Key technologies for improving resilience of super-large diameter shield tunnel affected by large variable loads underneath the Yellow River

Environmental conditions may have a significant impact on the construction and operation of underground works, and sudden changes in environmental conditions may lead to underground engineering damage. Considering the Yellow River crossing tunnel on Huanggang Road in Jinan as an example, this paper analyzes the mechanical performance of the lining of the shield tunnel with an external diameter of 16.8 m, which faces the challenge of large variable loads in operation stage, such as deep riverbed erosion and deposition, large-scale heightening of river embankment, seasonal changes in groundwater level, and key technologies for improving the resilience of tunnel structure are proposed accordingly. The results can not only assist in the construction of the super-large-diameter shield tunnel project in Jinan City but also provide technical support for other shield tunnel projects crossing rivers, lakes, and seas.


Introduction
The recent development in China's economy and society, in addition to rapid urbanization, has facilitated the demand for road tunnels in rivers, lakes, and seas.The key to the success of large-scale projects with huge investments and high risks is the technology chosen at the design stage of the project.Currently, the primary construction methods for tunnels under rivers, lakes, and seas are the cut-and-cover, shield, and immersed tube methods.The widespread integration of shield technology in road and rail transit tunnel projects in cities such as Shanghai, Guangzhou, Nanjing, and Wuhan, including projects such as the Shanghai Yangtze River Tunnel [1,2], Wuhan Yangtze River Tunnel [3], and Nanjing Yangtze River Tunnel [4], has resulted in the accumulation of rich design and construction experience [5][6][7][8][9].Thus, the shield method has become the mainstream choice for tunnel projects crossing rivers, lakes, and seas.
The construction and operation of underground work are constrained by environmental conditions, and sudden changes in these conditions generally lead to defects.For example, an accidental surface surcharge along a metro line in Shanghai caused excessive deformation of the tunnel lining, as shown in figure 1, resulting in concrete spalling, bolt fracture, joint expansion, and water leakage [10].The neighboring construction activities also pose safety hazards and risks associated with the structural performance degradation of tunnels during operation [11][12][13][14].In addition, riverbed erosion and seasonal changes in groundwater levels pose challenges to the safe operation of shield tunnels.
The outer diameter of the Yellow River crossing tunnel on Huangang Road in Jinan is 16.8 m, which is the largest cross section among the road and rail transit tunnels that are already built and under construction in China.During the operation stage, the tunnels face the challenges such as large variable loads, including deep riverbed erosion and deposition, large-scale heightening of river embankments, and seasonal changes in groundwater level.The resilience design of a shield tunnel requires a combination of the geological and environmental conditions wherein the tunnel is located, thus predicting the adverse effects on the tunnel structure caused by external factors, and allowing it to possess a certain buffer space, thus making it easier to repair and maintain its working performance after deterioration [15].To ensure the safety of super-large-diameter shield tunnels, this study analyzes the mechanical performance of the shield lining and proposes key technologies for improving the resilience of the shield tunnel structure.The results can not only assist in the construction of super large-diameter shield tunnel projects, but also provide technical support for other shield tunnel projects crossing rivers, lakes, and seas.

Project Overview 2.1. Overview of the Yellow River crossing tunnel
The Yellow River crossing tunnel on Huangang Road connects the core areas on both sides of the Yellow River in Jinan City.The shield machine of the project will be launched from the working shaft on the north bank of the Yellow River.Then, it will cross from north to south underneath the Shushan Reservoir debris basin, the Yellow River, the Jinan-Guangzhou Expressway, and some other important buildings, and will be received at the working shaft on the south bank.The floor plan and longitudinal profile of the shield tunnel project are shown in figures 2 and 3, respectively.The shield tunnel adopts the cross-sectional design of a single-tube, double-deck, six-lane dualcarriageway tunnel.The burial depth of the shield tunnel is 10.3-49.5 m.The outer diameter, inner diameter, and segment width of the shield tunnel were 15.2, 13.9, and 2.0 m, respectively.The segment ring was divided into 12 blocks, and the segments were connected using oblique bolts and assembled by stagger-jointed erection.The standard cross section of the crossing tunnel is shown in figure 4.

Geological conditions
According to a geotechnical investigation report, the shield machine crosses mainly through muddy clay strata.Small amounts of silt, silty sand, silver sand, and round gravels were found in the shield.Calcareous concretions and cemented sand consisting of muddy clay are also present, as shown in figures 5 and 6, respectively.

Challenges of large variable loads
The Yellow River, which is considered the mother river of China, possesses a large amount of sediment washed from upstream, resulting in the continuous elevation of the riverbed and the corresponding rise of the water level downstream, forming the famous suspended river.The Yellow River crossing tunnel on Huanggang Road, which uses a super-large-diameter shield machine to cross a suspended river, faces the challenge of large variable loads when it comes to tunnel structural design.

Deep riverbed erosion & deposition
The Yellow River flows upstream and downstream through the Loess Plateau, where severe soil erosion occurs.In the downstream, the Yellow River exhibits characteristics of less water and high sediment content.According to a flood assessment report, because the Xialangdi Reservoir became operational in 1997, it has played a significant role in water and sediment regulation, resulting in the continuous erosion of the riverbed downstream.However, as the sediment-detention capacity of the reservoir decreased and the sediment content in the water increased over time, the riverbed experienced deposition again.The average erosion depth of the Aishan-Lijin Reach, where the tunnel lies, is predicted to be 18.5 m in 100 years, while the riverbed will be elevated at a height of approximately 9 m from 2021 to 2121.The challenges of deep riverbed erosion and deposition in this tunnel project are illustrated in figure 7.In underwater shield tunnels, riverbed erosion and deposition result in changes in the buried depth, equivalent to unloading and surcharge, respectively.Surcharge causes the shield tunnel to exhibit a tendency to sink, and excessive surcharge can easily crush the concrete on both sides of the segment ring arch.The unloading causes the shield tunnel to exhibit a tendency to rise, and excessive unloading may lead to cracking or even fragmentation of the tunnel lining owing to inevitable dislocation during segment erection.In theory, the effects of unloading and surcharge on the shield tunnel can cancel each other out.However, due to the unpredictability of riverbed erosion and deposition, if permanent damage occurs, the safety of the shield tunnel will undoubtedly be a serious threat.

Large-scale heightening of river embankment
According to flood assessment reports and approval documents from relevant authorities, owing to the riverbed deposition of the Yellow River, the predicted flood water level for the Aishan-Lijin Reach was 43.00 m in 2121, which is approximately 9 m higher than the current water level.To ensure the safety of flood control, the Yellow River crossing tunnel faces the challenge of large-scale heightening of river embankments on both sides during its operational stage.The maximum elevation on the south bank is approximately 10 m, while that on the north side is approximately 12 m (see figure 8).The river embankment at the tunnel site was first built in the 1890s in the form of stone pitching without any pile foundation, as shown in figure 9. Therefore, the adverse impact of the heightening load on the tunnel lining must be considered without any discount.Based on the shield tunnel experience, there is no precedent for the large-scale heightening of the river embankment above an operational shield tunnel.

Seasonal changes in groundwater level
Due to the mutual supply of underground water and the Yellow River, the groundwater level at the construction site changes continuously.At different times of the year, for example, during the wet season and low-water period, there may be significant differences in terms of the groundwater level.The change in groundwater level causes subsequent changes in the internal force, leading to changes in the deformation of the shield tunnel.If the groundwater level decreases by a large margin, the stress on the tunnel lining increases accordingly, which may lead to a creak or uneven deformation.For a project with a design working life of 100 years, it is necessary to consider the impact of water-level changes on the tunnel lining during the design stage.

Key technologies for improving resilience
In response to the challenges of the large variable loads mentioned above, there have been several studies on the mechanical performance of the lining of the Yellow River crossing tunnel, and key technologies have been proposed to improve the resilience of the tunnel.

Considering multiple working conditions when doing force analysis
During the structural design process of the shield tunnel, the impact of large variable loads consisting of deep riverbed erosion and deposition, large-scale heightening of the river embankment, and seasonal changes in the groundwater level are considered.Multiple working conditions were considered when performing the force analysis; for example, for the section where the tunnel is buried under the north bank, the force analysis for the construction stage considers the combination of loads induced by the current buried depth and groundwater level only, whereas for the operational phase, the loads resulting from the current buried depth and low groundwater level with the live load caused by the heightening of the river embankment are combined.A modified routine method was applied for the internal force calculation, and the soil arching effect was considered for deformation prediction.The tunnel lining buried under the north bank exhibited the most unfavorable force characteristics during the operational stage.The calculation results, seen in figure 10, indicate that the deformation after the heightening of river embankment is carried out during operational stage meets the control requirement of no more than 3‰D, where D is the outer diameter of the shield tunnel.The reinforcement design of the tunnel lining of this section was performed based on the force analysis results.[16,17] has shown that steel-fiber-reinforced concrete exhibits better resilient characteristics in terms of crack resistance, seismic resistance, and fatigue resistance than ordinary concrete.The characteristics of the materials selected for prefabrication help determine the performance, safety, and reliability of tunnel linings.To respond to the challenges of large variable loads, a steel fiberreinforced concrete lining is adopted for sections where there are long-term surcharge plans to improve the mechanical properties and crack resistance capacity.

Enhance the connection of tunnel lining to improve the overall rigidity
The connection of the tunnel lining was strengthened to improve the overall rigidity and control the deformation of the tunnel structure under large variable loads.The following measures were taken to promote segment connections.First, 34 shear pins are arranged between the segment rings.Second, the number of connecting bolts between the segment rings is doubled from 34 to 68.The distributions of shear pins and connecting bolts are shown in figure 11.Finally, steel plates were embedded in the segment rings for sections with long-term surcharge plans.The other steel plates were welded with the embedded plates before surcharging to improve the deformation resistance capacity of the segment joints, as shown in figure 12.

Conclusions
The Yellow River crossing tunnel on Huangang Road in Jinan City faces the challenges such as large variable loads, including deep riverbed erosion and deposition, large-scale heightening of river embankments, and seasonal changes in the groundwater level during the operation stage.This study analyzed the impact of large variable loads on tunnel linings and proposed key technologies to improve the resilience of shield tunnel structures.The main conclusions and suggestions are as follows.
(a) Large variable loads may cause sudden changes in the environmental conditions of shield tunnels, which may potentially lead to permanent damage to the tunnel lining and threaten the safety of the tunnel.
(b) To improve the resilience of the Yellow River crossing tunnel, it is essential to consider multiple working conditions in the force analysis.Applying a steel fiber-reinforced concrete lining and enhancing the segment connection may be an effective approach.
(c) The key technologies for improving the resilience of shield tunnel structures can also be applied to shield tunnels affected by other types of large variable loads.

Figure 1 .
Figure 1.Schematic of safety hazards caused by accidental surface surcharge in Shanghai Metro.

Figure 2 .
Figure 2. Floor plan of the Yellow River crossing tunnel.

Figure 3 .
Figure 3. Longitudinal profile of the Yellow River crossing tunnel.

Figure 8 .
Figure 8. Schematic of large-scale heightening of river embankment.

Figure 10 .
Figure 10.Force analysis and deformation prediction of lining buried under north bank: (a) bending moment, (b) axial force, and (c) deformation.

Figure 11 .
Figure 11.Distribution of shear pins and connecting bolts between segment rings.

Figure 12 .
Figure 12.Schematic of welding of steel plates in segment joints.