Analysis of the Influence of Adjacent New Pile Foundation Construction on the Structural Deformation of the In-service Subway Tunnels

Taking the construction project of a new bridge pile foundation above an in-service subway in Xi’an as an example, considering the influence of horizontal symmetrical construction of pile foundation hole forming on the deformation of different sections of asymmetric tunnels, a three-dimensional finite element geometric model of pile-tunnel-soil is established based on ABAQUS software. Through the numerical simulation analysis of the structural deformation behavior of the in-service subway tunnel under the two construction stages of pile foundation hole forming and concrete pouring, the horizontal and vertical deformation laws of the key points of the in-service subway tunnel caused by the construction of the adjacent new pile foundation are analyzed from different angles.The calculation results show that: the deformation of the interval tunnel is mainly caused by vertical displacement, resulting in overall settlement of the tunnel. However, the settlement value is within a controllable range and does not affect the normal operation of the in-service subway tunnel. The concrete pouring stage has a greater impact on subway tunnels. The research work has important reference value for the reasonable safety monitoring and control of similar construction projects.


Introduction
In recent years, with the acceleration of urbanization, the continuous expansion of urban scale and the rapid expansion of population, and traffic congestion problems of different degrees have appeared in countries all over the world.Therefore, the construction of subways and viaducts has become increasingly common, and it is inevitable that the influence of bridge pile foundation construction on in-service subway tunnels will occur.How to minimize the impact of pile foundation construction on subway tunnels is particularly important and has become a problem that must be studied in pile foundation construction [1].
Research in this field started earlier abroad.Ward [2,3] analyzed the displacement and deformation of the tunnel by studying the impact of a large excavation project in London on the tunnel below.Benton et al. [4] studied the deformation of tunnels caused by pile construction by analyzing the impact of pile group load on adjacent tunnels.However, the domestic research on this aspect is relatively late.Lou Xiaoming et al. [5] calculated the influence of pile-raft foundation of high-rise building on adjacent subway tunnel under vertical load, and analyzed the interaction of pile group foundation, as well as the distribution characteristics of vertical additional stress and vertical deformation; Dai Zhiping [1] used numerical simulation method to analyze the impact of the whole construction process of bored piles on adjacent interval tunnel.Yang Ping et al. [6] studied the influence of single pile and group pile construction on the ground displacement field and the deformation of adjacent in-service tunnel structure through numerical simulation.The above scholars have studied the impact of the construction of bored pile foundation on in-service tunnel from different perspectives, but have not considered the deformation effects on different sections of asymmetric tunnel at the stages of bored pile hole forming and concrete pouring.
In this paper, the horizontal and vertical displacement of the subway tunnel are compared and analyzed in two different angles by ABAQUS numerical simulation method for a practical project in Xi 'an.The reasons for the displacement change are analyzed, and some useful conclusions are drawn, which can provide some reference for the construction of bridge pile foundation above asymmetric tunnel in the future.

Overview of engineering background
The second phase line of a subway in Xi'an is located below Kunming Road and Kunming Second Road, which coincides with its line position.A bridge across the Taiping River was built on Kunming No.2 Road (Fengdong New City-Yuzhang No.4 Road) at the pile number BKO+209.215.The construction of the new bridge affects the subway Fu ~Dou shield interval.The standard span of the new bridge is 25m.The upper structure adopts steel box girder, and the foundation adopts bored pile foundation.The pile diameter is 1.2m and the pile length is 35m.The main body of the tunnel adopts shield method, with a construction length of 1584.581m.The inner diameter of the interval tunnel is 5.4m, the thickness of the segment is 0.3m, and the outer diameter of the tunnel is 6m.The minimum net distance between the pile foundation construction and the tunnel structure is 3.32m.The position relationship between the bridge pile foundation and the in-service interval tunnel is shown in figure .1.According to the construction design drawings and project safety impact assessment report, the monitoring range of the influence area of the new bridge construction on the subway is 80m.One monitoring section is arranged every 10m in this area, and five monitoring points are arranged in each section.The horizontal and vertical displacement monitoring points of the tunnel structure are set at the tunnel roof, as shown in figures 2 and figure 3.
According to the requirements of the pre-assessment report of the safety impact of the new bridge project near the subway structure, the monitoring warning values of the subway tunnel is set, as shown in table.1.Note: The control value of the change rate of the structural deformation of the subway tunnel is 1mm / day.

The basic assumptions 1)
Assuming that the soil is a continuous homogeneous medium, the soil layer is an ideal elasticplastic material, and obeys the Mohr-Coulomb criterion; 2) The in-service subway tunnel structure and shield segments adopt linear elastic materials, and the specific parameters are shown in table.2; 3) When constructing the initial stress field, only the stress generated by the gravity of the soil is taken as the initial stress, without considering the effects of water pressure and horizontal geostress; 4) The shield segment and concrete bored piles are in contact with the soil, in line with the conditions of deformation coordination; 5)Ignoring the time effect of the construction process, it is believed that the deformation of subway tunnel is only related to the construction stage [7].

Constitutive relation model of materials
In practice, the soil is rarely completely elastic.The elastic theory can only fully determine the stressstrain response of soil in a very small stress space, beyond which plastic deformation will occur.Therefore, the soil adopts the Mohr-Coulomb model, and its calculation formula is: tan Where, is the internal friction angle of soil, and c is the cohesion of soil.n  , n  are the shear stress and normal stress on the slip surface, respectively.The stress-strain curve of soil is shown in figure 4.
The linear elastic material adopts the linear elastic model, which is a model where stress is proportional to strain.The stress-strain curve of the linear elastic model is shown in figure 5.

The determination of finite element geometric model
The three-dimensional finite element software ABAQUS was used for numerical simulation analysis.
There are three components in this model, which are soil, concrete bored pile and shield segment.The model has a length of 100m, a width of 80m, and a height of 50m, which means the tunnel layout direction (X-axis) is taken as 100m, the stratum burial depth (Y-axis) is taken as 50m, and the Z-axis is taken as 80m.The vertical fixed constraint ( UY = 0 in Y direction ) is applied at the bottom of the model, the normal displacement constraint (UX = 0 in X direction, UZ = 0 in Z direction) is applied around the model, and the surface is a free surface without any constraint [8,9].The model has contact between soil and the tunnel, as well as between soil and bored piles, using surface to surface contact.The soil is set as the main surface, and the outer surface of the tunnel and bored pile is set as the secondary surface: Among them, the tangential behavior is set to isotropic Coulomb friction, and the friction coefficient is 0.3; the normal behavior is set to "hard" contact and the constrain execution method to "penalty".The geometric model and boundary conditions are shown in figure 6, the shield segment and pile arrangement of the subway tunnel are shown in figure 7, and the contact relationship between the various components of the model is shown in figure 8.

Determination of construction sequence of pile foundation
According to the " Technical Code for Building Pile Foundations " [10] and relevant experience, in the continuous construction of bored piles, when the pile spacing is less than 2 times the pile diameter and less than 2.5 m, interval excavation should be adopted.This is mainly to prevent damage, disturbance, and extrusion of adjacent piles.The construction distance between adjacent pile foundations is mainly affected by the stratum where the pile foundation is located.The pile foundation of this project is symmetrical about the horizontal symmetry axis, and the horizontal pile spacing (the center distance of adjacent piles) is 2.2m and 22.8m, respectively.Because 2.2m is less than 2.5m, it should be excavated at intervals.The specific construction sequence is shown in figure 9.
Because the subway tunnel has been built before the pile foundation construction, the pile foundation is the follow-up construction.Before the pile foundation construction, the soil completes natural settlement, generates the self-weight stress field and displacement field, and resets the generated displacement field to zero [11].This model simplifies the construction process of pile foundation, only considering the two construction stages of hole forming and concrete pouring.The hole forming stage is divided into six working conditions, and the displacement in the in situ stress balance before working condition 1 is reset to zero (as the initial stress state).The specific working conditions are shown in table.3.

Deformation analysis of in-service tunnel under different sections
Through the analysis of the results of numerical simulation, it is not difficult to obtain the maximum vertical and horizontal deformation values of the in-service subway tunnel structure.It can be seen from the variation law of the curves shown in figures 10 and figure 11 that the vertical deformation of the in-service subway tunnel structure caused by the hole forming stage and the concrete pouring stage is sinking.The displacement change roughly shows a trend of decreasing first, then increasing and finally decreasing, that is, the displacement increases first, then decreases and finally increases.The maximum subsidence values of the left and right lines appear in in the section 5 of the working condition 7. The working condition 7 is the pouring concrete stage, and the disturbance to the tunnel reaches the maximum, while the section 5 is the middle position of the whole tunnel, which is greatly affected by the surrounding soil.The maximum subsidence value of the left line is 5.0285mm, and the maximum subsidence value of the right line is 5.326mm.The left and right lines of the tunnel are asymmetric about the center.The right line is 3.92m away from the center, and the left line is 4.08m away from the center.The right line is closer to the center than the left line, and is more affected by the middle pile.Therefore, the subsidence value of the right line is greater than that of the left line.
By analyzing the curve variation law of figure 12 and figure 13, it can be seen that the horizontal deformation of the in-service subway tunnel structure caused by the hole forming stage and the concrete pouring stage is offset to the left line, and the displacement change of the left line generally shows a trend of rising first and then decreasing, that is, the displacement increases first and then decreases, mainly reflected in the decrease.The displacement change of the right line roughly shows a trend of decreasing first and then increasing, that is, the displacement decreases first and then increases, mainly reflected in the increase.With the continuous change of working conditions, the influence on the left line of the tunnel gradually decreases, while the impact on the right line of the tunnel gradually increases.The maximum offset value of the left line is 0.55216mm, and the maximum offset value of the right line is 0.29508mm.As the monitoring points selected by the left line are more inclined towards the construction side, the deformation value of the left line is greater than that of the right line.According to figures.10-13, it can be seen that the influence of the hole forming stage and the concrete pouring stage in the construction of the adjacent new pile foundation on the displacement of the in-service subway tunnel structure is sinking and offset to the left line.The vertical displacement of the left and right lines of the tunnel shows an increasing trend with the change of the working conditions, while the horizontal displacement shows an opposite trend with the change of the working conditions.The hole forming stage and the concrete pouring stage lead to the change of soil stress state, and the soil rebounds [12], resulting in the vertical and horizontal deformation of the subway tunnel structure.
The formula for calculating the springback deformation [13] is: 0.5 0.04 0.54 29.17 0.167 12.5( ) Where, q is the ground overload, t is the depth of enclosure embedding, c is the cohesion of soil,  is the internal friction angle of soil andγis the gravity of the soil.
The increase in vertical deformation rate of the subway tunnel during the concrete pouring stage (working conditions 6-7) indicates that the disturbance to the stratum is greater in this stage.

Deformation analysis of in-service tunnel under different working conditions
By analyzing the curve variation law of figures 14 and 15, it can be seen that the vertical deformation trend of the left and right lines of the subway tunnel is roughly the same, and the deformation is sinking.The displacement change of working condition 1 is not obvious, roughly showing a straight line, and the vertical displacement change of the other working conditions shows a trend of decreasing first and then increasing, that is, the displacement increases first and then decreases, reaching the peak at the section 5 of working condition 7. Condition 7 is the stage of concrete pouring, with the maximum disturbance to the soil, while Section 5 is the middle position of the whole tunnel and is greatly affected by the surrounding soil, so the displacement here is the largest.The maximum subsidence value of the left line reaches 5.0285 mm, and the maximum subsidence value of the right line reaches 5.326 mm.
By analyzing the curve variation law of figures 16 and 17, it can be seen that the horizontal deformation of the in-service subway tunnel structure is offset to the left line.The maximum deformation of the left line of the tunnel occurs in the section 1 of working condition 2, and the maximum deformation value is 0.55216 mm.The maximum deformation of the right line of the tunnel occurs in section 5 of working condition 7, and the maximum deformation value is 0.29508 mm.Although the left line is farther from the construction areas on both sides than the right line, the monitoring points selected on the left line are closer to the construction than the right line, the horizontal deformation of the left line is greater.Through the analysis of the results of numerical simulation, it is not difficult to find that the vertical and horizontal deformation of the in-service subway tunnel structure is sinking and offset to the left line.Due to the uplift deformation of the soil caused by the excavation of the hole, the excavation will produce the free surface, which will lead to the horizontal displacement deformation of the tunnel structure facing the hole under the action of the soil pressure difference on both sides.Under the action of the uplift at the bottom of the hole, the soil around the hole will also undergo settlement deformation, resulting in vertical deformation of the tunnel.

Conclusion
Through ABAQUS numerical simulation and result analysis, the main conclusions are as follows: 1) In the process of bored pile construction, the impact of the hole forming stage on the subway tunnel is small, while the impact of the concrete pouring stage on the subway tunnel is large.
2) Under the construction of adjacent new pile foundation, the horizontal and vertical deformation of the interval tunnel occurs.The horizontal deformation is offset to the left line, and the vertical deformation is sinking.
3) On the whole, the vertical displacement of the interval tunnel is greater than the horizontal displacement, and the overall deformation is dominated by vertical displacement, which causes the overall settlement of the tunnel.4) The vertical displacement is controlled within 5.5mm, the horizontal displacement is controlled within 0.6mm, and the deformation values are all within the normal range of Tab.1, so the pile foundation construction of bored piles has little effect on the subway tunnel.

Figure 1 .
Figure 1.Location Relationship Diagram of Bridge Pile Foundation and In-service Interval Tunnel.

Figure 2 .
Figure 2. Plane position relationship diagram of measuring points in the influence area of bridge pile foundation construction.

Figure 3 .
Figure 3.The schematic diagram of measuring point section layout in the influence area of bridge pile foundation construction.

Figure 4 .
Figure 4. Stress-strain Curve of Soil.Figure 5. Stress-strain Curve of Linear Elastic Model.

Figure 5 .
Figure 4. Stress-strain Curve of Soil.Figure 5. Stress-strain Curve of Linear Elastic Model.

Figure 6 .
Figure 6.The Schematic Diagram of Geometric Model and Boundary Condition.

Figure 7 .
Figure 7.The Schematic Diagram of Shield Segment and Pile Layout of Subway Tunnel.

Figure. 9
Figure.9The schematic Diagram of the Construction Sequence of Bridge Pile Foundation.

Figure 10 .
Figure 10.The Comparison Curve of Vertical Displacement of Left Line of Tunnel under Different Cross-sections with Changes in working conditions.

Figure 11 .
Figure 11.The Comparison Curve of Vertical Displacement of Right Line of Tunnel Under Different Cross-sections with Changes in working conditions.

Figure 12 .
Figure 12.The Comparison Curve of Horizontal Displacement of Left Line of Tunnel Under Different Cross-sections with Changes in working conditions.

Figure 13 .
Figure 13.The Comparison Curve of Horizontal Displacement of Right Line of Tunnel Under Different Cross-sections with Changes in working conditions.

Figure 14 .
Figure 14.The Comparison Curve of Vertical Displacement of Left Line of Tunnel Under Different Cases with Changes in Crosssections.

Figure 15 .
Figure 15.The Comparison Curve of Vertical Displacement of Right Line of Tunnel Under Different Cases with Changes in Cross-sections.

Figure 16 .
Figure 16.The Comparison Curve of Horizontal Displacement of Left Line of Tunnel Under Different Cases with Changes in Cross-sections.

Figure 17 .
Figure 17.The Comparison Curve of Horizontal Displacement of Right Line of Tunnel Under Different Cases with Changes in Cross-sections.

Table 1 .
Interval Tunnel Monitoring Warning Value

Table 2 .
Linear elastic material parameter table

Table 3 .
Calculate the Operating Conditions