Study on the Influence of Construction Undercrossing Existing Station at Zero Distance in Confined Water

Abstract

Based on the Ciqu station project of Beijing Metro Line 17, this paper studies the influence of metro proximity construction in confined water stratum on the existing station, and puts forward relevant deformation control measures to solve the technical problem of new station zero distance under the complex geological conditions. Based on the systematic study of decompression and precipitation, embedment depth of ground wall and ground load of existing station, it is found that the deformation of existing station can be well controlled by taking precise precipitation, appropriately increasing embedment depth of ground wall and ground load of existing station at the same time, so as to achieve safe construction. Studies have shown that in the case of the existing station structure with deformation joints, the internal force of the structure is reduced by 58% compared with that of the station without deformation joints, The allowable deformation of existing station structure with reserved deformation joints is more than 3 times higher than that without reserved deformation joints, The existence of deformation joints improves the anti-damage ability of existing stations.

1. Introduction

With the accelerated urbanization, higher utilization of underground space, more transfer nodes in rail transit line, and increasingly dense rail transit network, a great many approaching construction appear in the subway station, such as new station adjacent to or under the existing station. The above factors bring great difficulty and raise the risk in subway construction. Over the years, massive research are carried out on approaching construction according to more and more practical cases [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Given the underground construction is complex and variable, there has yet been a unified theory of the implications of the approaching construction on existing structure, meanwhile, the confined groundwater makes the deformation pattern of approaching construction and the influence on the existing structure becomes more complicated. Hence, relying on related cases, the study on deformation control technology of approaching construction with confined groundwater is of important significance.

Table 1. Cases of adjacent foundation pit engineering.

Project Name	Excavation Depth/m	Adjacent Structure	Deformation of the Structure/mm
			Horizontal	Vertical
Jing’an Temple Station of Shanghai Metro Line 7	23.4	Shield tunnel	3.2	−1.3
Zhuhai City Plaza in Guangzhou	21	Mining tunnel	6.9	-
Foundation pit of a building in Beijing	12	Auxiliary structure of station	1.8	−5.3
A hospital in Singapore	15	Shield tunnel	6	3.8
A comprehensive building in Guangzhou	18	Subway station	5.8	−6.8
A control center in Hangzhou	13	Shield tunnel	1.52	−4.11
Qianhaiwan station of Shenzhen Metro Line 5	18	Subway station	4.9	−1.5
Yueyang International Plaza in Shanghai	15	Subway station	5.3	3.5
Provident fund building in Suzhou Industrial Park	12.2	Shield tunnel	1.05	6.5

shows the statistics of the foundation pit approaching projects in the major cities in China and other countries in recent years.

Subway approaching construction will inevitably cause deformation and damage the safety of existing structures. For the existing stations with reserved deformation joints, the influence of the joints on deformation control is controversial in the academic field, which requires further research. Based on the Ciqu station construction of Beijing Metro Line 17, the mechanical effect of existing stations with reserved deformation joints is deeply studied in this paper.

Although domestic and foreign scholars [21,22,23,24,25,26,27,28] have conducted plenty of research on foundation pit and yielded rich results, ranging from the surface deformation pattern after excavation, the deformation prediction of foundation pit construction, the influence of excavation on existing structure and protection measures, to the mechanism of dewatering seepage of foundation pit, and made many valuable research results. Nevertheless, previous studies mostly focus on the subsidence of surrounding strata and adjacent structure, while the rising building enclosure and adjacent existing structure induced by the excavation of foundation pit with confined groundwater are rarely studied with less construction cases.

2. Construction Overview

2.1. Project Introduction

This paper takes the Ciqu station project along Beijing Metro Line 17 as the studied case. The horizontal net distance between the enclosure structure of the newly built station and the existing station is only 2.8 m, which is about 9 m deeper than the bottom plate of the existing station. There is a cross transfer with the existing Yizhuang line in Ciqu station, the station consists of open excavation section and concealed excavation section, the open section is sited in the north and south sides of the existing Ciqu station, while the concealed section lies in the middle of the station and directly cross through the existing station. The general layout diagram of the new station and the existing station is shown in Figure 1.

The newly built Ciqu station is a kind of island station, in which the open excavation at both ends is the underground three-layer box frame structure, and the central part that underpasses the existing line is flat top and straight wall under concealed excavation construction. The total length of the station is 415 m, the standard section is 23.1 m in width, the structure net height is 22 m, the bottom plate buried depth of the station center line mileage is 25 m, and the absolute elevation of the rail roof is 1.963 m. The existing station is divided into two separate foundation pits, the total length of the foundation pit is 286.1 m, the depth of the existing station is 25.85 m and the width is 30 m, the depth of the ground wall is set to 13.8 m, which is inserted into the water barrier. The existing station is an open double-layer three-span frame structure with net height of 13.2 m, net width of 20.7 m and bottom plate buried depth of 16.2 m.

2.2. Geology and Hydrogeological Conditions

The formation of the Ciqu station is mainly the Quaternary Holocene alluvial sediment layer, which was reed pond in 1970s, thus this location is rich in groundwater. The stratum soil layer of the project is divided into: artificial fill stratum (Qml), mainly including mixed soil, plain fill, clay and sandy silt; Quaternary Holocene alluvial layer (Q4al+pl), mostly consists of silty clay and sandy clay, uneven soil and local thin layer of clayey silt; Quaternary Pleistocene alluvial layer (Q3al+pl), mainly is fine sand soil, saturated and uneven soil, thin layer with local fine sand soil. According to the physical and mechanical properties of Ciqu station, the strata is divided into 9 general layers, and the geological section is shown in Figure 2.

The Ciqu station is largely affected by phreatic water-confined water and confined water. The groundwater level is located above the bottom plate of the station structure, and the buried water level is shallow. According to the burial depth and groundwater dynamic change characteristics, it can be divided into upper stagnant water, phreatic water, interlayer phreatic water-confined water and confined water. The hydrogeological profile of the new station is shown in Figure 3.

3. Establishment of Numerical Model and Selection of Calculation Parameters

The corresponding numerical model is established in accordance with the engineering situation of the Ciqu station. The model contains both stations and foundation pits on both sides of the north and south. The diameter of the model is: 600 m in length, 200 m in width and 70 m in height. The modified M-C (Mohr-Coulomb) model in MIDAS/GTS is applied in this paper. Compared to the M-C model, the modified MC model consists of a nonlinear elastic and elastic-plastic model with no effect between shear and compression yield, thus correcting the abnormality of pit bottom and surface uplift in the M-C model, and eliminating the problem of abnormal soil uplift in the M-C model [29].

The displacement boundary condition of the model limits the displacement of five surfaces of the model, and the ground surface is a free boundary.

In the model, the rock and soil are simulated by hexahedral solid unit; the ground wall adopts dewatering plate unit in simulation, considering the relative slip between concrete and soil, interface unit between the ground wall and soil is added; the concrete support, steel support, column and waist beam are built by implanted beam unit that can automatically couple with the solid unit.

The stratum in the foundation pit includes miscellaneous fill, sandy silt, fine sand, silty clay, fine sand and silty clay from top to bottom. The mechanical parameters are selected according to the practical construction, and the recommended values of relevant rock and soil mechanical parameters are shown in

Table 2. Proposed table of geotechnical mechanical parameters.

Soil Layer	Compression Modulus/MPa	Severe/kN/m3	Poisson Ratio	Internal Friction Angle/°	Cohesion/kPa	Thickness/m
Miscellaneous fill	2	19.5	0.4	10	5	2
Sandy silt	6	19.5	0.4	25	8	8.5
Silty fine sand	14	21	0.38	28	0	3.2
Silty clay	7	19.8	0.4	15	20	8.7
Fine sand	12	21.5	0.4	32	0	12.3
Silty clay	9	20	0.41	20	30	25.3

.

Different from the M-C model, the main parameters of the modified M-C model involve triaxial test secant stiffness, the unloading elastic modulus, and primary consolidation loading test tangent stiffness. Thus, the soil mechanical parameters in the modified M-C model should be converted according to the

Table 3. Modified Mohr-Coulomb model parameter empirical value table.

Region	${\mathit{E}}_0$	${\mathit{E}}_{50\mathbf{ref}}$	${\mathit{E}}_{\mathbf{oedref}}$	${\mathit{E}}_{\mathbf{urref}}$
Beijing	$\beta E_{\mathrm{s}}$	$\beta E_{\mathrm{s}}$	$\left(0.5\text{–}1.0\right)E_{50\mathrm{ref}}$	$\left(2\text{–}4\right)E_{50\mathrm{ref}}$
Shanghai	$\beta E_{\mathrm{s}}$	$\left(0.9\text{–}1.0\right)E_{\mathrm{s}}$	$\left(0.7\text{–}1.2\right)E_{50\mathrm{ref}}$	$\left(4\text{–}9\right)E_{50\mathrm{ref}}$
Tianjin	$\beta E_{\mathrm{s}}$	$\left(1.5\text{–}2.0\right)E_{\mathrm{s}}$	$\left(1.0\text{–}1.5\right)E_{50\mathrm{ref}}$	$5E_{50\mathrm{ref}}$

Note: $E_0$: deformation modulus; $E_{\mathrm{s}}$: compression modulus; $E_{50\mathrm{ref}}$: triaxial test secant stiffness; $E_{\mathrm{urref}}$: unloading elastic modulus; $\beta$: conversion coefficient, $\beta =\frac{1-2{\nu }^2}{1-\nu }$ ($\nu$ Poisson ratio); $E_{\mathrm{oedref}}$: primary consolidation loading test tangent stiffness.

.

The obtained rock mechanical parameters and supporting structure parameters in the model according to the conversion are shown in

Table 4. Geotechnical mechanics parameter table.

Soil Layer	Constitutive Model	${\mathit{E}}_{50\mathbf{ref}}/\mathbf{MPa}$	${\mathit{E}}_{\mathbf{oedref}}/\mathbf{MPa}$	${\mathit{E}}_{\mathbf{urref}}/\mathbf{MPa}$
Miscellaneous fill	Modified M-C	3	3	9
Sandy silt	Modified M-C	11	11	33
Silty fine sand	Modified M-C	27	27	81
Silty clay	Modified M-C	13	13	39
Fine sand	Modified M-C	22	22	66
Silty clay	Modified M-C	15	15	45

and

Table 5. Table of mechanical parameters of supporting structures.

Structure	Constitutive Model	Elastic Modulus E/MPa	Severe/kN/m3	Poisson Ratio
Diaphragm wall	Elastic	24,000	25	0.167
Crown beam	Elastic	24,000	25	0.167
Concrete support	Elastic	24,000	25	0.167
Steel support	Elastic	212,000	78.5	0.31
Column	Elastic	212,000	78.5	0.31
Waist beam	Elastic	24,000	25	0.167

.

The existing station is C30 concrete. In light of the structural strength reduction during operation, the concrete strength is valued by the actual strength multiplied by the safety factor 0.8, and the elastic modulus is 24 GPa.

The model excavation is carried out in line with the actual construction procedure, with simultaneous symmetrical excavation on both sides, the 3-dimensional model is shown in Figure 4.

The simulation of groundwater is achieved by setting the head boundary conditions. Phreatic water is simulated by setting the total head and the confined water by setting the pressure head. As for the simulation of dewatering processes, it can be defined by the percolation boundary function. The dewatering in the pit is applied for the shallow ground water and confined water to simulate the water level drop by passivation head boundary conditions; the deep confined water is pumped by dewatering well to simulate the dewatering process [30].

The layer is stratified, and different stratum parameters are set according to different aquifers. At the same time, the corresponding interface unit is added between the aquifer and the water barrier layer according to the permeability of the weak water permeable layer to simulate the effect of the water barrier layer. Based on the geotechnical exploration report of Ciqu station, the values of the permeability parameters of the water-bearing layer and the water-resisting layer are shown in

Table 6. Calculating model main geotechnical parameters table.

Number	Aquifer Type	$\mathbf{Water}\mathbf{Storage}\mathbf{Rate}{\mathit{S}}_{\mathbf{s}}/\mathbf{m}{}^{-1}$	Horizontal Permeability $\mathbf{Coefficient}{\mathit{K}}_{\mathbf{x}}/\mathbf{m}·\mathbf{d}{}^{-1}$	Vertical Permeability $\mathbf{Coefficient}{\mathit{K}}_{\mathbf{y}}/\mathbf{m}·\mathbf{d}{}^{-1}$
1	Diving	2.356 × 10−4	0.003	0.002
2	Diving-Confined water (III)	6.495 × 10−5	1.31	1.62
3	Aquitard	1.369 × 10−4	0.005	0.002
4	Confined water (IV)	7.513 × 10−5	3	1
5	Aquitard	1.986 × 10−4	0.0004	0.002
6	Confined water (V)	6.927 × 10−5	2.32	1.57
7	Aquitard	2.013 × 10−4	0.0005	0.0002
8	Confined water (VI)	8.581 × 10−5	2	1
9	Aquitard	2.136 × 10−4	0.0001	0.0002
10	Confined water (VII)	8.627 × 10−5	2.24	1.42
11	Aquitard	2.014 × 10−4	0.0002	0.0003

.

4. Analysis on the Impact of Confined Water Approaching Construction on the Existing Station

During the construction in confined water formation, the foundation pit excavation gives rise to unloading, then the water pressure pushes the formation moves up, which is likely to occur construction accidents without control. The ground wall and other envelope can block water seepage, thus change the foundation pit seepage flow and hydraulic gradient, reduce the additional water amount of confined water layer, and control the existing structure and formation settlement.

4.1. Effect of Confined Water on the Deformation of the Existing Station

Three construction conditions are supposed to analyze the influence of confined water on the existing station.

Condition I: consider phreatic water (II), interlayer phreatic water—confined water (III), confined water (IV) and confined water (V);

Condition II: consider phreatic water (II), interlayer phreatic water—confined water (III), confined water (IV), confined water (V) and confined water (VI);

Condition III: consider phreatic water (II), interlayer phreatic water-confined water (III), confined water (IV), confined water (V), confined water (VI) and confined water (VII).

30 measuring points are selected at the bottom of the south wall for analysis, and the deformation curve of the existing station is shown in Figure 5.

As can be seen from Figure 5, the upward deformation caused by confined water increases the structure deformation of the three construction conditions. The deformation led by excavation is 1.89 mm, and the deformation caused by confined water (V) and above groundwater is 2.76 mm, the deformation caused by confined water (VI) is 2.07 mm, and that led by confined water (VII) is 2.94 mm. Groundwater is the major reason for the floating deformation of the existing station. Foundation pit excavation breaks the relative balance between the upper soil and the confined water top force. Under the action of the confined water top force, the formation is constantly upward, resulting in the floating of the existing station, in which the confined water (VI) and confined water (VII) exert a great impact on the station.

The existence of multi-layer pressure aquifer is the main cause of the floating structure. Since the groundwater is only drained in the pit, the existing structure outside the pit is still affected by the multi-layer confined water top support force, so that the existing structure appears seriously floating after the excavation of the foundation pit. The maximum up floating point cumulative floating curve is indicated in Figure 6.

As shown in Figure 6, with the occurrence of confined water (VI) and confined water (VII), the bearing force of existing stations ascends, so the floating value and floating rate increase after earthwork excavation; in the process of soil excavation, the unloading effect of the third layer and the fourth layer is remarkable, thus the structure floating is larger.

4.2. Effect of Depressurization Dewatering on Station Deformation

According to the actual situation, the drainage is conducted in the pit, given the influence of confined water (VI) and confined water (VII), it is calculated into three conditions.

Condition I: confined water (VI) pumping water of 180 m³/d 5 days;

Condition II: confined water (VI) pumping water of 180 m³/d for 5 days; confined water (VII) pumping water of 210 m³/d for 5 days;

Condition III: confined water (VI) pumping water of 180 m³/d for 5 days; confined water (VII) pumping water of 420 m³/d for 5 days.

The confined water (VII) adopts the dewatering outside the pit, and the dewatering well is 5 m away from the edge of the pit, which is arranged every 20 m. The calculation results are shown in Figure 7.

In Figure 7, the depressurization dewatering of confined water (VI) and (VII) can effectively control the floating of existing stations. The maximum floating of the existing structure under the three working conditions is 7.78 mm, 5.11 mm, 4.17 mm, respectively. After decompression and dewatering, the confined water support force of the upper stratum decreases, and the confined water level also causes the decline of the upper groundwater level, which consolidate the soil particles under the effective stress, thus causing the formation deformation and reducing the floating of the station. Dewatering not only reduces the deflection in the middle of the station, but also the settlement at both ends, lessening the deformation difference between the middle of the station and both ends of the station, and protecting the station structure. At the same time, it can be seen that there is a hydraulic connection between the pressure aquifer, and convection exists in the two aquifers during dewatering.

4.3. Effect of Embedded Depth of Diaphragm Wall on Station Deformation

Based on above analysis, this section adopts the method of variables control to study the effect of different embedded depths on the deformation of the station, the simulation includes the following three conditions.

Condition I: the embedded depth of the diaphragm wall is 30 m, with only partition in confined water (V);

Condition II: the embedded depth of the diaphragm wall is 36 m, inserted into the confined water (VI), without partition;

Condition III: the embedded depth of the diaphragm wall is 40 m, with partition in confined water (VI).

The deformation curves of the existing station under different construction conditions are shown in Figure 8.

According to Figure 8, the floating of existing stations decreases as the embedded depth of the diaphragm wall increases. With the addition of the wall depth, the groundwater seepage descends and the wall deformation decreases. When the diaphragm wall embedded depth is 40 m, the floating wall and surrounding surface deformation are effectively controlled under the barrier effect of the wall on ground water, thus alleviating the impact on the existing station.

4.4. Effect of Existing Station Upper Stacking on Station Deformation

The pile load is mainly carried out in the upper part of the existing station. According to Technical Code for Safe Construction of Building Deep Foundation Pit Engineering [31], the pile load around the foundation pit does not exceed 18 kPa, the pile load is mainly divided into three conditions: the upper pile load is 6 kPa, 12 kPa and 18 kPa, respectively. The deformation curve of the station under different pile loads according to numerical simulation is shown in Figure 9.

Based on the analysis of Figure 9, for every average increase of 6 kPa, the station floating decreases by 0.4 mm, and the station floating of 18 kPa is 2.51 mm, and the station floating is effectively controlled.

4.5. Comparison and Analysis of Monitoring Results and Numerical Results

In order to analyze the differences between the numerical simulation and the practical engineering, the monitoring data and the numerical simulation data are compared. The deformation comparison map of the south side of the existing station is shown in Figure 10, the horizontal deformation comparison map of the ground wall is indicated in Figure 11, the floating duration curve of the top of the ground wall is shown in Figure 12, and Figure 13 includes the surface deformation comparison map.

In Figure 10, Figure 11, Figure 12 and Figure 13, the measured deformation data of the station have the same change trend, and the measured value at the foot of the ground wall are “kicking”, indicating that the presence of confined water is harmful to the wall.

Meanwhile, the top of the wall floating is large, due to the depressurization dewatering, the wall top floating decreases in the excavation process, the measured data is greater than the numerical simulation data, this is because in the actual pumping process, the groundwater table will appear recovery period with time; Except for fluctuations in certain points, the surface deformation monitoring data is relatively consistent with the numerical simulation data, which can reflect the deformation pattern of foundation pit excavation of confined water strata through numerical simulation analysis.

5. Analysis of the Construction Mechanical Effect of Existing Stations with Preset Deformation Joint

The corresponding numerical model is established according to the construction situation of Ciqu station. The diameter of the model is: 540 m × 300 m × 60 m, as shown in Figure 14, the existing station deformation joint position is shown in Figure 15. The model and material parameters selection are included in Chapter 2.

5.1. Analysis of the Existing Station Displacement

The maximum vertical displacement is the left part of the deformation joint, which is 16.07 mm. Due to the incoherent concrete and steel bars on the left and right sides of the deformation joint, the vertical displacement of the station on both sides of the deformation joint is different, and the existence of the deformation joint affects the force transmission within the station. The displacement level along the existing station direction is low, and the maximum displacement level is 2.6 mm. The elastic modulus of the filler inside the deformation joint is generally smaller than that of the reinforced concrete structure. When the deformation occurs, it can absorb energy, reduce the internal force of the concrete structure, and prevent the cracking of the permanent structure concrete and the internal steel corrosion.

5.2. Stress Analysis of the Existing Station

When the deformation joint is set up in the existing station, the steel bar at the deformation joint is divided. At this time, the inner filling will be transferred with the force, but part of the energy dissipation will be produced. The maximum stress value occurs at the bottom plate of the subway station on the left side of the deformation joint, 1.5 m away from the deformation joint. The maximum value is 1149.23 kN/m², the minimum position is distal, and the minimum value is 279.86 kN/m². Subway station has no deformation seam in the central part, at this time it is a whole. The minimum stress value occurs at the top position in the middle of the existing station of 652.23 kN/m², and the maximum stress value is found at the bottom plate position of the same plane, which is in the same position with the deformation joint. It can be seen that the stress level of the station is lower than that of the station without the deformation joint, and the maximum value can be reduced by 58%, which can effectively reduce the permanent structural stress level of the station floor and roof.

5.3. Analysis of the Carrying Capacity and Additional Internal Force of the Existing Station

5.3.1. Carrying Capacity of the Structure

According to the Code for Design of Concrete Structures (GB50010-2010) [32], the concrete materials, structural dimensions and matched steel bars of the existing structures, as well as the design data of the existing structures, the lateral and longitudinal bearing capacity of the existing structures are calculated, respectively. The lateral carrying capacity of the existing structures is shown in

Table 7. Transverse carrying capacity of existing stations.

Structural Position		Plate Thickness	Actual Reinforcement	Bending Moment M (kN·m)
				Strength Control	Crack Control
Roof	Center pillar support	0.7 m	φ25@100	887.1	658.7
	Mid span	0.7 m	φ25@100	887.1	658.7
	Side wall support	0.7 m	φ25@100	887.1	658.7
Side wall		0.6 m	φ25@100	745.4	553.5
Floor		1.0 m	φ25@100	1334.4	941.7

Note: When the crack width is calculated according to the standard combination, the maximum allowable crack width is 0.2 mm on the soil side facing the structure and 0.3 mm on the back soil side.

.

On the grounds of the calculation results, when the central deformation joint is set in the subway station, the maximum bending moment occurs at the bottom plate of the subway station on the left side of the deformation joint, 1.5 m from the deformation joint. The maximum value is 1149.23 kN/m², which is lower than the transverse bearing capacity of the bottom plate of the existing station and higher than the transverse bearing capacity of the crack test. Taken the station without deformation seam as a whole, with a large vertical displacement of 9.96 mm, the maximum stress occurs at the bottom plate of the same plane, 2707.58 kN/m², far exceeding the bearing capacity of the bottom plate of the existing station. The subway station structure will be damaged, cause permanent damage to the structure and reduce the service life of the station.

5.3.2. Additional Internal Force of the Structure

The “load-structure” calculation model is established for the typical section structure above the new tunnel. The typical structural cross-section of the existing Ciqu station in Yizhuang line is shown in Figure 16. A brief diagram of the load calculation is recorded in Figure 17.

Both for single and double section structures, ground overload and surrounding soil pressure are borne by its roof and side walls. To truly simulate the contact between the side wall and the bottom plate and the soil, the pressure-only soil spring is used to simulate this constraint, and the stiffness of the soil spring at the side wall adopts the weighted average of the horizontal base bed coefficient of the soil layer of the structure, and the vertical base bed coefficient of the soil layer is used in the stiffness of the soil spring at the bottom plate. The computational model and the constrained type of the existing structures are shown in Figure 18.

The initial and additional internal force values of the existing structures are calculated, respectively, as shown in Figure 19 and Figure 20.

After the existing structure of the grid unit one by one, when the existing station is affected by the early open foundation pit engineering, the maximum uplift value is 9.96 mm, in addition to the structure at the bottom of the side wall at the moment value beyond the maximum allowable moment of crack control, the majority of the nodes does not exceed the maximum allowable moment of crack control structure. Considering that the bending moment value of the node is beyond the small and the rigid domain effect of the node. Existing stations do not exceed the maximum allowable bending moment value of the strength controlled by the structure, but some nodes have extremely approached the maximum allowable value.

6. Conclusions

Through the influence of the deformation of the existing stations and the adjacent construction of confined water formation, the following conclusions can be obtained:

(1)

When the construction of the new subway is adjacent to the existing station, the existence of multi-layer confined water is the main reason for the serious floating of the existing station. The confined water brought by groundwater (5) and the above is 2.76 mm, the floating caused by confined water (6) is 2.07 mm, and the floating led by confined water (7) is 2.94 mm;

(2)

With the depressurization dewatering, increasing the embedded depth of the ground connection wall and conducting the upper pile load can effectively control the floating of the station. On this basis, it improve the embedded depth of the diaphragm wall to 40 m, completely form a partition in the confined water (6), the floating value of the station is reduced to 4.44 mm, with the timely upper loading in the construction process, the floating value of the station can be controlled within 2 mm;

(3)

The existence of the deformation joint enhance the station damage resistance, the existing deformation joint station structure, internal force within the station structure descends by 58%, thus improve the station safety redundancy, the reserved deformation joint structure allows more than 3 times of deformation than that without reserved deformation joint, it proves that the deformation joint can effectively protect the existing station;

(4)

The monitoring data is basically consistent with the numerical simulation data, and the structural deformation mechanism can be reflected through the numerical simulation analysis. In the practical construction process, the construction should be carried out under the guidance of both site monitoring and numerical simulation.

7. Patents

• A protection device for precipitation well (No 2020206288314);
• A floating control method for existing stations adjacent foundation pit construction in confined water strata (No 202010534877.4);
• A buried wellhead structure (No 2020206288329).