Abstract
This paper aims to address the issue of disturbance caused by excavation tunnel construction on nearby dense pipelines. Relying on the actual project of Nanchang Metro, the three-dimensional finite element numerical simulation method is used to establish a multi-condition numerical model. At the same time, key influencing factors such as the clear distance, intersection angle, surrounding rock parameter, and construction method are considered. The mechanical response of the dense pipeline adjacent to the dug tunnel was systematically studied, and the influence of key factors on the mechanical behavior of the pipeline was analyzed. The results show that when using the CRD method for construction and grouting reinforcement of the surrounding strata of the tunnel, the maximum settlement of the vault is located at the vault of the right guide tunnel, and the maximum settlement value is located at the bottom of the rainwater pipe directly above the tunnel. The vertical displacement, maximum principal stress, and minimum principal stress of the pipeline are arranged in descending order for the full-face, stepped method, and CRD method. The greater the disturbance to the soil caused by the construction method, the more unfavorable the effect on the pipeline.
1. Introduction
During the construction of urban subway systems, it is inevitable that the dense network of underground pipelines will be affected. The subway construction may cause accidents such as excessive deformation, cracking, and even damage to underground pipelines, which seriously threaten their normal service life and use. Furthermore, damage to water supply and drainage pipelines can lead to an increase in groundwater in the surrounding soil of the tunnel, posing a threat to tunnel construction.
The research methods for underground pipeline deformation mainly include theoretical calculations, numerical simulations, and model experiments. Based on the elastic foundation beam theory, Klar et al. [1-4] have derived a theoretical and analytical formula for calculating pipeline deformation caused by tunnel excavation [5]. Theoretical, analytical formulas of Klar and Vorster were revised through model experiments and discrete element tests [6]. Derived analytical formulas to accurately describe the stress and deformation of underground pipelines when subjected to arbitrary displacement load conditions [7]. Using the Pasternak foundation model, a two-stage analysis method was employed to establish the pipeline equilibrium differential equation, effectively predicting pipeline displacement resulting from water leakage during tunnel construction [8]. To predict the deformation of the existing pipeline caused by shield machine excavation, the researchers simplified the pipeline as an Euler-Bernoulli beam resting on a Pasternak foundation beam. They employed the finite difference method (FDM) and established equations to accurately estimate the deformation of the pipeline [9]. Assuming that the deformation of the upper pipeline caused by tunnel excavation conforms to a Gaussian distribution, we introduced the variational energy principle to investigate the deformation of the overlying pipeline under small clearances induced by tunnel excavation and obtained a more convenient calculation method for such deformation. With the aid of numerical simulation, Kshama et al. [10] Investigated the mobilization of uplift resistance in dense sand through finite element analysis in this study [11]. The impact of pipeline seepage on surface deformation during tunnel excavation is being investigated [12]. Based on the measured data, the impact of shallowly buried tunnel construction on the overlying pipeline is analyzed using superposition method. Large-scale grouting measures are implemented to mitigate pipeline deformation caused by tunneling activities [13]. The impact of shield tunnel sewage pipes on stratum movement under complex conditions is analyzed, and the deformation of the pipes can be effectively mitigated through gap filling during excavation [14]. The stress and deformation of grouted reinforced materials during tunnel excavation through the pipeline are analyzed comprehensively [15]. The impact of deep foundation excavation on the displacement of underground pipelines is simulated, followed by an analysis of hazardous areas and a study of the combination of risk factors that pose the greatest threat.
The aforementioned studies have advanced research on the mechanisms underlying the impact of tunnel excavation on adjacent pipelines. However, these studies have primarily focused on single pipelines, and there is a lack of research on densely clustered pipelines. Therefore, this article relies on the concealed excavation of the Nanchang Metro station entrance and exit tunnels to establish a three-dimensional finite element model for densely clustered pipelines passing underneath shallow-buried subway tunnels. By investigating the stress characteristics and deformation patterns of underground pipelines with varying depths, materials, and diameters when crossing densely clustered underground pipelines, a theoretical basis for pipeline protection has been established. Targeted pipeline protection measures have been proposed to provide a reference for the protection of similar underground pipelines in engineering projects based on the research findings.
2. Engineering Background
2.1. Project Overview
Dengbu Station on Line 3 of the Nanchang Metro is located at the intersection of Yingbin Avenue and Yangguang Road, with the route running north to south along Yingbin Avenue. Yingbin Avenue has four lanes in each direction, with one non-motorized vehicle lane on each side, while Yangguang Road has four lanes in each direction. Multiple bus routes intersect in this area, and the concealed excavation section spans the main traffic roads, as shown in Figure 1 for the horizontal position. The cross-sectional form of the tunnel structure is a straight wall arch, with an excavation width of 8.3 m and an excavation height of 6 m. In order to guarantee construction safety and control surface and pipeline deformation, the CRD (cross diaphragm) method is used for tunnel construction, as shown in Figure 2.
Within the concealed excavation tunnel scope, the following pipelines are present from east to west in the overhead direction: DN500 concrete sewage pipe at a depth of 3.55 m, DN1500 concrete rainwater pipe at a depth of 3.27 m, DN800 cast iron pipe for water at a depth of 2.5 m, and a 300 × 300 mm PVC conduit with nine openings for low-voltage electrical cables (communication cable) buried at a depth of 2.38 m. DN500 concrete sewage pipe conduit and DN1500 concrete rainwater conduit were found to be distributed beneath the existing traffic evacuation roadways. The thickness of the overburdened soil on the tunnel arch excavated by the cut-and-cover method was measured to be 4.275 m. The deepest buried utility conduit was a sewage pipe located 3.55 m below the ground level, with a minimum clear distance of 0.725 m from the tunnel excavation. The distribution of utility conduits within the construction impact zone is shown in Figure 3.
2.2. Geological Profile
Within the exploration depth, the upper layers of the site’s stratigraphy consist of artificial fill (Qml) and Quaternary Holocene alluvial deposits (Q3al). In contrast, the lower layers comprise the Eocene Nanyu Formation (Exn) bedrock. The stratigraphy is divided from top to bottom based on rock type and its engineering characteristics into the following layers: ①1 miscellaneous fill soil, ①2 plain fill soil, ③1 silty clay, ③3 medium sand, ③4 coarse sand, and ③5 grit sand, as illustrated in Figure 4.
3. The Establishment of Numerical Models
3.1. Numerical Model Size
The shallow tunnel has a total length of 23.6 m. It contains a densely packed segment of utility conduits with an overburdened soil thickness of approximately 4.2 m, where the excavation in this section is 12.8 m. A computational model was developed based on engineering data for the densely packed utility conduits and tunnels. To reduce boundary effects, the overall model was established with a length of 30 m, height of 25 m, and thickness of 12.8 m. The surrounding rock, tunnel support structure, and utility conduits were modeled using solid elements. The lateral and bottom surfaces of the model were fixed, restricting the normal displacement of these five surfaces. Meanwhile, the top surface represented the ground surface and was set as an open boundary. Based on the actual working conditions, the model is shown in Figure 5.
3.2. Parameter Selection of the Model
Based on the site investigation report, material parameters necessary for numerical simulation were selected. The numerical model classified the soil layers into miscellaneous fill, plain fill, powder quality clay, and medium sand. The specific parameters for each soil layer are presented in Table 1. Considering the situation, a linear elastic constitutive model was adopted for the underground utility model. The physical and mechanical parameters of the concrete sewage pipe, concrete rainwater pipe, PVC communication cable, and cast iron pipe for water are presented in Table 2.
The surrounding rock in the computational model is assumed to be continuous and uniform. Considering the buried pipelines’ actual geological conditions, three different sets of parameters for the surrounding rock were established. The mechanical parameters for the surrounding rock are shown in Table 3.
3.3. Numerical Calculation Scheme
Due to the factors affecting the deformation response of nearby dense pipelines during tunnel excavation, including the clear distance between the pipeline and the tunnel, the intersection angle, the surrounding rock parameter, and the construction method, each of these factors contains different levels of influence. Based on the position diagram of the tunnel and pipelines in Figure 2 and the basic engineering information, a numerical calculation model was established to analyze various factors’ influence on nearby dense pipelines’ mechanical properties. The specific calculation scheme is shown in Table 4. All working conditions do not consider additional grouting reinforcement measures. The net distances between the pipelines and the tunnel are 1.895 m for PVC communication cable, 1.725 m for cast iron pipe for water, 0.725 m for sewage pipe, and 1.005 m for rainwater pipe. Different excavation simulation methods are shown in Figure 6.
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3.4. Analysis of Feature Point Selection
3.4.1. Setting Field Monitoring Points
According to the requirement of vertical deformation monitoring, five direct monitoring points are arranged equidistant on the PVC communication cable (GXC6-1∼5), and GXC6-3 is located directly above the central axis of the tunnel. Three direct monitoring points (GXC5-1∼3) are arranged equidistant on the formation where the DN800 cast iron pipe for water is located, among which GXC5-2 is located directly above the central axis of the tunnel. Five direct monitoring points (GXC7-1∼5) are arranged equidistant on the formation where DN1500 concrete rainwater pipe is located, among which GXC7-3 is located directly above the central axis of the tunnel. Five direct monitoring points (GXC8-1∼5) are arranged equidistant on the ground where DN500 concrete sewage pipe is located, among which GXC8-3 is located right above the central axis of the tunnel, as shown in Figure 7. Construction site monitoring points are set, as shown in Figure 8. Tunnel structure construction is shown in Figure 9.
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3.4.2. Selection of Numerical Simulation Monitoring Sites
In the numerical simulation model, relevant characteristic points were selected to study the mechanical response characteristics of the pipeline, as shown in Figure 10. The vertical direction in the figure represents the tunnel, and the upper horizontal line represents the dense pipeline intersecting the tunnel at a 90-degree angle. The midpoint of the lower section of each pipeline, located at the centerline of the tunnel, was designated as the point with an X-axis value of 0. were selected on the pipeline as characteristic points for analysis, and all are located at the bottom of the pipeline. In the analysis of the mechanical response characteristics of the pipeline, the same feature points were selected as those when the crossing angle between the pipeline and the tunnel was 90° and when the crossing angles were 0°, 30°, and 60°.
4. Calculation Result Analysis
4.1. Comparative Analysis of Measured Values
Through the comparison and analysis of the actual monitoring points of each pipeline in the project and the numerical calculation results, compared with the maximum sedimentation value of measured value, PVC communication cable has an error of 1.84 mm; DN800 cast iron pipe for water has an error of 0.88 mm; DN1500 concrete rainwater pipe has an error of 0.1 mm; and DN500 concrete sewage pipe has an error of 0.8 mm. As shown in Figure 11, the errors are all very small, indicating that the numerical model is reasonable.
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4.2. Mechanical Characteristics of Tunnel Construction
4.2.1. Mechanical Properties of Surrounding Rock
According to the actual excavation method of the engineering project, the CRD method was used for excavation simulation calculation. Based on the calculation, it is found that there is a significant stress concentration phenomenon at the crown and bottom of the tunnel, where the stress at the crown increases while at the bottom, due to the excavation of the surrounding rock and the subsequent compression from other directions, the direction of stress changes. In the numerical analysis, it was found that there was significant stress concentration at the arch crown and tunnel bottom. The stress increased at the arch crown, while the stress direction changed at the arch bottom due to the squeezing from other directions after excavating the surrounding rock. The vertical displacement path of the surrounding rock coincided with the stress path, resulting in settlement of the soil above the arch bottom and the uplift of the soil at the arch bottom, presenting a “bottom heave” phenomenon. Based on the simulation results shown in Figure 12(a), the concentrated stress at the tunnel crown increased by approximately 100 kPa compared to the surrounding soil after the tunnel excavation. The concentrated stress at the tunnel invert was approximately 361 kPa. As for Figure 12(b), the maximum settlement of the tunnel crown occurred at the top of the right-side adit, which was approximately 11.9 mm, and the maximum uplift of the tunnel invert also occurred on the right side, which was approximately 12.8 mm.
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4.2.2. Mechanical Characteristics of Individual Pipelines
Pipeline deformation occurs because of soil loss caused by tunnel excavation. From the cloud diagram of the maximum principal stress of the pipeline, it can be concluded that the deformation of the pipeline is highly correlated with its maximum principal stress. The distribution law of the minimum principal stress of the pipeline is basically the same as that of the principal stress, which indicates that the deformation of the pipeline is also related to the minimum principal stress to a certain extent. After the excavation of the tunnel is completed, the maximum settlement value of each pipeline is as follows: that for PVC communication cable is 8.23 mm; that for DN800 cast iron pipe for water is 7.88 mm; that for DN1500 concrete rainwater pipe is 5.83 mm; and that for DN500 concrete sewage pipe is 10.18 mm. Cloud diagram of vertical displacement of dense pipelines after excavation is shown in Figure 13.
The maximum value of the maximum principal stress of each pipeline is as follows: that for PVC communication cable is 257.48 kPa; that for DN800 cast iron pipe for water is 26.62 MPa; that for DN1500 concrete rainwater pipe is 850.2 kPa; and that for DN500 concrete sewage pipe is 2.22 MPa. Cloud chart of maximum principal stress of dense pipelines after excavation is shown in Figure 14.
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The maximum absolute value of the minimum principal stress of each pipeline is as follows: that for PVC communication cable is 1.06 MPa; that for DN800 cast iron pipe for water is 28.77 MPa; that for DN1500 concrete rainwater pipe is 1.24 MPa; and that for DN500 concrete sewage pipe is 2.65 MPa. Cloud diagram of minimum principal stress of dense pipelines after excavation is shown in Figure 15.
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4.3. Dense Pipeline Is Affected by the Clear Distance of the Tunnel
Based on the results of scenarios 1, 2, 3, and 4 in Table 4, the vertical displacement, maximum principal stress, and minimum principal stress of the dense pipeline were analyzed to determine the effect of tunnel clearance on the response characteristics of the dense pipeline.
4.3.1. The Vertical Displacement Response Characteristics of Each Characteristic Point of the Pipelines
Figure 16 shows the maximum vertical displacement of each pipeline feature point at different net distances of 0.05D, 0.1D, 0.15D, and 0.2D. Under the same rock mass conditions, intersection angle, and excavation method, the vertical displacement of the pipeline increases as the net distance decreases.
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4.3.2. Academic Writing: The Response Characteristics of Maximum Principal Stress for Each Feature Point of the Pipeline Are Analyzed
Figure 17 shows the maximum values of the maximum principal stress at various characteristic points of the pipelines at different clear distances of 0.05D, 0.1D, 0.15D, and 0.2D. At a clear distance of 0.05D between the dense pipelines and the top of the tunnel, the maximum values of the maximum principal stress at each characteristic point of the pipelines are much higher than those at clear distances of 0.1D, 0.15D, and 0.2D because the upper soil layer of the dark excavation tunnel that each pipeline passes through has a reinforcement area consisting of grouting big pipe sheds and grouting small conduits. Under the same rock mass, intersection angle, and excavation method conditions, the maximum principal stress of the pipelines decreases as the clear distance increases.
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4.3.3. Minimum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
Figure 18 shows the maximum absolute value of the minimum principal stress for each pipeline characteristic point at different net distances of 0.05D, 0.1D, 0.15D, and 0.2D. Except for when the net distance is 0.05D, the response curves of the minimum principal stress for the water supply pipeline and other pipelines differ in shape. This is because the material of the water supply pipeline is cast iron, which has a higher stiffness, resulting in a much smaller minimum principal stress than the top and unexcavated areas on both sides.
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4.4. Effect of Cross Angle on Dense Pipeline
Using the calculation results from cases 5, 6, 7, and 8 in Table 3, the vertical displacement, maximum principal stress, and minimum principal stress of the dense pipeline were analyzed to determine the response characteristics of the dense pipeline to the intersection angle between the pipeline and the tunnel.
4.4.1. Vertical Displacement Response Characteristics of Each Pipeline Characteristic Point
The maximum vertical displacement of each pipeline at different crossing angles (0°, 30°, 60°, and 90°) under various net distances is shown in Figure 19. The maximum variation of vertical displacement for the communication cable ranging from 0° to 90° is 2.07 mm, while for the water supply pipeline, it is 4.28 mm. For the sewage pipeline, it is 3.59 mm, and for the rainwater pipeline, it is 3.59 mm. When the pipelines are parallel to the tunnel, their overall vertical displacement is approximately the same as the maximum vertical displacement. Under the same conditions of the surrounding rock, clear distance, and excavation method, the vertical displacement of the pipeline increases with decreasing crossing angle. In contrast, the crossing angle has a negligible effect on the maximum value of the vertical displacement but has a significant impact on the overall settlement of the pipelines.
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4.4.2. Maximum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
Figure 20 depicts the maximum principal stress curves of various pipeline characteristic points under different crossing angles and net clearances. The communication cable is a flexible pipeline made of PVC material. Its maximum principal stress characteristic value exhibits a slightly different variation pattern from the maximum principal stress characteristic values of cast iron water pipes, concrete sewage pipes, and rainwater pipes, as the crossing angle varies. For all pipelines, the maximum principal stress response is the smallest at a crossing angle of 0°, whereas the communication cable exhibits the maximum principal stress response at a crossing angle of 30°, while other rigid pipelines exhibit the maximum principal stress response at a crossing angle of 90°. Under the same conditions of the surrounding rock, net clearance, and excavation method, for rigid pipelines, the maximum principal stress response increases with the increase of crossing angle. In contrast, for flexible pipelines, the maximum principal stress response decreases with the increase of the crossing angle (except at 0°).
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4.4.3. Minimum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
Figure 21 depicts the minimum principal stress curves of various pipeline characteristic points under different crossing angles and net clearances. The rigid cast iron water pipe, which exhibits the maximum rigidity, shows a slightly different variation pattern in the minimum principal stress characteristic value as the crossing angle varies, compared to the minimum principal stress characteristic values of PVC communication cables, concrete sewage pipes, and rainwater pipes. For all pipelines, the minimum principal stress response remains stable at a relatively small value at a crossing angle of 0°, while the cast iron water pipe exhibits the minor minimum principal stress response at a crossing angle of 90°, and other rigid or flexible pipelines exhibit the most minor minimum principal stress response at a crossing angle of 0°. Under the same conditions of the surrounding rock, net clearance, and excavation method, for pipelines with relatively low rigidity or flexibility, the minimum principal stress response increases with the increase of crossing angle, while for pipelines with high rigidity, the minimum principal stress response decreases with the increase of crossing angle (except at 0°).
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4.5. Influence of Surrounding Rock Parameters on Dense Pipeline
The results of calculations for scenarios 9, 10, and 11 in Table 3 were selected to analyze the dense pipeline’s vertical displacement, maximum principal stress, and minimum principal stress. The objective was to determine the response characteristics of the dense pipeline concerning various parameters of the surrounding rock, including density, elastic modulus, Poisson’s ratio, cohesion, and internal friction angle.
4.5.1. Vertical Displacement Response Characteristics of Each Pipeline Characteristic Point
As shown in Figure 22, the maximum vertical displacement curves of various pipeline characteristic points are presented under different parameter rock conditions (rock parameters: type 1 < type 2 < type 3). Different types of pipelines have different values of vertical displacement caused by the construction of the tunnel excavation, but as the strength of surrounding rock increases, these differences will become smaller. Moreover, the asymmetrical settlement curve caused by the CRD method will gradually become symmetrical with the increase of rock parameters. Under the same net distance, cross angle, and excavation method conditions, the vertical displacement of the pipeline decreases with the increase of rock parameters.
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4.5.2. Maximum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
Figure 23 shows the maximum principal stress of various types of pipelines at different parameters of surrounding rock conditions (where the rock parameters are compared as type 1 < type 2 < type 3). Under the same clear distance, intersection angle, and excavation method conditions, the maximum principal stress of flexible pipelines increases as the surrounding rock parameters increase, while the maximum principal stress of rigid pipelines decreases as the surrounding rock parameters increase.
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4.5.3. Minimum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
Figure 24 shows the minimum principal stress curve of various pipeline characteristic points under different surrounding rock conditions, where the surrounding rock parameters are ranked as follows: type 1 < type 2 < type 3. Under the same conditions of net distance, crossing angle, and excavation method, the minimum principal stress of the pipeline decreases with an increase in the strength of the surrounding rock parameters.
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4.6. The Influence of Construction Parameters on Dense Pipeline
The calculation results of working conditions 12, 13, and 14 in Table 3 were selected to analyze the vertical displacement, maximum principal stress, and minimum principal stress of the dense pipeline, and the response characteristics of different construction methods (CRD method, up and down step method, and full section method) to the dense pipeline were obtained.
4.6.1. Vertical Displacement Response Characteristics of Each Pipeline Characteristic Point
As shown in Figure 25, under the same conditions of surrounding rock, net distance, and crossing angle, the vertical displacement of the pipeline increases with an increase in the disturbance of the soil by the construction method.
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4.6.2. Maximum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
As shown in Figure 26, under the same conditions of the surrounding rock, net distance, and crossing angle, the maximum principal stress of the pipeline increases with an increase in the disturbance of the soil by the construction method.
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4.6.3. Minimum Principal Stress Response Characteristics of Each Pipeline Characteristic Point
As shown in Figure 27, the minimum principal stress response curve of the characteristic points of the water supply pipeline differs in form from those of other pipeline types. This is attributed to the fact that the water supply pipeline is made of a higher strength material, i.e., cast iron, which offers more excellent resistance to the deformation of the surrounding rock compared to other lower strength pipelines. Consequently, the minimum principal stress at the bottom of the water supply pipeline is significantly smaller than that at the top and in the unexcavated areas on both sides. Under the same conditions of the surrounding rock, net distance, and crossing angle, the pipeline’s minimum principal stress increases with the construction method’s disturbance of the soil.
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5. Conclusion
Based on the entrance and exit tunnel project of Dengbu Station of Nanchang Metro Line 3, this paper establishes a multi-condition numerical model with numerical calculation method by considering key influencing factors such as the clear distance, intersection angle, surrounding rock parameter, and construction method. The mechanical response of the dense pipeline adjacent to the dug tunnel was studied, and the influence of key factors on the mechanical behavior of the pipeline was analyzed. The results show the following:(1)When the CRD method was used to construct and grout the surrounding strata of the tunnel, the maximum settlement of the tunnel roof was located at the roof of the right guide tunnel, at approximately 11.9 mm. The measured settlement values of characteristic points of each pipeline are consistent with the simulated values, and the maximum settlement value is located at the bottom of the rainwater pipe directly above the tunnel. The error between the measured value and the simulated value is small, which reflects that the numerical model is more reliable.(2)Through numerical simulation, the influences of key factors such as the clear distance, intersection angle, surrounding rock parameter, and construction method on the mechanical properties of close neighbor dense pipeline are analyzed. When other factors are fixed, the vertical displacement of pipeline, the maximum principal stress, and the minimum principal stress all decrease with the increase of the clear distance, cross angle, and surrounding rock strength.(3)The vertical displacement, maximum principal stress, and minimum principal stress of the pipeline are arranged in descending order of magnitude for full-face, stepped method, and CRD methods, respectively. The greater the soil disturbance caused by the construction method, the more adverse the impact on the pipeline.
Data Availability
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was funded by the National Natural Science Foundation of China (nos. 52278397 and 52168055), the Natural Science Foundation of Jiangxi Province (20212ACB204001), and the “Double Thousand Plan” Innovation Leading Talent Project of Jiangxi Province (jxsq2020101001).