Abstract
This research was undertaken to predict the result of vault settlement during the excavation of super-large-span tunnels in soft rock strata, which are prone to deformation and hard to predict aftermaths. Numerical simulation was applied to tackle with vault problems during excavation of super-large-span soft rock tunnels such as proneness to deformation and hard prediction on vault settlement. As a result, a model for continuous prediction of vault settlement as the tunnel face keeps advancing was proposed, with influencing factors such as excavation span and tunnel depth taken into consideration, and put forward the optimization of super-large-span variable-section tunnels construction scheme. The results show that the vault settlement becomes fast at the beginning of the excavation because its surrounding rocks are characterized by weakness, incompleteness, and poor stability. At the initial stage of excavation, the varying tunnel depth had a greater impact on vault settlement, indicating that the primary support has to be enhanced at the beginning of the excavation of a vault with weak surrounding rocks. As the excavation of pilot tunnels proceeds, the high-stress zone transfers to the deep part of the surrounding rocks. The tunnel vault settlement model is capable of rapidly predicting the vault settlement at locations other than the observation points. The final settlement amount is affected by the excavation span, the tunnel depth, and the basic mechanical parameters of surrounding rocks. The key to the construction of a super-large-span and variable cross-section tunnel lies in the cross-section varying parts where the stress is concentrated, thus requiring reinforcement and frequent monitoring. The research methods and analysis may provide basic data and technical means for the control of the vault settlement of large-span tunnels in soft rock strata.
1. Introduction
In the 21st century, rapid progress has been made in the construction of highways and railways in China, and many tunnels have been constructed, such as the New Guanjiao Tunnel along the Qinghai-Tibet Railway and the Qingdao Jiaozhou Bay Undersea Tunnel. However, as the traffic volume continues to increase, single-bore tunnels with two lanes can no longer satisfy the demand, and thus more tunnels in China and even worldwide are developing towards supersized cross-sections. The definition of super-large-span tunnel has various forms. According to the section division standard of the International Tunnel Association, the tunnel whose excavation area is greater than 100 m2 is defined as the super-large-span tunnel [1] and the tunnel whose excavation span is greater than 14 m is defined as the super-large-span tunnel in the design specification of Chinese railway tunnel TB10003-2005 [2]. Because the excavation of a super-large-span tunnel may cause a great disturbance to the strata [3–6], the overlying strata of the tunnel are deformed, the surrounding rock stress is redistributed, and the surrounding rock at the vault and sidewalls of the tunnel undergoes certain deformation. However, the surrounding rock at the vault is first disturbed by excavation, and its change trend is more obvious than that of the sidewalls, and the surrounding rock at the vault is prone to collapse and collapse, and the amount of settlement at the vault cannot be predicted, so it is especially important to study the prediction of settlement of the surrounding rock at the vault generated by the tunnel excavation process.
Scholars worldwide have made a series of studies on the regularity of strata deformation and settlement. Field experiments [7–10], numerical simulation [11–15], empirical formula calculation [16–18], and physical model tests [19–21] are mainly applied to analyze the regularity of deformation and settlement of surrounding rocks during excavation. After analyzing a mass of surface deformation data and related engineering data, Peck put forward the conclusion that the ground surface settlement is normally distributed and proposed a formula for estimating ground surface settlement distribution and the maximum settlement amount [22]. Later, Attewell and Woodman [23] and Lou and Liu [24] modified and supplemented that formula. Rowe et al. proposed an estimation technique for the deformation of tunnels in soft soil based on Peck’s theory and applied the technique to parametric research [25]. Xu et al. put forward a prediction model of deformation of existing tunnel and stratum caused by construction based on the relationship between stochastic medium theory and Peck formula [26]. She and He made a 3D elastoplastic numerical simulation of the surrounding weak rock sections of tunnels and concluded that the settlement values distributed from the vault to the surface show a curve trend from large to small, with the changes being sharp at first and mild later, which indirectly verified the reliability of Rowe’s deformation estimation technique [27]. Through on-site monitoring and surveying of tunnels, Zhang et al. found that the vault settlement and the internal displacement of the surrounding rocks have the same trend, but the cumulative settlement is less than the displacement of the surrounding rock [28]. Shao et al. conducted research on the deformation characteristics of the surrounding rocks of super-span tunnels, sorted a large amount of monitoring and measurement data about the surrounding rocks’ deformation, and applied numerical simulation software to analyze the deformation laws of the surrounding rocks of super-large-span tunnels, thereby offering instructive suggestions to the safe construction of super-large-span tunnels [29]. Based on the construction mechanics theory of adjacent tunnels, Luo et al. analyzed the interaction mechanism of each guide tunnel by the subexcavation method and proposed the calculation method of surrounding rock pressure of super-large-span highway tunnel based on subexcavation method [30]. Tu et al. researched the excavation of shallow large-span subway tunnels with their upper parts in soft rock strata and lower parts in hard rock strata, confined by rock mass conditions, and built the kinematic model to predict the strata’s collapse state, which can be used to accurately determine the slip failure surface and the ground settlement curve [31]. Wang et al. showed, through indoor model tests, that the deeper buried soil layer settles more widely, and the settlement decreases gradually with a reducing buried depth, which is generally consistent with Peck’s theory [32]. However, most of the research on the deformation and settlement of the surrounding rocks of tunnels focuses on surface settlement of small cross-section tunnels, with relatively few on the deformation and settlement of the surrounding rocks at tunnel vaults.
For this purpose, taking the Qizishan Tunnel in the Changjiang Road Southward Extension Project in Suzhou City as the object, the authors conducted a large cross-section excavation simulation experiment by numerical simulation. Combining surface fitting modeling and analysis, we have proposed a model to predict vault settlement as the tunnel face keeps advancing, with influencing factors such as excavation span and depth taken into account and optimized the construction scheme of the superspan variable-section tunnel, thus offering a basis and reference for the construction of super-large-span tunnels.
2. Project Description
The Qizishan Tunnel is located in Mudu Town and Yuexi Town, Wuzhong District, Suzhou City, Jiangsu Province. The tunnel has a total length of 6.428 kilometers, with its surrounding rocks mainly composed of Grades IV and V, and the soil on the surface being silty clay, in a soft rock strata composed of moderately-weathered sandstone and cracked moderately-weathered sandstone. There are 5 types of cross-sections in the large-span section of the subsurface excavation tunnel area, and their sizes gradually reduced to the standard values from small to large mileage. The maximum buried depth of the tunnel is 190 meters, the minimum buried depth 18 meters, the average buried depth of the large-span section 37.5 meters, and the maximum excavation span 29.6 meters. The rendering of the Qizishan Tunnel is shown in Figure 1.
3. Three-Dimensional Numerical Simulation
3.1. Calculation Models and Parameters
In the analysis using the three-dimensional finite element numerical software MIDAS GTS NX, the surrounding rocks were considered to be homogeneous in both elasticity and plasticity, and the isotropic Mohr–Coulomb elastic-plastic model was used for calculation. 3D solid element, 2D embedded truss element, and 2D plate element were used to simulate surrounding rocks, anchor bolts, and primary support, respectively. The sizes of the models are as follows: the tunnel is 150 meters transversely (greater than 5 times the bore diameter), rises vertically upward to the surface (with an average buried depth of 37.5 meters), and descends 40 meters vertically downward (2 times the bore diameter) and 48 meters longitudinally. The constraint conditions are as follows: the surface has a free boundary without any constraint, and displacement constraints perpendicular to the boundary surface are given on the left and right boundaries and the bottom surface, respectively. To get an accurate solution, the target surface area was analyzed using a total of 52,398 fine elements in the vicinity of the tunnel structures with 35,737 nodes. The calculation models are shown in Figure 2, and the physical and mechanical parameters are shown in Table 1.
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3.2. Simulation of Excavation Steps
As shown in Figure 3, the 14-step excavation method that combines the double-side heading method commonly used in large-span tunnel construction with the cross diaphragm (CRD) method was used to divide a super-large section into smaller sections for excavation, and the role of surrounding rocks’ self-stability given full play while reducing their deformation, thus ensuring construction safety at the super-large-cross-sections.
The simulation was started with geo-stress balance and zeroing of displacement field and velocity field, then excavated pilot tunnels A to N in sequence, with excavation intervals of 4.8 meters, and cyclic excavation footage of 0.8 meters. After two cycles of excavation, primary supports and temporary supports were set up. Only when a pilot tunnel was excavated and provided with temporary supports could the excavation of the next pilot tunnel start and remove the transverse temporary support above this guide hole. The longitudinal distance between each excavation step was staggered, and each guide hole was separated by 4.8 m longitudinal distance. After the excavation of two sections was completed and the initial support is closed into a loop, the temporary supports could be removed in the sequence of “from left to right and from top to bottom.” The excavation sequence and measuring points are shown in Figure 3.
3.3. Numerically Simulated Conditions
To reveal the influence of the tunnel’s excavation span, buried depth, and other factors on vault settlement at the measuring point as the tunnel face keeps advancing under different conditions, the authors set 8 excavation conditions, as shown in Table 2. The research was aimed to analyze the changes in vault settlement as the tunnel face keeps advancing with the same buried depth but different excavation spans under conditions 1 to 4 and with the same excavation span but different excavation depth under conditions 5 to 8. The positional relationship between the measuring point and the excavation span, buried depth, and excavation distance of the tunnel face is shown in Figure 4.
3.4. Calculation Results and Analysis of the Vault Settlement
3.4.1. Analysis of the Settlement of Surrounding Rocks in the Vault
Eight groups of excavation models for the Qizishan Tunnel under the 8 excavation conditions mentioned above were built, and the data of vault settlement are obtained as the tunnel face keeps advancing under each kind of excavation condition. In regard to the model groups 1 to 4, the tunnel’s buried depth is 37.5 meters, the settlement of tunnel vaults with different excavation spans under the action of an advancing tunnel face is shown in Table 3, and the corresponding settlement changes are shown in Figure 5. In regard to the model groups 5 to 8, the excavation span is 30 meters, the settlement of tunnel vaults with different buried depths under the action of an advancing tunnel face is shown in Table 4, and the corresponding settlement changes are shown in Figure 6.
Figure 5 shows that when the tunnel’s buried depth is constant, when the tunnel face keeps advancing, the varying trends of vault settlement are all the same, even with different excavation spans. Before the tunnel face advances to 4.8 meters, the settlement values of tunnel vaults with different excavation spans register almost similar values. However, with the increase of the excavation span, the settlement of the tunnel vault also increases, and the trends of the vault settlement changes also increase. These facts imply that when the tunnel’s buried depth is constant, changes in the excavation span have less impact on the vault settlement at the beginning of the tunnel excavation. Before the tunnel face advances to 7.2 meters in the tunnel with an excavation span of 18 meters, the vault experienced a rapid settlement and deformation. However, it became stable after the tunnel face advanced to 28.8 meters, with a maximum vault settlement value of 2.50 centimeters. For the tunnel with an excavation span of 22 meters, the vault settlement and deformation became stable after the tunnel face advanced to 43.2 meters, with a maximum vault settlement value of 3.21 centimeters. However, for the tunnel with an excavation span of 30 meters, the vault settlement and deformation are still drastic even after the tunnel face advances to 48 meters, with the maximum vault settlement value of 4.80 centimeters showing no sign of any stable stage. It is clear that, with the increasing of the tunnel excavation span, the timing of the vault settlement reaching a relatively stable state is postponed. When the excavation span is reduced from 30 meters to 18 meters, the vault settlement value decreases by 2.30 centimeters.
Figure 6 shows that when the tunnel excavation span is constant, the varying trends of vault settlement, as the tunnel face keeps advancing, are the same, even with different buried depths. When the tunnel face starts advancing, the settlement values of tunnel vaults at different buried depths are quite different. Moreover, with the increase of the buried depth, the settlement of the tunnel vault also increases, and the trends of the vault settlement changes also grow. These facts imply that when the tunnel’s excavation span is constant, changes in the buried depth have a huge effect on the vault settlement at the beginning of tunnel excavation. Before the tunnel face advances to 9.6 meters in the tunnel with a buried depth of 27.5 meters, the vault sees a rapid settlement and deformation which, however, becomes stable after the tunnel face advances to 38.4 meters, with a maximum vault settlement value of 3.09 centimeters. For a tunnel with a buried depth of 37.5 meters, the vault settlement and deformation become stable after the tunnel face advances to 43.2 meters, with a maximum vault settlement value of 3.92 centimeters. However, for a tunnel with a buried depth of 57.5 meters, the vault settlement and deformation are still drastic even after the tunnel face advances to 48 meters, with a maximum vault settlement value of 5.68 centimeters, showing no sign of a stable stage. It is evident that with the increase of the tunnel’s buried depth, the timing of the vault settlement reaching a relatively stable state is postponed. When the buried depth is reduced from 57.5 meters to 27.5 meters, the vault settlement value decreases by 2.59 centimeters.
The growth rate of tunnel vault settlement at this measuring point gradually decreases with the increasing excavation distance between the measuring point and the tunnel face, despite different excavation spans or buried depths, indicating that the farther the measuring point is from the tunnel face, the smaller the influence is on the vault settlement at the measuring point, and settlement tends to be gradually stable. At the beginning when the tunnel face starts advancing, the vault settlement values corresponding to different buried depths see drastic differences, but those corresponding to different excavation spans are fairly similar, indicating that changes in buried depths at the beginning of excavation have a greater impact on vault settlement in weak surrounding rocks.
3.4.2. Von Mises Stress Analysis of Vault’s Surrounding Rocks
In Zone I in Figures 7 and 8, it is evident that after the excavation steps are carried out in A to C in Figure 3, the stress in the surrounding rocks of the pilot tunnels A to C is relaxed, forming a stress-relaxation zone, while the high-stress radiates around the pilot tunnels, slightly increasing the Von Mises stress in the surrounding rocks at the measuring point. Zone I in Figure 7 shows that after the pilot tunnels A to C are excavated in the tunnel with the same buried depth, the changes of stress in the surrounding rocks under different excavation spans are the same, all increasing by 50 to 70 kPa. From Zone I in Figure 8, it is evident that with a constant tunnel excavation span but different buried depths, the stress in the vault’s surrounding rocks is inconsistent under different working conditions: greater buried depths bring higher stress in the surrounding rocks, while, after the pilot tunnels A to C are excavated, the changes of stress in the surrounding rocks under different buried depth are the same, all increasing by 34 to 46 kPa.
In Zone II in Figures 7 and 8, it is evident that since the measuring point is above Pilot Tunnel D where the rock mass is weak, the rock cannot withstand the sharply increasing stress after the excavation step of Pilot Tunnel D is carried out, thus resulting in plastic deformation. The stress in the surrounding rocks around the measuring point relaxes, forming a zone with less stress, while the high-stress transfers to the deep part of the surrounding rocks. Therefore, when the tunnel face advances from 0 to 4.8 meters, the stress in the surrounding rocks at the measuring point drops sharply. Since the overall erection of temporary supports is to be completed after one excavation cycle is done, the supporting effect of the temporary supports cannot be fully manifested before the tunnel face advances to 4.8 meters, and it is impossible to stop the sharp decrease of the stress at the measuring point at the moment.
Zone III in Figures 7 and 8 shows that when the tunnel face advances from 4.8 to 9.6 meters, the surrounding rocks at the measuring point continue to settle. Since the temporary supports which have now been installed press on the surrounding rocks in the vault, the settlement slows down, and the stress in the surrounding rocks at the measuring point shows an uptrend.
In Zone IV of Figures 7 and 8, it is evident that when the tunnel face continues to advance until the tunnel excavation is completed, the stress in surrounding rocks at the measuring point is still in a downward trend due to excavation disturbance, and the temporary supports are dismantled following the erection sequence at this point, which releases the stress in the surrounding rocks in the vault for the second time. With the advancing of the tunnel face, the stress in the surrounding rocks in the vault decreases and eventually becomes stable. It is evident from Zone IV of Figure 7 that the stress in the surrounding rocks in the vault of a tunnel with an excavation span of 18 meters tends to be stable after the tunnel face advances to 24 meters, with the minimum stress in the surrounding rocks being 125.34 kPa; that with an excavation span of 22 meters, the stress tends to be stable after the tunnel face advances to 33.6 meters, with the minimum stress in the surrounding rocks being 47.87 kPa; that with an excavation span of 30 meters, the stress tends to be stable after the tunnel face advances to 43.2 meters, with the minimum stress in the surrounding rocks being 13.62 kPa. Those facts indicate that as the excavation span increases, the timing of the vault settlement reaching a stable state comes later. In other words, as the excavation span increases, the stability of stress in the surrounding rocks reduces, implying that a larger excavation span leads to a lower carrying capacity of the surrounding rocks in the vault of a tunnel. It is evident from Zone IV in Figure 8 that the stress in the surrounding rocks in the vault of a tunnel with a constant excavation span tends to be stable as the tunnel face keeps advancing; the stress with a buried depth of 27.5 meters tends to be stable after the tunnel face advances to 43.2 meters, with the minimum stress in the surrounding rocks being 19.52 kPa; that with a buried depth of 57.5 meters, the stress tends to be stable after the tunnel face advances to 24 meters, with the minimum stress in the surrounding rocks being 12.76 kPa. Those facts indicate that as the buried depth increases, the timing of the vault settlement reaching a stable state comes in advance, and the stress in the surrounding rocks in the vault grows correspondingly.
4. Model to Predict Vault Settlement
By inputting the vault settlement data corresponding to Figure 5 into TableCurve 3D, the authors used three-dimensional surface fitting to establish a vault settlement model for a tunnel with a buried depth of 37.5 meters, with the excavation distance between the measuring point and the tunnel face as well as the excavation span taken as the independent variables, and the vault settlement as the dependent variable. The model featuring a fitting degree of 99.51% and a fitting error of 5.73% is shown as follows:where means the vault settlement of the tunnel (with a buried depth of 37.5 meters); means the excavation distance between the measuring point and the tunnel face; and means the excavation span of the tunnel.
The three-dimensional plot of the fitting surface model is shown in Figure 9. From Figure 9 and Fitting Surface Model (1), it can be noted that the vault settlement at the measuring point is influenced by both the excavation distance from the measuring point to the tunnel face and the excavation span; the increase of these two factors leads to larger vault settlements. Therefore, the vault settlement rate of a tunnel with an increasing excavation distance is much smaller than that of a tunnel with a growing excavation span. In other words, the excavation span imposes a greater impact on the vault settlement of a tunnel after the excavation proceeds to a certain distance.
Taking the excavation distance between the measuring point and the tunnel face as well as the buried depth as independent variables, and the vault settlement as the dependent variable, the authors used the same method to establish a vault settlement model for a tunnel with an excavation span of 30 meters. The model, featuring a fitting degree of 99.80% and a fitting error of 5.81%, is shown as follows:where means the vault settlement of the tunnel (with an excavation span of 30 meters); means the excavation distance between the measuring point and the tunnel face; and means the buried depth of the tunnel.
The three-dimensional plot of the fitting surface model is shown in Figure 10. From Figure 10 and Fitting Surface Model (2), it can be noted that the vault settlement at the measuring point is influenced by both the excavation distance from the measuring point to the tunnel face and the buried depth; the increase of these two factors leads to larger vault settlements. Consequently, the vault settlement rate of a tunnel with an increasing excavation distance is much smaller than that of a tunnel with a growing buried depth. In other words, the buried depth imposes a greater impact on the vault settlement of a tunnel after the excavation proceeds to a certain distance.
The above-given two models can not only visually reflect the influence of excavation distance, excavation span, and buried depth of a tunnel on the vault settlement but also be applicable to reflect vault settlement at areas other than the measuring point. Moreover, the model formula can be used to continuously predict the vault settlement of a tunnel, thus indirectly enabling the visibility of vault settlement and better realize the fine design and efficient and safe construction of super-large cross-section tunnel.
5. Theoretical Verification
It is noted from mass data of tunnel engineering practices that the vault settlement at the measuring point eventually becomes stable when the tunnel excavation proceeds to a certain distance from the measuring point. Therefore, it can be assumed that the vault settlement value is constant when the excavation distance between the measuring point and the tunnel face is infinite. Hence, in order to verify the model’s reliability and whether conformity to the above assumption, formulas (3) and (4) are processed as follows:
Formula (3) indicates that when the distance between the measuring point and the tunnel face reaches a certain value as the excavation proceeds, the tunnel vault settlement model at the measuring point tends to be a one-variable function with the excavation span as the independent variable. This indicates that an increasing excavation distance has a gradually decreasing influence on the vault settlement at the measuring point until the influence is null eventually, and the final settlement of the vault is only related to the excavation span. When the excavation span is a fixed value, the settlement value of the vault is also fixed, which is in line with the above assumption.
Formula (4) indicates that when the distance between the measuring point and the tunnel face reaches a certain value as the excavation proceeds, the tunnel vault settlement model at the measuring point tends to be a one-variable function with the buried depth as the independent variable. This indicates that an increasing excavation distance has a gradually decreasing influence on the vault settlement at the measuring point until the influence is eventually null, and the final settlement of the vault is only related to the buried depth. When the buried depth is a fixed value, the settlement value of the vault is also fixed, which is in line with the above-given assumption.
6. Optimization of the Method Statement for Super-Large-Span Variable-Section Tunnels
By studying the mechanical properties of the construction of super-large-span tunnels, the authors drew preliminary conclusions on the distribution law of the stress and the deformation of surrounding rocks during tunnel construction. Taking the Qizishan Tunnel as the object to summarize control measures for the construction of super-large-span tunnels, the authors analyzed key nodes in the construction of variable-section tunnels to provide references for the safe construction of similar projects.
Plugging walls can be used for smooth transition among cross sections of a super-large-span tunnel. Figure 11 shows the schematic diagram of the plugging walls applied between the lining of a super-large-span tunnel and the lining of a multiarch section, among large-span section linings, and between a large-span section lining and a standard section lining.
The primary supports at the conversion between a multiarch section and a large-span section must be made of C25 or the anticorrosion steel fiber-reinforced shotcrete with a thickness of 30 centimeters. Such primary supports have to be provided with a row of I20b H-beams, at a lateral spacing of 100 centimeters. To prevent the tunnel face from protruding and the vertical H-beams from toppling, prestressed anchors of A32 × 6 millimeters are welded to the H-beams. The prestressed anchors are 8.0 meters long and arranged at a lateral spacing of 100 centimeters, a longitudinal spacing of 70 centimeters, and a designed minimum prestress of 100 kN. The secondary lining is made of C35 or C45 waterproof reinforced concrete with a thickness of 95 centimeters, for which two layers of C25 rebar must be arranged at a spacing of 20 × 20 centimeters; in addition, the C12 rebar has to be used as tie rods, at a spacing of 40 × 40 centimeters and erected in quincunx. The Grade V cement grout is used for grouting, with a water-cement ratio (weight ratio) of 1 : 1, and a grouting pressure of 0.5 to 1.0 MPa. The sectional and plan views of the plugging walls at section conversions are shown in Figures 12 and 13.
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The primary support for the plugging walls at large-span section conversions must be made of the C25 steel fiber-reinforced shotcrete in a thickness of 20 centimeters. Such plugging walls have to be reinforced by 6-meter-long grouting ducts with a specification of A42 × 4 millimeters, arranged in quincunx at a spacing of 50 × 50 centimeters (lengthways × circular). The secondary lining is made of C35 or C45 waterproof reinforced concrete in a thickness of 80 or 70 centimeters, for which two layers of C25 rebar must be arranged at a spacing of 20 × 20 centimeters; furthermore, C12 rebars have to be used as tie rods, at a spacing of 40 × 40 centimeters and erected in quincunx.
The 14-step excavation method that integrates the double sidewall heading method and the cross diaphragm (CRD) method is applied to divide a section for excavation. Due to the large excavation section, and the surrounding rock deformation is obvious, in order to reinforce the support for a superspan section, arch support, primary support of the first layer, primary support of the second layer, and secondary lining have to be used together, as shown in Figure 14.
The arch protection frame is divided into 10 segments according to the length of the arch support and the pilot tunnels and is arranged against the pilot tunnels—two segments, respectively, at the left and the right pilot tunnels and three at the two pilot tunnels in the middle, respectively. The arch frame of the primary support of the first layer is divided into 10 segments. The schematic diagram of the arch support frame is shown in Figure 15.
According to the data monitored and measured, the increasing rates of vault settlement and surrounding convergency value are reduced after applying the 14-step excavation method, adding arch protection support, using plugging walls at section conversions for construction, and other means. Thus, the stability of the surrounding rocks is improved, and the construction safety is guaranteed.
7. Conclusions
By establishing excavation models for the Qizishan Tunnel under different excavation conditions using three-dimensional numerical simulation, the models for continuously predicting the vault settlement are established by using three-dimensional surface fitting software, which indirectly realizes the visibility of the tunnel vault settlement and optimizes the construction plan of the superspan variable-section tunnel according to the change law of surrounding rock stress and displacement. The specific conclusions are listed as follows:(1)The vault settlement at the beginning is fast. Based on the analysis of the numerical simulation results, the vault settles fast at a high settling rate at the beginning, no matter different excavation spans or buried depths, which conforms to the surrounding rock’s characteristics such as weakness, incompleteness, and poor self-stabilization.(2)Changes in buried depth have a greater impact on vault settlement at the beginning of excavation. When the tunnel face advances, the vault settlement values corresponding to different buried depths have drastic differences, indicating that the primary supports have to be reinforced for weak surrounding rocks at the beginning of tunnel excavation when the buried depth increases.(3)The stress in the surrounding rocks at the measuring point increases slightly first and then sharply decreases. When tunnel excavation starts, the stress in the surrounding rocks around the pilot tunnels is relaxed, while the high-stress zone diffuses around the pilot tunnels, slightly increasing the stress in the surrounding rocks at the measuring point. As the excavation of other pilot tunnels proceeds, the high-stress zone transfers to the deep part of the surrounding rocks, and the surrounding rocks around the measuring point are subject to plastic deformation, so the stress in the surrounding rocks at the measuring point drops sharply.(4)The vault settlement model can be used to forecast vault settlement in zones other than the measuring point and accordingly realize continuous prediction on the vault settlement of a tunnel.(5)The key to super-large-span variable cross-section tunnel engineering lies in the cross-section conversion, featuring large excavation cross sections, complex structures, many construction procedures, poor stability of surrounding rocks, and high safety risks. Therefore, reinforcement and frequent monitoring are required to ensure workplace safety.
Data Availability
Some or all data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of CHINA (Grant no. 52009076), Young Elite Scientists Sponsorship Program by CAST (Grant no. 2021QNRC001), Youth Innovation Team of Shandong Higher Education Institutions (Grant no. 2022KJ21), and Young Elite Scientists Sponsorship Program of Shandong Province (Grant no. SDAST2021qt06).