The construction of urban tunnels (mainly subway projects and various municipal underground projects) is often located in the dense areas of buildings, roads and underground pipelines, which leads to the increasingly prominent environmental pollution problems of various projects in urban tunnel construction. Therefore, in urban tunnel construction, it is necessary to ensure that the impact of construction on existing facilities is within the allowable range. In particular, the types, pipe materials, joint types and initial stresses of various underground pipelines are different, and the protection standards of different departments are also different, which increases the difficulty of pipeline protection in tunnel construction.
As a new topic of urban environmental protection, many scholars at home and abroad have done a lot of work on the study of the influence of urban underground construction on adjacent pipelines, and have drawn many meaningful conclusions, which provide a certain theoretical basis for scientifically evaluating the influence of urban tunnel construction on adjacent pipelines. This paper summarizes the research status and progress of the influence of urban tunnel construction on adjacent pipelines, and puts forward some views on further research.
2. Research status at home and abroad
2. 1 Initial stress of underground pipeline
The stress of underground pipeline before urban tunnel excavation is called pipeline initial stress [1], which is the result of the combined action of internal working pressure, overlying soil pressure, static and dynamic load, installation stress, early ground movement and environmental impact. Generally speaking, uneven settlement caused by insufficient compaction of pipeline installation cushion or other reasons will increase the stress of pipeline section or increase the joint angle. Different pressures inside and outside the pipeline will lead to circumferential stress in the pipeline section; The effect of overburden pressure and dynamic and static load will make the section of the pipe section tend to be elliptical, and it will be accompanied by the change of the stress of the pipe section. Similarly, different buried soil layers will also lead to different stress states of pipelines: for example, burying pipelines in soil layers with large temperature difference will cause strain of pipelines, and the change of soil humidity around pipelines will also cause corrosion of pipelines, thus reducing the strength of pipelines.
Taki and O'Rourke analyzed the internal pressure, temperature stress, repeated load and installation stress acting on cast iron pipe, and calculated the typical values of tensile stress and bending strain of low-pressure pipe under comprehensive action. It is considered that when the longitudinal bending strain of the pipeline is 0.02% ~ 0.04%, the initial stress acting on the pipeline is roughly the corresponding stress value [2]. Researchers from Utah State University in the United States conducted stress, strain and stress relaxation tests on spiral ribbed steel pipes, low-strength ribbed steel pipes and PVC pipes, and reached corresponding conclusions [3]. Domestic scholars have studied the bearing load and axial stress of various pressure pipelines, and put forward the theoretical method and corresponding calculation formula of initial stress [4].
2.2 Interaction between pipeline and surrounding soil
In the process of tunnel construction, the uneven settlement and horizontal displacement of underground pipeline caused by the disturbance of surrounding soil will produce additional stress. At the same time, because the stiffness of the pipeline is about 1000 ~ 3000 times that of the soil, it will inevitably resist the movement of the surrounding soil. Attewell thinks that the influence of soil movement caused by tunnel construction on pipeline can be determined from the relative spatial position of tunnel excavation direction and pipeline. When the direction of tunnel excavation is perpendicular to the direction of pipeline extension, the influence on the pipeline is mainly manifested in the increase of pipeline bending stress and joint angle caused by the longitudinal displacement of soil around the pipeline. When the tunneling direction is parallel to the pipeline extension direction, the influence on the pipeline is mainly manifested in the axial tension and compression of the surrounding soil. The resistance of pipeline to soil movement is mainly related to pipeline diameter, stiffness, joint type and position [1].
Because most underground pipelines are buried at a small depth (generally within 1.5m), it can usually be assumed that when the pipe diameter is small, the underground pipelines have no resistance to the movement of the surrounding soil and will deform along the movement track of the soil. Some research results also show the feasibility of this hypothesis [2]: Carder and Tayor used full-scale experiments to study the behavior changes of cast iron pipes with a buried depth of 0.75m and a diameter of 100㎜ when placed in different soils under the influence of adjacent excavation. The test results show that the trajectory of the pipeline is consistent with that of the soil in the stratum. Nath used three-dimensional finite element simulation to analyze the response of cast iron pipes with a diameter of 75㎜~ 450㎜ to adjacent excavation under the condition of buried depth1.0m. The analysis results show that cast iron pipes with a diameter less than 150㎜ have little resistance to ground movement. Ahmed et al. used two-dimensional and three-dimensional finite elements to simulate the influence of deep trench excavation on adjacent cast iron pipelines. Assuming that there is no relative displacement between pipelines and surrounding soil, the additional strain of pipelines is calculated to be less than the allowable ultimate strength of cast iron pipelines. They believe that if there is no relative displacement between the pipeline and the surrounding soil under the influence of adjacent construction, the influence of construction on the pipeline can be ignored. In Molnar et al.' s research on the influence of deep foundation pit excavation on adjacent underground pipelines in Chicago Lurie Medical Research Center project, it is assumed that the predicted deformation value of underground pipelines with a diameter of 150 ㎜ ~ 500 ㎜ is consistent with the field measured data.
However, when the diameter of underground pipeline increases to a certain extent, it will resist the movement of surrounding soil, which also increases the risk of pipeline damage. Jiang Hongsheng, a domestic scholar, has studied the influence of shield construction on the combined sewer pipe with an upper diameter of 3.6m and the treatment measures [5]. However, Attewell believes that although the large-diameter pipeline will increase the stress of the pipe body when it resists the movement of soil, it will not produce a large additional stress in the pipe section because of the large strength of the pipeline itself (mainly for gray iron pipelines) [1]. Generally speaking, attention should be paid to large-diameter pipelines in tunnel construction, especially those with severe ground movement, fragile pipelines and joints and long service life.
2.3 Failure modes and allowable deformation values of underground pipelines
When investigating the main failure modes of underground pipelines under the action of stratum movement and deformation, there are generally two situations: first, the pipeline section cracks or even breaks under the action of additional tensile stress, resulting in the loss of working ability; Second, the pipe section is intact, but the joint corner of the pipe section is too large, so the joint cannot be kept closed and leakage occurs. The damage of pipeline may be mainly controlled by one of the modes, or two kinds of damage may occur at the same time: for welded plastic pipe and steel pipe, because of their high joint strength, the safety of pipeline can only be predicted by calculating their maximum bending stress; However, for cast iron pipes and ductile iron pipes, especially for cast iron pipes that run for a long time, both failure modes may occur because of their poor tensile strength and insufficient flexibility at the joints.
The document [1] defines the failure mode of underground pipelines caused by tunnel construction: 1. Hoses (mainly steel pipes and plastic pipes) are excessively deformed due to yield or buckling, and the pipe sections are broken; Second, the main failure modes of rigid pipes (mainly brittle gray iron pipes) are (1) section fracture caused by longitudinal bending, (2) radial cracking caused by circumferential deformation of pipe sections, and (3) leakage caused by excessive rotation of pipe sections. Gao Wenhua believes that the damage of welded long steel pipes is mainly controlled by the bending stress of the pipeline caused by the stratum decline; For pipes with joints, the damage is mainly determined by the allowable opening value△ and the allowable longitudinal and transverse bending strength of the pipes [6].
In the process of tunnel excavation, in order to ensure the safety of adjacent pipelines, the general practice at present is to control the settlement of pipelines, the inclination of the surface and the opening value of pipe joints. The determination of these control values is based on several specifications and engineering practice experience, which has a considerable degree of reliability. However, in practical engineering applications, it is difficult to measure the deformation and strain of underground pipelines and the joint angle of flexible joint pipelines. Moreover, because there is no unified theoretical control standard, the determination of these control values is arbitrary and lacks theoretical research results. On the basis of previous research results, Molnar gives the allowable bending stress and allowable joint angle of various pipelines by comparing theoretical calculation with measured data, which can provide reference for further research [2].
2.4 Deformation under the influence of underground pipeline tunnel construction
There are many factors that affect underground pipelines in tunnel construction. Accurate theoretical analysis of underground pipeline stress and deformation is the basis of underground pipeline protection research. At present, there are two main methods to calculate the stress and deformation of underground pipelines: analytical method and numerical simulation method.
2.4. 1 analysis method
Attewell puts forward an evaluation method of the influence of tunnel construction on structures and pipelines based on Winker elastic foundation model. According to the different positions of pipelines and the direction of stratum movement, the bending stress and joint angle of pipelines during vertical and parallel stratum movement are calculated respectively, and the different reaction behaviors of large-diameter and small-diameter pipelines under stratum movement are studied, and the practical application feasibility of theoretical analysis is discussed, and the pipeline design method is given, which is an earlier systematic research achievement [1]. Liao He also proposed two methods to treat underground pipelines as flexible pipes and rigid pipes based on the elastic foundation beam theory [7]. The calculation model is shown in figure 1, and the displacement equation of underground pipeline is established as follows:
Where:, k is the bed coefficient;
Ep- elastic modulus of pipeline;
Ip- moment of inertia of pipeline section;
Q- the pressure acting on the pipeline.
For flexible underground pipelines, they think that studying the stress and deformation of such pipelines in the process of ground settlement can analyze the differential settlement value of pipeline foundation or the curvature of settlement curve that each pipeline may bear from the aspects of joint opening value, longitudinal bending and lateral stress of pipe joint.
According to the elastic foundation beam theory, Takada Shiro et al. modeled four cases of underground pipelines affected by foundation settlement, and put forward design formulas for calculating the maximum bending deformation, joint angle and maximum joint elongation of pipelines [8]. According to Winker's foundation reaction model, Duan Guangjie discussed the influence of stratum loss caused by different tunnel construction methods on surrounding underground pipelines. Under the influence of radial deformation and axial deformation of the stratum at the pipeline, the relationship between the maximum ground settlement and the parameters such as pipeline deformation, strain and rotation angle when the pipeline is perpendicular to the tunnel axis and parallel to the tunnel axis is summarized respectively [9]. Gao Wenhua used Winker elastic foundation beam theory to analyze the vertical displacement and horizontal displacement of underground pipeline caused by foundation pit excavation, and derived the corresponding calculation formula. The factors that cause the deformation of underground pipelines, such as foundation coefficient, settlement zone length and surface settlement corresponding to underground pipelines, are discussed. Different pipeline deformation control standards and safety evaluation standards are given [6]
Based on the following two assumptions, firstly, it is assumed that the pipeline is continuous and flexible. When the pipeline moves with the soil, it only bends on the pipe section and does not turn at the joint. Because the axial displacement of the pipeline section is very small, it is considered that the pipeline does not bear axial strain when moving, and the pipeline bending obeys Bernoulli-Naville theory. Secondly, assuming that the pipe section is rigid, the displacement caused by the movement of the pipeline is all provided by the joint angle, and the joint does not produce resistance moment, allowing the joint to rotate freely, and the joint angle only produces in the longitudinal direction, that is to say, the moment on the pipeline is zero. Molnar derived the formulas for calculating the bending stress and joint angle of underground pipeline when it moves around soil, which are [2] respectively:
Calculation formula of (1) bending stress:
Fig. 2 Calculation model of pipeline bending stress [2]
(2)
Where: σ i refers to the bending stress at point I of the pipeline;
E- elastic modulus of pipeline;
Xi and zi- are the transverse and longitudinal distances from the fiber outside the pipeline to the neutral axis, respectively.
Z "(yi) and X" (yi)-are the longitudinal and transverse curvatures of the pipeline at point I, respectively.
(2) Calculation formula of joint angle:
Fig. 3 Calculation model of pipe joint angle [2]
(3)
Where: ε ji-the lateral displacement difference between point I and point J on the pipeline;
ρ ji-settlement difference between point I and point J on the pipeline;
Lji-length of pipe section;
For the same pipeline, the above two critical states are analyzed respectively, and the calculated values are compared with the allowable values, so that the safety state of the pipeline can be predicted.
2.4.2 Numerical simulation method
Using numerical simulation method, the interaction between ground displacement and pipeline caused by tunnel excavation can be well considered, and satisfactory results can be obtained.
Ahmed used the finite element model to calculate the additional bending stress of the underground pipeline when it was excavated near the deep foundation pit, and suggested that the maximum bending strain of the cast iron pipe caused by the surrounding stratum movement should be 0.05% and that of the ductile iron pipe should be 0. 15%[2].
Li Dayong, Gong Xiaonan and Zhang Tuqiao considered the coupling effect of foundation pit retaining structure, soil and underground pipeline, and established a three-dimensional finite element model of underground pipeline, soil and foundation pit retaining structure [10]. The influence of pipeline material, buried depth, distance from foundation pit, soil quality of underlying layer and elastic modulus ratio of pipeline to surrounding soil on underground pipeline is analyzed. Based on the tensile force, bending moment and torque generated by the sealing rubber ring in a single flexible interface, the stress and deformation of underground pipeline adjacent to the flexible interface in foundation pit engineering are studied, and the tensile force P of the pipeline flexible interface is obtained. The safety identification methods of underground pipelines and engineering monitoring and protection measures of underground pipelines are summarized [1 1][ 12]. Wu Bo and Gao Bo [13] based on the ANSYS software platform, simulated the underground pipeline as a three-dimensional elastic foundation beam, established a three-dimensional finite element analysis model of the coupling effect of tunnel supporting structure, soil and underground pipeline, simulated and analyzed the construction process, predicted the safety of underground pipeline, and gave the evaluation standard of pipeline safety.
2.5 ground movement and deformation caused by urban tunnel construction
Since Peck system put forward the empirical formula for predicting the surface subsidence trough in tunnel construction, many scholars have made in-depth and systematic research on the surrounding environmental geotechnical problems caused by tunnel construction, and Attewell and others have summarized this [1]. The theoretical analysis method proposed by Loganathan et al., Wei-I Chou and Antonio has achieved good results in predicting the displacement of the ground surface and strata caused by excavation [14] [664] Domestic scholars Liu Jianhang and Hou Xueyuan studied the ground settlement caused by shield construction and put forward corresponding prediction methods [16]. Xu Yongfu and Sun Jun discussed the influence of shield tunneling on the surrounding soil. Application of artificial intelligence neural network technology in prediction of shield tunneling disturbance and ground movement [17][ 18]. And Liu used the random medium theory to predict the ground movement and deformation caused by urban tunnel construction, and achieved ideal prediction results. Through the accurate prediction of surface displacement caused by tunnel excavation, it provides a theoretical basis for further study on the influence of tunnel construction on underground pipelines [19].
3. There is a problem
The research on the influence of urban tunnel construction on adjacent pipelines is a comprehensive subject involving municipal engineering, tunnel and underground engineering, engineering risk assessment and other disciplines. At present, the research depth is far from enough, and the initial stress, pipe-soil interaction, allowable value of pipeline deformation, stress and deformation calculation of underground pipeline need to be further deepened.
(1) The initial stress of underground pipeline is controlled by internal working pressure, earth pressure, static and dynamic load, installation stress, early ground movement and environmental impact. Although the research on single load is relatively perfect at present, the initial stress of pipeline is the result of the comprehensive action of the above forces, and the state of initial stress can not be accurately reflected by simple superposition. At present, the estimation of initial stress is mostly determined by experience. When conditions change, the original experience cannot be simply copied. Therefore, it is necessary to establish an effective calculation theory of pipeline initial stress, which provides a theoretical basis for determining the allowable value of pipeline deformation.
(2) At present, in the study of pipe-soil interaction, most scholars still assume that the pipe-soil is in close contact and there is no relative displacement. This assumption is suitable for small-diameter pipelines with good engineering properties of buried soil, but it is no longer applicable because large-diameter pipelines will obviously resist the movement of surrounding soil. Similarly, if the soil water content in the layer where the pipeline is located is large, there will be relative displacement between the pipeline and the soil when the soil moves.
(3) The allowable deformation value of pipeline should be determined by comprehensively considering the factors such as pipe material, pipe diameter, joint type, pipeline function, running time, relative position of pipeline and tunnel, tunnel construction method and so on. However, the current subway code basically gives a maximum allowable ground subsidence value (generally within 3㎝). Although this is reliable, the allowable value is not determined according to the specific situation, which not only fails to give full play to the self-supporting capacity of the pipeline, but also limits the tunnel construction progress and increases the project investment.
(4) At present, the stress and deformation calculation of pipelines are mostly based on the analysis results of Winker elastic foundation beam theory proposed by Attewell et al. in 1986, and most numerical simulations also simplify underground pipelines into foundation beams. The conclusions obtained in this way tend to be conservative and inappropriate in some cases; The calculation of pipe joint angle is mostly based on the back analysis of the calculation results of elastic foundation beam. Whether this method is suitable or not remains to be discussed because the pipeline deformation is imposed on the joint to make it "generate" angle. Moreover, the current analysis almost takes the ground movement and deformation caused by tunnel construction as input conditions to calculate the response of the pipeline, without considering the tunnel excavation and pipeline response as a whole, and lacks systematic analysis results.
Look forward
With the continuous development of social economy, the continuous growth of population and the relative shrinkage of space, people gradually turn their attention to the utilization of underground space. The development of underground space has become an important means and development trend for human beings to expand their living space, and more and more engineering environmental problems need to be studied [20]. The protection of adjacent underground pipelines in urban tunnel construction is expected to start from the following aspects in order to achieve systematic results in the future.
(1) Underground pipelines in urban areas are complex in distribution and various in types, so general survey should be done before tunnel construction (Guangzhou and other big cities have already conducted general survey of underground pipelines and established underground pipeline information system [2 1]). For pipelines with short running time, good pipe quality and small pipe diameter, the restriction standard can be relaxed; Protective measures should be strengthened for cast iron pipes that run for a long time, especially for cast iron pipes that are rigidly connected in the early stage. Because they can only bear a small joint angle and the tensile capacity of the pipe section is very poor, it is necessary to check whether they reach the limit from two aspects. Special research should be carried out on large-diameter pipelines.
(2) With the development of computer technology, the stress and strain analysis of pipeline displacement caused by tunnel can be considered by numerical simulation, and the tunnel and pipeline are considered as a system-the tunnel construction and pipeline deformation are calculated as a whole. In this way, the overall analysis of "tunnel-pipeline" can be realized by using different elements to simulate different soil, pipe-soil contact relationship and pipeline types and considering different tunnel construction methods. (3) It is necessary to combine theoretical analysis, test and field monitoring to accurately predict the initial stress and allowable deformation of the pipeline and scientifically evaluate the harm caused by tunnel construction to underground pipelines.
(4) The study of underground pipeline protection in urban tunnel construction is a systematic project, involving many disciplines and complicated influencing factors. Ignoring one aspect may lead to the destruction of pipelines. The expert system can absorb the intelligent knowledge of experts in various fields, and transform the professional model into a knowledge model, so as to make a more comprehensive, objective and accurate analysis and research on the protection of underground pipelines. Therefore, the establishment of underground pipeline protection expert system is helpful to concentrate the research results of pipeline protection and provide help for further development.
(5) To accurately evaluate the influence of tunnel construction on adjacent pipelines, we must closely combine social and economic conditions. In addition to theoretical analysis, testing and monitoring, engineering risk assessment system can also be introduced to assess the environmental problems caused by tunnel construction, taking into account many factors such as environmental protection, safety, post-cost caused by pipeline damage and so on.
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