Study on the Division of the Affected Zone under Construction Unloading and Its Construction Sequence of the Multiline Parallel River-Crossing Pipe Jacking

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Introduction
Because of many advantages such as the little impact on the environment, the small construction area, the fast construction speed, and the high degree of mechanization, slurry balance pipe jacking has become an important construction method of underground pipeline construction through rivers, roads, buildings, etc. [1][2][3][4]. Due to the factors of the topographical conditions, functional requirements, and others, there are more and more multiline parallel pipe-jacking projects with small clear distance D and thin overburden thickness Hs, such as four parallel pipe jacking under the Guan River [5,6] and three-hole parallel adjacent jacking in the drainage project of Meilan International Airport [7]. Te construction process of multiline parallel pipe jacking is the mechanical process in which an incomplete structure with a gradual change in geometric shape and material properties is subjected to the change in construction loads in time and space. Early construction pipe jacking will impact on the surrounding environment of postconstruction pipe jacking by infuencing the displacement and stress of its surrounding strata, while postconstruction pipe jacking will impact on early construction pipe jacking that has been completed during the construction process [8][9][10][11][12]. Te repeated cross infuence of multiple construction for multiline parallel pipe jacking will make the stress repeatedly adjusted and form the stress feld closely related to the spatial distribution of pipe jacking with a signifcant space-time efect and group-hole efect [13,14]. Tey have important theoretical signifcance and engineering application value to explore the self-stability ability of the surrounding strata during the construction of multiline parallel pipe jacking and defne the reasonable construction sequence.
Te main methods for studying on the surrounding stratum self-stabilizing capacity and the deformation characteristics of the multiline parallel pipe-jacking construction include the theoretical analysis method [15][16][17][18][19], model test method [20,21], in situ test method [22,23], and numerical computational method [24][25][26][27][28][29]. At present, the numerical calculation method has become an increasingly important tool in urban underground engineering research due to its advantages of the high speed, high accuracy, low cost, time-saving and labor-saving, and strong adaptability to complex conditions and processes. Te fnite element strength reduction method (FESRM) is one of the numerical calculation methods. Because of many advantages of the strict mechanical basis, such as being quantifable, visible, dynamic, and intuitive, FESRM has been widely used in the self-stability evaluation of surrounding rock stability of tunnels and underground projects in recent years [30][31][32][33][34][35][36].
At present, the research on the multiline parallel pipejacking construction mainly focuses on the deformation of surrounding strata and pipe-jacking segments, construction parameters, and other aspects. Te research on the subject of the afected zone and construction sequence is mainly carried out in combination with specifc projects, which is not systematic. Combined with engineering construction practice of the multiline parallel pipe-jacking project for the north city drainage and food control project through Chu River in Hefei China, the strength reduction method has been utilized to study the surrounding stratum FOS for pipejacking construction unloading under diferent overburden thickness Hs, diferent clear distance D, and diferent river water depth H w . Te afected zone division idea and the construction sequence determination method of multiline parallel pipe-jacking construction are summarized and improved in this paper. Te research results provide a case support and the theoretical basis for the multiline parallel river-crossing pipe-jacking construction with the thin overburden thickness and the narrow clear distance.

Mechanical Mechanism of Multiline Parallel Pipe-Jacking
Construction. Te construction method of slurry balance pipe jacking is carried out by using the pipe-jacking machine located in the starting well through the main pushing system. Te cutter head of the pipe-jacking machine cuts the stratum in front of the pipe-jacking face and then discharges the stratum soil through the mud treatment system and transportation system. At the same time, under the continuous pushing of the main pushing system, the subsequent segments are pushed following the pipe-jacking machine one by one, until reaching the receiving well. In addition, the drag reduction slurry shall be properly injected behind the pipejacking segment according to the actual situation, and the whole pipe-jacking construction process is completed. Te acting force acted on surrounding strata during pipe-jacking construction mainly includes the jacking force acting on the position of the pipe-jacking face and the frictional force between the pipe-jacking segment and the surrounding strata, as shown in Figure 1. When the pipe-jacking project is under normal construction, the stratum stress state near the pipejacking head in front of the machine is extremely complex due to the efect of the pipe-jacking force. Te stratum stress state in front of the pipe-jacking machine increases due to the extrusion efect of the front additional thrust of the pipejacking face. After the pipe-jacking machine passes, as the outer diameter of the pipe-jacking machine is slightly larger than the outer diameter of the pipe segment, the stratum loss is formed between the subsequent pipe segment and the surrounding strata. Te stratum moves towards the gap, and then, stress difusion occurs. Simultaneous grouting behind the pipe segment causes the surrounding strata to be encapsulated. When pipe-jacking construction is completed, the surrounding stratum consolidation settlement will produce under the action of weight. During the whole process of pipejacking construction, the stress states of the pipe-jacking strata around are in a continuous dynamic change, which causes the continuous change of stratum displacement. When the overburden thickness is thin, the deformation of the strata around the pipe extends to the surface and causes surface deformation.
When the clear distance between the early and the later stages of pipe-jacking construction is large, the stratum disturbance during the early stage has no impact on the later stage; meanwhile, the disturbance of the strata during the later stage has no impact on the early stage. When the clear distance between the early and the later stages of pipejacking construction is small, the later stage of pipejacking construction is completed in the strata afected by the disturbance of the early stage. At the same time, the stratum disturbance generated by the later stage has an impact on the early stage. Te mutual infuence degree becomes stronger as the clear distance decreases. When the clear distance between multiple parallel pipe-jacking is small, the change in the stratum stress state has been caused by the second pipe-jacking construction generating additional stress on the surrounding stratum of the frst pipe jacking, which will change the stress and displacement. Tis efect is greater on the side close to each other and less on the side far away from each other, as shown in Figure 2(a). Te third pipe-jacking construction has the same phenomenon, which generates the new equilibrium state and reacts on the frst pipe jacking, the second pipe jacking, and the third pipe jacking through the stress state. Among them, the impact between two jacking pipes is greater when they are close to each other and less when they are far away from one side, as shown in Figure 2(b). In this case, three construction sequence problems arise (see Figure 3). When there are 4 pipejacking operations, the relative position relationship between pipes increases to 6 forms. When there are npipejacking operations, the relative position relationships between pipes increase to C 2 n forms.

Erection of workings
Frictional resistance Head-on resistance Pipe jacking Pipe jacking machines Trust force Figure 1: Mechanical principle of pipe-jacking construction. 2 Advances in Civil Engineering

Project
Overview. Te north city drainage and food control project is located in Hefei, China. Te total length of the project line is 7015 m. Y25-Y26 of this project pipeline network crosses the main channel of Chu River. In this section, the width of the river channel is 55 m, the width is 25 m, the embankment height is 9.46 m, the slope is 45°, and the river depth is in the range of 2.0∼5.0 m, as shown in Figure 4(a). Slurry balance mechanical pipe-jacking construction is adopted. Tree circular reinforced concrete pipes with an inner diameter of 3.0 m and an outer diameter of 3.6 m are used for pipe jacking. Te clear distance between two adjacent pipe jacking is 2.8 m. Te stratum where pipe jacking crosses is silty clay. Te thickness of the pipe-jacking covering layer in the riverbed section is 3.0 m, as shown in Figure 4(b). Te vertical horizontal distance from the riverside of the pipe-jacking working shaft to the edge of the valley is about 10 m. Te project site belongs to the undulating plain landform of Jianghuai, and the microgeomorphology is the hillock with a concave groove. Te regional geological structure belongs to the southern edge of the North China Plateau, and the secondary tectonic unit belongs to the Hefei Basin. Te stratum distribution from top to bottom is as follows: fll soil, 0.5-1.5 m; silty chalky clay, 1.2-2.7 m; clay, 5.9-7.5 m; chalky clay, 4.8-6.0 m; and strongly weathered mudstone, 1.7-2.6 m, and the following are medium and slightly weathered mudstones. Te physical and mechanical parameters of each stratum are shown in Table 1.

Study on the Division of the Affected
Zone under Construction Unloading of Multiline Parallel River-Crossing Pipe Jacking

Finite Element Strength Reduction
Method. Due to the high aspect ratio, the problem of multiline parallel pipejacking construction unloading can be considered as a plainstrain one. Te failure complies with the Mohr-Coulomb criterion which can be expressed as shown in the following formula: where I 1 is the frst invariant of the stress tensor, J 2 is the second invariant of the partial stress tensor, c is cohesion, φ is the angle of internal friction, and θ σ is Lodder's angle. Te fnite element strength reduction method (FESRM) is a method that combines the strength reduction technique, the principle of ultimate equilibrium, and the principle of elastic-plastic fnite element calculations. First, the state of force and deformation under the original parametric working condition of the stratum was calculated. Ten, the stratum strength parameters c and φ were simultaneously discounted according to equation (2) to obtain a new set of strength parameters c′ and φ′, and they were used as the new material strength parameters for calculation. Finally, the calculation was carried out by continuously adjusting the  Advances in Civil Engineering discount factor k until the surrounding strata were in ultimate equilibrium, and the critical rupture surface was obtained; at the same time, the discount factor k of the material was FOS. Te calculation principle of FESRM [23] is shown in Figure 5: where k is the discount factor.

Computational Model.
Finite element numerical calculation software of Midas GTS NX was utilized to study the surrounding stratum self-stability characteristics of the pipe-jacking construction. Te numerical calculation was based on the plane strain problem. Pipe jacking took a circle with an outer diameter of 3.6 m. Te distance between the left and the right boundaries of the calculation model was more than 3 times of the pipe-jacking outer diameter, and the horizontal displacement constraint was applied. Te distance between the lower boundary and the bottom of the  calculation model was more than 3 times of the pipe-jacking outer diameter, and the vertical displacement constraint was applied to the lower boundary, with the upper surface free. Te model boundary calculation cell grid was set to 1.0 m longitudinally and horizontally, and the pipe perimeter grid was 0.5 m, as shown in Figure 6. Te calculation used the DP4 equivalence Mohr-Coulomb yield criterion. Jacking construction unloading took place in the one-time fullsection excavation. Te initial stress took into account the self-weight of the ground soil and river water pressure, and no other construction process factors were considered. Numerical calculations of the physical and mechanical parameters of the strata, the dimensions of pipe jacking, and the relative position relationships were based on dependent engineering parameters as basic data and were extended to the general case on this basis. Te method of controlling single variable analysis was adopted while discussing the self-stability characteristics of the unloading stratum during pipe-jacking construction; that is, when one calculation parameter changed, other calculation parameters remained unchanged.

Study on the Surrounding Stratum Self-Stability Characteristics of the Single-Line Pipe-Jacking Construction.
In this study, H w was taken as 2.0 m, 5.0 m, and 8.0 m, respectively, and the stratum of pipe-jacking crossing was silty clay stratum. Te FOS calculation results of the pipe-jacking construction surrounding strata under diferent Hs are shown in Table 2.
From Table 2, it was shown that the overall change trend of FOS increased frst and then decreased with an increase in Hs under the same conditions of other factors (see Figure 7). It illustrated the problem of the surrounding stratum selfstabilizing ability for pipe-jacking construction increased frst and then decreased with an increase in Hs. Te fundamental reason for the above phenomenon was that the self-stability characteristics of the pipe-jacking construction surrounding strata were mainly determined by the relative relationship between the stratum strength itself and the surrounding stratum redistribution stress caused by pipeline construction unloading. Te surrounding stratum redistribution stress by pipeline construction unloading was positively related to the stratum initial stress itself under the same conditions of other factors. Te stratum initial stress was composed of the efect for stratum self-weight and for the river water self-weight. With an increase in stratum depth, the efect of the former on the stratum initial stress gradually increased, while the latter gradually decreased. Tus, the stratum initial stress decreased frst and then increased with an increase in stratum depth. Furthermore, the surrounding stratum redistribution stresses the same rule. Te stratum strength itself was basically unchanged. Terefore, the ratio of stratum strength to the surrounding stratum redistribution stress increased frst and then decreased.
FOS could quantitatively evaluate the degree of surrounding stratum self-stability by pipe-jacking construction unloading. Taking FOS � 1.25 as limitation, the calculation results of the surrounding stratum minimum critical overburden thickness Hs min d, the maximum critical overburden thickness Hs max r, and the single-line pipe-

Advances in Civil Engineering
jacking construction under diferent river water depth H w are shown in Table 3. Te mathematical ftting equations between Hs min H w and Hs max H w are shown in equations (3) and (4), respectively: Te basic goal of an excellent underground space development plan was to make full use of the stratum selfstability ability, minimize the engineering auxiliary measures, and pay attention to the economy while ensuring safety. Te realization of this goal mainly depends on the geological conditions of the site, the understanding of these geological conditions, and the ability to use them as design.
Taking Hs min Hs max as the upper and the lower limitation, respectively, the surrounding stratum self-stability distribution zone of single-line pipe-jacking construction unloading is shown in Figure 8. For the specifc environmental conditions of pipe-jacking construction, there are many advantages for the pipe-jacking vertical section that was designed in the surrounding stratum self-stability distribution zone, such as reducing the construction safety risk and construction difculty.

Study on the Surrounding Stratum Self-Stability Characteristics for the Two-Line Parallel Pipe-Jacking Construction.
In this study, H w was taken as 2.0 m, 5.0 m, and 8.0 m, respectively, Hs was taken as 3.0 m, and the stratum of pipejacking crossing was the silty clay stratum. Te FOS calculation results of the pipe-jacking construction surrounding strata under diferent D are shown in Table 4.
From Table 4, it was shown that the overall change trend of FOS increased frst and then decreased with an increase in D under the same conditions of other factors (see Figure 9). With an increase in D, the potential failure pattern of the surrounding strata for double-line parallel pipe-jacking construction unloading transited from the double pipejacking overall collapse to the gradual separation collapse, and the fnal collapse pattern was consistent with that of single pipe-jacking (see Figure 10). It was demonstrated that when D was small, the mutual infuence was strong of double-line parallel pipe-jacking construction unloading. With D increased, the mutual infuence was gradually weakened, and when D increased to a certain extent, it would be no longer afecting each other.
Taking FOS � 1.25 as the limitation, the calculation results of the surrounding stratum minimum critical clear distance D min for the double-line pipe-jacking construction under diferent buried depths Hs are shown in Table 5. Te mathematical ftting equation among the minimum critical clear distance D min , the overburden thickness Hs, and the river water depth H w of double-line parallel pipe-jacking construction unloading is shown in equation (5). Te spatial feature distribution map among them is drawn in Figure 11, which provided the theoretical basis for the cross-sectional design of the multiline parallel pipe-jacking construction: 3.4. Study on the Surrounding Stratum Self-Stability Characteristics for the Tree-Line Parallel Pipe-Jacking Construction. In this study, H w was taken as 2.0 m, 5.0 m, and 8.0 m, respectively, Hs was taken as 3.0 m, and the stratum of pipe-jacking crossing was the silty clay stratum. Tree working conditions were taken for the construction sequence, which were 1# ⟶ 2# ⟶ 3#, 2# ⟶ 1# ⟶ 3#,    Advances in Civil Engineering and 1# ⟶ 3# ⟶ 2# (see Figure 9). Te FOS calculation results of the pipe-jacking construction surrounding strata under diferent D are shown in Table 6. From Table 4, it was shown that with the pipe-jacking number increased, FOS tends to decrease gradually on the whole, and the smaller the D was, the more signifcant the trend was. When the pipe-jacking construction was completed, FOS was not afected by the construction sequence. However, when the sequence was diferent, FOS in the construction process was diferent (see Figure 12). Te overall change trend of FOS increased frst and then decreased with an increase in D under the same conditions of other factors (see Figure 13). With an increase in D, the potential failure pattern of the surrounding strata for threeline parallel pipe-jacking construction unloading transited from the double pipe-jacking overall collapse to the gradual separation collapse, and the fnal collapse pattern was consistent with that of single pipe jacking (see Figure 14). It was demonstrated that when D was small, the mutual infuence was strong of three-line parallel pipe-jacking construction unloading; with D increased, the mutual infuence was gradually weakened, and when D increased to a certain extent, it would be no longer afecting each other.
Taking FOS � 1.25 as the limitation, the calculation results of the surrounding stratum minimum critical clear distance D min for the three-line pipe-jacking construction under diferent buried depths Hs are shown in Table 7. Te mathematical ftting equation among the minimum critical clear distance D min , the overburden thickness Hs, and the     river water depth H w of double-line parallel pipe-jacking construction unloading is shown in equation (6). Te spatial feature distribution map among them is drawn in Figure 15:

Dividing Mutual Infuence Zones of Multiline Parallel
Pipe-Jacking Construction Unloading. Te surrounding stratum self-stability degree of multiline parallel pipejacking construction unloading could be determined quantitatively by FOS, and the mutual infuence degree of them could be determined quantitatively by the change in FOS. For the convenience of expression, when the multiline parallel pipe-jacking construction has been completed, FOS was uniformly called FOS-M. Meanwhile, the single-line pipe-jacking construction has been completed, and FOS was uniformly called FOS-S. Taking FOS � 1.25 as the limitation, based on the surrounding stratum self-stability degree and the mutual infuence degree, the surrounding stratum zone of the multiline parallel pipe-jacking construction unloading could be divided.
When FOS-M ≤ FOS-S < 1.25, it meant that the mutual infuence would occur during multiline parallel pipe-jacking construction unloading, and the surrounding stratum zone       met the self-stability requirement. Tus, the surrounding stratum zone was called the mutual infuence selfstability zone. When FOS-M � FOS-S > 1. 25, it meant that the mutual infuence would not occur during multiline parallel pipejacking construction unloading, while the surrounding stratum zone met the self-stability requirement. Tus, the surrounding stratum zone was called the nonmutual infuence self-stability zone.

Study on the Construction Sequence of the
Multiline Parallel River-Crossing Pipe Jacking 4.1. Basic Assumptions. Te stratum initial stress had been changed by pipe-jacking construction, which led to a series of complex physical and mechanical efects. It was not only related to the physical properties of the stratum but also closely related to the construction method, construction process, and relevant construction parameters. When using numerical software for calculation, it was difcult to take all factors into account to completely refect the construction process. Terefore, proper simplifcation should be carried out during numerical calculation. It could not only meet the requirements of calculation software but also make the numerical calculation result refect the construction process well. According to the actual situation of the supporting project, the numerical calculation was based on the following assumptions in this paper: (1) All strata were homogeneous, continuous, and isotropic ideal elastic-plastic material. Te infuence of groundwater is ignored in the calculation process. Te initial stress only considers the stratum selfweight stress. Stratum settlement was only considered due to pipe-jacking construction, and the consolidation settlement was ignored. (2) Jacking pressure of the pipe-jacking excavation surface was applied to the whole excavation surface in the form of a circular uniformly distributed load. Te pressure was taken as the lateral static earth pressure at the center of the pipe-jacking excavation surface. According to the project actual situation, the pressure was 88.5 kpa in this paper, as shown in Figure 16(a). (3) Te infuence of grouting pressure on the construction process was not considered. Te grouting unit around pipe jacking was distributed along the radial and equal thickness of the segment. Te elastic modulus was taken as 1/50 of the original formation unit, and the thickness of the grouting equivalent layer was 2 cm, as shown in Figure 16(b). (4) Te pipe joint material was an isotropic linear elastic body, and the indirect head efect of a pipe joint was ignored. Te friction resistance between pipeline and stratum acted on the outer surface of the pipe casing and the inner surface of the soil around the pipe, with the same size and opposite direction. Te frictional resistance was a certain value and evenly distributed along the pipe-jacking direction. Te frictional resistance was achieved by setting a friction coefcient on the contact surface, and the value was 3.5 kpa in this paper, as shown in Figure 16(c).

Calculation Model and Implementation Process.
Te length, the width, and the height of the numerical calculation model for the multiline parallel pipe-jacking construction were 80 m, 80 m, and 30 m, respectively. Te left and right boundaries of the calculation model were subject to horizontal displacement constraints, the lower boundary was subject to vertical displacement constraints, and the upper surface was free. Te pipe-jacking segment was regarded as an elastic material, and the thickness was 0.30 m. Te shell of the pipe-jacking machine adopted the linear elastic model. Te material property adopted a 2D plate element, and the thickness was 0.06 m, as shown in Table 8. Te material properties of strata and segments adopted 3D unit entities, which were divided into meshes by geometry and then expanded by 2D meshes. Te calculation unit grid around pipe jacking was set to 0.5 m in both vertical and horizontal directions, while the model boundary was 2.0 m. Te values of the stratum physical and mechanical parameters are shown in Table 1, and the calculation model is shown in Figure 17.
Te numerical calculation process of the multiline parallel pipe-jacking construction was divided into the following stages: pipe-jacking excavation, pipe-jacking advance, and segment application. Te specifc steps were as follows: (1) Te initial boundary conditions of the model and the stratum self-weight were applied, the initial displacement was cleared, and the analysis of the initial stress feld was started. (2) Te jacking pressure of the pipe-jacking face was applied, the function of the stratum excavated by pipejacking construction was deactivated, and the friction resistance was activated; at the same time, the casing of the pipe jacking machine was activated. (3) Te shell of the pipejacking machine was deactivated, the pipe-jacking machine was jacked, and the pipe-jacking segment was applied. (4) Te stratum of the next pipe segment was excavated, and the above steps were repeated.

Calculation and Analysis.
Tere were diferences in the degree of disturbance to the surrounding strata caused by the diferent jacking sequences of the multiline parallel pipejacking construction, which made the stratum displacement diferent. It was very important to control the stratum displacement by relying on the pipe-jacking project passing through the river channel. Te cumulative vertical displacement of the stratum under various working conditions is shown in Figure 17. Displacement monitoring points were set at the river bottom surface section at the river center, which was perpendicular to the pipe-jacking axis. Te surface displacement monitoring points directly above 1#, 2#, and 3# pipe jacking were marked as S1, S2, and S3 in turn (see Figure 18).
From Figure 19, it was shown that the overall law of stratum displacement at S1, S2, and S3 points in the construction steps under diferent construction sequences was basically consistent. On the whole, the stratum uplifted in the front of the pipe-jacking cutter head and subsided in the rear. Te former was caused by the jacking force of the pipejacking face, and the latter was caused by the formation loss caused by the cutter head of the pipe-jacking machine slightly larger than the outer diameter of the segment. Te mutual infuence of stratum displacement was very strong caused by the three-line parallel pipe-jacking construction.
Te calculation results of the pipe jacking in the construction sequence of "1# ⟶ 2# ⟶ 3#" were taken as examples for detailed explanation. During the construction of 1# pipe jacking, with the distance between the cutter head and the river center continuously approaching, the stratum displacement at S1, S2, and S3 was continuously uplifted and the value gradually increased. When the cutter head was pushed directly below the river center, the uplift of the stratum displacement at S1, S2, and S3 reached the maximum. When the cutter head passed through the river center, the stratum displacement at S1 rapidly sank to the negative value (settlement), and the displacement at S2 slightly sank. With the distance between the cutter head and the river center getting further away, the stratum displacement at S1, S2, and S3 was becoming slower and tending to be stable. During the construction of 2# pipe jacking, with the distance between the cutter head and the river center continuously approaching, the stratum displacement at S2 and S3 was continuously uplifted and the value gradually increased, while S1 showed the rising trend, but the value was small. When the cutter head was pushed directly below the river center, the uplift of the stratum displacement at S2 and S3 reached the maximum, while S1 rose to the maximum value. When the cutter head passed through the river center, the stratum displacement at S2 rapidly sank to a negative value (settlement), while S1 and S3 slightly sank. With the distance between the cutter head and the river center getting further away, the stratum displacement at S1, S2, and S3 was   becoming slower and tending to be stable. 3# pipe-jacking construction had little impact on the stratum displacement at S1. During the construction of 3# pipe-jacking, with the distance between the cutter head and the river center continuously approaching, the stratum displacement at S3 was continuously uplifted and the value gradually increased, while S2 showed the rising trend, but the value was small. When the cutter head was pushed directly below the river center, the uplift of the stratum displacement at S3 reached the maximum, while S2 rose to the maximum value. When the cutter head passed through the river center, the stratum displacement at S3 rapidly sank to a negative value (settlement), while S2 slightly sank. With the distance between the cutter head and the river center getting further away, the stratum displacement at S2 and S3 was becoming slower and tending to be stable. When the frst, the second, and the third pipe-jacking has been completed, respectively, in the sequence of 1# ⟶ 2# ⟶ 3#, 2# ⟶ 1# ⟶ 3#, and 1# ⟶ 3# ⟶ 2#, the calculation results of river bottom stratum deformation at the river bottom surface section in the river center, which was perpendicular to the pipe jacking axis, are shown in Figure 20.
From Figure 20, it is shown that the overall law of stratum displacement at the river center section under diferent construction sequences was basically consistent. Te mutual infuence of stratum displacement was very strong caused by the three-line parallel pipe-jacking construction. When the single-line pipe-jacking construction was completed, the horizontal surface displacement curve was approximately in a normal distribution. Te maximum settlement was located at the pipe-jacking axis, and the maximum settlement value was 18.73 mm and gradually decreased from the axis to both sides. When the double-line pipe-jacking construction was completed and the clear distance D was narrow (in the sequence of 1# ⟶ 2# or 2# ⟶ 1#), the horizontal surface displacement curve showed the partial v-shaped distribution, and the surface settlement maximum value was 25.10 mm, which was located between two pipe jacking and in the pipe-jacking side that was constructed frst. When the clear distance D was wide (in the sequence of 1# ⟶ 3# or 3# ⟶ 1#), the horizontal surface displacement curve showed the partial wshaped distribution. When the three-line parallel pipejacking construction was completed in the sequence of 1# ⟶ 2# ⟶ 3#, 2# ⟶ 1# ⟶ 3#, and 1# ⟶ 3# ⟶ 2#, the

Construction Sequence.
Tere was no construction sequence problem of the multiline parallel pipe-jacking in the mutual infuence non-self-stability zone and the nonmutual infuence self-stability zone. Te impact of the multiline parallel pipe-jacking construction in the mutual infuence self-stability zone on the surrounding stratum was not that the simple superposition of the individual single-line pipejacking construction. Early construction pipe jacking would impact on the surrounding strata of postconstruction pipe jacking by infuencing the displacement and stress, while postconstruction pipe jacking would impact on early construction pipe jacking that has been completed during the construction process. Diferent construction sequence had diferent impacts on the surrounding strata. Pipe-jacking construction should avoid or reduce the mutual infuence of the adjacent pipe-jacking construction as much as possible. Terefore, the reasonable selection of the pipe-jacking construction sequence was related to the safety, efciency, and even the success or failure of the project construction to a certain extent. Te construction sequence scheme of the multiline parallel pipe-jacking construction for the north city drainage and food control project through Chu River was established according to the following steps. First, we ensure that technology was feasible, safe, and reliable. Ten, the characteristics of stratum deformation caused by the multiline parallel pipe-jacking construction were comparatively analyzed, and the working condition with the minimum stratum deformation was taken. Ten, comparative analyses were carried out for construction organization, construction period, project economy, and other factors. Finally, the jacking sequence of "1# ⟶ 3# ⟶ 2#" as the optimal sequence was confrmed. Te project was ofcially launched on June 10 in 2022, and the construction was successfully

Conclusions
(1) It was revealed that the variation law of the surrounding stratum self-stability characteristics for single-line pipe-jacking construction unloading varies with the overburden thickness Hs. Te surrounding stratum minimum critical overburden thickness Hs min and the maximum critical overburden thickness Hs max of single-line pipe-jacking construction unloading were obtained. Te surrounding stratum self-stability distribution zone of single-line pipe-jacking construction unloading was drawn, which provided the theoretical basis for the vertical-sectional design of river-crossing pipe jacking with overburden thickness Hs drastic change.
(2) It was revealed that the variation law of the surrounding stratum self-stability characteristics for multiline parallel pipe-jacking construction unloading varies with the clear distance D. Te mathematical ftting equations among the minimum critical clear distance D min , the overburden thickness Hs, and the river water depth H w of multiline parallel pipe-jacking construction unloading were obtained. Te spatial feature distribution map among the minimum critical clear distance D min , the overburden thickness Hs, and the river water depth H w was drawn, which provided the theoretical basis for the cross-sectional design of the multiline parallel pipe-jacking construction. (3) Based on the surrounding stratum self-stability degree and the mutual infuence degree of multiline parallel pipe-jacking construction unloading, the surrounding stratum zone was divided into the mutual infuence nonself-stability zone, the mutual infuence self-stability zone, and the nonmutual infuence self-stability zone. (4) Combined with the relying project, the construction sequence of "1# ⟶ 3# ⟶ 2#" was concluded to be the optimal construction sequence through the comparative analysis of the ground deformation characteristics caused by the multiline parallel pipejacking construction and the combination of various factors such as engineering safety, economy, and convenience.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.