipe, and the socket interface has a small anti-deformation capacity at the interface. Part of the main pipe network adopts pipe jacking construction, and the caisson during the construction period is generally reserved for use as inspection wells. The caisson blade foot is about 2 percent lower than the bottom of the pipe5 m。The connection between the caisson and the pipeline is generally rigidly connected, and the sinking of the well will drive the synchronous sinking of the pipeline. If the downhole sinking exceeds the deformation capacity of the flexible interface at the pipe section, it will inevitably cause structural failure of the pipeline. 1.2 Shield section 12.The longitudinal section of the 1 section tunnel line adopts an energy-saving slope, that is, after the line comes out of the station, the two ends of the station maintain the same 2 slopes as the station range, and then descend with a steep slope and a smooth gentle slope, and then ascend with a gentle slope and a steep slope, and the longitudinal slope of the interval tunnel is V-shaped. The buried depth of the top surface of the shield section structure is 9 20 m. The longitudinal slope of the interval line is shown in Figure 1. 
Figure 1 Schematic diagram of the longitudinal slope of the section line According to the above slope change and elevation relationship, the obvious conflict between the section and the sewage pipeline is basically near the two ends of the station. 1.2.2. The interval tunnel structure adopts reinforced concrete single-layer segmentation with an inner diameter of 55 m, segment outer diameter 62 m, segment width 12 m。1.3 The relationship between the tunnel and the sewage pipe in the subway section is small, and the height difference between the shield and the sewage main in the first phase of the first phase of Metro Line 4 is small, including the following sections (see Figure 2). (1) Guangshi Road Station - Qingshi Road Station. At the place near Qingshi Road Station, it crosses the D1000 sewage pipe of the main passage of the Chengbei Factory, and the elevation of the bottom of the sewage pipe is about -15 m, buried at a depth of about 45 m。Due to the intrusion of the sewage pipe into the station, it has been permanently relocated to the outside of the fence before the construction of the station, and the relocated sewage pipe will be passed down during the construction of the shield section, and the elevation of the center of the tunnel at the crossing is about -98 m, the clear distance between the tunnel and the pipeline is about 52 m。The relative relationship between the two is shown in Figure 3 and Figure 4. 
Figure 2 The relationship between the tunnel and the sewer pipe in some sections
Fig.3 Plane relationship diagram of the sewage pipeline under the Guangqing section
Fig.4 Profile relationship diagram of the sewage pipeline under the Guangqing section (2) The section between Sports Center Station and Yinxiu Road Station. Underpass the D1000 sewage pipe of the main channel of the current Lucun plant, and the elevation of the bottom of the pipe is -180 m, the top of the interval structure is -459 m, spacing 279 m。(3) Yinxiu Road Station - Lihu Park Station section. Underpass the D1350 sewage pipe of the main channel of the current Lucun plant, and the elevation of the bottom of the pipe is -376 m, the top of the interval structure is -565 m, spacing 189 m。(4) Business Center Station - Gonghu Avenue Station. Underpass the D1200 sewage pipe of the main channel of the current Taihu New Town Plant, and the elevation of the bottom of the pipe is -337 m, the top of the interval structure is -609 m, spacing 270 m；In addition, the sewage pipe is a pipe jacking construction, and the caisson doubles as an inspection well during operation. The caisson blade is about 1 percent from the top of the rail40 m (see Figure 5, Figure 7). 
Fig.5 Plane relationship between the tunnel and the sewage pipe in the Shanggong section
Fig.6 Diagram of the vertical relationship between the tunnel and the sewage pipe in the Shanggong section
Fig.7 Diagram of the relationship between the tunnel and the sewage pipe caisson in the Shanggong section2.Interval crossing sewer control standardsDue to factors such as the uneven settlement of the foundation, the current sewage pipeline should have produced a certain amount of deformation, and it is difficult to determine the size of the amount. For the sake of prudence, the amount of deformation that has been generated should be fully estimated when calculating the impact of the interval on the sewer pipeline. There are no clear provisions on the allowable deformation of sewage pipes. The Code for Construction and Acceptance of Water Supply and Drainage Pipeline Engineering (GB 50268 2008) stipulates the maximum deflection angle of the curved section encountered during pipeline laying, which shall be regarded as the maximum allowable deflection angle of the flexible interface of the pipeline (see Table 1). Understood in this way, the resulting displacement deviation corresponding to the deflection angle can be regarded as the maximum allowable deformation of the pipe. For example, for the D1200 reinforced concrete pipe in the Shanggong section, see Table 1, the maximum allowable deflection angle during installation is 1°. According to the section of 2 m of the pipe section, the corresponding maximum deformation is 349 mm, which can be used as the allowable settlement control value of the sewage pipe. Considering the completed settlement, the maximum settlement of the sewage pipe during the construction period of the interval shield is controlled by no more than 10 mm. The following takes the Shanggong interval as an example to calculate and analyze the impact of shield on sewage pipes. Table 1 specifies the permissible rotation angle for mounting interfaces along curves
3.Analysis of the impact of shield tunneling on sewage pipes3.1. The impact analysis of the shield tunnel through the deep buried sewage pipe during the construction of the interval, the factor that causes the settlement and deformation of the sewage pipeline is mainly the soil loss caused by the shield jacking, and the deformation is proportional to the soil loss, so if you want to control the deformation, you must control the soil loss rate. The relationship between the deformation of the pipeline and the soil loss rate was analyzed, so as to find out the soil loss rate corresponding to the settlement of 10 mm of the sewage pipeline. The following are according to the soil loss rate of 05% and 035% are checked. 3.1.1. The rate of soil loss is 0The 5% control was calculated using the finite element model of plane strain. In the calculation, the two-dimensional finite element model is established with PLAXIS, and the soil is regarded as a hardened elastoplastic model (HS). The calculation simulates the construction conditions with full support applied to the bottom of the geometry and vertical sliding constraints on both sides of the model, while the surface of the model is treated as a free boundary (see Figure 8). 
Fig. 8 The finite element model calculates that the maximum vertical displacement of the soil is 1774 mm, the maximum vertical displacement of the sewage pipe is 1398 mm (see Fig. 9, Fig. 10). 
Fig. 9 Vertical displacement cloud of soil during double-line penetration.
Fig. 10 After the tunnel is excavated, the stress release and rebound deformation of the surrounding rock of the tunnel will cause the ground subsidence. It is calculated that the maximum differential settlement rate of the pavement after the breakthrough of the double-lane tunnel is 022%, based on the length of each sewage pipe 2 m, the maximum differential settlement value of the pipe section is 44 mm (see Figure 11).
Fig. 11 Pavement settlement diagram under each construction stage From the above calculations, it can be seen that the maximum settlement value of the sewage pipeline caused by tunnel construction is about 1398 mm, the maximum differential settlement rate of the pavement after the breakthrough of the double-lane tunnel is 022%, the maximum differential settlement value for the 2 m section is 44 mm, which does not meet the limit requirements of the sewage pipe settlement standard. 3.1.2. The rate of soil loss is 0The 35% control is calculated after modeling, and the maximum vertical displacement is 1217 mm, the vertical displacement of the sewage pipe is up to 757 mm。After the breakthrough of the double-lane tunnel, the maximum differential settlement rate of the road surface is 016%, the maximum differential settlement value of the 2 m section is 32 mm。From the above calculations, it can be seen that the construction control is further strengthened to control the soil loss rate to 0Within 35%, it can meet the limit requirements of the sewage pipeline settlement standard. 3.2. Analysis of the impact of the shield tunnel crossing the sewage pipe caisson using the same model and calculation principle to analyze the impact of the shield tunnel crossing the sewage caisson in the Shanggong section (see Fig. 12).
Fig. 12 The maximum vertical displacement of the left and right lines of the FE model after the construction of the tunnel is 1693 mm, which occurred at the surface soil above the cavern of the tunnel on the left line. The maximum vertical displacement of the sewage caisson is 1025 mm (see Fig. 13, Fig. 14).
Fig. 13 Vertical displacement cloud of soil during double-line penetration.
Fig. 14 Vertical displacement contour diagram of sewage pipe caisson when double lines are penetratedThe above calculation shows that the loss rate in the soil layer is controlled to be 0In the case of 5%, the maximum settlement value of the sewage pipe caisson caused by the shield construction of the Shanggong section tunnel is about 1025 mm, which cannot meet the deformation bearing capacity of sewage pipes. In order to ensure the safety of construction, it is necessary to further strengthen the construction control and reduce the soil loss rate, so as to reduce the impact of shield construction on sewage pipes and caissons. 3.3. After the above calculation and analysis, the loss rate of the soil layer is 0In the case of 5%, the settlement deformation of the D1200 sewage pipeline and its caisson caused by the shield construction in the Shanggong interval exceeds the allowable deformation. In this regard, the method of further strengthening construction control and reducing the soil loss rate can be adopted, so as to reduce the impact of shield construction on sewage pipes and caissons; It can also increase the relocation and modification scope of sewage pipes and caissons to reduce the investment caused by the increase in shield construction measures, and also further reduce the impact of sewage pipes on the tunnel section. After technical and economic comparison, Shanggong finally chose the plan to increase the scope of sewage relocation. 4.ConclusionWhen the distance between the main sewage pipe of large diameter and the shield interval is small, or when the distance between the caisson blade foot and the shield interval is small for the pipeline constructed by pipe jacking, the soil layer loss caused by the shield construction will cause the uneven settlement of the sewage pipeline; If it is not properly controlled, it will cause leakage, pollute groundwater, and affect the construction and operation safety of subway sections. Therefore, for large-diameter sewage pipelines, strict calculation and special design must be carried out as a risk source in the early design of the subway, so as to minimize the interaction between the subway and the sewage pipeline. During the construction of the subway shield, the soil loss rate is strictly controlled to reduce the settlement deformation of the sewage pipeline. From: Urban Roads and Bridges and Flood Control.