Analysis of Disturbance and Safety Risk Assessment of Shallow-Buried Pressure Pipelines Utilizing the Shield Tunneling Method

Abstract

With the rapid development of urban rail transit, the impact of shield tunneling on existing pipelines is increasing. To protect pipeline safety, this research focuses on the complex pipelines in the Shaluo shield tunneling section, utilizing FLAC3D numerical simulation software to investigate the deformation characteristics of cast iron pipelines during shield construction. Additionally, it quantifies the influence of pipeline materials on deformation and establishes the pipeline safety risk grading system. Safety assessment of pipelines based on the research. The research indicates that (1) The deformation difference between the tops of the pressure and pressureless pipeline is less than 1 mm, suggesting that pipeline deformation is minimally influenced by pressure. The deformation is the largest at the entrance and gradually decreases along the direction of excavation, indicating that the deformation has an obvious hysteresis effect. (2) The threefold variation in maximum deformation among pipelines of different materials during shield tunneling indicates the high sensitivity of pipeline material properties to shield construction processes. (3) By analyzing and discussing the literature and local norms, the deformation value of the pipeline is taken as the evaluation index. And the pipeline assessment system is established. (4) Cast iron pipelines at the start of the shield have the highest safety, and concrete pipelines at the beginning of the shield are the lowest.

1. Introduction

In the process of urbanization, the construction of underground tunnels is continuously expanding. The shield tunneling method is the mainstream construction technique [1] which is prone to triggering disturbance and stress redistribution of the surrounding soil during the construction process. Such changes may cause deformation, cracking, or even damage to shallow-buried pressure pipelines, jeopardizing their normal operation. Municipal pipelines, as key carriers of urban water supply, drainage, and energy transmission, shoulder the important responsibility of maintaining the basic functions of the city, ensuring the livelihood of its residents, and promoting social and economic development [2]. Its safety cannot be compromised. Therefore, conducting disturbance analysis on shallow-buried pressure pipelines to accurately assess [3] and effectively manage the impacts of shield tunneling construction has become a critical issue that needs to be addressed.

Extensive research has been conducted globally on pipeline disturbance and safety assessment during underground construction, utilizing analytical methods [4,5,6,7] model experiments [8,9,10,11] and numerical simulation. Numerical simulation, favored for its low cost, high flexibility, and strong visualization capabilities, is particularly prevalent. Deng et al. [12], Hu et al. [13], and Zhao et al. [14] used numerical simulation methods to investigate the problems related to the deformation caused by shield tunnels to obtain better research results. Numerous scholars have researched the factors influencing pipeline deformation. Shen et al. [15], Chen et al. [16], and Sun et al. [17] established a numerical model to analyze the effects of construction conditions and construction methods on pipeline deformation. Wang et al. [18]; Ren et al. [19]; Zhang et al. [20] established a numerical model to analyze the effects of burial depth, the relative position of pipelines, pipelines diameter, and soil quality on pipeline deformation. Sun et al. [21] analyzed the variation rule of pipeline settlement and deformation and longitudinal stress state after tunnel penetration under the conditions of different spacing between pipelines and tunnels, different pipeline materials, and different relative positions of pipelines and tunnels. Due to the complexity of the underground pipeline deformation research environment and the interactions among multiple entities, several scholars have conducted coupled simulations. Guan et al. [22]; Wu et al. [23] analyzed pipeline-soil-tunnel interactions. Weng et al. [24] studied the dynamic response of concrete pipelines under multi-field coupling. The purpose of studying the influencing factors of pipeline deformation and its characteristics is to provide a basis for the establishment of a pipeline safety assessment system, so as to protect pipeline safety more effectively. In the field of pipeline safety assessment, Cen et al. [25] proposed a method based on a combination of the Slacks-Based Measure model and the Malmquist model to investigate the effectiveness of pipeline safety assessment. Tang et al. [26] constructed the index system for evaluating the safety of high-steel-grade natural gas pipelines based on the 10 influencing factors with the highest frequency of occurrence. Liu et al. [27] constructed a pipeline safety risk assessment system based on on-site monitoring data and pipeline control standards. Wang et al. [28] based on the pipe-soil relative stiffness and material strength criterion established the method of taking the maximum allowable settlement of pipelines with rigid interfaces by analytical derivation. The factors influencing pipeline deformation and the establishment of a safety system are inherently linked to the monitoring of structural deformation. In the field of pipeline monitoring, Ni et al. [29]; Bao et al. [30] have applied distributed fiber optic sensing technology and autoregressive moving average models for pipeline health monitoring. Li et al. [31] studied an automatic monitoring device for the vertical displacement of an oil pipeline based on liquid pressure. For pipeline displacement detection in this research, field measurements of pipeline displacements were made using existing depth displacement gauges, taking into account the time constraints of the construction schedule. To ensure the reliability of this research, the measured data were subsequently compared and validated with numerical simulation results.

The research results of many scholars have provided a rich theoretical basis for understanding the influencing factors of pipeline deformation, and the current research has covered many aspects such as construction conditions and construction methods, pipeline burial depth, pipeline relative position, pipeline diameter, and soil quality, etc. These studies are of great significance for the in-depth understanding of the complexity of pipeline deformation. Despite the richness of the existing research results, based on the literature research, the search was analysed with keywords such as pipeline, shield, and disturbance analysis; the existing studies seldom take internal pressure and pipeline materials as variables to explore their effects on pipeline deformation in depth. Meanwhile, the impact of shield excavation on soil varies significantly under different geological conditions, which leads to significant changes in the deformation pattern of shallow buried pipelines, and thus, the pipeline assessment system needs to be adjusted and optimized accordingly. Therefore, the current research on pipeline deformation risk assessment systems is still insufficient under specific complex geological conditions, especially when the shield traverses soft overburden and hard underlying strata. The innovation of this thesis is to analyze the deformation characteristics of pipelines by taking the internal pressure and materials of pipelines as variables, and to establish a deformation risk assessment system for shallow buried pipelines under complex geological conditions by shield crossing based on the deformation characteristics.

Therefore, in this paper, numerical simulation is used. Firstly, an efficient and reliable numerical model is developed. Second, the effects of pipeline material properties and internal pressure on pipeline deformation are analyzed. Finally, a risk assessment method for pipelines is proposed in the shield traversing the specific complex geological conditions of soft overburden and hard underlying strata, and the research also quantified the influence of pipeline material on its deformation based on the evaluation system. The findings of this research provide valuable technical support for the construction and risk management of related projects.

2. Numerical Simulation Methods

2.1. Engineering Overview

The project is located in the coastal area of Qingdao, China, where shield tunneling traverses the composite stratum characterized by soft overburden and hard underlying strata, resulting in high engineering risks. The pipelines in the Shalao section are predominantly located within Workers’ New Village, at the Shazikou Tax Office, and along Laoshan Road. We use the process shown in Figure 1. The pressure pipelines in this section primarily consist of water supply, sewage, and gas lines. Because the special geological conditions of the soft overburden and hard underlying strata in this zone are more typical, and the type of pipeline is rich, the risk level is high (Level I), which can meet the research needs. At the same time, the research data can be obtained directly during the construction of the shield structure in this section during the study period, and the conclusions of the study can be directly applied to the protection and rating of the pipelines in this section. Therefore, this interval was chosen as the study area for this study. The majority of the pipelines are circular in cross-section, and the material is mainly cast iron and concrete. The pipeline diameters range from 100 mm to 1000 mm, with approximately 1000 mm being the most common. The burial depth of the pipelines predominantly falls within the range of 1 to 2 m. Due to the complexity of the soil layer information at the site, a realistic on-site simulation could not be achieved. Therefore, the simulation is considered to be carried out under the most unfavorable conditions, i.e., 50% of the shield machine passing through the soft overburden and hard underlying strata, and the information about the thickness of the soil layer and the position of the shield is shown in the simplified diagram of the simulation in Figure 2.

Theoretical calculations are challenging due to the complex interactions of the geotechnical medium. FLAC3D (6.0), a continuous medium mechanics analysis software developed by ITASCA, is specifically designed to analyze the mechanical properties of geotechnical materials and is widely used in various applications. The software includes a variety of built-in models for constitutive modeling and allows users to customize their own constitutive models using the Fish(6.0) programming language. Therefore, this research utilizes FLAC3D software to simulate tunnel construction in the presence of existing underground pipelines.

2.2. Model Parameters

The geometric parameter of the calculation model is designed based on the previously mentioned dimensions. The excavation diameter of the tunnel is 6 m, and the clear distance between the two tunnels is 12 m. Based on the boundary effect analysis, the extent of ground disturbance caused by shield construction is primarily confined to the region within 1.0D to 3.0D (where D is the tunnel’s outer diameter) surrounding the tunnel. Therefore, in numerical simulations, boundary dimensions were set to a distance of 3.0D to 5.0D from the tunnel structure, by St. Venant’s principle, to ensure that the model adequately captures the extent of the disturbance impact area. This approach simultaneously mitigates the influence of boundary effects on computational accuracy and enhances overall computational efficiency. The model takes the shield tunnel and the surrounding geotechnical body as the research object. It utilizes the combination of embedded radical-cylinder gradient brick mesh, cylindrical mesh, embedded cylindrical-shell mesh, and brick mesh to complete the construction of the geometric model. The model mesh change picture is shown in Figure 3. Based on practical conditions, the model is designed with a width of 84.4 m and a height of 46.2 m. The length of the tunneling direction is an integer multiple of the number of excavation rings, taken as 120 m. This yields the computational model with dimensions of 120 m × 84.4 m × 46.2 m. The maximum mesh size is 2 m, with finer mesh sizes applied in critical regions. A related study on the problem of mesh size accuracy, Han et al. [32] can shows that it is sufficient. Regarding the element shape in this paper, the built-in hexahedral element body in FLAC3D is used in this paper, which can track the stress transfer and change more accurately due to its larger number of faces and it can better maintain the symmetry and continuity of the stresses in space, which makes the prediction of the shear surface more in line with the physical process of the actual soil damage. Therefore, it can more accurately simulate the geometry and stress distribution of the soil, and thus more accurately determine the location of the shear surface when the limit state is reached. The model comprises a total of 825,600 elements and 838,593 nodes. Each side of the tunnel is equipped with 80 excavation segments, each segment 1.5 m in length. The specific model is shown in Figure 4.

The project interval comprises a total of seven strata, listed from top to bottom as follows: plain fill, fine sand, silty clay, coarse gravelly sand, strongly weathered rock, moderately weathered rock, and slightly weathered rock. To enhance computational efficiency and address convergence issues, the strongly weathered and moderately weathered rock layers are combined and simplified for analysis. Material parameters such as density, elastic modulus, and Poisson’s ratio are derived from geological survey data before construction. The volume modulus and shear modulus are calculated based on formulas to simulate the elastic-plastic deformation of the rock and soil around the tunnel.

The Mohr-Coulomb model is a common model used in numerical simulations. For example, Su et al. [33] used the model to conduct an investigation in a related field. However, the model has some limitations, which are based on the ideal elastic-plastic theory, and it is difficult to accurately describe the complex mechanical behaviors of soil bodies such as super-consolidated clay and dense sand, including strain softening and shear expansion characteristics. The model assumes that the modulus of elasticity of the soil body is a constant value, which cannot effectively reflect the nonlinear stress-strain relationship of the actual soil body in the loading process, and is prone to deviation when simulating large deformation conditions. Moreover, the structural nature of the soil body is not considered, and the simulated strength and deformation characteristics may be inaccurate for soils with obvious structural properties, such as in situ soils. The limitations of the Mohr-Coulomb model are not addressed in this study, and the main reasons for choosing this model are as follows: the stress-strain behavior of the Mohr-Coulomb model can be represented by a bilinear curve. This includes the linear elastic stage: before the yield point, the stress and strain are linear relationship, which is controlled by the linear behavior of elastic modulus and Poisson’s ratio. The fully plastic stage: after the yield point, the stress reaches the maximum value and remains unchanged, while the strain continues to increase. Therefore, the geotechnical body is modeled using the Mohr-Coulomb constitutive model. Pipeline, tube sheets, and the grouting layer are modeled using an isotropic elastic model. Although the actual structure of the grouting layer may have a certain degree of microscopic anisotropy, its macroscopic mechanical behavior tends to exhibit isotropy in engineering practice. For example, under normal shield construction and grouting pressure, the stress state suffered by the grouting layer is relatively complex, but its deformation and stress distribution do not show obvious anisotropic differences. In this case, the use of an isotropic elasticity model can, to a certain extent, ignore the complexity of the microstructure and focus on the overall mechanical properties, so as to obtain more accurate results. The soil and grouting layer are modeled using the solid elements. Tube sheets are modeled using the shell elements, and pipelines are modeled using the structure elements. The relationship between the elastic volume modulus K, elastic shear modulus G, the elastic modulus E, and Poisson’s ratio μ required by the Mohr-Coulomb model is shown in the following formulas. The specific parameters are shown in

Table 1. Actual stratum parameters.

Stratum	Elastic Modulus (106 Pa)	Poisson’s Ratio (μ)	Density (kg·m−3)	Bulk Modulus (K/MPa)	Shear Modulus (G/MPa)	C (kPa)	Φ (°)	Soil Layer Thickness (m)
plain fill soil	20	0.35	1800	22	7	19.2	18	4.3
fine sand	35	0.3	1750	29	13	0	22	1.7
silt clay	25	0.33	1970	25	9	23	20	1.1
coarse gravel sand	54	0.28	2050	41	21	80	35	6.0
strongly weathered rock	15,000	0.21	2500	8621	6198	500	40	7.2
moderately weathered rock	32,000	0.2	2650	17,778	13,333	2000	45	25.9
slightly weathered rock	94	0.22	2100	56.2	38.7	—	—	—
slurry	10,500	0.25	2500	7000	4200	—	—	—

.

$$K={\displaystyle \frac{E}{3(1-2\mu )}}$$

(1)

$$G={\displaystyle \frac{E}{2(1+\mu )}}$$

(2)

2.3. Load Boundary Conditions of the Model

In the initial phase, the shield has not yet been carried out; in the excavation phase, the shield is progressively carried out; and in the penetration phase, the shield is completed. The overall model only takes into account the effect of gravity. The reason is that the pipelines were buried before the tunnels. Under the influence of loads from buildings and other structures, the pipeline has undergone long-term deformation and reached a stable state, while the soil has also consolidated. Additionally, in actual engineering practice, roads within the influence range of the shield tunnel are temporarily closed to reduce project risks, so there is no dynamic surface load on the road. In summary, based on the actual site conditions, we have omitted the overloading of buildings and traffic loads in the numerical model. In the dynamic simulation of shield tunneling, it is essential to account for the earth pressure in the chamber and the grouting pressure of the shield machine. Based on parameters from actual construction conditions, the earth pressure in chamber pressure is set to 180 kPa, and the grouting pressure is set to 300 kPa. The model boundary condition is shown in Figure 5.

The boundary condition of the model is implemented through command flow in the FLAC3D software, primarily constraining the displacements of the model’s front, back, left, right, and bottom surfaces. These five surfaces are considered fixed, while the top surface of the model is designated as a free surface, indicating that there are no constraints at the ground surface. The overall constraints and loads of the model are shown in Figure 6.

The model implementation process is as follows: the model diameter is 6 m, with each excavation step length being 1.5 m, which is the width of one segment. The length of both the left and right tunnels is 120 m, and 160 cycles are required for each working condition to complete the excavation. The excavation process is shown in Figure 7. The soil bin pressure is realized by applying the pressure perpendicular to the tunnel face, and after each ring is excavated, the previous ring is grouted, and the grouting layer is simulated by a grouting layer with a thickness of 0.1 m, and the grouting pressure is realized by applying the face force perpendicular to the surrounding rock. The details of the shield tunneling process are shown in Figure 8. Regarding the model solution time, the simulation specifies that it stops when the error is less than 10⁻⁵, an accuracy that meets the requirements of the study.

3. Disturbance of Pressure Pipelines by Shield Tunneling Construction

According to the site conditions and the feasibility of the simulation calculations, a representative working condition was selected: the cast iron pressure pipeline with a diameter of 1000 mm and a burial depth of 1.8 m, to simulate the force and deformation of the pipeline under the state of 0.2 MPa when the pipeline is in normal operation, and to compare and analyze the disturbance of the pressure pipeline caused by the construction of the shield structure.

3.1. Analysis of Disturbance to Pressure Pipelines by Shield Tunneling Construction

The numerical model of the shield tunnel penetrating the existing pipeline is shown in Figure 9, which shows the displacement of the left tunnel and the right tunnel at the completion of construction.

Based on the three-dimensional numerical model, the influence of shield tunnel excavation on existing pressure pipelines is systematically analyzed. After achieving initial stress equilibrium, the pipeline deformation is 0. The status of the pipeline is shown in Figure 9a,b, where the vertical displacement of 0 is observed. Subsequently, the excavation of the left tunnel begins with the excavation step length consistent with actual construction, set to 1.5 m per step. The left tunnel excavation is completed after 80 steps. Upon completion of the left excavation, the settlement cloud maps of the pipeline are shown in Figure 9c,d. The overall pipeline will deform upward in the vertical direction, the pressure pipeline bulge is between 6.80 mm~7.60 mm, and the pressureless pipeline bulge is between 6.95 mm~7.74 mm. The analysis indicates that the excavation of the left tunnel disrupts the initial stress field and induces the formation of a settlement trough, with the left vault as the trough’s bottom. This settlement pattern is characterized by horizontal soil displacements, whereby the soil on the left tends to move laterally to the right, while the soil on the right tends to move laterally to the left. In the XZ direction. The left soil shifts downward to the right, while the right soil shifts downward to the left. The primary cause of this change is the unavoidable necessity to re-establish equilibrium in the stress field following damage, leading to a loosening of the soil layer above the pipeline. Initially subjected to compressive stress from the surrounding earth, the pipeline experiences a rebound as the vertical load diminishes, resulting in upward deformation observable on the displacement cloud map [34].

Following the completion of the left tunnel, the right tunnel is excavated sequentially in increments of the same step length until the excavation is fully connected. Unlike the left tunnel, which is constructed adjacent to and passes beneath the left side of the pipeline, the right tunnel directly intersects the pipeline beneath its center. After the right tunnel is completed, the vertical displacement cloud map of the pipeline is shown in Figure 9e,f. The pipeline as a whole deforms downward in the vertical direction, with the maximum vertical displacement of the pressure pipeline being −3.96 mm and that of the pressureless pipeline being −3.81 mm. The reason is that the pipeline is directly above the right tunnel, located in the centerline of the sinkhole, the settlement of the ground is larger, and the pipeline sinks with the ground, which is represented as a downward deformation on the displacement cloud map.

The morphologies of the pressure pipeline during the shield tunneling process are shown in Figure 10. The pipeline cross-section remains elliptical throughout all stages of shield tunneling, indicating that the pipeline is subjected to a pressure state. The morphology of the pipeline at halfway through the left tunnel and left tunnel completion is essentially identical, characterized by overlapping curves. Similarly, the morphology of the pipeline at halfway through the right tunnel and right tunnel completion is consistent. The downward deformation of the pipeline during the right tunnel excavation is greater than that during the left tunnel excavation, indicating that the impact of the right tunnel on the pipeline deformation is more significant than that of the left tunnel. The comparison shows that there is little difference in the pipeline form between the completion of the left tunnel and the right tunnel, and that the right tunnel has undergone overall settlement deformation compared to the left tunnel. The specific impact of shield construction on pipeline deformation is shown in Figure 11.

The deformation curve of the top of the pipeline is shown in Figure 11. It can be seen from Figure 11a,b that the presence or absence of pressure does not cause significant variation in the vertical deformation of the pipeline during the shield tunneling process. The theoretical support is as follows. First, according to the theory of thin-walled shells, under internal pressure, pipelines mainly generate axial and circumferential stresses. The magnitude of these stresses is related to the internal pressure, internal diameter, and wall thickness of the pipelines. For pipelines of a certain specification, when the internal pressure changes, the stress changes relatively little and remains within the elastic range, where stress is proportional to strain, and deformation is reversible. This limits the impact of internal pressure on deformation to some extent [35]. Second, from the perspective of yield criteria, before reaching the yield strength, the deformation of pipeline materials is primarily elastic, and changes in internal pressure have a relatively minor effect on deformation. Only when the internal pressure becomes sufficiently high, causing the pipeline’s materials to enter the plastic deformation stage, does the impact of internal pressure on deformation significantly increase. Therefore, for pipelines with high elastic modulus, the influence of internal pressure on deformation is relatively small [36]. In actual engineering, the working internal pressure of pipelines is usually much lower than the internal pressure corresponding to the material’s yield strength, so the impact of internal pressure on deformation is minimal. Moreover, in practical applications, pipelines are often constrained by external factors. These constraints limit the free deformation of the pipeline, suppressing its deformation under internal pressure. For example, buried pipelines, constrained by the soil, have their radial deformation limited, thereby reducing the overall impact of internal pressure on the deformation of the pipelines. Throughout the entire excavation of the left tunnel, the pipeline undergoes an upward deformation as a whole. The deformation remains consistent, gradually diminishing as excavation progresses. Specifically, the deformation during the first half of the left tunnel excavation is approximately 6 mm, which is 5~6 times greater than the deformation observed during the second half of the excavation. During the halfway right tunnel excavation, vertical downward deformation of the pipeline is observed. The maximum deformation occurs at the tunnel face and gradually diminishes in the direction of shield tunneling. Approximately 20 m behind the tunnel face, deformation stabilizes, indicating an obvious hysteresis effect in the pipeline deformation [37]. When the right tunnel is completed, the pipeline deformation in the 0 m to 90 m area is generally stable, around −2 mm. In the 90 m to 120 m range, the pipeline gradually transitions from upward deformation to downward deformation. It can be observed that the pipeline deformation at the shield tunnel starting point is greater at the halfway point of the right-line tunnel excavation than at the completion of the tunnel, indicating that the downward deformation of the tunnel has partially recovered. The reason is that during the initial stage, the pipeline deforms downward along with the surrounding soil. As the surrounding rock stress redistributes, the pressure on the top of the pipeline decreases, allowing some recovery of the pipeline’s elastic deformation, which leads to a reduction in the downward deformation. This indicates that the maximum deformation of the pipeline did not occur at the moment of completion of shield tunneling, but rather at the location close to the shield start shaft during shield tunneling.

3.2. Analysis of the Disturbance Caused by Shield Tunneling to Pipelines of Different Materials

This paper also studies the deformation behavior of concrete pipelines under the influence of shield tunneling, compares it with cast iron pipelines, and analyzes the disturbance caused by shield tunneling to pipelines of different materials. The established concrete pipeline model and the cast iron pipeline model differ only in materials. The deformation cloud maps of the cast iron pipeline and the concrete pipeline are shown in Figure 12.

A comparison of the displacement cloud maps of the two types of pipelines shows that there is a significant difference in their displacements during the shield tunneling process. In the initial state, the displacements of both types of pipelines were set to 0. Therefore, there is no discernible difference between the cloud maps in Figure 12a,b, indicating that neither exhibits any deformation. After the completion of the left tunnel shield excavation, the two pipelines exhibit opposite deformations. Figure 12c shows upward deformation for the cast iron pipeline, while Figure 12d shows downward deformation for the concrete pipeline. The reason for the opposite deformation law is mainly due to the different mechanical properties of cast iron and concrete. For cast iron pipelines, the material stiffness is larger, and the bending resistance is stronger, which can effectively resist the settlement deformation caused by the loss of strata. It can efficiently transform the grouting lifting force into overall uplift displacement. So, when the grouting effect dominates, it tends to be upward displacement. As for the concrete pipeline, the material stiffness is relatively low, the flexibility is large, and it adapts to the ground settlement trough through its own bending deformation. And the self-weight is large on the grouting lifting force response is mainly local bending, making it difficult to achieve overall uplift. So it tends to be displaced downwards. Shield disturbance generates upward (grouting lifting) and downward (sinkhole) soil displacement components. Different stiffness pipes have different ‘filtering’ and ‘response amplification’ mechanisms for the above displacement components, resulting in different final displacement directions. At specific locations below the shield crossing and directly above the disturbed zone, the high-stiffness pipe is more sensitive to uplift, and the flexible pipe is more sensitive to settlement. This phenomenon also reflects the deformation law of ‘the stiff one is easy to float, and the flexible one is easy’. After the completion of the right-line tunnel shield excavation, Figure 12e shows that the cast iron pipeline experiences settlement in some areas, while other parts still undergo upward deformation. In contrast, Figure 12f shows that the concrete pipeline exhibits overall downward deformation, with some regional variations in the pipeline’s deformation.

The deformation curve of the top axis of the concrete pipeline is shown in Figure 13. The left tunnel excavation causes the concrete pipeline to undergo downward deformation, with the main disturbance occurring during the first half of the tunnel excavation. From the halfway point of the left tunnel excavation to the completion of the left tunnel, the downward deformation of the pipeline is almost 0. As the construction progresses, the right tunnel begins excavation. During the halfway point of the right tunnel excavation, significant downward deformation of the pipeline is observed. In the second half of the right tunnel excavation, the impact on the tunnel entrance is relatively small, but there is a greater effect on the middle section of the pipeline, with the downward deformation of the pipeline in the middle section being approximately 12.7 mm.

The morphologies of cast iron and concrete pipelines during shield tunneling are shown in Figure 14. It can be seen that, compared to the initial state, there is no difference in the X direction between the two types of pipelines. The deformation is mainly concentrated in the Z direction. Pipelines made of different materials exhibit varying degrees of downward deformation. The vertical deformation at different positions along the pipeline also varies; specifically, deformation increases with the depth of burial when measured downward from the horizontal axis of the pipeline. And deformation decreases with increasing depth when measured upward from the horizontal axis. Additionally, the downward deformation of the concrete pipeline is greater than that of the cast iron pipeline.

The deformation curves of the top axial direction of pipelines with different materials are shown in Figure 15. A comparison shows that the overall morphologies of the pipeline axial deformation curves are essentially the same, with the difference being in the downward deformation amounts. The maximum downward deformation of the cast iron pipeline is 3.7 mm, while the maximum downward deformation of the concrete pipeline is −17.0 mm. There is a threefold difference in the maximum downward deformation between the two types of pipelines, indicating that the pipeline material has a strong sensitivity to shield tunneling. The observed differences between the two pipeline materials can be attributed to cast iron pipes exhibit superior stability and greater resistance to external disturbances, whereas concrete pipelines, with significantly lower stiffness, are more susceptible to the shield tunnel construction [38].

3.3. Model Validation and Error Analysis

On-site pipeline deformation measurement mainly adopts the deep displacement meter. According to Deep displacement in accordance with the 5 m interval arrangement of three, and the pipeline directly above the soil, a total of 72 deep displacement meters, monitoring data collected day by day, take the average value of the cumulative calculation. The numerical simulation can directly output the displacement value of each mesh point. A comparison of the measured deformation of the concrete pipeline and the numerical simulation results after the completion of the right tunnel is shown in Figure 16. The monitoring data are the cumulative displacement data of all sensors directly above and behind the tunnel when it was bored to 120 m. The monitoring data shows that the longitudinal settlement curve is “S” type, 0–20 m settlement is larger, 20–100 m slow settlement, 100–120 m rapid settlement, 100–120 m position, the shield disturbance is larger, so the instantaneous settlement rate is larger, 20–100 m, after the shield disturbance, the soil body has undergone stress redistribution, to reach the new stress equilibrium state so the settlement is slower and 0–20 m shows that the soil body is basically settled. It can be observed that the trends of both are in good agreement, indicating that the numerical simulation results can accurately reflect the actual deformation of the pipeline. However, there are still some errors. The analysis is as follows: first, there will be certain monitoring errors in the on-site measured data; second, numerical simulation is a simplification based on the on-site monitoring situation, not a perfect replication of it. For example, the horizontal simplification of strata and the homogenization assumption of soil both lead to rounding errors in numerical simulations. These factors collectively cause discrepancies between the simulated data and the monitored data. The above errors are mainly the result of simplifying the model after considering the difficulty of numerical modeling, the efficiency of numerical calculation, and the convergence of the model. From the perspective of numerical simulation results, the numerical outcomes show consistent trends with the measured data and have relatively small errors, which demonstrates that the numerical model established in this paper can simulate on-site conditions to some extent.

The reliability of the model simplification is illustrated as follows. As a numerical simulation software based on the finite difference method, FLAC3D can effectively simulate deformations in engineering. Due to convergence and computational efficiency, models are often simplified to some extent [39,40,41]. The omission of dynamic surface loads is the choice based on actual engineering conditions; uniform soil layers are commonly selected for numerical simulations for three reasons: First, the actual geological strata are undulating, and engineering surveys cannot cover all sections comprehensively, making it impossible to create a fully realistic model according to actual conditions. Second, the built-in modeling methods in FLAC3D cannot achieve extremely detailed modeling, while common third-party modeling techniques can provide detailed modeling but have issues with meshing, meaning they cannot export detailed mesh files for subsequent FLAC3D simulations. Finally, high-precision meshes involve large computational costs and difficulty in convergence. Regarding the reliability of this method, we modeled it based on actual working conditions. Model parameters are derived from the site survey report, while boundary conditions are established based on the actual conditions of the model. The site conditions were based on the actual conditions of the shield tunneling, with excavation carried out every 1.5 m per cycle. In terms of numerical modeling, parameter assignment, boundary condition setting, and simulation of site conditions, this paper refers to relevant existing research and employs scientific methods to ensure the reliability of numerical simulations. A comparative validation using field monitoring data further explains the method’s low error rate.

4. Safety Risk Assessment of Shallow-Buried Pressure Pipelines Using the Shield Tunneling Method

4.1. Classification of Pipeline Safety Risk Levels

Numerous safety assessment parameters are used for pipelines, with common indicators including stress-strain control, joint angle, and disconnection thresholds, as well as settlement displacement limits [42]. However, research on pipeline risk assessment criteria is limited, and a unified standard has not yet been established. Most of the projects will carry out expert meetings based on actual site conditions to establish corresponding pipeline parameter control standards. Some cities have formulated local technical standards to regulate the installation and control of the city’s municipal pipelines. For example, in Shanghai, the maximum displacement of gas pipelines should not exceed 15 mm, and for water supply pipelines, it should not exceed 30 to 50 mm; the local standard of Hubei Province stipulates that the maximum settlement of water supply pipelines should not exceed 30 mm.

As there is a strong link between deformation and the actual instability modelling of the pipeline, firstly, deformation can lead to pipeline failure, which may occur when the pipeline is subjected to loads exceeding its yield strength, and plastic deformation may occur. In the case of cast iron pipes, although they are tough, they may still crack or fracture under excessive deformation, leading to failure. Under normal operating conditions, pipelines may experience elastic deformation. If the elastic limit of the pipeline is exceeded, this may result in permanent deformation, affecting its sealing and load-carrying capacity, and eventually leading to failure. Accumulation of elastic deformation may lead to stress concentrations, which in turn may initiate localized failure of the pipeline, such as local buckling or rupture. In the case of concrete pipelines, plastic deformation usually manifests itself in the formation and expansion of cracks, which may eventually lead to rupture or failure of the pipeline. The failure mode of the pipeline can, in turn, affect the overall deformation behavior of the pipeline. For example, the development of cracks may lead to a reduction in local stiffness, making the pipeline more susceptible to deformation under subsequent loading. Changes in the failure mode may lead to an uneven distribution of stresses in the pipeline, which can trigger wider deformation, creating a vicious cycle. And the direct and convenient nature of detecting pipeline deformation, considering that strain accumulation at pipeline joints is often accompanied by settlement, this research primarily adopts pipeline deformation as the criterion for assessing pipeline damage. In the project, detection equipment can be arranged on site to monitor the deformation value of the pipeline, and numerical simulation means can be used in the design stage to simulate and calculate the deformation value of the pipeline. Wu et al. [43] divided urban water supply pipelines into five safety risk levels, with 10 mm, 20 mm, 30 mm, and 40 mm as critical points. Considering the macro-scale changes of pipelines in shield construction projects, the simulated grouting body in this research includes building voids, which leads to overly conservative calculation results. Thus, based on the numerical simulation data presented in this chapter, the results are multiplied by a safety factor of 2.0. Additionally, relevant literature is referenced to classify the safety risk levels of underground pipelines. Therefore, in this chapter, 10 mm, 20 mm, and 30 mm are selected as the critical points for classifying the pipeline safety risk levels, and corresponding status descriptions and protection measures are provided for each level. They are shown in

Table 2. Classification of safety risk levels based on pipeline deformation values as the control standard.

Safety Risk Level	Status Description	Protective Measures
Level I (safety)	The deformation value is less than 10 mm, and the pipeline is not significantly affected by construction disturbance, with its function remaining unaffected.	No protective measures are required.
Level II (basic safety)	The deformation value is between 10 mm and 20 mm, and the pipeline experiences minor deformation due to construction disturbance, but its function can still be achieved normally.	Simple protection: regularly monitor key pipeline sections to keep track of their condition.
Level III (risk warning)	The deformation value is between 20 mm and 30 mm, and the pipeline undergoes obvious deformation, reaching the deformation limit, with its function barely achieved.	Key protection: comprehensively monitor the pipeline and appropriately reinforce the local and surrounding strata.
Level IV (damage)	The deformation value exceeds 30 mm, and the pipeline deformation surpasses the safety standard, resulting in functional damage and necessitating immediate maintenance or replacement.	Professional protection: conduct a comprehensive inspection of pipeline damage and leakage points, promptly maintain damaged areas, and replace the pipeline if necessary.

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4.2. Safety Risk Assessment of Pipelines During Shield Tunneling Construction

Based on the pipeline safety risk levels defined above, a three-dimensional graph is used to assess the safety risk range of pipelines under different construction conditions of shield tunneling simulated in this research. The pipeline cross-section is taken as the X-axis, the pipeline extension direction as the Y-axis, and the pipeline deformation values as the Z-axis to create a three-dimensional graph, in order to investigate the distribution and morphological characteristics of each safety risk range of the pipeline.

The deformation of cast iron and concrete pipelines is shown in Figure 17. From Figure 17, the safety status of pipelines at different locations can be quickly identified and analyzed, allowing for the implementation of different protective measures. In the figure, blue represents the safety zone. Green represents the basic safety zone. Yellow represents the risk warning zone. And red represents the damage zone.

The deformation of the pipeline is classified as follows: deformation between 0 and 10 mm is considered a safe state (Level I), deformation between 10 mm and 20 mm is considered a basic safe state (Level II), deformation between 20 mm and 30 mm is in the risk warning zone (Level III), and deformation greater than 30 mm is in the failure zone (Level IV). There are significant differences in safety states between pipelines made of different materials. The division of safe zones along the pipeline primarily involves a gradient variation in the extension direction, with the safety status at each position within the same cross-section generally being consistent. Comparing the three-dimensional deformation graphs of pipelines made from different materials reveals that, although the deformation curves have similar shapes, the amounts of deformation differ significantly, leading to noticeable differences in corresponding safety levels. The overall safety levels for cast iron pipelines are classified as Level I and Level II, while the safety levels for concrete pipelines range from Level I to Level IV, with the majority falling into Levels II, III, and IV. This indicates that cast iron pipelines have a higher safety level during the excavation process, while concrete pipelines present higher risks. An analysis of the proportion of different safety level ranges for the two types of pipelines is shown in Figure 18.

From Figure 18, the safety level range of pipelines with different materials shows that the safety levels of the cast iron pipeline along its extension direction are Grade I and Grade II, accounting for 87.5% and 12.5%, respectively. The safety of the pipeline at the shield launch point is the highest, and the overall pipeline is minimally disturbed by construction. No measures need to be taken, or only simple protective measures should be implemented, for regular monitoring of key areas of the pipeline. The safety levels of the concrete pipeline along its extension direction are Grade IV, Grade III, Grade II, and Grade I, accounting for 7.5%, 82.5%, 7.5%, and 2.5%, respectively. The safety of the pipeline at the shield launch point is the lowest. The overall construction process of the pipeline is subject to significant disturbances, necessitating specialized protective measures and comprehensive detection of potential damage and leakage points along the pipeline.

5. Conclusions

This paper is based on the complex pipelines in the Shalao shield tunneling section, utilizing FLAC3D to simulate the disturbance of pipelines during the shield’s passage through soft overburden and hard underlying strata. A pipeline safety risk assessment method is proposed, emphasizing the analysis of the deformation characteristics of pressure-cast iron pipelines and the influence of pipeline material properties on deformation extent. The key findings are summarized as follows:

①

The presence or pressure less does not cause variations in the vertical deformation of the pipeline during the tunneling process. During the entire excavation process of the left tunnel, the pipeline experiences an overall upward deformation, with the amount of deformation gradually decreasing as excavation progresses; at the half way of the right tunnel excavation, a significant downward deformation of the pipeline can be observed, with the deformation extending gradually smaller along the pipeline, and deformation stopping behind the tunnel face, indicating a clear lag effect in the pipeline deformation. When the excavation of the right tunnel is completed, the pipeline deformation in the 0 m to 90 m area is basically stable, while in the 90 m to 120 m range, the pipeline gradually transitions from upward deformation to downward deformation. The maximum deformation of the cast iron pipeline does not occur at the moment when the shield tunneling is completed, but rather during the tunneling process, close to the position of the tunnel’s starting shaft.

②

Sensitivity of pipeline materials to shield tunneling is quite high. Both axial and lateral deformations of the pipeline show significant variations in concrete pipelines. The main reason for this phenomenon is that cast iron pipelines have better stability and can withstand larger disturbances, while the stiffness of concrete pipelines is much lower than that of cast iron pipelines, making them more susceptible to the impacts of shield tunnel construction.

③

By analyzing and discussing the literature and local norms, safety risk levels of pipelines can be determined using pipeline deformation values as criteria. A deformation of 0 to 10 mm is classified as a safe state (Level I), 10 to 20 mm as a basic safe state (Level II), 20 to 30 mm as a risk warning zone (Level III), and greater than 30 mm as a damage zone (Level IV). Corresponding safety measures for each risk level should also be proposed.

④

Through the assessment of the pipelines on site, it is found that the safety level of cast iron pipelines is I and II along the pipeline extension direction, accounting for 87.5% and 12.5% respectively, and the pipelines at the start of the shield have the highest safety. The safety level of concrete pipelines is Class IV, Class III, Class II, and Class I along the pipeline extension direction, accounting for 7.5%, 82.5%, 7.5% and 2.5% respectively, and the safety of pipelines at the beginning of the shield is the lowest.