Bearing Capacities and Failure Behaviors of F-Type Socket Joint in Rectangular Pipe Jacking Tunnel

Abstract

The joint bending test was carried out to study the bending mechanical property and deformation characteristics of the F-type socket joint in rectangular pipe jacking tunnels under the conditions of foundation settlement, construction disturbance and different upper loads. The supporting function of soils under different geological conditions on the rectangular pipe jacking was simulated by arranging different numbers of equivalent foundation springs at the bottom of the pipe. The test results show that the greater the foundation stiffness is, the greater the joint bending moment will be at the same loading displacement, which leads to greater joint opening deformation. When the pipe joint itself produces large deformation, the change rate of joint opening slows down, and the slope of the bending stiffness curve of the joint increases. The bending bearing capacity of the joint is closely related to the foundation stiffness. The greater the foundation stiffness is, the higher the bending bearing capacity.

1. Introduction

In recent years, with the rapid development of underground engineering, rectangular pipe jacking tunnels have often been used in underground crossings, municipal comprehensive pipe galleries, underground commercial space connecting channels and other projects. Advantages include a higher cross-section utilization rate, higher mechanization degree and less environmental pollution. At the same time, these tunnels demonstrate the development direction of construction technology for short and medium tunnels in future cities. However, the theoretical research on rectangular pipe jacking seriously lags behind the engineering practice, and theoretical guidance does not play its due role. Current research on rectangular pipe jacking tunnels is not comprehensive enough [1,2,3,4,5].

At present, an increasing number of scholars are paying attention to the jacking construction technology soil disturbance theory, sludge improvement technology, friction-reducing mud materials, surface deformation caused by tunnel construction and other issues [6,7,8].

Liu Wenjun et al. [9] used FLAC3D to analyze the water pressure and soil deformation of a subway tunnel under existing rectangular pipe jacking.

Liu Bo et al. [10] conducted a numerical simulation of the pedestrian passage under an existing subway tunnel by jacking method and analyzed the deformation of the tunnel and ground surface during the jacking process. Wei Gang et al. [11,12,13] used Mindlin’s solution to derive the equations of surface deformation and soil vertical deformation regarding the effect of the jacking process on surface settlement and analyzed the surface deformation law. Xiaolong Chen et al. [14], combined with an actual project, gave an overview of the key technology of rectangular pipe jacking construction, and analyzed the soil deformation caused by jacking. Xu Youjun et al. [15] derived a theoretical solution for ground surface deformation caused during rectangular pipe jacking construction, compared it with the field-measured data for verification and analyzed the causes of ground surface deformation with the obtained laws. Yang Xian et al. [16] conducted a theoretical and measured analysis of the jacking force of deeply buried pipe jacking in the pipe curtain preconstruction method, and proposed a formula for calculating the vertical earth pressure considering the soil arch effect and the soil holding force under the arch by combining Pratt’s theory and the Taishaji theory. Shi Peixin et al. [17] analyzed and theoretically derived the construction jacking force of large diameter curved pipe jacking by combining many jacking force influencing factors during pipe jacking. Xiang Antian et al. [18] studied the variation law and influencing factors of topping force and average frictional resistance with topping distance by combining the field measurement data. Wang Shuang et al. [19] divided the morphology of mud jackets in the process of pipe jacking construction and proposed a formula for calculating the frictional resistance by combining the divided morphology. Peng Ma et al. [20] analyzed the ground response characteristics of the same utilized cross-sectional area under rectangular pipes with different height and width ratios, and derived the law of geometric effects on the ground and its force state. Wei Gang et al. [21] conducted a theoretical study on the disturbance and crowding effect of soil in pipe jacking construction. Youjun Xu et al. [22] conducted an indoor test on mud ratio for the problem of poor friction reduction of mud in field pipe jacking construction. Peng Ma et al. [23] improved the existing method of calculating friction resistance of pipe jacking by combining field measurement data. Shuo Liu et al. [24] improved the traditional mud ratio and proposed a nano-modified mud ratio for rectangular pipe jacking with a large cross-section in an anhydrous sand layer. Xu Youjun et al. [25] used ABAQUS to study the joint shear test of rectangular top pipe joints by numerical simulation and investigated the force characteristics and sealing properties of the waterproof rubber ring at the joints.

Rectangular pipe joint construction usually employs F-type socket joints, which can meet the requirements of tunnel waterproofing and fine deformation. Nevertheless, the joint stiffness is much smaller than the stiffness of the tube body and is a weak part. Thus, the bending and deformation of rectangular pipe tunnel joints occur easily due to a series of factors such as disturbances during construction, changes in groundwater, and drastic variations in the load above the tunnel. Ultimately, the joint waterproofing failed, even causing damage and endangering the safety of the tunnel operation.

Based on the above analysis, the bending performance, deformation characteristics and damage mechanism of rectangular pipe jacking joints with F-type socket joints are still unclear and need to be further studied. To this end, indoor bending tests of F-type socket joints of rectangular pipe jacking were carried out and a series of numerical simulations were established. The results show that under the same loading displacement, the greater the foundation stiffness, the greater the bending moment borne by the joint, and the higher the joint bending capacity. The bending moment of the joint is mainly borne by the steel ring, and eventually the steel ring will be damaged by bending. The larger the foundation stiffness and the harder the strata, the smaller the joint bending deformation; the larger the section size of the pipe joint, the smaller the joint bending deformation; the larger the size of the steel collar, the smaller the joint bending deformation.

2. Full-Scale Experiments

2.1. Experiments Background

At present, with the development of urban rail transit, China’s rectangular pipe jacking engineering practice has achieved rapid development. Longitudinally connected with F-type socket joints is used in most rectangular pipe jacking tunnels. This type of joint is generally not equipped with bolts, which mainly play a waterproofing role and belong to flexible joints. When the uneven settlement of the foundation occurs, the bending deformation of the joint opening is very easy to occur at the joint location. The excessive longitudinal uneven deformation leads to the local failure of the joint, which greatly reduces the overall waterproof effect, resulting in leakage, water dripping, even sand and mud leakage and other diseases affecting normal use, endangering structural safety, and even causing engineering disasters. Therefore, this paper carries out relevant research on the bending resistance of the longitudinal joint of rectangular pipe jacking tunnel, explores the mode and characteristics of the joint bending failure, as well as the relationship between the joint stress and the joint opening. Through the indoor bending test of the rectangular pipe jacking tunnel F-type socket joint, the joint moment–relative rotation relationship is studied, and the joint moment–relative rotation model is established.

2.2. Test Overview Rectangle Pipe Jacking

To consider the research objectives and test loading conditions, the rectangle pipe jacking test section size was set at 1500 mm × 1625 mm × 1075 mm (length × width × height). F-type socket joint construction, as shown in Figure 1, mainly contains the pipe body concrete, a steel ring, and a waterproof rubber ring; the section size and reinforcement of the pipe section are shown in Figure 2. The assembly process through the steel ring extrusion socket rubber ring was set to complete, so as to achieve the purpose of waterproofing and resistance to deformation of the joint; the rubber ring section is shown in Figure 3. The pipe joint was cast in C50 concrete, and the steel ring was made of 10 mm thick Q235 steel ring, which was buried at the socket end of the pipe joint in the course of casting.

The mechanical property indexes of concrete materials, the mechanical property indexes of steel and the mechanical property indexes of hawser rubber ring are shown in

Table 1. Mechanical properties of test materials.

Material	Elastic Modulus (MPa)	Cube Compressive Strength fcu/ MPa	Yield Strength fy/ MPa	Extreme Strong fu/ MPa	Shore A Hardness/°	Tensile Strength/ MPa
concrete	34,500	53.1
Q235	19,600		216	352
HRB400	203,000		385	527
rubber					54	12.2

.

The bending test of the rectangular pipe joint was divided into three groups of conditions, and each group of conditions was assembled from two pipe joints, with a total length of 3 m. The horizontal restraint device was arranged outside the side wall of the pipe joint to induce the pipe joint bending deformation during the loading process, and playing a lateral protection role [26]. The test load was applied through the jack at the upper end of the counterforce frame, the distribution beam was used to transfer to the pipe joint, and finally, the joint bending test loading was completed; the experimental loading process is shown in Figure 2.

To simulate the vertical restraint effect of the stratum on the pipe joint in actual engineering, the vertical support of the stratum on the pipe joint was achieved by uniformly arranging the equivalent foundation springs at the bottom of the pipe joint. The restraint effect of different stratum conditions was achieved by changing the number of equivalent foundation springs. The correlation between the number of springs and the equivalent bed coefficient was based on the principle of equal foundation reaction force per unit displacement of the foundation, which is $n\cdot k=K_v\cdot \mathrm{S}$; the equivalent foundation stiffness can be calculated, where K_v is the number of equivalent foundation springs and S is the area of the pipe jacking base plate. The foundation springs were tested by loading tests and the single spring stiffness was obtained as $k =1734 \mathrm{kN}/\mathrm{m}$. The foundation springs were arranged in three ways, with 9, 6 and 4 springs uniformly arranged, respectively, as shown in Figure 3.

Circumferential strain gauges were arranged in the concrete and steel ring at the joint area of the pipe joint to measure the force deformation characteristics at the top and bottom plates, side walls and chamfers of the pipe joint. In addition, to study the cross-sectional deformation of the pipe joint during the test loading process, three horizontal, vertical and oblique guyed displacement meters were arranged at the joint in the form shown in Figure 4.

Joint shear test loading was displacement-controlled, and the loading system was divided into 14 levels. Considering that the loading of the initial joint force and deformation was small and later larger, the first four levels were done in increments of 5 mm, increasing to 3 mm per level from the fifth level onward. The test was stopped after displacement loading until the joint was damaged, and

Table 2. Test conditions.

Loading Conditions	Number of Springs/ Size	Equivalent Bed Coefficient ${\mathit{K}}_{\mathit{v}}$/(×103 kN-m−3)	Equivalent Stratum (Geology)
Option I	9	10.16	loose sandy soil
Option I	6	6.77	soft clay
Option 3	4	4.52	freshly filled soil

shows the design of loading conditions.

The purpose of this test is to study the force performance and deformation characteristics of F-type socket joints under bending deformation. Therefore, the upper load was uniformly transferred to both sides of the joint through the distribution beam, so that the test condition was close to pure bending to avoid excessive difference in the bending effect between the two sides of the pipe joint.

2.3. Analysis of Test Data

2.3.1. Failure Behaviors

During bending deformation of the top pipe, the concrete at the joint and the steel collar failed and was destroyed destroyed under the interactions. As can be seen from the foregoing, the middle of the top base plate and the middle of the side wall of the joint concrete was the maximum strain area, and in the loading process, when the loading was about 15 mm, the top base plate started to show fine cracks and produce large deformation, while when the loading was 25 mm, the concrete outside the middle of the side wall appeared to have fine cracks, and as the load continued to increase, the micro-cracks continued to extend and penetrate each other. In addition, during the bending process, the concrete of the top slab on both sides of the socket was extruded from each other, which led to the crushing of the concrete at the top slab. The damage characteristics of the concrete in each part are shown in Figure 5.

With the increasing bending deformation of the pipe joint, the interaction between the socket concrete and the steel ring was gradually enhanced, and the bending deformation of the pipe joint was mainly generated by the deformation of the steel ring as shown in Figure 6.

2.3.2. Stress Deformation of Pipe Joint

The force conditions of the pipe joint are shown in Figure 7, where F is the upper jack reaction force, and the $k_i{x}_j(i=j=1,2,3)$ term is the equivalent spring reaction force under the foundation, where $k_1 = 5202 \mathrm{kN}/\mathrm{m}$ and $k_2 = k_3=3468 \mathrm{kN}/\mathrm{m}$. According to the force balance ${\displaystyle \sum M=0}$, the joint bending moment M can be obtained, and the joint bending moment changes as shown in Figure 8.

From Figure 8, it can be found that under the same load displacement, the larger the foundation stiffness, the greater the moment of the pipe joint. The joint will be deformed under the upper load, and the amount of joint opening will change as shown in Figure 9.

As shown in Figure 9, the joint tension is closely related to the joint moment, the larger the joint bending moment, the larger the corresponding joint tension.

In addition, under the 9 springs, when the loading reached 15 mm, the trend of joint opening distance changes slowed down, and at the same time, it was observed that micro-cracks started to appear at the middle of the top and bottom plate of the pipe joint; with the increase of loading displacement, the micro-cracks continued to expand and penetrate, indicating that the upper applied displacement was not only borne by the deformation of the foundation spring and the deformation of the joint, but also partly borne by the deformation of the pipe joint, and thus the rate of increase of the joint tension slowed down.

Similarly, under 6 springs, when the loading reached 20 mm, small cracks started to appear at the middle of the top and bottom of the pipe joint, and the rate of change of the joint opening slowed down. At 4 springs, when the load reached 30 mm, a small crack started to appear at the middle of the top and bottom plate of the pipe joint and the rate of change of the joint tension slowed down.

The change of joint bending stiffness can be further obtained from the change of bending moment and tension at the joint, as shown in Figure 10.

As can be seen from Figure 10, the joint bending stiffness can be roughly divided into two stages. When the tensioning amount was less than 0.007 rad, the joint itself did not have large deformation, the joint stiffness was mainly controlled by the strength of the joint itself and the difference of the joint bending stiffness under each equivalent foundation was small. When the tensioning amount was greater than 0.007 rad, micro-cracks appeared at the axillary corners of the side walls of the pipe joint, and as the upper loading displacement continued to increase, the micro-cracks expanded further and penetrated, and the pipe joint itself started to deform more, leading to the slowdown of the joint tensioning amount and further increasing the slope of the curve.

2.3.3. Deformation of Pipe Joint Section

The deformation of the cross-section at the joint during loading was found by means of a guyed displacement meter. The value of the guyed displacement meter is positive when it is stretched and negative when it is shortened. The deformation of the joint in three directions (horizontal, vertical and diagonal) is shown in Figure 11.

The horizontal direction of the guyed displacement meter exhibited at different degrees of tensile deformation at the equivalent foundation spring. When the displacement reached 30 mm, the sidewall underwent outward bending, and correspondingly, fine cracks began to appear in the concrete on the outer side of the middle of the sidewall. In addition, under the same displacement, the greater the foundation stiffness, the greater the horizontal deformation produced by the pipe joint.

The vertical pull-wire displacement meter showed different degrees of shortening deformation at the equivalent foundation spring. The deformation characteristics also showed that the greater the foundation stiffness, the greater the deformation produced by the pipe joint. After the upper loading displacement reached 15 mm, the top bottom plate had already begun to deform, indicating that the top bottom plate was more obviously stressed than the side walls during the loading process. In the diagonal direction, there was no obvious deformation of the pipe section under the 9 springs, while the 6 springs and the 4 springs diagonal had different degrees of compression deformation with the loading displacement.

From Figure 11, it can be found that the deformation of the pipe joint in the horizontal direction and vertical direction was small, and the deformation in the oblique direction was large, indicating that the pipe joint had twisted deformation during the loading process. Under the 9 springs and 6 springs, no significant torsion was produced in the pipe section, while under the 4 springs, a torsional deformation was produced in the pipe section due to the eccentricity of the load.

3. Numerical Simulations

3.1. Model Size and Material Parameters

The overall model of rectangular pipe jacking tunnel joint consists of pipe joint, steel collar, distribution beam and foundation spring. The size of the single pipe section is 1.625 m × 1.075 m × 1.5 m, the wall thickness of the pipe section is 0.15 m, and the pipe section is made of C50 concrete. The socket segment is equipped with a steel collar, which is made of Q235 steel plate by cold-forming process. The reinforcement cage is designed with reinforcement according to the actual engineering conditions and embedded into the concrete of the pipe section. Except for the reinforcement, all other components are 3D solid units. See Figure 12 and Figure 13.

The plastic damage intrinsic model, provided in Abaqus, is used in the model, which assumes that concrete materials are damaged mainly by tensile cracking and crushing fragmentation, and the differences in material tensile and compressive properties is considered to simulate the irrecoverable material degradation induced by damage under low hydrostatic pressure.

The master–slave contact algorithm is used in the model, and the slave surfaces should be selected for finer meshing in order to converge the results while obtaining more accurate calculation results. Each component of the joint is mainly set up with surface-to-surface contact and binding constraints. The binding constraints is used in steel collar and socket concrete; surface-to-surface contact is set in the steel collar, socket concrete and adjacent pipe joints; and penalty friction and “hard” contact is set in the tangential behavior. The friction coefficient is 0.2, allowing separation after contact. See Figure 14.

The same loading method and boundary conditions are used for all working conditions of this model. The boundary conditions are mainly applied at the bottom and the left and right sides of the model. Different elastic constraints are applied at the bottom to simulate the support of different strata on the overall top tunnel; displacement constraints are applied at the left and right sides to ensure that the overall top tunnel does not twist. See Figure 15.

The displacement is applied by means of displacement application, and the displacement is evenly applied to the distribution beam, which is placed at about 500 mm to the left and right of the joint, and the load is transferred to the pipe joint through the distribution beam by control jacks, which induces the pipe joint to rotate, resulting in tension deformation of the joint, as shown in Figure 16. When loading, amplitude adjustment is required to load in a smooth analysis step to ensure the calculation accuracy.

3.2. Numerical Results and Comparisons to the Tests

The numerical simulation on rectangular pipe jacking tunnel joint segment was firstly conducted to verify the accuracy and applicability of the numerical model, as shown in Figure 17.

It can be seen from Figure 17 that the overall damage of the pipe joint showed that the top of the pipe was compressed, and the bottom of the pipe was separated under the action of bending moment. The plastic deformation occurred at the top of the pipe joint, resulting in the steel collar at the joint being extruded by the concrete at the socket segment, which deformed in flexure and buckled upward. The steel ring crushed concrete at the upper contact surface, and there was a large degree of concrete separation between the steel ring at the bottom of the pipe and the concrete at the socket segment. In this case, the pipe joint was deformed greatly because of the bottom of the pipe joint, resulting in structural damage and the calculation was stopped.

From the deformation characteristics of the model, it was found that the deformation of the top plate, bottom plate and side wall is more significant. Therefore, the deformation of the top plate, bottom plate and side wall at the joint was analyzed in detail by setting the junction and path to extract the required junction displacement at the socket segment, the socket segment and the steel ring.

The 14 units were divided spaced from left to right into equally in the jack segment, socket segment and steel ring at the joint of the top plate and bottom plate, and 15 nodes were set to extract their vertical displacements, and the deformation curves shown in Figure 18 are drawn. As shown in Figure 18 (the negative sign in the figure indicates the deformation direction downward), the vertical deformation was the largest in the center of the top plate, with a maximum value of 40 mm.

As shown in Figure 18, the deformation of the jack segment, socket segment and steel ring of the top plate joint of the top pipe was basically the same under the load, the vertical deformation of the middle part of the top plate was the largest, the vertical deformation of the chamfered position gradually decreased, the maximum difference in vertical displacement between the jack segment and the socket segment was 28.84 mm and the maximum difference in vertical displacement between the socket segment and the steel ring was 29.16 mm. The deformation law of jack segment, socket segment and steel ring at the joint was approximately the same as that at the top plate, the vertical deformation was the largest at the middle of the steel ring at the bottom plate, the vertical deformation gradually decreased at the chamfered position, the maximum difference in vertical displacement between the jack segment and the socket segment was 24.28 mm, and the maximum difference in vertical displacement between socket segment and steel ring was 29.26 mm. The above shows that the joint area of the top plate is more prone to deformation than other locations under the same load, so it is necessary to focus on protection research at the top plate of the joint area, when dealing with the safety issues such as the waterproofing and uneven settlement of rectangular pipe tunnel

The vertical load on the top plate was larger than the horizontal load on the side plate; the deformation of the side plate of the rectangular top pipe at the socket segment and the spigot segment was approximately wavy, from top to bottom, the side plate was concave and then convex, and then concave again, and returned to normal when it was close to the bottom plate. The maximum deformation of the side plate was 0.05 mm, which was much smaller than the minimum deformation of the top and bottom plates of the rectangular top pipe joint of 7.42 mm. Therefore, the deformation of the side plate can be approximately ignored compared with the top and bottom plates of the pipe joint.

When bending deformation occurred in rectangular pipe jacking tunnel, the steel ring was used as the main force-bearing member. Thus, the bending moment of the steel collar is regarded as the bending moment caused by the joint bearing the bending deformation. Different foundation stiffnesses represent different strata, where the vertical bed coefficient is 50,000 kN/m³ for gravel strata, 30,000 kN/m³ for silt strata and 15,000 kN/m³ for clay strata. Extracting the joint bending moment and joint relative turning angle under different foundation stiffness conditions, the joint bending moment–joint relative turning angle variation curve is obtained (see Figure 19).

To verify the accuracy of the numerical simulation, the simulated values were compared with the experimental data as shown in Figure 19. A rectangular pipe jacking tunnel joint computational model is established in this paper, and its bending moment and relative angle trend are in good agreement with the bending test data. The tension angle at the joint obtained from the simulation is generally larger than the experimental value, mainly because of the different test conditions and the existence of certain errors in the assembling of the test pieces.

4. Parametric Analysis

4.1. Analysis of Cross-Sectional Dimensions

According to the Technical Regulations for Rectangular Pipe Jacking Engineering (T/CECS716-2020), the cross-sectional dimensions of 1625 mm × 1075 mm, 6000 mm × 4300 mm, 7000 mm × 5000 mm, 7700 mm × 4500 mm, 9100 mm × 5500 mm and 10,400 mm × 7500 mm were selected for analysis and study to compare their joint bending moment–relative corner relationships. This is shown in Figure 20.

As can be seen from Figure 20, under the action of the joint bending moment, a section of the joint is compressed and the other end is opened, and the compression amount is much smaller than the opening amount. The maximum amount of joint opening with the joint bending moment is more significant, there is an approximately linear relationship for the different cross-sectional size of the pipe joint, the relative angle of the joint under the action of the joint bending moment and the law of change are basically the same, but the larger the cross-sectional size of the pipe joint, and the larger the joint bending moment and relative angle, the greater the rotational stiffness. It is calculated that for each increase of 1.3 m to 2 m in section size, the rotational stiffness increases from 160 kN-m/rad to 170 kN-m/rad.

4.2. Analysis of Steel Ring Dimensions

The effect of steel collar size on the rotational stiffness of the pipe joint is mainly in the concentrated steel collar length and thickness, and the lengths l = 360 mm, l = 340 mm, l = 320 mm, l = 300 mm, and l = 280 mm and t = 19 mm, t = 16 mm, t = 15 mm, t = 12 mm and t = 10 mm were selected here for comparative analysis [14]. Their M–θ relationship curves under the influence of length and thickness are shown in Figure 21 and Figure 22.

Before the relative angle of joint θ reaches 0.027 rad, the contact between the steel ring and the socket joint has not started, which is mainly manifested by the extrusion of concrete on both sides of the top joint and the “head knocking phenomenon”. When the relative angle of the joint reaches 0.122 rad, the joint bending moment continues to increase, the angle remains constant, and the steel collar and the concrete of the socket pipe joint are mutually extruded and slide to the upper edge of the chamfer. After that, the joint bending moment continues to increase linearly with the relative angle of rotation. This is because the steel ring at the top plate and the concrete at the socket of the pipe joint are extruded from each other, and gradually slide to the upper edge of the chamfer from the middle area of the pipe joint, while the lower part of the steel ring and the lower part of the pipe joint are disconnected. At this time, the joint bending moment continue to increase, and the angle remains unchanged.

In addition, the joint bending moment and rotational stiffness were minimized at a length of 280 mm and a thickness of 10 mm, and maximized at a length of 360 mm and a thickness of 19 mm. By fitting the above data, the results show that increasing the length of the ring or the thickness of the ring can increase the rotational stiffness, with an average increase of 16% for each 20 mm increase in the length of the ring and 5% for each 1 mm increase in the thickness of the ring.

5. Conclusions

Based on full-scale experiments and a series of numerical simulations, this paper presents the Bearing Capacities and Failure Behaviors of F-Type Socket Joint in Rectangular Pipe Jacking Tunnel. The failure processes, rotational angle, and bending stiffness of segments were monitored and analyzed in the full-scale tests. The influence law of different foundation stiffness on bearing characteristics of jacking pipe tunnel is clarified. The main research conclusions are as follows.

(1)

Under the same loading displacement, the greater the foundation stiffness, the greater the bending moment borne by the joint, and the higher the joint bending capacity. The bending moment of the joint is mainly borne by the steel ring, which will eventually be damaged by bending.

(2)

The change of joint opening is closely related to the change of joint bending moment; the larger the joint bending moment, the larger the corresponding joint opening. In addition, the larger deformation of the pipe joint itself will lead to a slower rate of change of the joint opening, resulting in an increase in the slope of the joint bending stiffness.

(3)

The deformation of the pull-wire displacement gauge at the joint, the concrete strain and the steel collar strain show the same change pattern, which all indicate that the joint tends to be flattened during the bending loading process.

(4)

The deformation of the steel collar is closely related to the deformation of concrete, and the two are in the same state of stress at the same position. In addition, the top bottom plate of the steel ring will be warped near the socket side, and the degree of warping is most obvious at the axillary corner of the top bottom plate due to stress concentration. When the bottom of the pipe joint is separated from the socket end, the structure is eventually deformed due to the large deformation of the bottom of the pipe joint.

(5)

Joint bending deformation is influenced by the foundation stiffness of the strata. The larger the foundation stiffness and the harder the strata, the smaller the joint bending deformation; the larger the section size of the pipe joint, the smaller the joint bending deformation; the larger the size of the steel collar, the smaller the joint bending deformation.