Research on spatiotemporal patterns of ground deformation caused by shallow buried tunnel construction in urban areas based on three-dimensional numerical simulation

According to incomplete statistics, since 2018, there have been as many as 15 geological safety accidents caused by urban subway construction in Guangzhou, resulting in significant loss of people’s lives and property and social impact. The main reason for this is insufficient research on the spatiotemporal patterns of ground deformation caused by shallow buried and underground excavation tunnel construction in complex disaster-prone environments and a lack of targeted geological risk prevention measures. This study combines various methods such as statistical analysis, on-site investigation and monitoring, and numerical simulation to reveal the spatiotemporal deformation laws of the ground caused by the construction of shallow buried and underground tunnels in cities. The results obtained can provide a reference for reducing or preventing ground accidents caused by urban subway construction, thereby better ensuring the safety of people’s lives and property.


Introduction
Since the reform and opening up, China has been experiencing rapid development, leading to overcrowding in urban living spaces.The development and utilization of underground space can effectively solve the current problem of traffic congestion in urban development.However, underground engineering construction can cause varying degrees of settlement and displacement.When the local layer movement and surface deformation exceed a certain limit, it will cause ground subsidence, leading to economic losses [1].The main reason for this is insufficient research on the mechanism of ground deformation caused by shallow buried and underground excavation tunnel construction in complex urban construction environments and a lack of targeted geological risk prevention measures [2][3].In recent years, with the rapid development of computer technology, numerical simulation has received increasing attention from industry professionals and researchers.Currently, numerical simulation methods have become a widely used method in the study of subway tunnel excavation [4][5].
The study takes a subway section in Guangzhou as the research area.It uses statistical analysis, onsite investigation and monitoring, and numerical simulation methods to study the spatiotemporal deformation laws of ground deformation or collapse caused by the construction of shallow buried tunnels in the city.

Overview of the Subway Project
A certain section of Guangzhou is located in Tianhe District, Guangzhou.The station runs southwestnortheast and crosses Tianhe Park to connect Huangpu Avenue and Zhongshan Avenue.The tunnel excavation adopts the CRD construction method, dividing the large section tunnel into four relatively independent small chambers for construction.The tunnel arch is about 14.5 meters above the ground.The monitoring section is located in Tianhe Park, with a length of about 100 meters.

Geological Conditions
The site is located in Tianhe Park, belonging to a plain area with flat terrain.The rock and soil layers in the site can be divided from top to bottom according to geological age, genetic type, and lithological characteristics.They can be divided into loose artificial fill layer, marine land interaction silty clay layer, terrestrial alluvial and alluvial silty clay layer, bedrock (sandstone, conglomerate), etc.The depth of the bedrock surface is about 20m, and the thickness of the upper soft soil layer is relatively large, Having geological conditions for ground subsidence and subsidence.

Threat Object Types
The subway tunnel is a major engineering project that crosses the Tianhe Park.The underground excavation section is adjacent to the water body on both sides, and there are scattered park buildings and dense pedestrian flow around.The ground settlement and collapse risks during tunnel excavation pose a serious threat to park visitors.Moreover, the tunnel is adjacent to the water body, and ground settlement and collapse may cause ground cracking, leading to water backflow into the tunnel, posing a threat to construction personnel and tunnel safety.

Geometric models
The selection of the calculation area for the overall model fully considers the boundary effects caused by tunnel excavation.Referring to relevant literature and practical experience, the principle is to take the horizontal geometric dimensions of the 13th Line tunnel as more than 2-3 times its tunnel diameter and the vertical geometric dimensions as more than 2-3 times, as shown in Figure 1.The geometric dimensions X, Y, and Z of the numerical model are 60m, 100m, and 40m, respectively.The model is divided into 246680 elements (hexahedral mixed grid).

Parameters
The soil layer structure analysed in this study includes miscellaneous fill, muddy soil, silty clay, silty clay, fully weathered clastic rock, strongly weathered argillaceous siltstone, and slightly weathered siltstone.The supporting structure mainly includes anchor rods, concrete, and steel.The specific physical and mechanical parameters and constitutive models of the rock and soil are shown in Table 1 and Table 2

Modeling
The construction steps and longitudinal profile of the tunnel construction process using the CRD method are shown in Figure 2. Based on the actual construction situation of the tunnel on site, this analysis includes 127 construction steps, mainly simulating the CRD method construction process.4, during the excavation process of the subway tunnel using the underground excavation method, the arch bottom of the tunnel experiences uplift, the arch top experiences settlement, and the ground surface experiences settlement.The vertical displacement of the ground increases with the construction step, that is, the tunnel is continuously excavated forward, and the settlement values of each measuring point continue to increase.As the distance between the tunnel construction section and the monitoring section decreases, the settlement value change caused by the same excavation length first increases, and the larger the increase is closer to the tunnel axis then decreases continuously and finally tends to stabilize.
2. As shown in figures 5, figure 6, the ground deformation caused by subway tunnel excavation is generally funnel-shaped, with the same horizontal distance from the tunnel axis.Before the excavation passes through the monitoring completely, the settlement value of the monitoring point on the left side of the tunnel is larger than that on the right side, and the difference in settlement values between the left and right sides first increases and then decreases; After the excavation completely passes through the monitoring surface (corresponding to the construction step of 93 steps, which is the position where the construction section coincides with the monitoring section), the settlement value of the monitoring point on the right side of the tunnel is larger than that on the left side.The difference in settlement values between the left and right sides remains basically stable.As the horizontal distance from the tunnel axis decreases, the left settlement value initially increases then decreases, and then continues to increase; The settlement value on the right side has been continuously increasing.
3. The construction step 126 is a cloud map of the geological deformation after the completion of the underground excavation method for the entire subway project.As shown in figure 7, in the vertical direction, the vertical displacement near the top of the tunnel is the largest, and the displacement change diverges from the tunnel centerline to both sides, away from the tunnel towards the surface direction, and the displacement change decreases.There is a clear symmetry on both sides of the tunnel excavation in terms of horizontal displacement changes.
4. There are some numerical differences between the calculated curve and the measured curve, but the variation trend of the two curves is the same.Before the excavation is completed through monitoring, the surface subsidence is rapid, and the settlement change rate of the two curves is fast.After the excavation completely passes through the monitoring surface, the surface settlement change slows down and tends to be stable, and the settlement change rate of the two curves slows down.When the construction section passes through the monitoring section, the difference between the calculated value and the measured value is the largest, which is 13.317mm, indicating that the calculation model is reasonable, and the calculation model established according to the aforementioned modeling parameters and methods can well reflect the engineering practice.At the same time, the maximum calculated value of surface subsidence does not exceed the warning value of 30mm and the control value of 40mm, which meets the control requirements.

Conclusion
Urban land is scarce, and the development and utilization of subways can greatly alleviate the problem of urban traffic congestion.However, the construction of shallowly buried subway tunnels faces complex geological environments and significant risks.Clarifying the temporal and spatial deformation patterns during the construction process can provide scientific guidance for disaster prevention and control.This article analyzed the deformation law of the subway tunnel excavation process using a geotechnical engineering survey, three-dimensional numerical simulation, and on-site monitoring.The following conclusions can be drawn: 1.The ground deformation caused by subway tunnel excavation is generally funnel-shaped in the vertical direction, with uplift at the bottom of the tunnel arch, settlement at the top of the arch, and surface settlement.In the horizontal direction, there is a clear symmetry on both sides of the tunnel excavation.
2. The deformation caused by tunnel construction varies with the construction distance, with the maximum vertical displacement near the top of the tunnel.The displacement changes diverge from the centerline of the tunnel towards both sides and decrease towards the surface direction away from the tunnel.

Figure 2 .
Numerical simulation of the tunnel construction process 4. Spatio-temporal deformation feature 1.As shown in Figures 3, figure

Figure 3 . 4 .
Figure 3. Displacement contour in Z direction Figure 4. Displacement contour in X direction

Table 1 .
. Soil layer parameters and constitutive relationship

Table 2 .
Material parameters and constitutive relation of supporting structure