S S symmetry
Article Risk Analysis and Control Factors Based on Excavation of a Large Underground Subway Station under Construction
Zhien Zhang 1,2,*, Mingli Huang 1,2 and Baohua Wu 3
1 School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China; [email protected] 2 Tunneling and Underground Engineering Research Center of Ministry of Education, Beijing 100044, China 3 China Railway 16th Bureau Group Co., Ltd., Beijing 100018, China; [email protected] * Correspondence: [email protected]; Tel.: +86-136-8311-4602
Received: 21 August 2020; Accepted: 24 September 2020; Published: 2 October 2020
Abstract: Considering the convenience of pedestrian transfer, reasonable structural stress and beautiful shape design, most subway stations adopt symmetrical design. At present, the new subway station is developing in the direction of a multidimensional space, as well as a large scale, and complex structure. Tunnel construction also presents unpredictability, coupling amplification and high risks. For example, a subway extension project involves construction, which would affect the normal use of the subway or damage its structure. Based on excavation of the largest underground subway station under construction in China, the Erligou station extension project (line 16 of Beijing Metro), and using theoretical analysis, numerical simulation, monitoring data, and other research methods, this paper quantitatively analyzes the risk of a large space station’s construction process on the adjacent existing station structure and track, as well as highlights key, high-risk sub-projects, or construction steps, combined with specific engineering measures to ensure safety during construction of a new station. The general rules concerning large space subway station construction are further summarized to provide reference for similar projects.
Keywords: large space subway station; undercutting; extension construction; risk analysis; control measures
1. Introduction Generally, subway stations are designed symmetrically. However, due to the increasing number of urban public transport trips, its construction also presents another trend: a multidimensional space, as well as a large scale, and complex structure. Its construction risk also presents unpredictability, coupling amplification and high risk [1,2]. A subway is usually built in the center of a densely populated city. Since a large number of subways, municipal bridges, roads, municipal pipelines, and high-rise buildings have been built in city centers, newly-built subways often pass through existing buildings and structures in a short distance. Table1 lists some engineering cases at Beijing Metro crossing existing lines [3–17]. If the stratum deformation caused by engineering construction is too large, it will cause excessive subsidence, differential subsidence, cracking, and even collapse of adjacent existing projects, and then cause irreparable economic property loss and casualties [18,19]. For example, in July 2003, an accident occurred in the tunnel between Pudong South Road and Nanpu Bridge Station of Shanghai Metro Line 4, resulting in three buildings on the ground, the fracture of surrounding roads, fracture of the Pujiang levee due to excessive deformation, as well as river water flowing back into the tunnel. In November 2008, 17 people died and more than 50 people were injured due to the collapse of the subway foundation pit at Hangzhou Fengqing Avenue. In July 2010, the collapse of Xisanqi station
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(line 8 of Beijing Metro) caused the rupture of an underground natural gas pipeline, and gas leakage, which affected the gas consumption of tens of thousands of nearby households [20–22]. Subway extension constructions take place close to existing lines. Existing stations are built into new subway stations by means of close-up extensions, connections, and vertical increases. These kinds of newly-built subway stations are very close to existing stations, and are combined with a large scale of undercut sections and obvious spatial effects, which significantly increase construction risks. Construction risks in subway extension projects are mainly due to excessive deformation of existing station structures caused by close-distance construction, which affects normal use of the subway or causes structural failure. Some scholars have carried out research, combined with specific projects, to analyze the mechanical behaviors of subway construction close to existing lines. Regarding Suzhou Street Station (line 16 of Beijing Metro), Jia Longfei [23] simulates the construction process of the new station by combining the stratum structure method and the load structure method, and analyzes the influence of the existing station on the stress deformation caused by the existing station. According to the deformation control standard of the existing structure of the urban rail transit, Jia Longfei [23] puts forward the deformation control value for the construction procedure of the new station, and gives engineering measures to control the deformation. Wu Zhijian [24] used ANSYS finite element analysis software, combined with the actual monitoring data, and obtained the deformation law of the concrete structure and track geometry size of the new station, parallel, crossing the existing station, and proposed construction safety control measures. According to the engineering characteristics of Xuanwumen Station (line 4 of Beijing Metro), passing under the existing station, Liu Lei et al. [25] put forward comprehensive construction technology of a large pipe shed and segmented backward curtain grouting for pre-reinforcement, tracking compensation grouting in the whole process of excavation and support, and block construction of secondary lining to ensure normal operation safety of existing lines. In view of engineering problems (i.e., tunnels crossing existing lines and deformation control), Chen Mengqiao [26] and Yang Guangwu [27] proposed technology used for synchronous jacking. This involves dynamically adjusting the jack layout and jacking force distribution based on the deformation monitoring data of the existing line. The technology is based on the principle of “zoning, phased, grouping and grading”. It was applied in the turn back line project of Dongzhimen Station at the Beijing Airport line after crossing line 13 of Dongzhimen Station. Relying on the Gongzhufen station (line 10 of Beijing Metro) closely passing through the Gongzhufen station of line 1, Tao Lianjin et al. [28], through theoretical analysis and numerical calculation, concluded that the stress release of the tunnel face is the main factor causing the settlement of the existing station. However, timely closing the initial support, adding temporary jack column, and back-filling grouting can effectively control the settlement deformation of the existing structure. Park, boo Seong et al. [29] introduced the design and construction method of the largest tunnel station in Seoul. The station is located 15 cm below the existing Metro Line 3, and an abandoned underground shopping center has been built in the highly congested area. In order to overcome the obstacles of adjacent construction, civil complaints, and traffic jams caused by site conditions, a construction method combining pipe shed method and honeycomb arch method is proposed and applied. The subway station has been successfully constructed without any damage to adjacent structures. Through a variety of advanced monitoring instruments, including distributed optical fiber strain sensing (DFOS), photogrammetry, and strain gauge instrumented tunnel bolts, Gue, C.Y., etc. [30], studied the mechanical and deformation behavior of the existing royal post tunnel during the new passenger tunnel construction, and guided the design and construction of the tunnel according to data feedback. Symmetry 2020, 12, 1629 3 of 18
Table 1. Engineering cases of Beijing Metro crossing existing railway.
Overview of Crossing Section Shape of Width * Minimum Number Crossing Mode Crossing Angle (◦) Construction Method New Metro Existing Metro New Station Height/(m * m) Spacing/m Xuanwumen Station Xuanwumen Station 1 [3,4] Flat top vertical wall 9.85 * 9 Crossing below 90 1.9 CRD method of line 4 of line 2 Pingguoyuan station Pingguoyuan station 2 [5] Flat top vertical wall 23.5 * 15 Crossing below 70 0 PBA method of line 6 of line 1 Gongzhufen station Gongzhufen station 3 [6] Flat top vertical wall 14. * 9.32 Crossing below 90 0 PBA method of line 10 of line 1 Jiaomen west station Jiaomen west station 4 Horseshoe shape 10.1 * 9.3 Crossing below 90 0.15 PBA method of line 10 of line 4 Suzhou Street Station Suzhou Street Station 5 [7] Flat top vertical wall 9.4 * 8.77 Crossing below 70 0 PBA method of line 16 of line 10 6 [8] Xidan station of line 4 Section of line 1 Horseshoe shape 9.9 * 9.17 Overhead crossing 90 0.5 CRD method Dongdan station Wangfujing Dongdan 7 [9] Single arch 23.7 * 7.17 Overhead crossing 90 0.6 Column hole method of line 5 section of line 1 Chongwenmen station Chongwenmen Beijing 8 [10] arch 24.2 * 11.4 Crossing below 90 1.98 Column hole method of line 5 station section of line 2 Chaoyang Gate Chaoyang Gate Station 9 [11] Dongdaqiao section Flat top vertical wall 6.3 * 7.52 Crossing below 90 0 PBA method of line 2 of line 6 Dongsi Chaoyang Gate 10 [12] Line 5 Dongsi station Flat top vertical wall 7.1 * 8.7 Crossing below 90 0 PBA method section of line 6 Chegongzhuang Chegongzhuang station 11 [13] pinganli section Horseshoe shape 6.6 * 6.9 Crossing below 90 2.47 Step method of line 2 of line 6 Guang-double section Double well station of 12 [14] Flat top vertical wall 6.2 * 6.5 Crossing below 90 0 Step method of line 7 line 10 Chongwenmen Ciqikou 13 Ciqikou station of line 5 Horseshoe shape 6.2 * 6.5 Crossing below 90 0.7 Step method section of line 7 Guomao Shuangjing Guomao Dawanglu 14 [15] Flat top vertical wall 6.1 * 7.84 Crossing below 90 1.08 PBA method section of line 10 section of line 1 Jiulongshan Dawang Dawang road Sihui 15 [16] Circular 6 * 6 Crossing below 90 2.15 Shield method road section of line 14 section of line 1 Futong Wangjing Wangjing to Wangjing 16 [17] Circular 6 * 6 Crossing below 73 1.9 Shield method section of line 14 west section of line 15 National Library to National Library of 17 Erligou section line 9—Baishiqiao Horseshoe shape 6.3 * 6.42 Crossing below 90 2.2 Step method of line 16 south section Xiyuan Wanquanhe 18 Section of line 4 Circular 6.4 * 6.4 Crossing below 56 4 Shield method section of line 16 Symmetry 2019, 11, x FOR PEER REVIEW 4 of 18 applied. The subway station has been successfully constructed without any damage to adjacent structures. Through a variety of advanced monitoring instruments, including distributed optical fiber strain sensing (DFOS), photogrammetry, and strain gauge instrumented tunnel bolts, Gue, C.Y., etc. [30], studied the mechanical and deformation behavior of the existing royal post tunnel during the new Symmetrypassenger2020 tunnel, 12, 1629 construction, and guided the design and construction of the tunnel according4 of to 18 data feedback. Researchers mostly mostly focus focus on on the the study study of construction of construction deformation, deformation, under aunder new subway a new crossing subway ancrossing existing an line,existing while line, research while onresearch the construction on the construction risk of crossing risk of existingcrossing lines, existing above, lines, is relativelyabove, is small,relatively especially small, for especially subway for extension subway projects extension involving projects large involving spaces andlarge large spaces sections. and large Using sections. Erligou stationUsing Erligou (line 16 station of Beijing (line Metro) 16 of asBeijing the engineering Metro) as the background, engineering this background, paper analyzes this thepaper influence analyzes of constructionthe influence processes of construction of new stationsprocesses (involving of new largestations spaces) (involving and the large resulting spaces) stress and and the deformation resulting onstress adjacent and deformation structures, andon adjacent track crossing structures, existing and lines,track bycrossing using existing the research lines, methods by using of the theoretical research analysis,methods of numerical theoretical simulation, analysis, numerical and monitoring simulation, data. and The monitoring construction data. risk The is quantifiedconstruction by risk force is andquantified deformation, by force and and the deformation, key construction and the process key construction sensitive to the process deformation sensitive of to existing the deformation structures, isof discovered.existing structures, Control is discovered. measures are Control taken measures to ensure are the taken normal to ensure use of the existing normal stationsuse of existing in the constructionstations in the process. construction process.
2. Overview of New Station Project
2.1. Project Overview The newnew ErligouErligou stationstation (line (line 16 16 of of Beijing Beijing Metro) Metro) is designedis designed symmetrically, symmetrically, and and it comprises it comprises of a totalof a total length length of 303 of m303 (including m (including a 113.2 a 113.2 m undercut m undercut double-layer double-layer section section at theat the north north end, end, a 49.5 a 49.5 m single-layerm single-layer undercut undercut section section at at the the middle, middle, and and a a 140.3 140.3 m m undercutundercut double-layerdouble-layer sectionsection at the south side), withwith aa clearclear spanspan ofof 28 28 m m and and a a clear clear height height 16.3 16.3 m, m, the the burial burial depth depth of of the the bottom bottom plate plate is 27.255is 27.255 m, and m, and the total the totalconstruction construction area is area 39,000 is m 39,0002. It is m currently2. It is currently the largest the underground largest underground excavation extensionexcavation subway extension station subway under station construction under construction in the country. in the The country. existing The Erligou existing Station Erligou of line Station 6 is a single-layerof line 6 is a single-layer underground underground excavation andexcavation separated and platform separated station, platform with station, a total with length a total of 174.2 length m andof 174.2 a buried m and depth a buried of 27.93 depth m of of the 27.93 station m of roof. the station The two roof. stations The two are “crossing”stations are transfer “crossing” stations transfer with astations minimum with distance a minimum of 0.675 distance m. The of station0.675 m. is The at a crossroads,station is at witha crossroads, many surrounding with many buildings surrounding and undergroundbuildings and pipelines, underground and thepipelines, construction and the environment construction is veryenvironment complicated. is very The complicated. plane position The of theplane new position Erligou of station the new is Erligou shown instation Figure is1 .shown in Figure 1.
Figure 1. Plane position of new Erligou station.
The newly-built Erligou station is composed of three main structures at both ends (double-layer three column four span structure) and middle (single-layer and double span structure), as well as auxiliary structures,structures, suchsuch asas transfertransfer channel, channel, external external hanging hanging hall, hall, and and air air duct, duct, as as shown shown in in Figure Figure2. 2. The north and south ends of the station are constructed by the Pile-Beam Arch (PBA) Method. The height of the excavation section is 18.36 m, the width is 29.40 m, and the covering depth is 8.90 m, as shown in Figure3. The middle section of the station crosses the existing Erligou Station (of line 6 above), which is constructed by the middle tunnel method. The excavation section is 19.8 m wide and 8.57–9.67 m high. The existing line 6, here, is a flat-topped, straight wall section ZB; the initial Symmetry 2020, 12, 1629 5 of 18
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Symmetryconcrete, 2019 as, shown11, x FOR in PEER Figure REVIEW4. 5 of 18
Note: 0—existing station, 1—south PBA construction section, 2—north PBA construction section, 3— transverse passage, 4—east side no.3 hanging hall, 5—east side no.4 hanging hall, 6—east side no.2 accessNote: 0 road,—existing 7—east station, side no.3 1— southaccess PBA road, construction 8—the overpassing section, 2 and—north concealed PBA construction excavation section,section, 93— no.3transverse air duct, passage, 10—west 4— eastside sideno.2 no.3hanging hanging hall, hall, 11— 5west—east side side no.5 no.4 hanging hanging hall, hall, 12 6——westeast sideside no.1no.2 accessaccess road, 713——eastwest side side no.3 no.4 access access road, road, 8— 14the— overpassingsouthwest entrance, and concealed 15—external excavation hanging section, hall 9 —1, 16no.3— northwestair duct, 10 entrance—west side and exit,no.2 17hanging—northwest hall, 11 new—west air shaft. side no.5 hanging hall, 12—west side no.1 access road, 13—west side no.4 access road, 14—southwest entrance, 15—external hanging hall 1, 16—northwest entrance and exit,Figure 17 —2. northwestStructural newlayout air of shaft. new station.
The north and south endsFigure of the station2. Structural are constructedlayout of new by station. the Pile-Beam Arch (PBA) Method. The2.2. height Geology of the excavation section is 18.36 m, the width is 29.40 m, and the covering depth is 8.90 m, asThe shown north in Figureand south 3. The ends middle of the section station of are the constructed station crosses by thethe Pile existing-Beam Erligou Arch (PBA)Station Method. (of line 6The above), heightThe maximumwhich of the is excavation constructed depth of thesection by stratum the is middle 18.36 revealed m, tunnel the in thiswidth method. survey is 29.40 The is 51 excavationm, m. and According the sectioncovering to the is depth19.8 drilling m is wide data8.90 andm,and as 8.57 indoor shown–9.67 geotechnical in m Figure high. 3The. The test existing middle results, line section the 6, stratum here, of the is is astation mainlyflat-topped, cro composedsses straight the existing of artificialwall Erligou section fill, Station siltyZB; the fine (of initial sand, line lining6silty above), clay, adopts andwhich a sand 350 is mmconstructed pebble. thick The C20 by Erligou grid the shotcretemiddle station, tunnel ofstructure, the newmethod. and line the 16,The issecond excavation mainly lining located section adopts in pebbleis a 19.8C40 andmoldedm wide silty concrete,andclay 8.57 layers.– as9.67 shown The m geologicalhigh. in Figure The profileexisting 4. is line shown 6, here, in Figure is a 5flat. -topped, straight wall section ZB; the initial lining adopts a 350 mm thick C20 grid shotcrete structure, and the second lining adopts a C40 molded concrete, as shown in Figure 4.
FigureFigure 3. 3. CrossCross section section of of main main structure structure of of the the PBA PBA section. section.
Figure 3. Cross section of main structure of the PBA section.
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Figure 4. Longitudinal section of the overpassing and concealed excavation section.
2.2. Geology The maximum depth of the stratum revealed in this survey is 51 m. According to the drilling data and indoor geotechnical test results, the stratum is mainly composed of artificial fill, silty fine
sand, silty clay, and sand pebble. The Erligou station, of the new line 16, is mainly located in pebble Figure 4. and silty clay layers.Figure 4. TheLongitudinal Longitudinal geological sectionsection profile of the is shownoverpassing in Figure and and concealed concealed 5. excavation excavation section. section.
2.2. Geology The maximum depth of the stratum revealed in this survey is 51 m. According to the drilling data and indoor geotechnical test results, the stratum is mainly composed of artificial fill, silty fine sand, silty clay, and sand pebble. The Erligou station, of the new line 16, is mainly located in pebble and silty clay layers. The geological profile is shown in Figure 5.
Figure 5. Geological section. Note: unit of number: m, O1 —silt fill, O1 1—miscellaneous fill, O2 —silt, Figure 5. Geological section. Note: unit of number: m, ①—silt fill, ①1—miscellaneous fill, ②—silt, O3 —silt, O3 3—silty sand, O4 3—silty fine sand, O5 —pebble, O6 —silty clay, O7 —pebble, O8 —silty clay, ③—silt, ③3—silty sand, ④3—silty fine sand, ⑤—pebble, ⑥—silty clay, ⑦—pebble, ⑧—silty clay, ⑧ O8 2 —silt, and 9 O9 —pebble. 2 —silt, and 9 ⑨—pebble. 3. Analysis of the Station Construction on the Stress and Deformation of Existing Structure 3. Analysis of the Station Construction on the Stress and Deformation of Existing Structure 3.1. DescriptionFigure 5. Geological of Calculation section. Model Note: unit of number: m, ①—silt fill, ①1—miscellaneous fill, ②—silt,
3.1. Description(1)③ Calculation—silt, ③ of3— Calculationsilty Assumption sand, ④ Model3—silty fine sand, ⑤—pebble, ⑥—silty clay, ⑦—pebble, ⑧—silty clay, ⑧ In2 this —silt, paper, and 9 MIDAS ⑨—pebble./GTS finite element software is used to establish the stratum structure model, (1) Calculation Assumption to analyze the influence of the construction stages of the newly-built M16 Erligou station on the station In this paper, MIDAS/GTS finite element software is used to establish the stratum structure structure3. Analysis and trackof the of Station line 6. Construction The purpose on of usingthe Stress the finite and Deformation element software of Existing is, not toStructure pay attention to model, to analyze the influence of the construction stages of the newly-built M16 Erligou station on the accuracy of the absolute value of the calculation results, but to find out the rules and trends by the station structure and track of line 6. The purpose of using the finite element software is, not to analyzing3.1. Description the relative of Calculation values ofModel deformation or stress. pay attention to the accuracy of the absolute value of the calculation results, but to find out the rules The(1) calculation Calculation assumes Assumption the following: and trends by analyzing the relative values of deformation or stress. In this paper, MIDAS/GTS finite element software is used to establish the stratum structure (1) During the construction of the new structure, the Erligou station of the existing metro line 6 only model, to analyze the influence of the construction stages of the newly-built M16 Erligou station on theconsiders station structure normal and operational track of line conditions, 6. The purpo andse of does using not the consider finite element earthquakes software and is, not civil to air paydefense attention conditions. to the accuracy of the absolute value of the calculation results, but to find out the rules (2)andIt trends is assumed by analyzing that the the soil relative layer isvalues homogeneous of deformation and horizontally or stress. distributed.
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The calculation assumes the following:
(1) During the construction of the new structure, the Erligou station of the existing metro line 6 only considers normal operational conditions, and does not consider earthquakes and civil air Symmetrydefense2020, 12conditions., 1629 7 of 18 (2) It is assumed that the soil layer is homogeneous and horizontally distributed. (2) Mechanical Model and Calculation Parameters The upper upper boundary boundary of of the the numerical numerical calculation calculation model model is the is surface, the surface, which iswhich 50 m inis Z 50 direction, m in Z direction,215 m in X215 direction, m in X direction, and 200 m and in Y200 direction, m in Y direction, as shown as in shown Figure in6. Figure Di fferent 6. Different constitutive constitutive models modelsare used are to used simulate to simulate different different materials, materials, a linear a elasticlinear modelelastic model is used is for used concrete, for concrete, and the and Mohr the MohrCoulomb Coulomb (M–C) (M model–C) model is used is for used soil. for The soil. model The model is divided is divided into 139,811 into 139,811 elements elements with 21,134 with 21,134 nodes. Thenodes. ground The overload ground overload is considered is considered as 20 kPa. as The 20 values kPa. of The physical values and of mechanicalphysical and parameters mechanical of parametersmaterials in of the materials model are in shownthe model in Table are shown2. in Table 2.
Figure 6. Computational model.
Table 2. Physical and mechanical parameters of materials. 3 ProjectProject h/mh/m E/MPaE/MPa vv cc//kPakPa φ/°ϕ /◦ γ (γKN/m(KN3/)m Constitutive) Constitutive Relation Relation 1—Silt,1 Fill—Silt, Fill 2.52.5 99 0.30.3 23.523.5 28 28 19.5 19.5 M–C M–C 2—Fine2— sandFine sand 44 25 25 0.330.33 00 33 33 21 21M–C M–C 3—Pebble3—Pebble layer 1layer 1 28.728.7 9090 0.20.2 00 45 45 21 21M–C M–C 4—Pebble4—Pebble layer 2layer 2 8.38.3 120120 0.20.2 00 45 45 21 21M–C M–C 5—Gravel layer 6.5 400 0.2 0 45 22 M–C 5—Gravel layer 6.5 400 0.2 0 45 22 M–C Initial support Thick 0.3 25,500 0.2 / / 25 Elastic Initial support Thick 0.3 25,500 0.2 // 25 Elastic Second lining Thick 0.5 32,500 0.2 / / 25 Elastic Second lining Thick 0.5 32,500 0.2 // 25 Elastic Note: h, E, v, c, φ and γ represent material thickness, elastic modulus, Poisson’s ratio, cohesion, Note: h, E, v, c, ϕ and γ represent material thickness, elastic modulus, Poisson’s ratio, cohesion, internal friction internalangle and friction gravity, angle respectively. and gravity, respectively.
3.2. Simulation Simulation of Construction Process According to the construction process of Erligou station, on the new M16 line, the calculation is divided into 15 construction stages for simulation, as shown in Table3 3..
Table 3. Description of construction simulation process.
ConstructionConstruction Stage Stage Description Description 1 Initial stage 1 Initial stage 2 Construction cross passage and PBA section 2 Construction cross passage and PBA section 3 Construction of no.3 and no.4 external hanging halls 34 ConstructionConstruction of no.3 of no.2 and and no.4 no.3 external access roads hanging halls 45 Integrated Construction interface of of no.2construction and no.3 southeast access roadscorner 56 Ground reinforcement Integrated by grouting interface in the of constructionoverpassing and southeast concealed corner excavation section 6 7 Ground reinforcementConstruction by groutingof overpassing in the and overpassing concealed excavation and concealed section excavation section 78 ConstructionFour connecting of channels overpassing under andthe construction concealed excavationundercut section section 89 Four connecting channelsConstruction under of theno.3 construction air duct undercut section 910 Construction Construction of no.2 and no.5 of no.3external air hanging duct halls 1011 ConstructionConstruction of no.2 of no.1 and and no.5 no.4 external access roads hanging halls 1112 ConstructionSouthwest entrance of no.1 and and exit no.4of construction access roads 1213 SouthwestConstruction entrance of external and exithanging of construction hall 1 13 Construction of external hanging hall 1 14 Northwest entrance and exit of construction 15 Construction of northwest exhaust and fresh air shaft Symmetry 2019, 11, x FOR PEER REVIEW 8 of 18
Symmetry 2020, 1214, 1629 Northwest entrance and exit of construction 8 of 18 15 Construction of northwest exhaust and fresh air shaft
3.3.3.3. Stress andand DeformationDeformation AnalysisAnalysis ofof ExistingExisting StationStation StructureStructure (1)(1) DeformationDeformation AnalysisAnalysis TheThe newly-builtnewly-built stationstation isis locatedlocated aboveabove thethe existingexisting stationstation ofof lineline 6;6; therefore,therefore, thethe deformationdeformation eeffectffect ofof thethe newnew stationstation constructionconstruction onon thethe existingexisting stationstation structurestructure isis mainlymainly (Z(Z direction)direction) verticalvertical deformation,deformation, andand only only the the vertical vertical deformation deformation can can bebe analyzed. analyzed. Since there there areare manymany simulated simulated constructionconstruction stages, stages, only only partial partial deformation deformation diagrams diagrams are listed, are as listed, shown as in Figureshown7 .in The Figure deformation 7. The resultsdeformation of each results construction of each stageconstruction are shown stage in Tableare shown4. in Table 4.
2 Construction cross passage and PBA section.
7 Construction of overpassing and concealed excavation sections.
FigureFigure 7. 7. AccumulatedAccumulated deformation diagram of of Z directiondirection of the the existing existing station station in in partial partial constructionconstruction stage.
Table 4. Statistics of vertical deformation of the existing line caused by each construction stage.
Maximum Cumulative Maximum Vertical Construction Stage Description Vertical Deformation of Deformation of Existing Existing Line/mm Line/mm 1 Initial stage 0 0
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Table 4. Statistics of vertical deformation of the existing line caused by each construction stage.
Maximum Cumulative Maximum Vertical Construction Stage Description Vertical Deformation of Deformation of Existing Line/mm Existing Line/mm 1 Initial stage 0 0 Construction cross passage and 2 0.987 0.987 PBA section Construction of no.3 and no.4 3 1.072 0.085 external hanging Hall Construction of no.2 and no.3 4 1.257 0.185 access roads Integrated interface of 5 1.404 0.147 construction southeast corner Ground reinforcement by 6 grouting in the overpassing and 1.404 0 concealed excavation section Construction of overpassing and 7 3.965 2.561 concealed excavation section Four connecting channels under 8 4.142 0.177 the construction undercut section 9 Construction of no.3 air duct 4.218 0.076 Construction of no.2 and no.5 10 4.303 0.085 external hanging halls Construction of no.1 and no.4 11 4.768 0.465 access roads Southwest entrance and exit 12 4.952 0.184 of construction Construction of external 13 4.831 0.121 hanging hall 1 − Northwest entrance and exit 14 4.829 0.002 of construction − Construction of northwest exhaust 15 4.819 0.01 and fresh air shaft − Note: positive value indicates upward floating and negative value indicates settlement, Maximum vertical deformation of existing line N = Maximum cumulative vertical deformation of existing line N—Maximum cumulative vertical deformation of existing line N-1.
The existing station is one of the risk sources in the construction environment of the new station. The construction of the new station will cause disturbance and deformation in the soil. The soil, as the transmission medium, will transfer the deformation to the existing station, causing the deformation of the existing station’s concrete structure. If the deformation is too large, it will cause the concrete structure to crack. According to the cooperative deformation principle, the track will also produce excessive deformation, which will affect the normal operation of the subway. This is consistent with the principle of the tuning fork resonance test. Air, as a medium, transmits acoustic energy, which makes the non-contact tuning fork vibrate. If the deformation is regarded as the direct factor causing the construction risk, the risk of adjacent construction can be quantitatively analyzed. It can be seen from Table4 that the construction of the new Erligou station will cause the overall uplift of the existing station. The maximum uplift of the existing structure is 4.952 mm, which occurs in the construction of the southwest entrance and exit in phase 12. The construction of the PBA section of main structure and the overpassing and concealed excavation section has obvious influence on structural deformation of the existing railway. In the seventh construction stage, the distance between the overpassing and concealed excavation section and the existing line is the closest, which has the greatest impact on the structural deformation of the existing line 6 tunnel, with upward floating of 2.561 mm. Therefore, the overpassing and concealed excavation section is a key part of the project to control the structural deformation of the existing line. Secondly, the PBA section of the main structure causes the existing line to float 0.987 mm. Due to Symmetry 2019, 11, x FOR PEER REVIEW 10 of 18 Symmetry 2020, 12, 1629 10 of 18 Symmetry 2019, 11, x FOR PEER REVIEW 10 of 18 mm. Due to the close distance between the PBA construction section, the existing line, and the large scale of underground excavation space, the construction disturbance has great influence on the themm. close Due distanceto the close between distance the between PBA construction the PBA construction section, the section, existing the line, existing and line, the largeand the scale large of deformation of the existing line tunnel structure. undergroundscale of underground excavation excavation space, the construction space, the construction disturbance hasdisturbance great influence has great on the influence deformation on the of
Figure 8. Contribution of each construction stage to the structural failure risk of the existing line 6 tunnel.Figure 8. Contribution of each construction stage to the structural failure risk of the existing line Figure 8. Contribution of each construction stage to the structural failure risk of the existing line 6 6 tunnel. tunnel. (2) Stress Analysis (2) Stress Analysis The(2) Stress position Analysis with the largest structural deformation of the existing line tunnel is selected for stressThe analysis, position as withshown the in largest Figure structural 9 and Table deformation 5. Due to of the the large existing number line tunnel of stress is selectednephogram for stress and analysis,The asposition shown with in Figure the largest9 and Tablestructural5. Due deformation to the large of number the existing of stress line nephogram tunnel is selected and little for littlestress color analysis, difference as shown between in Figure stress 9 diagrams,and Table Figure5. Due 9to on thely largeshows number the partial of stress stress nephogram Fxx before andand aftercolor construction, difference between at the place stress with diagrams, the largest Figure deformation9 only shows of the the existing partial structure, stress F xx andbefore the complete and after little color difference between stress diagrams, Figure 9 only shows the partial stress Fxx before and stressconstruction, difference at the is shown place with in Table the largest 5. deformation of the existing structure, and the complete stress diafterfference construction, is shown at in the Table place5. with the largest deformation of the existing structure, and the complete stress difference is shown in Table 5.
Initial stage Fxx Maximum stage of structural deformation Fxx
FigureFigureInitial 9. 9. CloudCloud stage chart chart Fxx of of internal internal force force of ofMaximum existing existing station station stage and and of section sectionstructural structure. deformation Fxx
Figure 9. Cloud chart of internal force of existing station and section structure.
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Symmetry 2019, 11, x FOR PEER REVIEW 11 of 18 Symmetry 2019, 11, x FOR PEER REVIEW 11 of 18 Table 5. Internal force value at the maximum displacement of Erligou station and section structure of Table 5. Internal force value at the maximum displacement of Erligou station and section structure of Tableexisting 5. lineInternal 6. force value at the maximum displacement of Erligou station and section structure of existing line 6. existing line 6. Direction Initial Stage/kPa Internal force Value at the Maximum Displacement/kPa Direction Initial Stage/kPa Internal force Value at the Maximum Displacement/kPa F Direction Initial7403.39 Stage/kPa Internal force Value at the 1760.71Maximum Displacement/kPa xx Fxx −7403.39 1760.71 Fxx − −7403.39 1760.71 Fyy Fyy 5861.64−5861.64 4043.69 4043.69 Fyy − −5861.64 4043.69 Fxy Fxy 157.759−157.759 −154.326154.326 Fxy − −157.759 −154.326−
ItIt cancancan bebebe seen seenseen from fromfrom Table TableTable5 that 55 thatthat during duringduring the wholethethe wholewhole construction constructionconstruction process processprocess of Erligou ofof ErligouErligou station, station,station, of metro ofof metrolinemetro 16, lineline the 16, maximum16, thethe maximummaximum structural structuralstructural internal internalinternal force of theforceforce Erligou ofof thethe station ErligouErligou of stationstation the existing ofof thethe metro existingexisting line metrometro 6, and lineline the 6,section6, andand thethe with sectionsection the largest withwith thethe structural largestlargest deformation, structuralstructural deformation,deformation, is 4043.69 kPa; isis 4043.694043.69 thus, it kPa;kPa; is necessary thus,thus, itit isis to necessarynecessary check the toto bearing checkcheck thecapacitythe bearingbearing of reinforcedcapacitycapacity ofof concrete reinforcedreinforced structure, concreteconcrete and structure,structure, focus on andand real-time focusfocus onon monitoring realreal--timetime monitoringmonitoring of this part ofof during thisthis partpart the duriconstructionduringng thethe constructionconstruction process to ensure processprocess the toto safety ensureensure of thethe the sasa existingfetyfety ofof thethe structure. existingexisting structure.structure. 4. Analysis on Construction Risk of the Overpassing and Concealed Excavation Section Close to 4.4. AnalysisAnalysis onon ConstructionConstruction RiskRisk ofof thethe OverpassingOverpassing andand ConcealedConcealed ExcavationExcavation SectionSection CloseClose toto the Existing Station thethe ExistingExisting StationStation 4.1. Description of Calculation Model 4.1.4.1. DescriptionDescription ofof CalculationCalculation ModelModel Midas finite element software was used to establish the three-dimensional calculation model. Midas finite element software was used to establish the three-dimensional calculation model. The x,Midas y, and finite z were element 49.5 m softwa90 mre was50 m, used respectively. to establish The the main three part-dimensional of the station calculation adopts a grid model. size The x, y, and z were 49.5 m× × 90 m× × 50 m, respectively. The main part of the station adopts a grid Theof 1.0 x, m, y, andand the z were external 49.5 soil m × size 90 graduallym × 50 m, transits respectively. to 4 m. The The main model part is divided of the station into 98,975 adopts nodes a grid and size of 1.0 m, and the external soil size gradually transits to 4 m. The model is divided into 98,975 164,794size of 1.0 elements m, and (Figure the external 10). The soil calculation size gradually assumption, transits constitutiveto 4 m. The model,model is and divided material into physical 98,975 nodes and 164,794 elements (Figure 10). The calculation assumption, constitutive model, and material nodesand mechanical and 164,794 parameters elements (Figure are the same10). The as thosecalculation in Section assumption, 3.1. constitutive model, and material physicalphysical andand mechanicalmechanical parametersparameters areare thethe samesame asas thosethose inin SectionSection 3.1.3.1.
Figure 10. Model drawing of the overpassing and concealed excavation section. Figure 10. Model drawing of the overpass overpassinging and concealed excavation section. 4.2. Simulated Construction Sequence 4.2.4.2. SimulatedSimulated ConstructionConstruction SequenceSequence According to the actual construction method of the sub-project, the simulation is divided into AccordingAccording toto thethe actualactual constructionconstruction methodmethod ofof thethe subsub--project,project, thethe simulationsimulation isis divideddivided intointo 1616 16 construction stages, as shown in Figure 11 and Table6. constructionconstruction stages,stages, asas shownshown inin FigureFigure 1111 andand TableTable 6.6.
FigureFigure 11.11. ConstructionConstruction sequence.sequence. Note: Note:O 1①and andO2 ②are are middle middle holes, holes,O3 and③ andO4 are ④ sideare side holes, holes,and Oand5 and ⑤ Figure 11. Construction sequence. Note: ① and ② are middle holes, ③ and ④ are side holes, and ⑤ 6 are side holes. Oand ⑥ are side holes. and ⑥ are side holes.
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Symmetry 2019, 11, x FOR PEER REVIEW 12 of 18 Table 6. Simulation construction stage. Table 6. Simulation construction stage. Construction Stage Description Construction Stage Description 1 Initial stress 1 Initial stress 2 Excavation of existing station 2 Excavation of existing station 33 DisplacementDisplacement clearing clearing 44 GroutingGrouting reinforcement reinforcement ofof soil mass mass of of tunnels tunnels 1 and 1 and 2 2 55 ExcavationExcavation of of soilsoil massmass of of tunnels tunnels 1 1and and 2 2 66 InitialInitial support support for tunnels 1 1and and 2 2 77 ConstructionConstruction of of secondary secondary lining of of tunnels tunnels 1 and 1 and 2 2 88 GroutingGrouting reinforcement reinforcement ofof soil mass mass of of tunnels tunnels 3 and 3 and 4 4 99 ExcavationExcavation of of soilsoil massmass of of tunnels tunnels 3 3and and 4 4 1010 InitialInitial support support for tunnels 3 3and and 4 4 1111 GroutingGrouting reinforcement reinforcement ofof soil mass mass of of tunnels tunnels 5 and 5 and 6 6 1212 ExcavationExcavation of of soilsoil massmass of of tunnels tunnels 5 5and and 6 6 1313 InitialInitial support support for tunnels 5 5and and 6 6 1414 ConstructionConstruction of base base plate plate 1515 ConstructionConstruction ofof secondary lining lining 1616 ExcavationExcavation ofof rectangularrectangular passage passage
4.3.4.3. Analysis Analysis of of Stress Stress and and Deformation Deformation Characteristics Characteristics of of Track Track Bed Structure (1)(1) Deformation Deformation Analysis Analysis AccordingAccording to to the calculation assumption, the the existing existing track track and and track track bed structure deform harmoniously,harmoniously, so so the the track track bed bed structure structure deformation deformation is used is used to represent to represent the track the track deformation, deformation, and theand stress the stress and anddeformation deformation characteristics characteristics of the of existing the existing line linein the in c theonstruction construction process process of the of overpassingthe overpassing and and concealed concealed excavation excavation section, section, using using the middle the middle hole holemethod, method, are analyzed. are analyzed. The correspondingThe corresponding monitoring monitoring points points are set are on set the on existing the existing station station structure, structure, as shown as shown in Figure in Figure 12, 12to, monitorto monitor the the vertical vertical displacement displacement of the of the station station in each in each construction construction stage. stage.
Figure 12. Selection of deformation measuring points of existing stations. Note: the red dots in the Figure 12. Selection of deformation measuring points of existing stations. Note: the red dots in the cloud image indicate the deformation monitoring points. cloud image indicate the deformation monitoring points. The vertical displacement of the main structure of the existing station is shown in Figures 13 and 14. It canThe be seenvertical from displacement the deformation of the curve main that structure due to of the the excavation existing station and unloading is shown ofin theFigures soil mass13 and of 14the. It new can station, be seen the from uplift the deformation deformation of curve the lower that stationdue to occurs,the excavation and the upliftand unloading amount is of positively the soil masscorrelated of the with new the station, excavation the uplift progress. deformation The overall of the uplift lower deformation station occurs, presents and athe reverse uplift peckamount curve. is positivelyThe construction correlated of primary with the support excavation and secondaryprogress. The lining overall forms uplift the main deformation structure presents of the new a reverse station, peckwhich curve. is equivalent The construction to loading of above primary the existing support station, and secondary which helps lining to reduceforms the upliftmain deformationstructure of theof the new existing station, station. whichBecause is equivalent the soil to betweenloading theabove two the stations existing is reinforcedstation, which by grouting, helps tothe reduce uplift the of upliftthe middle defor partmation of theof the curve existing will be station. reduced, Because and the the trend soil curvebetween of the the reverse two stations double is peck reinforced settlement by grouting,tank will the be formeduplift of finally. the middle The part influence of the ofcurve grouting will be reinforcement reduced, and onthe thetrend bottom curve of of thethe existingreverse doublestation peck structure settlement is small, tank and will its be general formed trend finally. is consistent The influence with thatof grouting of the top, reinforcement but there is on only the a bottomsingle reverse of the existing peck settlement station structure tank. is small, and its general trend is consistent with that of the top, but there is only a single reverse peck settlement tank.
Symmetry 2019, 11, x FOR PEER REVIEW 13 of 18 Symmetry 2020, 12, 1629 13 of 18 Symmetry 2019, 11, x FOR PEER REVIEW 13 of 18
Figure 13. Z-direction displacement of the monitoring point at the top of the existing station. Figure 13. ZZ-direction-direction displacement of the monitoring point at the top of the existing station.
Figure 14. Z-direction displacement of the monitoring point at the bottom of the existing station. Figure 14. Z-direction displacement of the monitoring point at the bottom of the existing station. TheFigure vertical 14. Z deformation-direction displacement of existing of stationthe monitoring structure point in eachat the constructionbottom of the sequenceexisting station. is shown in The vertical deformation of existing station structure in each construction sequence is shown in Table7. It can be seen from the table that the accumulated maximum floating point of the existing TableThe 7. It vertical can be deformation seen from the of tableexisting that station the accumulated structure in maximum each construction floating sequence point of theis shown existing in station was 5.473 mm, which occurred in the 12th construction stage. The maximum floating value of stationTable 7 was. It can 5.473 be mm, seen which from theoccurred table thatin the the 12th accumulated construction maximum stage. The floating maximum point floating of the valueexisting of the existing station in each construction stage was 3.3 mm, which also occurred in the 12th construction thestation existing was 5.473 station mm, in which each occurred construction in the stage12th construction was 3.3 mm, stage. whi Thech alsomaximum occurred floating in the value 12th of stage, accounting for 63.4% of the cumulative deformation of the existing station. Therefore, the soil constructionthe existing stationstage, accounting in each construction for 63.4% of stage the wascumulative 3.3 mm, deformation which also of occurred the existing in the station. 12th excavation of the side tunnels, 5 and 6, is the stage with the greatest risk of causing excessive deformation Therefore,construction the stage, soil excavation accounting of the for side 63.4% tunnels, of the 5 and cumulative 6, is the stage deformation with the ofgreatest the existing risk of causing station. of the existing station structure. In the actual construction process, the deformation monitoring during excessiveTherefore, deformationthe soil excavation of the of existingthe side tunnels, station structure.5 and 6, is the In thestage actual with the construction greatest risk process, of causing the the construction stage should be strengthened and reinforcement measures should be taken in time. deformationexcessive deformation monitoring o fduring the existing the construction station structure. stage should In the be actualstrengthened construction and reinforcement process, the (2) Stress Analysis measuresdeformation should monitoring be taken during in time. the construction stage should be strengthened and reinforcement During the construction of the new station, the maximum and minimum principal stresses of the measures should be taken in time. existing station structure are shown in Figure 15. The maximum principal stress occurs at the top Table 7. Vertical deformation of existing station structure. plate of the left line, with the maximum value of 12.848 MPa, which is greater than the design value of Table 7. Vertical deformation of existing station structure. Maximum Cumulative Maximum Vertical C40Construction concrete axial tensile strength of 1.71 MPa, and there is a risk of concrete cracking. In addition, Description VerticalMaximum Deformation Cumulative of DeformationMaximum ofVertical Existing theConstruction maximumStage compressive strength of the reinforced concrete is 288.1 MPa, which is smaller than the Description VerticalExisting Deformation Station/mm of DeformationStation/mm of Existing designStage value of reinforced concrete. 1 Initial stress Existing0.000 Station/mm Station/mm0.000 21 ExcavationInitial of existing stress station 9.1630.000 9.1630.000 32 ExcavationDisplacement of existing clearing station 0.0009.163 0.0009.163 Grouting reinforcement of soil mass 43 Displacement clearing −0.0050.000 −0.0050.000 Groutingof reinforcement tunnels 1 and of 2 soil mass 4 Excavation of soil mass of tunnels 1 −0.005 −0.005 5 of tunnels 1 and 2 1.577 1.582 Excavation of soiland mass 2 of tunnels 1 5 1.577 1.582 6 Initial supportand for tunnels2 1 and 2 2.041 0.464 Construction of secondary lining of 76 Initial support for tunnels 1 and 2 1.0842.041 −0.9570.464 Constructiontunnels of secondary 1 and 2 lining of 7 1.084 −0.957 tunnels 1 and 2
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Table 7. Vertical deformation of existing station structure.
Maximum Cumulative Maximum Vertical Construction Stage Description Vertical Deformation of Deformation of Existing Station/mm Existing Station/mm Symmetry 2019, 11, x FOR PEER REVIEW 14 of 18 1 Initial stress 0.000 0.000 2 Excavation of existing station 9.163 9.163 Grouting reinforcement of soil mass 8 3 Displacement clearing1.032 0.000−0.052 0.000 Groutingof tunnels reinforcement3 and 4 of soil 4 Excavation of soil mass of tunnels 3 0.005 0.005 9 mass of tunnels 1 and 2 1.934− 0.902− Excavationand of4 soil mass of tunnels 5 1.577 1.582 10 Initial support for tunnels1 and 3 2 and 4 2.221 0.287 Grouting reinforcement of soil mass 11 6 Initial support for tunnels 1 and 22.173 2.041−0.048 0.464 Constructionof tunnels 5 and of secondary 6 lining 7 Excavation of soil mass of tunnels 5 1.084 0.957 12 of tunnels 1 and 2 5.473 −3.3 Groutingand reinforcement6 of soil 13 8 Initial support for tunnels 5 and 6 5.038 1.032 −0.4350.052 mass of tunnels 3 and 4 − 14 ConstructionExcavation of of base soil plate mass of tunnels 4.293 −0.745 9 1.934 0.902 15 Construction of secondary3 and 4lining 4.533 0.24 16 10Excavation Initial of support rectangular for tunnelspassage 3 and 45.201 2.2210.668 0.287 (2) Stress Analysis Grouting reinforcement of soil 11 2.173 0.048 During the constructionmass of ofthe tunnels new station, 5 and 6 the maximum and minimum principal− stresses of Excavation of soil mass of tunnels the existing12 station structure are shown in Figure 15. The maximum5.473 principal stress occurs 3.3 at the top 5 and 6 plate of the13 left line, with Initial the support maximum for tunnels value 5of and 12.848 6 MPa, which 5.038 is greater than the design0.435 value − of C40 concrete14 axial tensile Construction strength of of 1.71 base MPa, plate and there is a risk 4.293 of concrete cracking.0.745 In addition, − the maximum15 compressive Construction strength of of secondary the reinforced lining concrete is 288.1 4.533 MPa, which is smaller 0.24 than the design value16 of reinforcedExcavation concrete. of rectangular passage 5.201 0.668
Maximum principal stress diagram
Minimum principal stress diagram
Figure 15. Stress analysis of existing station structure.
5. Risk Control of Construction Close to Existing Station
5.1. Deformation Control Measures for Construction Close to Existing Line Deformation is the direct factor leading to the failure of the existing station concrete structure. In the construction process of the new station, it is necessary to control the deformation to ensure the normal use of the existing station. The deformation includes the structural deformation of the new station itself, the surrounding soil deformation, and the concrete structure deformation of the existing station. The construction of the new station is the source of deformation, and soil is the medium to
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5. Risk Control of Construction Close to Existing Station
5.1. Deformation Control Measures for Construction Close to Existing Line Deformation is the direct factor leading to the failure of the existing station concrete structure. In the construction process of the new station, it is necessary to control the deformation to ensure the normal use of the existing station. The deformation includes the structural deformation of the new station itself, the surrounding soil deformation, and the concrete structure deformation of the existing station.Symmetry The 2019 construction, 11, x FOR PEER of REVIEW the new station is the source of deformation, and soil is the medium15 of 18 to transfer deformation/risk. Therefore, deformation can be effectively controlled from the source and transfer deformation/risk. Therefore, deformation can be effectively controlled from the source and propagation path. propagation path. The source: (1) changes the construction method, adopts a multi-pilot tunnel excavation to change The source: (1) changes the construction method, adopts a multi-pilot tunnel excavation to the tunnelchange working the tunnel face working from face a large from a section large secti toon a small to a small section, section, reduces reduces the the spacespace effect effect caused caused by construction,by construction, and then and reducesthen reduces the the deformation deformation (Figure (Figure 16 16)).. (2) (2) Support Support measures, measures, such such as initial as initial support,support, radial radial bolt, bolt, foot foot lock lock bolt, bolt, and and advance advance small pipe pipe are are adopted adopted to provide to provide support support resistan resistancece fromfrom outside, outside, bear bear part ofpart the of surrounding the surrounding rock rock pressure pressure caused caused by excavation by excavation unloading, unloading, and and restrain soil deformationrestrain soil deformation around the around tunnel the excavation tunnel excavation contour contour line. line.
FigureFigure 16. 16.Excavation Excavation of multiple multiple pilot pilot tunnels. tunnels.
In theIn aspectthe aspect of theof the transmission transmission path: path: thethe cement slurry, slurry, or or cement cement water water glass glass double double liquid liquid slurry,slurry, is injected is injected into theinto soil the tosoil improve to improve the cohesion the cohesion c and c internaland internal friction friction angle angle of the of soil,the soil, to enhance to the anti-deformationenhance the anti- abilitydeformation of the ability soil body of the itself, soil reducebody itself, the soilreduce pressure the soil exerted pressure on exerted the existing on the station structureexisting due station to the structure soil deformation, due to the and soil controldeformation, the deformation and control ofthe the deformation existing stationof the existing structure. station structure. 5.2. Analysis on Deformation Monitoring of Existing Track Station 5.2. Analysis on Deformation Monitoring of Existing Track Station Whether the engineering measures adopted are effective, and whether the construction risks Whether the engineering measures adopted are effective, and whether the construction risks are are controlledcontrolled or or not not needs needs to tobe verified be verified by the by actual the actualmonitoring monitoring data. The data. vertical The deformation vertical deformation of track of trackstructure structure in Erligou in Erligou station, station, Baishiqiao Baishiqiao south station south— station—ErligouErligou station section, station and section, Erligou andstation Erligou stationChegongzhuang Chegongzhuang West Weststation station section section of the ofexisting the existing metro line metro 6 is linemonitored. 6 is monitored. The rangeThe is K5 range + is K5 +009.000~K5009.000~K5 + +310.000310.000 for forthe theright right line, line, 301 m 301 for m single for single track, track,K4 + 991.000~K5 K4 + 991.000~K5 + 310.000+ for310.000 the left for the left line,line, and and 319319 mm forfor singlesingle track. track. Figures Figures 17 17 and and 18 18 show show the the monitoring monitoring range range of the of track’s the track’s vertical vertical deformationdeformation of the of the existing existing station station and and the the data data picturespictures taken taken by by staff sta ffduringduring field field monitoring. monitoring. It can be seen from Figure 19 that, due to the influence of the construction of the new Erligou station, the existing line floats upward, and the deformation trend is high in the middle and low on both sides. The distance between the station and the upper undercutting section is closer than that of the sections on both sides, so the space–time effect caused by the construction is greater, and the floating degree is more obvious. The accumulated maximum uplift value of the left line is 1.34 mm, and that of the right line is 1.63 mm, which meets the requirements of the existing line structure and track deformation in the technical code for monitoring of Urban Rail Transit Engineering (GB 500911-2013) and the enterprise standard technical standard of Beijing Metro Operation Co., Ltd. guidelines for maintenance of public works (QB (J)/BDY (a) xl003-2009).
Figure 17. Monitoring range of track vertical deformation of the existing station.
Symmetry 2019, 11, x FOR PEER REVIEW 15 of 18 transfer deformation/risk. Therefore, deformation can be effectively controlled from the source and propagation path. The source: (1) changes the construction method, adopts a multi-pilot tunnel excavation to change the tunnel working face from a large section to a small section, reduces the space effect caused by construction, and then reduces the deformation (Figure 16). (2) Support measures, such as initial support, radial bolt, foot lock bolt, and advance small pipe are adopted to provide support resistance from outside, bear part of the surrounding rock pressure caused by excavation unloading, and restrain soil deformation around the tunnel excavation contour line.
Figure 16. Excavation of multiple pilot tunnels.
In the aspect of the transmission path: the cement slurry, or cement water glass double liquid slurry, is injected into the soil to improve the cohesion c and internal friction angle of the soil, to enhance the anti-deformation ability of the soil body itself, reduce the soil pressure exerted on the existing station structure due to the soil deformation, and control the deformation of the existing station structure.
5.2. Analysis on Deformation Monitoring of Existing Track Station Whether the engineering measures adopted are effective, and whether the construction risks are controlled or not needs to be verified by the actual monitoring data. The vertical deformation of track structure in Erligou station, Baishiqiao south station—Erligou station section, and Erligou station Chegongzhuang West station section of the existing metro line 6 is monitored. The range is K5 + 009.000~K5 + 310.000 for the right line, 301 m for single track, K4 + 991.000~K5 + 310.000 for the left line,Symmetry and2020 319, 12 m, 1629for single track. Figures 17 and 18 show the monitoring range of the track’s vertical16 of 18 deformation of the existing station and the data pictures taken by staff during field monitoring.
Symmetry 2019, 11, x FOR PEER REVIEW 16 of 18
Symmetry 2019, 11, x FOR PEER REVIEW 16 of 18 Figure 17. MonitoringMonitoring range range of of track vertical deformation of the existing station.
Figure 18. On-site monitoring of track deformation of the existing station.
It can be seen from Figure 19 that, due to the influence of the construction of the new Erligou station, the existing line floats upward, and the deformation trend is high in the middle and low on both sides. The distance between the station and the upper undercutting section is closer than that of the sections on both sides, so the space–time effect caused by the construction is greater, and the floating degree is more obvious. The accumulated maximum uplift value of the left line is 1.34 mm, and that of the right line is 1.63 mm, which meets the requirements of the existing line structure and track deformation in the technical code for monitoring of Urban Rail Transit Engineering (GB 500911- 2013) and the enterprise standard technical standard of Beijing Metro Operation Co., Ltd. guidelines for maintenance of public works (QB (J)/BDY (a) xl003-2009). Figure 18. OnOn-site-site monitoring of track deformation of the existing station.
It can be seen from Figure 19 that, due to the influence of the construction of the new Erligou station, the existing line floats upward, and the deformation trend is high in the middle and low on both sides. The distance between the station and the upper undercutting section is closer than that of the sections on both sides, so the space–time effect caused by the construction is greater, and the floating degree is more obvious. The accumulated maximum uplift value of the left line is 1.34 mm, and that of the right line is 1.63 mm, which meets the requirements of the existing line structure and track deformation in the technical code for monitoring of Urban Rail Transit Engineering (GB 500911- 2013) and the enterprise standard technical standard of Beijing Metro Operation Co., Ltd. guidelines for maintenance of public works (QB (J)/BDY (a) xl003-2009).
FigureFigure 19. 19.Monitoring Monitoring datadata ofof tracktrack vertical deformation deformation of of the the existing existing station. station.
Figure 19. Monitoring data of track vertical deformation of the existing station.
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6. Conclusions Through the above research, we can draw the following conclusions: (1) Construction risks may occur in metro extensions due to adjacent existing structures. The risk of underground excavation in large sections and large space subway stations has the following characteristics: classification, zoning, and time sharing. (2) There are many types of tunnel construction risks. There are many types of tunnel construction risk. Different risk types have different quantitative evaluation indexes and different monitoring instruments. The analysis of construction risk must be specific to a certain risk event in order to be targeted. For example, the risk of structural failure of adjacent existing stations caused by the construction of new stations is studied. (3) During the construction of subway stations, there are risk zones. The upper-pass and undercut sections are high-risk areas, which contribute 50.38% to the risk of structural failure of the existing station. It is necessary to strengthen monitoring and safety measures. The size of the construction space and the proximity distance are the two most important factors that affect risk zoning. (4) The risk of a single sub-project during the construction process is dynamic and a function of time. In the construction process of the upper-crossing undercut section, the soil excavation of the side tunnels, 5 and 6, has the greatest impact on the deformation of the adjacent existing line, accounting for 63.4% of the cumulative deformation of the existing station. It is necessary to strengthen monitoring during the high-risk construction period. The best way to control risk is to “break the whole into parts” and prevent and control them step-by-step.
Author Contributions: Conceptualization, M.H. and Z.Z.; data curation, B.W.; funding acquisition, M.H.; investigation, Z.Z.; software, Z.Z.; supervision, M.H.; validation, B.W.; writing—original draft, Z.Z.; writing—review & editing, Z.Z. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the financial support provided by the National key R&D program funding (2018YFC0808701). Conflicts of Interest: The authors declare no conflict of interest.
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