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In diesem Kapitel wird der Einfluss des Kiesschildvortriebs unter bestehenden Tunneln auf die Verformung untersucht, wobei der Schwerpunkt auf der Absenkung der Oberfläche und der Verformung des bestehenden Tunnels liegt. Die Studie untersucht drei entscheidende Faktoren: Kreuzungswinkel, Nettoabstand und unterste Verlustrate. Durch eine Kombination aus Modellversuchen und numerischen Simulationen liefert die Forschung eine umfassende Analyse, wie diese Faktoren die Verformung sowohl der Oberfläche als auch des bestehenden Tunnels beeinflussen. Die Ergebnisse zeigen, dass mit zunehmender Verlustrate sowohl die vertikale Verformung des bestehenden Tunnels als auch die Absenkung der Oberfläche signifikant zunehmen. Umgekehrt verringert die Vergrößerung der Nettodistanz den maximalen Verformungswert und verlangsamt die Verformungsrate. Die Studie unterstreicht auch die erheblichen Störungseffekte, die durch den ersten Aushub in der linken und rechten Linie des neuen Tunnels verursacht wurden. Beim Vergleich der Daten aus numerischen Simulationen und Modellversuchen zeigen die Ergebnisse ein hohes Maß an Übereinstimmung und bestätigen die Ergebnisse. Die Untersuchung kommt zu dem Schluss, dass die Oberflächenablagerung und die Tunnelverformung zwar die Warnstandards überschritten haben, aber noch keine unmittelbare Gefahr darstellen. Diese detaillierte Analyse bietet Experten, die mit Tunnelbau und Infrastrukturentwicklung zu tun haben, wertvolle Einsichten und liefert ein tieferes Verständnis der Verformungsmechanismen und der Faktoren, die sie beeinflussen.
KI-Generiert
Diese Zusammenfassung des Fachinhalts wurde mit Hilfe von KI generiert.
Abstract
In recent years, the rapid development of urban rail transit in China has led to an increasingly dense subway network. The construction of new subway tunnels frequently requires crossing many buildings in the city and existing infrastructure such as tunnels. Especially in the construction of Wumi Road Station of Kunming Metro Line 5, the construction phenomenon of crossing the existing structure is particularly significant. Based on this background, this paper uses the combination of model test and numerical simulation to explore the construction deformation law of shield tunneling under the existing tunnel in sandy cobble stratum of Kunming subway. After in-depth research on the three key factors of crossing angle, tunnel clearance, and formation loss rate, we have drawn the following conclusion: whether using a 60°oblique crossing method or a 90° straight crossing method when passing through the existing tunnel, their final impact on the surface and the existing tunnel is roughly the same. Further analysis reveals a significant negative correlation between the final deformation of the surface and existing tunnels, as well as the net distance between tunnels. That is, the larger the net distance between tunnels, the smaller the deformation; there is a positive correlation with the formation loss rate; that is, the higher the formation loss rate, the greater the deformation. With the increase of distance, the range of vertical deformation and surface settlement will also increase. In addition, there is a linear relationship between the formation loss rate and the final deformation value.
1 Introduction
With the continuous increase of infrastructure area, the contradiction between urban transportation resources and the speed of urban development has gradually become prominent, which has become a major problem in municipal construction. As the surface space is compressed due to the rapid development, which cannot meet the growing demand of urban road construction, more and more cities turn their eyes to underground and take the development of underground rail transit as the primary choice to relieve the urban traffic pressure [1‐7].
In the actual situation of the shield tunnel underpass construction, the core mechanism that causes a series of deformation problems is due to the interaction between the new tunnel, the soil, and the existing structure. The soil bears the disturbance caused by the construction and transmits the displacement field generated by this disturbance to the existing structure [8]. As a common research method, model test plays an important role in exploring the deformation law of structure and formation in the construction process. For example, Zhu [9] et al. used the scale model test to explore the law of formation settlement with time and explain the effect of double-line tunnel superposition. Chen Renpeng et al. [10] used the self-made shield machine model to systematically study the potential impact of different buried depths on the boundary bearing force and the surface settlement of dry sand formation. Fang Qian et al. [11], based on the model test, deeply studied the influence mechanism of particle grading and other factors in the construction of sand formation and found that the change of particle size has a significant direct impact on the settlement value. Liu Xinjun et al. [12] combined with the engineering background of Nanjing Metro Line 5 through the existing Line 1, comprehensively elaborated the 3D spatial deformation characteristics of the orthogonal area by using model test and numerical simulation methods.
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This paper takes the engineering practice of the shield under the existing Line 3 tunnel of Kunming Metro Line 5 as the background and carries out systematic research and analysis on the deformation of the shield under construction combined with model test and numerical simulation, hoping to provide reference for subsequent and similar engineering cases.
2 Research on Puncture Deformation Law Based on Model Test
2.1 Introduction of the Test Support Project
This chapter studies the potential impact on the existing line 3 tunnel and the surface subsidence of Kunming Metro Line 5. The simulation study analyzes the specific influence law of shield construction in the left and right lines of Line 5, especially when the tunnel of Line 5 (the buried depth of the vault is 23.5 m) intersects the existing tunnel of Line 3 (the buried depth of the vault is 14.9 m). In addition, the spacing between the left and right tunnels of Line 5 is precisely designed to be 8.8 m, while the left and right tunnels of Line 3 is 8.6 m. Note that the net distance between the two tunnels is 2.1 m and 2.2 m, respectively. When the shield machine of Line 5 tunnel crosses the tunnel of Line 3, the crossing angle between them is 62.8°, which brings some technical challenges to the construction process. In the actual construction, we use the “from left to right” order to ensure the safety and efficiency of the construction.
In order to simplify the test process and enhance the feasibility and efficiency of the study, we unified some parameters in the simulation test, setting the spacing between Line 5 and Line 3 to 8.7 m. At the same time, the spacing between the upper and lower tunnels was also simplified to 2.2 m. In addition, the crossing angle between the two tunnels was simplified and set to 60° to simulate the analysis. These adjustments are designed to ensure that the core objectives of the study are unaffected, while improving the convenience and efficiency of trials.
2.2 Determination of the Model Similarity Ratio
In the design of this model test, we fully referred to the research results of many scholars [12‐15] and the similar theories mentioned above, aiming to comprehensively and comprehensively explore the influence law of multiple key indicators on the construction of shield tunneling through the method of dimensional analysis. These key indicators include geometric size (L), material bulk density (γ), compression modulus (Es), and Poisson's ratio (μ). Based on these considerations, we formulated the similar parameters of the model trials and listed them in detail in Table 1.
Table 1
Expression table for each similar parameter of the model test
Using the exponential method to set the geometric similarity constant and substituting the geometric similarity constant into the expression of stress, we derive the following dimensional relation:
Similarly, the parameters in Table 1 into the displacement expression to obtain the relationship between the similarity constants as follows:
$$C_{E} = C_{\sigma } = C_{\gamma } C_{l}$$
(9)
$$C_{F} = C_{E} C_{l}^{2}$$
(10)
After fully considering the purpose and conditions and space in the test, we selected the following similar parameters: geometric similarity constant Cl is set to 60, and the compression modulus and stress similarity constant CE and Cσ are set to 60, to ensure that the physical quantity in the same proportional relationship between the model and the prototype. Meanwhile, meanwhile, because the material density is less sensitive to the model scaling, we set the similarity constant of the material density Cγ to 1; that is, the model is consistent with the material density in the prototype. The parameters determined are similar as shown in Table 2.
Table 2
Table of similar ratios of each parameter
Parameter
Displacement
Modulus of elasticity
Poisson ratio
Unit weight
Stress
Meet an emergency
How much
Ratio of similitude
60
60
1
1
60
60
60
2.3 Design and Manufacture of the Test Model Box
In order to further study the settlement law of the existing tunnel and the ground surface, we set the long side of the model box as a section perpendicular to the excavation direction of the tunnel, while the short side is consistent with the excavation direction of the tunnel. To minimize this effect, we are careful to limit the effect of tunneling on the surrounding strata to an area of 3 to 5 times the tunnel diameter (D). Such a design ensures that we can more accurately observe and analyze the dynamic changes of the settlement curve, thus providing a deep understanding of the formation response mechanism during tunneling. After careful planning and calculation, careful planning, and accurate measurement, we finally determined the size of this test model to be 1.80 m × 1.00 m × 1.20 m, ensuring its high degree of accuracy and reliability.
After a thorough study of the design documentation and reference drawings (as shown in Figs. 1 and 2), we found that a 250 mm spacing was used for the Line 3 tunnel and a 288 mm spacing for the new Line 5 tunnel. In the model simulation, the center of the existing Line 3 tunnel is 313 mm away from the top of the model, while the center of the new Line 5 tunnel is farther away from the top of the model, specifically 456 mm. These precise dimensional parameters are the key factors to ensure the safe and efficient construction of shield construction.
The overall design of the model box takes into account the quality and stiffness, ensures its strong carrying capacity and stability (Fig. 3), and thus effectively reduces the error caused by the deformation of the box. And a working window is specially left on the front side of the model box, as shown in Fig. 4.
Fig. 3
Model test box (laboratory instrument, obtain a license)
In order to comprehensively explore the impact of surface subsidence in the construction process of double-line shield, this experiment was carefully planned and specially set up two measuring lines A and B. The two test lines are arranged in strict accordance with the criteria of being parallel to the tunnel of Line 3, aiming to ensure that the collected data is both accurate and highly representative. During the detailed layout process, line A is accurately set 218 mm from the long edge of the front edge of the model box. We place the measuring points numbered AD1 to AD11 from left to right. At the same time, the measuring line B is accurately placed directly above the left line of Line 3362 mm away from the long edge of the front of the model box. Similarly, we also set the measuring points numbered BD1 to BD11 on this line from left to right. This well-designed layout and numbering method not only provides a clear reference for the subsequent surface subsidence monitoring, but also provides an orderly reference basis for data analysis.
In terms of the measuring point layout, we followed the symmetrical layout principle based on the center line of the tunnel of Line 5. We specially put the no. 6 measuring point accurately in the central position, as the core reference point of the whole measuring line. Then, we used this central point as a benchmark and evenly distributed five measuring points on the left and right sides, with the aim of comprehensively and meticulously monitoring the subsidence of the surface around the tunnel. The distance between each adjacent measuring point was precisely set to 132 mm to ensure the accuracy and consistency of the data. In conclusion, each measuring line contains 11 sets of measuring points. The specific layout can be shown in Figs. 5 and 6.
Fig. 5
Arrangement diagram of surface measuring line and measuring points
In this test, we specially arranged two measuring lines, respectively, located at the left and right axis of Line 3. In order to accurately capture the settlement change, these two measuring lines are arranged at the vault position of Line 3, with the center line of Line 5 as the reference axis. The adjacent spacing between the measurement points is accurately controlled as 132 mm to ensure data continuity and accuracy. There are 11 measuring points set on each measuring line, shown in Figs. 7 and 8.
Fig. 7
Layout diagram of tunnel measuring line and measuring point of the existing left line
In order to explore deeply the formation loss rate, the formation loss rate caused by the crossing angle, and how the net distance between the tunnels affects the surface settlement and the deformation of the existing tunnels, we carefully planned six sets of test conditions. Detailed settings of each group working condition have been arranged in Table 3.
Table 3
Model test conditions
Number
Through the angle
Formation loss rate (%)
Clear distance (D)
1
90°
0.75
0.4
2
60°
1.5
0.4
3
60°
2.0
0.4
4
60°
0.75
0.4
5
60°
0.75
0.8
6
60°
0.75
1.2
3 Analysis of the Model Test Results
3.1 Analysis of the Land Surface Settlement Results
Figures 9 and 10 intuitively shows A variety of test conditions, double tunnel mining work after the completion of A, B two line recorded by the surface subsidence data, these data reveals the specific situation of the settlement for us, help us to further analyze and understand the influence of tunneling on the surface, for we analyze the influence of tunneling on the surrounding environment provides an important data support.
The following conclusions can be drawn from the drawing:
(1)
When comparing the surface settlement data after excavation, the settlement amount of each point of measurement line A is generally significantly more than that of the corresponding point of measurement line B. This difference stems from the strong resistance of the existing tunnel to stratigraphic disturbances, which actually strengthens the original formation and effectively limits the diffusion of the displacement field, and thus has relatively little surface subsidence in the line B area.
(2)
It can be seen from the figure that the settlement peak of measurement line A and measurement line B both converge at the measurement points near the center line of the new tunnel. The settlement is negatively correlated with the distance from the center line; that is, the closer to the center line, the settlement and settlement speed increase. This surface settlement pattern presents a similar “V” groove, which intuitively reveals that in the process of tunnel excavation, the influence on the surrounding soil mainly focuses on the adjacent area of the excavation axis. That is, with the gradual narrowing of the distance from the excavation point, the disturbance degree of the soil is gradually enhanced. The effect is mainly limited to approximately 5 times the tunnel diameter (5D) from the excavation center.
(3)
The test results show that under the condition of condition 3, the cumulative settlement of each measuring point of line A and B reaches the peak, while the trough is in condition 6. Comparing conditions 1 and 4, it is found that when the angle increases from 60° to 90°. The emergence of this settlement pattern is largely due to the parallel arrangement of the measuring lines A and B along the long side of the model box. In particular, at the crossing angle of 60°, the contact length between the measuring line and the tunnel is extended, so that the influence of the tunnel excavation on each measuring point becomes more significant and prominent.
(4)
By comparing the data curves of working conditions 2, 3, and 4, we find that after fixing the crossing angle and the tunnel distance, the increase of the formation loss rate leads to the increase of the settlement value of the measurement points on the two measurement lines A and B. The mechanism behind this increase is that the increase of the formation loss rate leads to the expansion of the gap between the soil and the tunnel segment, which weakens the supporting conditions of the surrounding rock, thus aggravating the degree of surface subsidence.
(5)
After comparing the data curves of working conditions 4, 5, and 6, we draw an obvious conclusion: when the formation loss rate and crossing angle are constant, the increase of net distance will lead to the decrease of the settlement of the measuring point near the center line. With the increase of the net distance of the tunnel, the position of the existing tunnel is relatively moved down, and the soil layer in the middle is thickened. This thickened soil layer can form a more obvious soil arch effect, which effectively strengthens the support ability of the stratum and then reduces the disturbance and subsidence of the tunnel excavation on the surface.
3.2 Analysis of the Vertical Displacement Results of the Existing Tunnel
Figure 11 clearly shows the cumulative vertical displacement changes of the existing tunnel vault on the left line after the completion of the double-line tunnel excavation under different test conditions. Accordingly, Fig. 12 intuitively reflects the cumulative vertical displacement trend of the existing tunnel vault on the right line under the same working conditions.
The following conclusions can be drawn from the figure above:
(1)
After a detailed analysis of different construction conditions, we find that the final displacement values of the two tunnels are very close and within a similar numerical range. It is worth noting that in condition 6, the settlement curve is obviously “U” type, while in the rest of the construction conditions, the displacement curve is more “W” type.
(2)
After observing the similar settlement rules and similar settlement values of the existing tunnels on the left and right lines, we decided to focus on each working condition of the left line for detailed analysis. By comparing the displacement curves of working conditions 1 and 4, we observed two important phenomena: first, the maximum displacement value of both is concentrated in the LS5 measuring point. When the double-line tunnel excavation is completed, the cumulative vertical displacement change of the existing tunnel vault on the left line is 0.195 mm, while the right line is 0.200 mm.
(3)
After comparing the curve data of working conditions 2, 3, and 4, it is found that the maximum displacement value of the vault is concentrated in the LS5 measurement point under all working conditions. Specifically, in condition 2, the LS5 displacement is 0.284 mm; in condition 3, the displacement increases to 0.388 mm; in condition 4, the LS5 displacement is 0.203 mm. It shows that if the factors are the same, the increase of the formation loss rate will directly lead to a significant increase of the cumulative settlement value of the existing tunnel.
(4)
After the detailed curve comparative analysis of conditions 4, 5, and 6, we draw the following conclusions: in conditions 4 and 5, the maximum displacement of the vault appears in the measuring point of LS5, that is, the central axis of the left line of the new tunnel. Specifically, the displacement value of condition 4 is 0.202 mm, while the displacement value of condition 5 is 0.163 mm. This result shows that the displacement on the left central axis of the new tunnel is significant under similar construction conditions. However, in working condition 6, we observed a change in the position of the maximum displacement, transferred to the LS6 test site, the center line of the existing tunnel. The displacement value for this point is 0.132 mm. This change indicates that the center line of the existing tunnel may be more affected under specific construction conditions (such as the change of crossing angle) and requires special attention during the construction process.
3.3 Establishment of the 3D Numerical Simulation Model
Focusing specifically on the three core factors of tunnel net distance, crossing angle, and formation loss rate, we systematically analyze their key effects on the shield construction process and accurately evaluate the extent of these effects. This study provides an important basis for us to better understand the deformation mechanism during the shield construction process. In view of this, to address such problems more efficiently and comprehensively assess the construction impact, we plan to introduce numerical simulation methods as a validation and complementary means to the experimental results.
In order to ensure the accuracy of the simulation results and minimize the potential impact of the boundary conditions, based on the principle of Saint-Vernan, a relatively large simulation boundary range, that is, 3 to 5 times the outer diameter of the shield machine as the boundary size of the simulation. At the same time, according to the size setting of the previous chapter model test, we carefully determined the size of the FLAC 3D 3 D model as 106 m 62 m 70 m to ensure that the simulation results can accurately reflect the deformation in the actual engineering.
In the model setting, we adopt the Mohr–Coulomb criterion to describe the strength characteristics of each soil layer. At the same time, the lining and isogeneration structures are regarded as isotropic ideal elastomers as shown in Fig. 13.
3.4 Layout Scheme of Surface Survey Line and Existing Tunnel Survey Line
On the basis of previous research, we realize that existing tunnels have some mitigation effect on surface subsidence. In order to further explore this influence mechanism and according to the research practice of selecting the most unfavorable conditions in engineering practice, this section will specifically study the surface subsidence deformation of the existing tunnel.
According to the results of the previous section, we observed that the left and right lines of the existing tunnel were similar in the final settlement, and the settlement change law during excavation also showed a high degree of consistency. In order to simplify the research content and avoid repeated analysis, we decided to study the left line of the existing tunnel as the main measurement line in the following discussion, so as to deeply analyze the influence of different factors on the vertical deformation of the existing tunnel.
4 Analysis of the Numerical Simulation Results
4.1 Simulation Analysis of the Surface Impact
As shown in Fig. 14, the adjustment of the tunnel net distance did not change the “V” pattern unique to the final settlement curve of the surface. It is worth noting that directly above the axis of the new shield tunnel is the most significant deformation of the existing tunnel. Through in-depth data analysis, we found that when the net distance is set to 0.2D, the excavation of the tunnel leads to be within 10.5D; when the net distance increases to 3D, this influence range is significantly expanded to about 12.3D. This obvious trend shows that as the net distance of the tunnel increases, the influence range of the surface transverse deformation will increase accordingly.
Fig. 14
Decurve of surface line under different net distance
As shown in Fig. 15, when examining the influence of different net distance conditions on surface settlement, we observed that the maximum settlement value of the surface reached 17.64 mm when the net distance was 0.2D, while the minimum settlement value decreased to 6.78 mm when the net distance was expanded to 3D. The final surface settlement curve clearly shows a trend: the surface settlement decreases significantly as the net distance increases, but the rate of reduction gradually slows down. Importantly, even under the most unfavorable conditions, the surface settlement value does not exceed the specified 20 mm warning value. This finding shows that in the context of dependent engineering, the impact of changes in net distance conditions on surface settlement is in a controllable state and will not cause safety risks.
Fig. 15
Decurve of surface line under different formation loss rates
As shown in Fig. 16, for the underpass project discussed in this paper, we observed a remarkable phenomenon: the maximum settlement point in the surface survey line is always located precisely directly above the center line of the two new tunnels. This observation shows that the “V” shape of the final surface settlement value curve remains unchanged regardless of how the formation loss rate is adjusted. Further analysis shows that there is an approximate linear growth relationship between the formation loss rate and the surface settlement. The detailed analysis shows that under the condition of strictly controlled formation loss rate of 0.25%, the ground surface transverse deformation caused by the excavation of the double-line shield tunnel is mainly concentrated in the range of 9.7D. However, as the formation loss rate climbs to 1.5%, the influence boundary of the ground surface transverse deformation extends outward to an area of approximately 12.1D. This data change intuitively reveals a rule: with the increase of the formation loss rate, the range of influence of the surface transverse deformation will gradually expand.
Fig. 16
Deformation of existing tunnels under different net distance conditions
In the deep study of the influence of formation loss rate on surface subsidence, we reveal a remarkable law: with the increase of formation loss rate, the final surface subsidence also increases significantly. Specifically, when the formation loss rate climbs to 1.5%, the maximum surface subsidence value is 24.66 mm; otherwise, when the formation loss rate remains low at 0.25%, the minimum surface subsidence value is only 6.75 mm. It is particularly noteworthy that once the formation loss rate η reaches or exceeds the critical value of 1.25%, the formation settlement value will exceed the early warning range, which poses a potential threat to the safety and stability of the project. Therefore, in the actual engineering operation, the formation loss rate must be strictly controlled to ensure that it always remains within the safety threshold, so as to ensure the smooth progress and long-term safety of the project.
4.2 Simulation Analysis of the Effects on Existing Tunnels
For the underpass engineering under specific formation conditions, the tunnel net distance has a significant influence on the deformation characteristics of the existing tunnel. Specifically, the deformation curve of the existing tunnel shows a typical “W” pattern when the net distance d is between 0.2D and 1D, and when the net distance d is greater than or equal to 1D. A deeper analysis shows that the adjustment of the tunnel net distance is directly related to the influence range of the transverse deformation of the existing tunnels. Especially in the net distance of 0.2D, the construction of the double-line shield tunnel will mainly concentrate the transverse deformation of the existing tunnel within a range of about 9.7D. However, as the tunnel net distance increases to 3D, this range of influence expands significantly extended to a region of about 11.2D. This obvious change trend clearly shows that the increase of the tunnel net distance will directly lead to the gradual increase in the influence range of the transverse deformation.
To study the specific effect of the net tunnel distance on the vertical displacement of the existing tunnel in detail, we focused on analyzing the data of the maximum vertical displacement data of the tunnel arch bottom under different working conditions, as shown in Figs. 17, 18 and 19. After careful analysis, we draw the following important conclusions: as the net distance of the tunnel gradually increases from 0.2D to 3.0D, the vertical displacement value of the characteristic point of the arch bottom above the left line axis shows a significant downward trend. Specifically, the maximum vertical displacement value was gradually reduced from the initial 13.91 mm, until reaching a minimum value of 5.62 mm. Of particular concern is that the final displacement values of the existing tunnel change significantly in the narrow range of net distance of 0.2D to 0.4D, and these values exceed the warning value of tunnel settlement control. This finding warns us that additional engineering control measures must be taken to effectively limit the vertical displacement of existing tunnels and ensure construction safety and engineering stability. Although the vertical displacement value decreases when the net distance exceeds 2.0D, there is still a significant displacement phenomenon. This result clearly shows that there is a limitation of relying solely on increasing the tunnel net distance to control the vertical displacement of existing tunnels, and that other and more refined engineering measures are needed to achieve more efficient displacement control.
From the detailed observation data in Fig. 17, we learn a key phenomenon: the change of the formation loss rate has little significant effect on the shape of the vertical displacement curve of the existing tunnel arch bottom, which always maintains the typical “W” type characteristics. Moreover, we note a remarkable rule that the maximum deformation of the existing tunnel is always precisely located directly above the axis of the new shield tunnel. Further analysis reveals a clear correlation between the formation loss rate and the influence range of the transverse deformation of the existing tunnel. Specifically, when we control the formation loss rate at 0.25%, the impact of the excavation of the double-line shield tunnel on the transverse deformation of the existing tunnel is roughly limited to the range of 9.8D. However, when the formation loss rate rises to 1.5%, this influence range increases significantly to about 12.1D. This data change directly proves that the increased formation loss rate will lead to a corresponding expansion of the influence range of the transverse deformation of the existing tunnel.
Fig. 17
Deformation of existing tunnels under different formation loss rates
When studying the effect of the formation loss rate on the vertical displacement of the existing tunnel, we noticed that the vertical displacement value of the existing tunnel gradually increased from 3.98 mm to 10.56 mm in the range of 0.25% to 0.75%. This continuous and stable growth pattern indicates that the degree of deformation of the existing tunnel has not reached the predetermined safety threshold when the formation loss rate is controlled below 0.75%. Therefore, in this interval, the shield construction does not have a direct and significant impact on the safety of the existing tunnel.
However, the situation turns when the formation loss rate η touched or exceeded the limit of 0.75%. At this time, the maximum vertical displacement value of the existing tunnel has exceeded the predetermined safety warning value. In particular, when the formation loss rate climbs to 1.5%, the maximum displacement value of the existing tunnel is up to 21.68 mm, which significantly exceeds the control standard. When the parameters in shield construction are improperly selected, such as the soil bin pressure and grouting pressure is low, or the adjustment of shield posture is unreasonable, it may lead to a sharp increase in the formation loss rate, thus posing a serious threat to the safe operation of the existing tunnel.
5 Comparative Analysis of Numerical Simulation and Model Test
5.1 Comparative Analysis of Land Surface Subsidence
When evaluating the situation of surface settlement, we selected the data of model test line A to compare with the surface line data obtained by numerical simulation. To present these data more intuitively, we plotted them as shown in Fig. 18
Fig. 18
Comparison of the final surface displacement values
By observing the data curves in the figure, we can clearly see that the model test line A is highly similar to the numerical simulated surface line in morphology, which reflects the consistency between the two. Specifically, the maximum settlement of the model test line A is 16.67 mm, while the maximum settlement of the numerical simulated surface line is 14.35 mm. Although the results of the model test are slightly smaller than those of the numerical simulation, the difference between the two is only small, less than 20%, which basically meets the relevant accuracy requirements.
From the diagram, the model test line A and the numerical simulated surface test line show similar characteristics in the settlement trend, but there is still a subtle difference between the two. This difference is mainly due to their different design considerations in the simulation methods. In the numerical simulation, to simplify the calculation, the soil is regarded as an elastic material, in the model test. Moreover, the operations involved in the test, such as repeated filling, digging, are difficult to ensure completely consistent conditions for each test, which inevitably introduces additional errors. In contrast, the numerical simulations can avoid the errors caused by such human factors. Comparative analysis of the existing tunnel deformation.
In view of the deformation of the existing tunnel, we specially selected the comparative analysis of the left vault data in the model test and the data of the left vault of the existing tunnel in the numerical simulation, and drew these data points as a curve, as shown in Fig. 19
According to the comparison of the data in Fig, the shape of the deformation curve of the left line vault in the model test and the existing tunnel arch of the left line vault in the numerical simulation is almost identical, showing a small difference. Specifically, the maximum displacement value measured by the model trial is 12.03 mm, while the numerical simulations give results of 10.60 mm. Despite the subtle difference between the two values, it is noteworthy that this difference represents only 11.95% of the maximum displacement value of the model trial, well below the threshold of 20%. Therefore, we can think that this result still meets the relevant requirements and further verify the validity and high consistency of the model test and the numerical simulations in predicting the tunnel deformation.
The difference in the results between the model test and the numerical simulation is mainly due to the difference in the excavation method and the selection of tunnel materials. In the model test, due to the influence of the actual pulling effect and the lack of real grouting process simulation, the maximum displacement value of the left line vault is slightly higher than the result of the numerical simulation. This difference reflects the possible dynamic effect and uncertainty under the real construction conditions.
6 Conclusion
In this paper, the three angles of crossing angle, net spacing, and bottom loss rate are studied, and the following rules of existing tunnel and surface settlement caused by shield tunnel crossing are obtained.
(1)
When the formation loss rate gradually climbed from 0.25% to 1.5%, we observed that the vertical deformation of the existing tunnel and the influence of surface subsidence on the transverse and longitudinal sections were expanded. This increase is not only reflected in the expansion of the deformation range, but also in the significant increase of the maximum deformation value and the obvious acceleration of the deformation rate.
(2)
With the gradual increase of the net distance from 0.2D to 3D, the vertical deformation of the existing tunnel and the surface settlement in the transverse and longitudinal sections have been expanded. During this change, the maximum deformation value decreases gradually, and the rate of deformation slows down.
(3)
After the time-course curve analysis, we observed that in the left and right lines of the new tunnel, the first excavation caused significant disturbance effects on the surface and the existing tunnel.
(4)
Comparing the data of numerical simulation and model test, we find that although there are slight differences between the two values, there is a high degree of consistency in the key data points and the changing trends shown.
(5)
Under the condition of simulating the actual construction environment, we detected that the surface settlement value reached 14.24 mm and found that the settlement value of the existing tunnel was 10.53 mm, which has exceeded the warning standard but has not yet constituted a danger.
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