# Research on Construction Safety Risk Assessment of New Subway Station Close-Attached Undercrossing the Existing Operating Station

Authors: Zhenhua Luo; Li Zeng; Haize Pan; Qijun Hu; Bo Liang; Jianqiang Han
Journal: Mathematical Problems in Engineering
 (2019)

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## Abstract

With the large-scale construction of urban rail transit, it will lead to the intersection and transfer of various lines, resulting in more transfer stations. The transfer station is a collection point for multiple subway lines, which is difficult to construct and has a high construction risk. The construction of the new subway station and the operation of the existing subway station are mutually influenced during the close-attached undercrossing construction. Considering the two objectives of ensuring the smooth operation of the existing subway station and the safe construction of the new subway station, this paper comprehensively analyzes the possible safety risk factors during the construction of the new subway station close-attached undercrossing the existing operating station and identifies 75 preliminary risk factors by means of literature review and on-site investigation. Then the Delphi Method and Entropy Weight Method are used to screen the preliminary risk factors, and the main risk factors with greater influence are retained, so that 49 key risk factors are obtained. According to the list of key risk factors, a safety risk assessment index system including 2 first-level indexes, 12 second-level indexes, and 49 third-level indexes is established. Based on the index system, this paper establishes a safety risk assessment model by using Analytic Hierarchy Process (AHP) and Fuzzy Matter Element Method (FMEM). The model first calculates the weight of each index by using AHP, calculates the comprehensive correlation degree of each index by using FMEM, classifies the risk grade of each index according to the comprehensive correlation degree, and determines the risk grade of the project. Finally, the safety risk assessment model is applied to the Dongdalu Station project of Chengdu Rail Transit Line 8. The result shows that the risk grade of this project is moderate risk, which is basically consistent with the actual situation, indicating that the model has good practicability. In this paper, a new safety risk assessment model for subway close-attached undercrossing construction is proposed, which fills the gap in the field of safety risk assessment for the construction of the new subway station close-attached undercrossing the existing operating station.


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## Body

## 1. Introduction

With the continuous progress of urbanization in China, the population density and traffic pressure are also increasing. In order to solve the problem of urban traffic congestion, subway construction has become the preferred solution for major cities in China. By the end of 2018, there were 37 cities with urban rail transit in mainland China, with a total length of 5539.19 km [1]. Compared with other projects, the subway project has more outstanding concealment, construction complexity, and uncertainty of stratum conditions and surrounding environment, which increases the difficulty of construction technology and the risk of construction. According to statistics, during the period from 2002 to 2016 (statistics to March 2016), 246 accidents occurred in the construction of subways in China. There were 238 accidents with clear and specific location records, of which subway station projects accounted for 57% and subway section projects accounted for 43% [2]. For example, on November 15 of 2008, the site collapse of Xianghu Station of Hangzhou Subway resulted in 21 deaths and a direct economic loss of 49.61 million yuan [3]. It can be seen that, in the construction of underground engineering, accidents occur frequently and the situation is very serious. Therefore, the necessity and urgency of implementing safety risk management in underground engineering construction is very obvious.Risk analysis is a tool that can establish an active safety strategy by investigating potential risks [4]. The application and research on safety risk management of underground engineering have been actively carried out worldwide. Kampmann [5] used risk assessment technology to propose 10 types of risk for the Copenhagen Subway project, including more than 40 types of disasters, and proposed a specific classification system for the possibility of events and their impact results. Based on the actual project, through the safety risk analysis and assessment, Burland.JB [6, 7] calculated the possible damage caused by the buildings along the line during the design and planning stage and gave corresponding reinforcement measures. Later, the research result was applied to the Jubillee line in London, which ensured the smooth construction of the project. In 2004, the International Tunnel Association (ITA) wrote the Guidelines for Tunnel Risk Management [8], which provided a set of reference standards and methods for tunnel risk management and affirmed the practicability of safety risk assessment.In terms of specific engineering examples, due to the safety risk assessment before construction and good construction control measures during construction, in a South Korean project where the subway station undercrossed the existing subway, the settlement of the existing subway line was only 3mm during the whole construction process [9], which is lower than the 10mm required by China’s national normative standards [10]. In the project where the northbound tunnel section of the I-93 Interstate Highway in Boston, USA, passed through the South Station of Red Line, through the 24-hour monitoring of the construction process, the safety risks of the entire construction process were accurately assessed and responded, and the normal operation of the existing station and its ancillary facilities was guaranteed [11]. Cao Zhen [12] formed a safety risk system for shield construction of Xi'an Subway Tunnel according to four aspects of engineering geology and hydrogeology, load factor, construction factor, and external factor. Through the analysis of shield construction safety of Xi’an Subway Line 2, three key risk sources, existing railway, moat arch bridge, and ancient city wall, were obtained. The construction safety risk grade can be reduced from grade I to grade IV by the proposed reasonable construction plan. Zhang L [13] conducted a risk assessment of the project based on the importance index of the possibility and the fuzzy importance to reveal the key method of reducing the risk limit. Wang ZZ [14] used the Fuzzy Comprehensive Bayesian Network (FCBN) which combined with Fuzzy Comprehensive Evaluation Method (FCEM) and Bayesian Network (BN) to analyze the risk of the subway project. The comparison between the analysis result and the example proved that this method can be effective to estimate the risk grade of the subway construction project. Peng Yuhan [15] formed a set of assessment system combining the risk factors of engineering characteristics and environmental characteristics, made a risk assessment on the Suzhou Street Station Project of Beijing Subway Line 16, and finally gave corresponding countermeasures for different risk grades. Wang F [16] identified the causes of land subsidence by statistical model based on adaptive Correlation Vector Machine (aRVM) and put forward risk countermeasures for high-risk factors, which effectively ensured the normal operation of the construction.With the large-scale construction of urban rail transit, it will bring about the intersection and transfer of various lines, resulting in more transfer stations. During the construction of the close-attached undercrossing engineering of transfer stations, the new subway construction and the existing subway structure will affect each other. The construction of the new subway project will affect the safety of the existing station structure, and the existing lines will also affect the construction safety of the new subway project [17].With the increase of subway crossing projects, some scholars have begun to explore the research on risk management of subway crossing projects. Based on the risk dynamic analysis method, Guan Jifa [18] constructed a risk assessment system for a tunnel undercrossing the existing subway and put forward the idea that the settlement amount (the uplift amount) and settlement rate (the uplift rate) of the existing subway structure baseplate should be used as the main control indexes in the subway crossing project. The risk control system proposed by Yang Wendong [19] was mainly based on technical indexes and supplemented by nontechnical indexes. An underground line undercrossing a station of Subway Line 4 in Beijing was taken as an example to verify that the risk control system can effectively avoid and deal with risks. Li X [20] made a research on the safety control framework for shield tunneling in close proximity to the operating subway tunnels, proposed that the use of remedial grouting measures can effectively correct the excessive settlement problem in close-attached undercrossing projects, and suggested using the optical fiber measurement to dynamically monitor the project.Because of the characteristics of underground engineering, such as large investment, long construction period, complex construction specialty, and many unpredictable risk factors, it is very important to carry out reasonable and effective risk management for underground engineering. In recent years, many scholars have studied the safety risk management of underground engineering, but there are few studies on the safety risk management of subway crossing engineering. At present, some scholars have studied the risk management of tunnels undercrossing existing subway stations, but there is no research on the risk management of the new subway station close-attached undercrossing the existing operating station. In the crossing project of new subway station close-attached undercrossing the existing operating station, both the normal operation of the existing station and the normal construction of the new station should be guaranteed, which makes the construction of the crossing project more complex and risky than the ordinary subway project. Therefore, it is necessary to put forward a risk assessment system suitable for the subway crossing project of new subway station close-attached undercrossing the existing operating station.


## 2. Establishment of the Construction Safety Risk Assessment Index System for New Subway Station Close-Attached Undercrossing the Existing Subway Station

### 2.1. Research Framework

The construction of new subway station close-attached undercrossing the existing station has the characteristics of complex technology and high risk, so it is of great significance to carry out risk assessment. How to successfully complete the construction of the new subway station under the normal operation of the existing subway station and how to ensure the two objectives of the smooth operation of the existing subway station and the safe construction of the new subway station are major technical problems faced by such crossing projects. Therefore, establishing a risk assessment model for the new subway station close-attached undercrossing the existing station can provide a theoretical basis for safety risk management of similar projects, help construction units to effectively control the risks of similar projects, and prevent or reduce the possible adverse effects in the construction process.The research ideas of this paper are as follows: the first step is to identify preliminary risk factors by means of literature searching and on-site investigation. The second step is to use Delphi Method and Entropy Weight Method to screen the preliminary list of risk factors and retain the major risk factors that have a greater impact, so as to obtain the list of key risk factors. The third step is to use the list of key risk factors to construct a risk assessment index system and to divide the risk grade range of each index according to national norms and expert opinions. Fourthly, according to the established assessment index system, the appropriate assessment method is selected to establish a risk assessment model. In the assessment model, the weight of each index is calculated by Analytic Hierarchy Process (AHP), and then the comprehensive correlation degree of each index corresponding to different risk grades is calculated by Fuzzy Matter Element Method (FMEM). According to the comprehensive correlation degree, the risk grade of each index is graded, and the risk grade of the project is determined. Finally, the corresponding risk control measures are put forward for the indexes with higher risk grade in the assessment result. The specific research framework flow chart is shown in Figure1.Figure 1
Research framework flow chart.


### 2.2. Identifying Preliminary Risk Factors

To ensure the smooth operation of the existing subway station and the safe construction of the new subway station is the purpose of establishing and applying the risk assessment model. Therefore, the first-level indexes of the assessment index system are considered from two aspects: the existing subway station and the new subway station. Through on-site investigation of similar projects and consulting the “Technical Specification for Urban Rail Transit” promulgated by the Ministry of Housing and Urban-Rural Development of the People's Republic of China [10, 21–28], combined with the opinions of relevant experts, a list of preliminary risk factors is finally determined, which includes 2 first-level indexes, 12 second-level indexes, and 75 third-level indexes. The list of preliminary risk factors is shown in Table 1.Table 1
Preliminary risk factors of the new subway station close-attached undercrossing the existing station.
First-level index Second-level index A Existing subway station A1 Station structure A2 Track structure A3 Power supply system A4 Water supply and drainage system A5 Ventilation and signal system B New subway station B1 Construction monitoring of existing station B2 New space enclosing structure B3 Engineering dewatering B4 Foundation pit excavation B5 Surrounding rock and soil mass and surrounding environment B6 Construction technology management of main structure B7 Green constructionThese 12 second-level indexes in Table1 include 75 third-level indexes, which are composed as follows:( 1 ) Station structure (A1): A11: radial displacement of the station structure, A12: absolute settlement value of station structure, A13: horizontal displacement of the station structure, A14: station structure floating, A15: differential settlement of station structure deformation joint, and A16: crack width of station structure( 2 ) Track structure (A2): A21: track gauge, A22: orbital height difference (Vector value), A23: vibration velocity, A24: transverse elevation difference of track, and A25: void amount of track-bed( 3 ) Power supply system (A3): A31: area power outage renovation, A32: circuit failure, and A33: fault of power supply equipment( 4 ) Water supply and drainage system (A4): A41: system blocking, A42: mechanical fault, and A43: pipeline damage( 5 ) Ventilation and signal system (A5): A51: equipment sampling pass rate, A52: maintenance pass rate, A53: equipment failures, A54: operating rate of workers with diseases, A55: protective equipment completion rate, A56: equipment renewal rate, and A57: equipment pending repair rate( 6 ) Construction monitoring of existing station (B1): B11: information monitoring equipment failures, B12: accuracy rate of feedback real-time data, B13: stress of column structure, B14: horizontal and vertical displacement of retaining pile (wall) and top of slop, B15: monitoring frequency, B16: axial force of struts, B17: anchor tension, B18: ground settlement, B19: clearance convergence of shaft support structure, B110: monitoring hole of groundwater level, B111: distance from excavation face to monitoring point or monitoring section, and B112: vertical displacement of column structure( 7 ) New space enclosing structure (B2): B21: survey spot arrangement spacing, B22: stress of the retaining wall, and B23: lateral displacement of space enclosing structure( 8 ) Engineering dewatering (B3): B31: stratum filtering flow, B32: precipitation depth, B33: sand content control, and B34: aquifer characteristics( 9 ) Foundation pit excavation (B4): B41: foundation pit protection, B42: foundation pit excavation plan, B43: maintenance of foundation pit dewatering system, and B44: depth of foundation pit( 10 ) Surrounding rock and soil mass and surrounding environment (B5): B51: design of station structure, B52: stratum and geological condition, B53: site classification, B54: soil liquefaction grade, B55: buried depth, B56: surface settlement, B57: underground pipeline monitoring, B58: effect of water level reduction on stratum, and B59: impact of surrounding buildings on construction( 11 ) Construction technology management of main structure (B6): B61: stiffness and deformation reinforcement of main structure, B62: stiffness of initial support, B63: quality of secondary lining, B64: technical training for construction personnel, B65: mechanical operation, B66: construction personnel management, B67: emergency plan, and B68: safety drill( 12 ) Green construction (B7): B71: public building green rate, B72: ground traffic control, B73: ratio of underground building area to total land area, B74: noise control, B75: sewage control, B76: light pollution control, B77: dust control, B78: construction trash control, B79: impact on business climate, B710: underground space lighting, and B711: the proportion of the weight of building materials produced locally to the total weight of building materials


### 2.3. Selecting Key Risk Factors

In the list of preliminary risk factors, the occurrence of some risk factors may have a greater adverse impact on the project, and even the consequences may be unacceptable to managers; such risks are the key risk factors. Project managers should put limited time, energy, and funds on the prevention and control of key risk factors, so how to sort out and select key risk factors from the list of many preliminary risk factors is crucial.For multivariate and multilevel index system, Entropy Weight Method is an ideal method to objectively determine index weights, which can effectively use the information contained in each index to measure the difference between indexes, and avoid the subjectivity of weight assignment [29–31]. Based on the comprehensive consideration of the characteristics and the risk management objectives of the subway crossing project, this paper consults relevant technologies and management experts to sort out the preliminary risk factors and uses Entropy Weight Method to select and identify key risk factors.


#### 2.3.1. Obtaining Raw Data

The risk factors in the preliminary risk factor list are classified according to different risk degrees, and the risk degree classification standards are shown in Table2.Table 2
Risk degree classification standards.
Risk degree value High risk 5 Moderately high risk 4 Moderate risk 3 Moderately low risk 2 Low risk 1 Between the two adjacent degrees Median value of adjacent valuesThe subway project has the characteristics of high cost and complex technology, involving many specialties and participating units. In order to avoid the subjective one-sidedness of the investigation caused by the small number of experts and the single source of experts, six types of units, such as owner, geological prospecting unit, designing unit, construction unit, supervision unit, and subcontractor, are investigated on the spot. A total of 30 experts are selected from these six types of units for consultation. The number of experts from various units is shown in Figure2. In practice, if 30 experts cannot be invited for some objective reasons, at least 6 experts from different units should be invited.Figure 2
Number of experts from various units.Firstly, 30 experts are invited to score and classify the risk degree of 75 preliminary risk factors according to the risk degree classification standards in Table2. Secondly, according to the different units, the average value of experts’ scores in each unit is calculated, and six average values from six units are obtained. Thirdly, the evaluation value matrix At (t denotes the ordinal number of second-level indexes, t = 1, 2, …, 12) is calculated. In the At matrix, the number of rows is the number of expert source units, and the number of columns is the number of third-level indexes included in each second-level index. Because the arithmetic mean of the expert scores is calculated separately according to the six types of units, At is a matrix of six rows and n columns, of which the value of n is the number of third-level indexes included in each second-level index.For example, the second-level index “station structure” has six third-level indexes, so its evaluation value matrixA1 is(1)A1=4.542342.543.52.53.54.524.54334.5244343.53.544.5433.534.53.53433


#### 2.3.2. Standardization of Raw Data

The evaluation matrix is standardized to eliminate the dimensional differences among the indexes. If the evaluation value is bigger, the better, the standardized formula (2) can be used. If the evaluation value is smaller, the better, the standardized formula (3) can be used. aij is an element corresponding to the ith row and the jth column in the evaluation value matrix At. rij is the standardization result of aij:(2)rij=aij-min/jaijmax/jaij-min/jaij(3)rij=max/jaij-aijmax/jaij-min/jaijBecause the evaluation value of risk factors in subway construction belongs to the smaller the better type, formula (3) is adopted to standardize the evaluation value matrix. For example, matrix R1 is the standardization result of the evaluation value matrix A1 (station structure), which is as follows:(4)R1=0121113231134120101212123111210001001231301120113


#### 2.3.3. Determining Index Entropy Weight

The evaluation value weightpij of the index is(5)pij=rij∑i=1mrij,j=1,2,…,nThe entropy valueej of the index is(6)ej=-k∑i=1mpij×ln⁡pij,j=1,2,…,nAmong them,k=1/ln⁡m, when the evaluation value pij×ln⁡pij=0, pij×ln⁡pij=0.The entropy weightwj of the index is(7)wj=1-ej∑j=1n1-ejIn order to solve the problem of numerous indexes and complicated calculation, formulas (6) and (7) are compiled into Excel program. By inputting the evaluation value weight pij of each index into Excel program, the entropy value ej and entropy weight wj of each index can be obtained.According to formula (5), the evaluation value weight matrix P1 of the second-level index “station structure (A1)” is(8)P1=0174152718151327151703100172152714310131741500013002714110027215038110By inputting the value of matrixP1 into Excel program, the entropy value ej and entropy weight wj of each third-level index can be obtained. The calculation results are shown in Table 3.Table 3
Entropy value and entropy weight of each third-level index in the second-level index “station structure (A1).”
Third-level Index Entropy Valueej Entropy Weightwj A11 Radial displacement of the station structure 0.613 0.326 A12 Absolute settlement value of station structure 0.865 0.114 A13 Horizontal displacement of the station structure 0.873 0.107 A14 Station structure floating 0.754 0.207 A15 Differential settlement of station structure deformation joint 0.869 0.111 A16 Crack width of station structure 0.840 0.135Referring to the calculation results in Table3, according to the characteristics that the smaller the entropy weight is, the larger the weight is, those indexes with smaller entropy weight are selected as the key risk factors. For example, four indexes with smaller entropy weight in Table 3 are taken as key risk factors, namely, A12, A13, A15, and A16, and A11 and A14 are deleted. Similarly, the entropy value and entropy weight of other third-level indexes are calculated, and the key risk factors are determined. From this, a list of key risk factors is obtained. The calculation results are shown in Table 4.Table 4
List of key risk factors.
Second-level index Key risk factor A1 Station structure A12 Absolute settlement value of station structure, A13 Horizontal displacement of the station structure, A15 Differential settlement of station structure deformation joint, A16 Crack width of station structure  A2 Track structure A22 Orbital height difference (Vector value), A24 Transverse elevation difference of track, A25 Void amount of track-bed  A3 Power supply system A31 Area power outage renovation, A32 Circuit failure  A4 Water supply and drainage system A41 System blocking, A43 Pipeline damage  A5 Ventilation and signal system A51 Equipment sampling pass rate, A53 Equipment failures, A56 Equipment renewal rate, A57 Equipment pending repair rate  B1 Construction monitoring of existing station B11 Information monitoring equipment failures, B12 Accuracy rate of feedback real-time data, B14 Horizontal and vertical displacement of retaining pile(wall) and top of slop, B16 Axial force of struts, B17 Anchor tension, B18 Ground settlement, B19 Clearance convergence of shaft support structure, B110 Monitoring hole of groundwater level  B2 New space enclosing structure B21 Survey spot arrangement spacing, B23 Lateral displacement of space enclosing structure  B3Engineering dewatering B31 Stratum filtering flow, B33 Sand content control  B4 Foundation pit excavation B42 Foundation pit excavation plan, B44 Depth of foundation pit  B5 Surrounding rock and soil mass and surrounding environment B51 Design of station structure, B52 Stratum and geological condition, B53 Site classification, B54 Soil liquefaction grade, B55 Buried depth, B58 Effect of water level reduction on stratum, B59 Impact of surrounding buildings on construction  B6 Construction technology management of main structure B61Stiffness and deformation reinforcement of main structure, B62 Stiffness of initial support, B63 Quality of secondary lining, B65 Mechanical operation, B66 Construction personnel management, B67Emergency plan  B7 Green construction B72 Ground traffic control, B74 Noise control, B75Sewage control, B76 Light pollution control, B77 Dust control, B78 Construction trash control, B79 Impact on business climateIn summary, after the list of preliminary risk factors including 75 risk factors was screened by Delphi Method and Entropy Weight Method, 26 risk factors with relatively less impact were deleted, and a list of key risk factors including 49 risk factors is obtained.


### 2.4. Establishing the Construction Safety Risk Assessment Index System

According to the selected list of key risk factors (Table4), a construction safety risk assessment index system of the new subway station close-attached undercrossing the existing operating station is established. The index system is divided into 2 first-level indexes: the existing subway station and the new subway station, including 12 second-level indexes and 49 third-level indexes, as shown in Table 5.Table 5
Construction safety risk assessment index system of new subway station close-attached undercrossing the existing station.
First-level index Second-level index Third-level index Risk Grade High risk Medium risk Low risk A Existing subway station A1 Station structure A 12 Absolute settlement value of station structure (mm) ≥7 ≥3, < 7 < 3 A 13 Horizontal displacement of the station structure (mm) ≥5 ≥3, < 5 < 3 A 15 Differential settlement of station structure deformation joint (mm) ≥2 ≥1, < 2 < 1 A 16 Crack width of station structure (mm) ≥0.2 ≥0.1, < 0.2 < 0.1 A2 Track structure A 22 Orbital height difference (Vector value) (mm) ≥4 ≥2, < 4 < 2 A 24 Transverse elevation difference of track (mm) ≥4 ≥2, < 4 < 2 A 25 Void amount of track-bed (mm) ≥5 ≥3, < 5 < 3 A3 Power supply system A 31 Area power outage renovation (h/times) ≥2 ≥0.5, < 2 < 0.5 A 32 Circuit failure (times/ month) ≥3 ≥1, < 3 < 1 A4 Water supply and drainage system A 41 System blocking (h/times) ≥5 ≥2, < 5 < 2 A 43 Pipeline damage (times/month) ≥3 ≥1, < 3 < 1 A5 Ventilation and signal system A 51 Equipment sampling pass rate (%) < 60 ≥60, < 90 ≥90 A 53 Equipment failures (times/month) ≥30 ≥15, < 30 < 15 A 56 Equipment renewal rate (%) ≥7 ≥3, < 7 < 3 A 57 Equipment pending repair rate (%) ≥10 ≥5, < 10 < 5  B 11 Information monitoring equipment failures (times/month) ≥10 ≥5, < 10 < 5  B New subway station B1 Construction monitoring of existing station B 12 Accuracy rate of feedback real-time data (%) < 90 ≥90, < 95 ≥95 B 14 Horizontal and vertical displacement of retaining pile (wall) and top of slop (m) 20 < Control point spacing 10 < Control point spacing ≤20 Control point spacing≤10 B 16 Axial force of struts (pieces) Number of struts < 3 3≤ Number of struts < 5 Number of struts ≥5 B 17 Anchor tension (pieces) Number of anchors < 3 3≤ Number of anchors < 5 Number of anchors≥5 B 18 Ground settlement (m) 8 < Control point spacing 3 < Control point spacing ≤8 Control point spacing ≤3 B 19 Clearance convergence of shaft support structure (m) 5 < Monitoring section spacing 3 < Monitoring section spacing ≤5 Monitoring section spacing ≤3 B 110 Monitoring hole of groundwater level (pieces) Number of monitoring holes < 4 4≤Number of monitoring holes < 6 6≤Number of monitoring holes B2 New space enclosing structure B 21 Survey spot arrangement spacing (m) ≥15 ≥10, < 15 < 10 B 23 Lateral displacement of space enclosing structure (mm) ≥20 ≥10, < 20 < 10 B3 Engineering dewatering B 31 Stratum filtering flow (permeability coefficient cm/s) ≥10-4 ≥10-6, < 10-4 <10-6 B 33 Sand content control (10000−1) 1/1 < Fine sand content 1/2 < Fine sand content ≤1/1 Fine sand content ≤1/2 B4 Foundation pit excavation B 42 Foundation pit excavation plan Unreasonable Reasonable and partially meeting standards. Reasonable and meeting standards B 44 Depth of foundation pit (m) ≥20 ≥10,<20 < 10 B 51 Design of station structure Insufficient survey, no precise calculation and design Moderately sufficient survey, calculation and design Full survey, accurate calculation and design  B New subway station B5 Surrounding rock and soil mass and surrounding environment B 52 Stratum and geological condition Complex topography and geomorphology, strong development of adverse geological conditions Moderately complex topography and geomorphology, moderate development of adverse geological conditions Simple topography and geomorphology, good geological condition B 53 Site classification Class III Class II Class I B 54 Soil liquefaction grade Serious Moderate Slight B 55 Buried depth (m) ≥16 ≥10,<16 < 10 B 58 Effect of water level reduction on stratum (m) Inadequate dewatering for foundation excavation Groundwater level drops to 0-1m below foundation pit bottom Groundwater level drops below the foundation pit bottom to more than 1m B 59 Impact of surrounding buildings on construction Distance within 0.7 times the buried depth of baseplate Distance within 0.7-1 times the buried depth of baseplate Distance within 1-2 times the buried depth of baseplate B6 Construction technology management of main structure B 61 Stiffness and deformation reinforcement of main structure Unreasonable grouting parameters, not meeting the actual construction Moderately reasonable grouting parameters, meeting the actual construction Reasonable grouting parameters, in line with the actual construction B 62 Stiffness of initial support (N/mm2) Bond strength of sprayed concrete < 0.6 0.6≤ Bond strength of sprayed concrete<0.8 Bond strength of sprayed concrete≥0.8 B 63 Quality of secondary lining Not meeting the requirements, bad effect Moderately meeting the requirements, better effect Meeting the requirements, good effect. B 65 Mechanical operation Improper mechanical operation Moderately proper mechanical operation Proper mechanical operation B 66 Construction personnel management Unqualified in construction operation, no job disclosure Qualified in construction operation, with job disclosure and Skilled in construction operation, with job and daily training periodic training disclosure and daily training B 67 Emergency plan No emergency plan, no emergency drill plan Lack of maneuverability and failure to organize drills as planned Standardized and targeted and success to organize drills as planned. B7 Green construction B 7 2 Ground traffic control (thousand vehicles/h) ≥5 ≥3,<5 < 3 B 7 4 Noise control (dB) 70 < Daytime, 70≤ Daytime<60, Daytime ≤ 60, 55 < Nighttime 55≤ Nighttime<45 Nighttime ≤ 45 B 7 5 Sewage control (mg/L) Suspend solid ≥400 250≤ Suspend solid<400 Suspend solid<250 B 7 6 Light pollution control (W/m2) ≥11 ≥10,<11 < 10 B 7 7 Dust control (m) Enclosure height<2 2≤Enclosure height<2.5 2.5≤Enclosure height B 7 8 Construction trash control (%) Percentage of recovery and reuse<20 20≤Percentage of recovery and reuse<30 30≤Percentage of recovery and reuse B 7 9 Impact on business climate (ten thousand person-time/month) < 10 ≥10,<20 ≥20By consulting to the relevant national technical specifications and engineering site data, the risk grade of the third-level indexes in the construction safety risk assessment index system is divided into three levels: high risk, medium risk, and low risk, as shown in Table5.


## 2.1. Research Framework

The construction of new subway station close-attached undercrossing the existing station has the characteristics of complex technology and high risk, so it is of great significance to carry out risk assessment. How to successfully complete the construction of the new subway station under the normal operation of the existing subway station and how to ensure the two objectives of the smooth operation of the existing subway station and the safe construction of the new subway station are major technical problems faced by such crossing projects. Therefore, establishing a risk assessment model for the new subway station close-attached undercrossing the existing station can provide a theoretical basis for safety risk management of similar projects, help construction units to effectively control the risks of similar projects, and prevent or reduce the possible adverse effects in the construction process.The research ideas of this paper are as follows: the first step is to identify preliminary risk factors by means of literature searching and on-site investigation. The second step is to use Delphi Method and Entropy Weight Method to screen the preliminary list of risk factors and retain the major risk factors that have a greater impact, so as to obtain the list of key risk factors. The third step is to use the list of key risk factors to construct a risk assessment index system and to divide the risk grade range of each index according to national norms and expert opinions. Fourthly, according to the established assessment index system, the appropriate assessment method is selected to establish a risk assessment model. In the assessment model, the weight of each index is calculated by Analytic Hierarchy Process (AHP), and then the comprehensive correlation degree of each index corresponding to different risk grades is calculated by Fuzzy Matter Element Method (FMEM). According to the comprehensive correlation degree, the risk grade of each index is graded, and the risk grade of the project is determined. Finally, the corresponding risk control measures are put forward for the indexes with higher risk grade in the assessment result. The specific research framework flow chart is shown in Figure1.Figure 1
Research framework flow chart.


## 2.2. Identifying Preliminary Risk Factors

To ensure the smooth operation of the existing subway station and the safe construction of the new subway station is the purpose of establishing and applying the risk assessment model. Therefore, the first-level indexes of the assessment index system are considered from two aspects: the existing subway station and the new subway station. Through on-site investigation of similar projects and consulting the “Technical Specification for Urban Rail Transit” promulgated by the Ministry of Housing and Urban-Rural Development of the People's Republic of China [10, 21–28], combined with the opinions of relevant experts, a list of preliminary risk factors is finally determined, which includes 2 first-level indexes, 12 second-level indexes, and 75 third-level indexes. The list of preliminary risk factors is shown in Table 1.Table 1
Preliminary risk factors of the new subway station close-attached undercrossing the existing station.
First-level index Second-level index A Existing subway station A1 Station structure A2 Track structure A3 Power supply system A4 Water supply and drainage system A5 Ventilation and signal system B New subway station B1 Construction monitoring of existing station B2 New space enclosing structure B3 Engineering dewatering B4 Foundation pit excavation B5 Surrounding rock and soil mass and surrounding environment B6 Construction technology management of main structure B7 Green constructionThese 12 second-level indexes in Table1 include 75 third-level indexes, which are composed as follows:( 1 ) Station structure (A1): A11: radial displacement of the station structure, A12: absolute settlement value of station structure, A13: horizontal displacement of the station structure, A14: station structure floating, A15: differential settlement of station structure deformation joint, and A16: crack width of station structure( 2 ) Track structure (A2): A21: track gauge, A22: orbital height difference (Vector value), A23: vibration velocity, A24: transverse elevation difference of track, and A25: void amount of track-bed( 3 ) Power supply system (A3): A31: area power outage renovation, A32: circuit failure, and A33: fault of power supply equipment( 4 ) Water supply and drainage system (A4): A41: system blocking, A42: mechanical fault, and A43: pipeline damage( 5 ) Ventilation and signal system (A5): A51: equipment sampling pass rate, A52: maintenance pass rate, A53: equipment failures, A54: operating rate of workers with diseases, A55: protective equipment completion rate, A56: equipment renewal rate, and A57: equipment pending repair rate( 6 ) Construction monitoring of existing station (B1): B11: information monitoring equipment failures, B12: accuracy rate of feedback real-time data, B13: stress of column structure, B14: horizontal and vertical displacement of retaining pile (wall) and top of slop, B15: monitoring frequency, B16: axial force of struts, B17: anchor tension, B18: ground settlement, B19: clearance convergence of shaft support structure, B110: monitoring hole of groundwater level, B111: distance from excavation face to monitoring point or monitoring section, and B112: vertical displacement of column structure( 7 ) New space enclosing structure (B2): B21: survey spot arrangement spacing, B22: stress of the retaining wall, and B23: lateral displacement of space enclosing structure( 8 ) Engineering dewatering (B3): B31: stratum filtering flow, B32: precipitation depth, B33: sand content control, and B34: aquifer characteristics( 9 ) Foundation pit excavation (B4): B41: foundation pit protection, B42: foundation pit excavation plan, B43: maintenance of foundation pit dewatering system, and B44: depth of foundation pit( 10 ) Surrounding rock and soil mass and surrounding environment (B5): B51: design of station structure, B52: stratum and geological condition, B53: site classification, B54: soil liquefaction grade, B55: buried depth, B56: surface settlement, B57: underground pipeline monitoring, B58: effect of water level reduction on stratum, and B59: impact of surrounding buildings on construction( 11 ) Construction technology management of main structure (B6): B61: stiffness and deformation reinforcement of main structure, B62: stiffness of initial support, B63: quality of secondary lining, B64: technical training for construction personnel, B65: mechanical operation, B66: construction personnel management, B67: emergency plan, and B68: safety drill( 12 ) Green construction (B7): B71: public building green rate, B72: ground traffic control, B73: ratio of underground building area to total land area, B74: noise control, B75: sewage control, B76: light pollution control, B77: dust control, B78: construction trash control, B79: impact on business climate, B710: underground space lighting, and B711: the proportion of the weight of building materials produced locally to the total weight of building materials


## 2.3. Selecting Key Risk Factors

In the list of preliminary risk factors, the occurrence of some risk factors may have a greater adverse impact on the project, and even the consequences may be unacceptable to managers; such risks are the key risk factors. Project managers should put limited time, energy, and funds on the prevention and control of key risk factors, so how to sort out and select key risk factors from the list of many preliminary risk factors is crucial.For multivariate and multilevel index system, Entropy Weight Method is an ideal method to objectively determine index weights, which can effectively use the information contained in each index to measure the difference between indexes, and avoid the subjectivity of weight assignment [29–31]. Based on the comprehensive consideration of the characteristics and the risk management objectives of the subway crossing project, this paper consults relevant technologies and management experts to sort out the preliminary risk factors and uses Entropy Weight Method to select and identify key risk factors.


### 2.3.1. Obtaining Raw Data

The risk factors in the preliminary risk factor list are classified according to different risk degrees, and the risk degree classification standards are shown in Table2.Table 2
Risk degree classification standards.
Risk degree value High risk 5 Moderately high risk 4 Moderate risk 3 Moderately low risk 2 Low risk 1 Between the two adjacent degrees Median value of adjacent valuesThe subway project has the characteristics of high cost and complex technology, involving many specialties and participating units. In order to avoid the subjective one-sidedness of the investigation caused by the small number of experts and the single source of experts, six types of units, such as owner, geological prospecting unit, designing unit, construction unit, supervision unit, and subcontractor, are investigated on the spot. A total of 30 experts are selected from these six types of units for consultation. The number of experts from various units is shown in Figure2. In practice, if 30 experts cannot be invited for some objective reasons, at least 6 experts from different units should be invited.Figure 2
Number of experts from various units.Firstly, 30 experts are invited to score and classify the risk degree of 75 preliminary risk factors according to the risk degree classification standards in Table2. Secondly, according to the different units, the average value of experts’ scores in each unit is calculated, and six average values from six units are obtained. Thirdly, the evaluation value matrix At (t denotes the ordinal number of second-level indexes, t = 1, 2, …, 12) is calculated. In the At matrix, the number of rows is the number of expert source units, and the number of columns is the number of third-level indexes included in each second-level index. Because the arithmetic mean of the expert scores is calculated separately according to the six types of units, At is a matrix of six rows and n columns, of which the value of n is the number of third-level indexes included in each second-level index.For example, the second-level index “station structure” has six third-level indexes, so its evaluation value matrixA1 is(1)A1=4.542342.543.52.53.54.524.54334.5244343.53.544.5433.534.53.53433


### 2.3.2. Standardization of Raw Data

The evaluation matrix is standardized to eliminate the dimensional differences among the indexes. If the evaluation value is bigger, the better, the standardized formula (2) can be used. If the evaluation value is smaller, the better, the standardized formula (3) can be used. aij is an element corresponding to the ith row and the jth column in the evaluation value matrix At. rij is the standardization result of aij:(2)rij=aij-min/jaijmax/jaij-min/jaij(3)rij=max/jaij-aijmax/jaij-min/jaijBecause the evaluation value of risk factors in subway construction belongs to the smaller the better type, formula (3) is adopted to standardize the evaluation value matrix. For example, matrix R1 is the standardization result of the evaluation value matrix A1 (station structure), which is as follows:(4)R1=0121113231134120101212123111210001001231301120113


### 2.3.3. Determining Index Entropy Weight

The evaluation value weightpij of the index is(5)pij=rij∑i=1mrij,j=1,2,…,nThe entropy valueej of the index is(6)ej=-k∑i=1mpij×ln⁡pij,j=1,2,…,nAmong them,k=1/ln⁡m, when the evaluation value pij×ln⁡pij=0, pij×ln⁡pij=0.The entropy weightwj of the index is(7)wj=1-ej∑j=1n1-ejIn order to solve the problem of numerous indexes and complicated calculation, formulas (6) and (7) are compiled into Excel program. By inputting the evaluation value weight pij of each index into Excel program, the entropy value ej and entropy weight wj of each index can be obtained.According to formula (5), the evaluation value weight matrix P1 of the second-level index “station structure (A1)” is(8)P1=0174152718151327151703100172152714310131741500013002714110027215038110By inputting the value of matrixP1 into Excel program, the entropy value ej and entropy weight wj of each third-level index can be obtained. The calculation results are shown in Table 3.Table 3
Entropy value and entropy weight of each third-level index in the second-level index “station structure (A1).”
Third-level Index Entropy Valueej Entropy Weightwj A11 Radial displacement of the station structure 0.613 0.326 A12 Absolute settlement value of station structure 0.865 0.114 A13 Horizontal displacement of the station structure 0.873 0.107 A14 Station structure floating 0.754 0.207 A15 Differential settlement of station structure deformation joint 0.869 0.111 A16 Crack width of station structure 0.840 0.135Referring to the calculation results in Table3, according to the characteristics that the smaller the entropy weight is, the larger the weight is, those indexes with smaller entropy weight are selected as the key risk factors. For example, four indexes with smaller entropy weight in Table 3 are taken as key risk factors, namely, A12, A13, A15, and A16, and A11 and A14 are deleted. Similarly, the entropy value and entropy weight of other third-level indexes are calculated, and the key risk factors are determined. From this, a list of key risk factors is obtained. The calculation results are shown in Table 4.Table 4
List of key risk factors.
Second-level index Key risk factor A1 Station structure A12 Absolute settlement value of station structure, A13 Horizontal displacement of the station structure, A15 Differential settlement of station structure deformation joint, A16 Crack width of station structure  A2 Track structure A22 Orbital height difference (Vector value), A24 Transverse elevation difference of track, A25 Void amount of track-bed  A3 Power supply system A31 Area power outage renovation, A32 Circuit failure  A4 Water supply and drainage system A41 System blocking, A43 Pipeline damage  A5 Ventilation and signal system A51 Equipment sampling pass rate, A53 Equipment failures, A56 Equipment renewal rate, A57 Equipment pending repair rate  B1 Construction monitoring of existing station B11 Information monitoring equipment failures, B12 Accuracy rate of feedback real-time data, B14 Horizontal and vertical displacement of retaining pile(wall) and top of slop, B16 Axial force of struts, B17 Anchor tension, B18 Ground settlement, B19 Clearance convergence of shaft support structure, B110 Monitoring hole of groundwater level  B2 New space enclosing structure B21 Survey spot arrangement spacing, B23 Lateral displacement of space enclosing structure  B3Engineering dewatering B31 Stratum filtering flow, B33 Sand content control  B4 Foundation pit excavation B42 Foundation pit excavation plan, B44 Depth of foundation pit  B5 Surrounding rock and soil mass and surrounding environment B51 Design of station structure, B52 Stratum and geological condition, B53 Site classification, B54 Soil liquefaction grade, B55 Buried depth, B58 Effect of water level reduction on stratum, B59 Impact of surrounding buildings on construction  B6 Construction technology management of main structure B61Stiffness and deformation reinforcement of main structure, B62 Stiffness of initial support, B63 Quality of secondary lining, B65 Mechanical operation, B66 Construction personnel management, B67Emergency plan  B7 Green construction B72 Ground traffic control, B74 Noise control, B75Sewage control, B76 Light pollution control, B77 Dust control, B78 Construction trash control, B79 Impact on business climateIn summary, after the list of preliminary risk factors including 75 risk factors was screened by Delphi Method and Entropy Weight Method, 26 risk factors with relatively less impact were deleted, and a list of key risk factors including 49 risk factors is obtained.


## 2.3.1. Obtaining Raw Data

The risk factors in the preliminary risk factor list are classified according to different risk degrees, and the risk degree classification standards are shown in Table2.Table 2
Risk degree classification standards.
Risk degree value High risk 5 Moderately high risk 4 Moderate risk 3 Moderately low risk 2 Low risk 1 Between the two adjacent degrees Median value of adjacent valuesThe subway project has the characteristics of high cost and complex technology, involving many specialties and participating units. In order to avoid the subjective one-sidedness of the investigation caused by the small number of experts and the single source of experts, six types of units, such as owner, geological prospecting unit, designing unit, construction unit, supervision unit, and subcontractor, are investigated on the spot. A total of 30 experts are selected from these six types of units for consultation. The number of experts from various units is shown in Figure2. In practice, if 30 experts cannot be invited for some objective reasons, at least 6 experts from different units should be invited.Figure 2
Number of experts from various units.Firstly, 30 experts are invited to score and classify the risk degree of 75 preliminary risk factors according to the risk degree classification standards in Table2. Secondly, according to the different units, the average value of experts’ scores in each unit is calculated, and six average values from six units are obtained. Thirdly, the evaluation value matrix At (t denotes the ordinal number of second-level indexes, t = 1, 2, …, 12) is calculated. In the At matrix, the number of rows is the number of expert source units, and the number of columns is the number of third-level indexes included in each second-level index. Because the arithmetic mean of the expert scores is calculated separately according to the six types of units, At is a matrix of six rows and n columns, of which the value of n is the number of third-level indexes included in each second-level index.For example, the second-level index “station structure” has six third-level indexes, so its evaluation value matrixA1 is(1)A1=4.542342.543.52.53.54.524.54334.5244343.53.544.5433.534.53.53433


## 2.3.2. Standardization of Raw Data

The evaluation matrix is standardized to eliminate the dimensional differences among the indexes. If the evaluation value is bigger, the better, the standardized formula (2) can be used. If the evaluation value is smaller, the better, the standardized formula (3) can be used. aij is an element corresponding to the ith row and the jth column in the evaluation value matrix At. rij is the standardization result of aij:(2)rij=aij-min/jaijmax/jaij-min/jaij(3)rij=max/jaij-aijmax/jaij-min/jaijBecause the evaluation value of risk factors in subway construction belongs to the smaller the better type, formula (3) is adopted to standardize the evaluation value matrix. For example, matrix R1 is the standardization result of the evaluation value matrix A1 (station structure), which is as follows:(4)R1=0121113231134120101212123111210001001231301120113


## 2.3.3. Determining Index Entropy Weight

The evaluation value weightpij of the index is(5)pij=rij∑i=1mrij,j=1,2,…,nThe entropy valueej of the index is(6)ej=-k∑i=1mpij×ln⁡pij,j=1,2,…,nAmong them,k=1/ln⁡m, when the evaluation value pij×ln⁡pij=0, pij×ln⁡pij=0.The entropy weightwj of the index is(7)wj=1-ej∑j=1n1-ejIn order to solve the problem of numerous indexes and complicated calculation, formulas (6) and (7) are compiled into Excel program. By inputting the evaluation value weight pij of each index into Excel program, the entropy value ej and entropy weight wj of each index can be obtained.According to formula (5), the evaluation value weight matrix P1 of the second-level index “station structure (A1)” is(8)P1=0174152718151327151703100172152714310131741500013002714110027215038110By inputting the value of matrixP1 into Excel program, the entropy value ej and entropy weight wj of each third-level index can be obtained. The calculation results are shown in Table 3.Table 3
Entropy value and entropy weight of each third-level index in the second-level index “station structure (A1).”
Third-level Index Entropy Valueej Entropy Weightwj A11 Radial displacement of the station structure 0.613 0.326 A12 Absolute settlement value of station structure 0.865 0.114 A13 Horizontal displacement of the station structure 0.873 0.107 A14 Station structure floating 0.754 0.207 A15 Differential settlement of station structure deformation joint 0.869 0.111 A16 Crack width of station structure 0.840 0.135Referring to the calculation results in Table3, according to the characteristics that the smaller the entropy weight is, the larger the weight is, those indexes with smaller entropy weight are selected as the key risk factors. For example, four indexes with smaller entropy weight in Table 3 are taken as key risk factors, namely, A12, A13, A15, and A16, and A11 and A14 are deleted. Similarly, the entropy value and entropy weight of other third-level indexes are calculated, and the key risk factors are determined. From this, a list of key risk factors is obtained. The calculation results are shown in Table 4.Table 4
List of key risk factors.
Second-level index Key risk factor A1 Station structure A12 Absolute settlement value of station structure, A13 Horizontal displacement of the station structure, A15 Differential settlement of station structure deformation joint, A16 Crack width of station structure  A2 Track structure A22 Orbital height difference (Vector value), A24 Transverse elevation difference of track, A25 Void amount of track-bed  A3 Power supply system A31 Area power outage renovation, A32 Circuit failure  A4 Water supply and drainage system A41 System blocking, A43 Pipeline damage  A5 Ventilation and signal system A51 Equipment sampling pass rate, A53 Equipment failures, A56 Equipment renewal rate, A57 Equipment pending repair rate  B1 Construction monitoring of existing station B11 Information monitoring equipment failures, B12 Accuracy rate of feedback real-time data, B14 Horizontal and vertical displacement of retaining pile(wall) and top of slop, B16 Axial force of struts, B17 Anchor tension, B18 Ground settlement, B19 Clearance convergence of shaft support structure, B110 Monitoring hole of groundwater level  B2 New space enclosing structure B21 Survey spot arrangement spacing, B23 Lateral displacement of space enclosing structure  B3Engineering dewatering B31 Stratum filtering flow, B33 Sand content control  B4 Foundation pit excavation B42 Foundation pit excavation plan, B44 Depth of foundation pit  B5 Surrounding rock and soil mass and surrounding environment B51 Design of station structure, B52 Stratum and geological condition, B53 Site classification, B54 Soil liquefaction grade, B55 Buried depth, B58 Effect of water level reduction on stratum, B59 Impact of surrounding buildings on construction  B6 Construction technology management of main structure B61Stiffness and deformation reinforcement of main structure, B62 Stiffness of initial support, B63 Quality of secondary lining, B65 Mechanical operation, B66 Construction personnel management, B67Emergency plan  B7 Green construction B72 Ground traffic control, B74 Noise control, B75Sewage control, B76 Light pollution control, B77 Dust control, B78 Construction trash control, B79 Impact on business climateIn summary, after the list of preliminary risk factors including 75 risk factors was screened by Delphi Method and Entropy Weight Method, 26 risk factors with relatively less impact were deleted, and a list of key risk factors including 49 risk factors is obtained.


## 2.4. Establishing the Construction Safety Risk Assessment Index System

According to the selected list of key risk factors (Table4), a construction safety risk assessment index system of the new subway station close-attached undercrossing the existing operating station is established. The index system is divided into 2 first-level indexes: the existing subway station and the new subway station, including 12 second-level indexes and 49 third-level indexes, as shown in Table 5.Table 5
Construction safety risk assessment index system of new subway station close-attached undercrossing the existing station.
First-level index Second-level index Third-level index Risk Grade High risk Medium risk Low risk A Existing subway station A1 Station structure A 12 Absolute settlement value of station structure (mm) ≥7 ≥3, < 7 < 3 A 13 Horizontal displacement of the station structure (mm) ≥5 ≥3, < 5 < 3 A 15 Differential settlement of station structure deformation joint (mm) ≥2 ≥1, < 2 < 1 A 16 Crack width of station structure (mm) ≥0.2 ≥0.1, < 0.2 < 0.1 A2 Track structure A 22 Orbital height difference (Vector value) (mm) ≥4 ≥2, < 4 < 2 A 24 Transverse elevation difference of track (mm) ≥4 ≥2, < 4 < 2 A 25 Void amount of track-bed (mm) ≥5 ≥3, < 5 < 3 A3 Power supply system A 31 Area power outage renovation (h/times) ≥2 ≥0.5, < 2 < 0.5 A 32 Circuit failure (times/ month) ≥3 ≥1, < 3 < 1 A4 Water supply and drainage system A 41 System blocking (h/times) ≥5 ≥2, < 5 < 2 A 43 Pipeline damage (times/month) ≥3 ≥1, < 3 < 1 A5 Ventilation and signal system A 51 Equipment sampling pass rate (%) < 60 ≥60, < 90 ≥90 A 53 Equipment failures (times/month) ≥30 ≥15, < 30 < 15 A 56 Equipment renewal rate (%) ≥7 ≥3, < 7 < 3 A 57 Equipment pending repair rate (%) ≥10 ≥5, < 10 < 5  B 11 Information monitoring equipment failures (times/month) ≥10 ≥5, < 10 < 5  B New subway station B1 Construction monitoring of existing station B 12 Accuracy rate of feedback real-time data (%) < 90 ≥90, < 95 ≥95 B 14 Horizontal and vertical displacement of retaining pile (wall) and top of slop (m) 20 < Control point spacing 10 < Control point spacing ≤20 Control point spacing≤10 B 16 Axial force of struts (pieces) Number of struts < 3 3≤ Number of struts < 5 Number of struts ≥5 B 17 Anchor tension (pieces) Number of anchors < 3 3≤ Number of anchors < 5 Number of anchors≥5 B 18 Ground settlement (m) 8 < Control point spacing 3 < Control point spacing ≤8 Control point spacing ≤3 B 19 Clearance convergence of shaft support structure (m) 5 < Monitoring section spacing 3 < Monitoring section spacing ≤5 Monitoring section spacing ≤3 B 110 Monitoring hole of groundwater level (pieces) Number of monitoring holes < 4 4≤Number of monitoring holes < 6 6≤Number of monitoring holes B2 New space enclosing structure B 21 Survey spot arrangement spacing (m) ≥15 ≥10, < 15 < 10 B 23 Lateral displacement of space enclosing structure (mm) ≥20 ≥10, < 20 < 10 B3 Engineering dewatering B 31 Stratum filtering flow (permeability coefficient cm/s) ≥10-4 ≥10-6, < 10-4 <10-6 B 33 Sand content control (10000−1) 1/1 < Fine sand content 1/2 < Fine sand content ≤1/1 Fine sand content ≤1/2 B4 Foundation pit excavation B 42 Foundation pit excavation plan Unreasonable Reasonable and partially meeting standards. Reasonable and meeting standards B 44 Depth of foundation pit (m) ≥20 ≥10,<20 < 10 B 51 Design of station structure Insufficient survey, no precise calculation and design Moderately sufficient survey, calculation and design Full survey, accurate calculation and design  B New subway station B5 Surrounding rock and soil mass and surrounding environment B 52 Stratum and geological condition Complex topography and geomorphology, strong development of adverse geological conditions Moderately complex topography and geomorphology, moderate development of adverse geological conditions Simple topography and geomorphology, good geological condition B 53 Site classification Class III Class II Class I B 54 Soil liquefaction grade Serious Moderate Slight B 55 Buried depth (m) ≥16 ≥10,<16 < 10 B 58 Effect of water level reduction on stratum (m) Inadequate dewatering for foundation excavation Groundwater level drops to 0-1m below foundation pit bottom Groundwater level drops below the foundation pit bottom to more than 1m B 59 Impact of surrounding buildings on construction Distance within 0.7 times the buried depth of baseplate Distance within 0.7-1 times the buried depth of baseplate Distance within 1-2 times the buried depth of baseplate B6 Construction technology management of main structure B 61 Stiffness and deformation reinforcement of main structure Unreasonable grouting parameters, not meeting the actual construction Moderately reasonable grouting parameters, meeting the actual construction Reasonable grouting parameters, in line with the actual construction B 62 Stiffness of initial support (N/mm2) Bond strength of sprayed concrete < 0.6 0.6≤ Bond strength of sprayed concrete<0.8 Bond strength of sprayed concrete≥0.8 B 63 Quality of secondary lining Not meeting the requirements, bad effect Moderately meeting the requirements, better effect Meeting the requirements, good effect. B 65 Mechanical operation Improper mechanical operation Moderately proper mechanical operation Proper mechanical operation B 66 Construction personnel management Unqualified in construction operation, no job disclosure Qualified in construction operation, with job disclosure and Skilled in construction operation, with job and daily training periodic training disclosure and daily training B 67 Emergency plan No emergency plan, no emergency drill plan Lack of maneuverability and failure to organize drills as planned Standardized and targeted and success to organize drills as planned. B7 Green construction B 7 2 Ground traffic control (thousand vehicles/h) ≥5 ≥3,<5 < 3 B 7 4 Noise control (dB) 70 < Daytime, 70≤ Daytime<60, Daytime ≤ 60, 55 < Nighttime 55≤ Nighttime<45 Nighttime ≤ 45 B 7 5 Sewage control (mg/L) Suspend solid ≥400 250≤ Suspend solid<400 Suspend solid<250 B 7 6 Light pollution control (W/m2) ≥11 ≥10,<11 < 10 B 7 7 Dust control (m) Enclosure height<2 2≤Enclosure height<2.5 2.5≤Enclosure height B 7 8 Construction trash control (%) Percentage of recovery and reuse<20 20≤Percentage of recovery and reuse<30 30≤Percentage of recovery and reuse B 7 9 Impact on business climate (ten thousand person-time/month) < 10 ≥10,<20 ≥20By consulting to the relevant national technical specifications and engineering site data, the risk grade of the third-level indexes in the construction safety risk assessment index system is divided into three levels: high risk, medium risk, and low risk, as shown in Table5.


## 3. Assessment Approach

### 3.1. Superiority of the Method

After the establishment of the construction safety risk assessment index system, how to choose a reasonable and effective assessment method is very important to establish a construction safety risk assessment model. After consulting the relevant literature [32–38], this paper uses Analytic Hierarchy Process (AHP) and Fuzzy Matter Element Method (FMEM) to evaluate the construction safety risk of the new subway station close-attached undercrossing the existing station qualitatively and quantitatively. Firstly, the weights of the indexes are calculated by AHP, and then FMEM is used to determine the risk grade of each index and the project.In order to facilitate the research of problems with multiple contents, it can be transformed into quantitative problems and then studied. AHP is a common method to solve such problems. For the complex index system, the weight of the index occupies an important position. Using AHP to calculate index weight can reduce subjective factors and improve the scientific nature of weight determination.Fuzzy Matter Element Method (FMEM) was proposed by Chinese scholar Cai Wen in the 1980s. The main point of this method is to describe things with “object, features and quantized values.” Matter element refers to the basic elements of an orderly group composed of these three elements. If the object has n characteristics and corresponding quantized values, it is called n-dimensional matter element. Matter element analysis is to study the matter element and its changes, mainly to study the incompatibility of the real world. The purpose of adding fuzzy mathematics to matter element analysis is to deal with the quantity ingredient with fuzziness in matter element, and the combination of these two is fuzzy matter element analysis [35–38].The application of FMEM in risk assessment has a number of advantages. Firstly, it can describe the complex risk assessment index comprehensively and clearly. Secondly, it can describe the fuzzy state of the assessed object in the form of fuzzy subset vector and objectively evaluate various indexes. Thirdly, it can be used not only for the comprehensive assessment of subjective risk factors to solve fuzzy phenomena with unclear boundaries and that are difficult to quantify in various indexes, but also for the comprehensive assessment of objective risk factors to make the assessment process clear and scientific.


### 3.2. Construction Safety Risk Assessment Model Based on AHP and FMEM

The construction of new subway station close-attached undercrossing the existing subway station should not only ensure the safe construction of the new subway station, but also ensure the normal operation of the existing subway station. Its risk assessment can be seen as a problem with two incompatible objectives. Combining AHP and FMEM, the construction safety risk assessment model is constructed. The detailed calculation process is shown in Figure3. The main calculation steps are as follows.Figure 3
Calculation process of the construction safety risk assessment model.


#### 3.2.1. Calculating Index Weight

AHP is used to calculate index weight. Firstly, the judgement matrix of all levels of indexes is constructed by synthesizing the judgement of experts. Secondly, whether the judgement matrix passes the consistency test is observed. If it passes the test, the weight of each index in the judgment matrix can be obtained.


#### 3.2.2. Determining Matter Element

In the construction safety risk assessment model, FMEM is used to establish the matter element composed of “risk assessment, assessment indexes, and quantized values.” Risk assessmentN, risk assessment feature c, and feature quantified value x together constitute the matter element to be assessed, which is(9)Rx=Nxc1x1c2x2⋮⋮cnxn


#### 3.2.3. Determining Matter Element Matrix of Classical Domain and Joint Domain

The classical domain matter element matrix of the construction safety risk assessment can be expressed as(10)Roj=Noj,ci,xoji=Nojc1aoj1,boj1c2aoj2,boj2⋮⋮cnaojn,bojnwhere Roj is the classical domain matter element; Noj is the jth level of the classified risk grade, j=(1,2,⋯,n); ci is the ith assessment index; (aoji,boji) is the quantized value range corresponding to the jth risk grade, that is, the classical domain.The joint domain matter element matrix of the construction safety risk assessment can be expressed as(11)Rp=Np,Ci,Vpi=Npc1ap1,bp1c2ap2,bp2⋮⋮cnapn,bpnwhere Rp is the joint domain matter element; (api,bpi) is the quantized value range of the feature ci; p is the overall level of risk grade.


#### 3.2.4. Calculating the Correlation Function and Correlation Degree to Determine the Risk Grade of Each Third-Level Index

The index correlation functionK(x) of construction safety risk assessment is defined as(12)Kxi=-ρX,XoXo,X∈XoρX,XoρX,Xp-ρX,Xo,X∉Xo(13)Where:ρX,Xo=X-12ao+bo-12bo-ao(14)ρX,Xp=X-12ap+bp-12bp-ap(15)Xo=bo-aowhere ρ(X,Xo) is the distance between point X and finite interval Xo=ao,bo; ρ(X,Xp) is the distance between point X and finite interval Xp=ap,bp; X is the quantized value of matter element to be assessed; Xo is the quantized value range of the classical domain matter element; XP is the quantized value range of the joint domain matter element.


#### 3.2.5. Calculating the Comprehensive Correlation Degree and Determining the Risk Grade of the Project

The comprehensive correlation degreeKj(Nx) of Nx with respect to risk grade j is as follows:(16)KjNx=∑i=1nωiKjXiwhere Kj(Nx) is the comprehensive correlation degree; Kj(Xj) is the single correlation degree j=(1,2,⋯,m); ωi is the weight of each index. If(17)Kji=max⁡KjXi,j=1,2,⋯,nthen the ith index belongs to the jth grade of construction safety risk assessment. If(18)Kjx=max⁡KjNx,j=1,2,⋯,nthen Nx belongs to the jth grade of construction safety risk assessment.When0<Kj(Nx)<1, it indicates that the unit to be assessed meets the requirements of the assessment standard; when -1<Kj(Nx)<0, it indicates that the unit to be assessed does not meet the requirements of a certain grade of assessment standard, but the unit to be assessed has the conditions to convert to the assessment standard, and the smaller the value is, the easier it is to convert; when KjNx<-1, it indicates that the unit to be assessed does not meet the requirements of a certain grade of assessment standard and does not have the conditions to convert to the assessment standard.


## 3.1. Superiority of the Method

After the establishment of the construction safety risk assessment index system, how to choose a reasonable and effective assessment method is very important to establish a construction safety risk assessment model. After consulting the relevant literature [32–38], this paper uses Analytic Hierarchy Process (AHP) and Fuzzy Matter Element Method (FMEM) to evaluate the construction safety risk of the new subway station close-attached undercrossing the existing station qualitatively and quantitatively. Firstly, the weights of the indexes are calculated by AHP, and then FMEM is used to determine the risk grade of each index and the project.In order to facilitate the research of problems with multiple contents, it can be transformed into quantitative problems and then studied. AHP is a common method to solve such problems. For the complex index system, the weight of the index occupies an important position. Using AHP to calculate index weight can reduce subjective factors and improve the scientific nature of weight determination.Fuzzy Matter Element Method (FMEM) was proposed by Chinese scholar Cai Wen in the 1980s. The main point of this method is to describe things with “object, features and quantized values.” Matter element refers to the basic elements of an orderly group composed of these three elements. If the object has n characteristics and corresponding quantized values, it is called n-dimensional matter element. Matter element analysis is to study the matter element and its changes, mainly to study the incompatibility of the real world. The purpose of adding fuzzy mathematics to matter element analysis is to deal with the quantity ingredient with fuzziness in matter element, and the combination of these two is fuzzy matter element analysis [35–38].The application of FMEM in risk assessment has a number of advantages. Firstly, it can describe the complex risk assessment index comprehensively and clearly. Secondly, it can describe the fuzzy state of the assessed object in the form of fuzzy subset vector and objectively evaluate various indexes. Thirdly, it can be used not only for the comprehensive assessment of subjective risk factors to solve fuzzy phenomena with unclear boundaries and that are difficult to quantify in various indexes, but also for the comprehensive assessment of objective risk factors to make the assessment process clear and scientific.


## 3.2. Construction Safety Risk Assessment Model Based on AHP and FMEM

The construction of new subway station close-attached undercrossing the existing subway station should not only ensure the safe construction of the new subway station, but also ensure the normal operation of the existing subway station. Its risk assessment can be seen as a problem with two incompatible objectives. Combining AHP and FMEM, the construction safety risk assessment model is constructed. The detailed calculation process is shown in Figure3. The main calculation steps are as follows.Figure 3
Calculation process of the construction safety risk assessment model.


### 3.2.1. Calculating Index Weight

AHP is used to calculate index weight. Firstly, the judgement matrix of all levels of indexes is constructed by synthesizing the judgement of experts. Secondly, whether the judgement matrix passes the consistency test is observed. If it passes the test, the weight of each index in the judgment matrix can be obtained.


### 3.2.2. Determining Matter Element

In the construction safety risk assessment model, FMEM is used to establish the matter element composed of “risk assessment, assessment indexes, and quantized values.” Risk assessmentN, risk assessment feature c, and feature quantified value x together constitute the matter element to be assessed, which is(9)Rx=Nxc1x1c2x2⋮⋮cnxn


### 3.2.3. Determining Matter Element Matrix of Classical Domain and Joint Domain

The classical domain matter element matrix of the construction safety risk assessment can be expressed as(10)Roj=Noj,ci,xoji=Nojc1aoj1,boj1c2aoj2,boj2⋮⋮cnaojn,bojnwhere Roj is the classical domain matter element; Noj is the jth level of the classified risk grade, j=(1,2,⋯,n); ci is the ith assessment index; (aoji,boji) is the quantized value range corresponding to the jth risk grade, that is, the classical domain.The joint domain matter element matrix of the construction safety risk assessment can be expressed as(11)Rp=Np,Ci,Vpi=Npc1ap1,bp1c2ap2,bp2⋮⋮cnapn,bpnwhere Rp is the joint domain matter element; (api,bpi) is the quantized value range of the feature ci; p is the overall level of risk grade.


### 3.2.4. Calculating the Correlation Function and Correlation Degree to Determine the Risk Grade of Each Third-Level Index

The index correlation functionK(x) of construction safety risk assessment is defined as(12)Kxi=-ρX,XoXo,X∈XoρX,XoρX,Xp-ρX,Xo,X∉Xo(13)Where:ρX,Xo=X-12ao+bo-12bo-ao(14)ρX,Xp=X-12ap+bp-12bp-ap(15)Xo=bo-aowhere ρ(X,Xo) is the distance between point X and finite interval Xo=ao,bo; ρ(X,Xp) is the distance between point X and finite interval Xp=ap,bp; X is the quantized value of matter element to be assessed; Xo is the quantized value range of the classical domain matter element; XP is the quantized value range of the joint domain matter element.


### 3.2.5. Calculating the Comprehensive Correlation Degree and Determining the Risk Grade of the Project

The comprehensive correlation degreeKj(Nx) of Nx with respect to risk grade j is as follows:(16)KjNx=∑i=1nωiKjXiwhere Kj(Nx) is the comprehensive correlation degree; Kj(Xj) is the single correlation degree j=(1,2,⋯,m); ωi is the weight of each index. If(17)Kji=max⁡KjXi,j=1,2,⋯,nthen the ith index belongs to the jth grade of construction safety risk assessment. If(18)Kjx=max⁡KjNx,j=1,2,⋯,nthen Nx belongs to the jth grade of construction safety risk assessment.When0<Kj(Nx)<1, it indicates that the unit to be assessed meets the requirements of the assessment standard; when -1<Kj(Nx)<0, it indicates that the unit to be assessed does not meet the requirements of a certain grade of assessment standard, but the unit to be assessed has the conditions to convert to the assessment standard, and the smaller the value is, the easier it is to convert; when KjNx<-1, it indicates that the unit to be assessed does not meet the requirements of a certain grade of assessment standard and does not have the conditions to convert to the assessment standard.


## 3.2.1. Calculating Index Weight

AHP is used to calculate index weight. Firstly, the judgement matrix of all levels of indexes is constructed by synthesizing the judgement of experts. Secondly, whether the judgement matrix passes the consistency test is observed. If it passes the test, the weight of each index in the judgment matrix can be obtained.


## 3.2.2. Determining Matter Element

In the construction safety risk assessment model, FMEM is used to establish the matter element composed of “risk assessment, assessment indexes, and quantized values.” Risk assessmentN, risk assessment feature c, and feature quantified value x together constitute the matter element to be assessed, which is(9)Rx=Nxc1x1c2x2⋮⋮cnxn


## 3.2.3. Determining Matter Element Matrix of Classical Domain and Joint Domain

The classical domain matter element matrix of the construction safety risk assessment can be expressed as(10)Roj=Noj,ci,xoji=Nojc1aoj1,boj1c2aoj2,boj2⋮⋮cnaojn,bojnwhere Roj is the classical domain matter element; Noj is the jth level of the classified risk grade, j=(1,2,⋯,n); ci is the ith assessment index; (aoji,boji) is the quantized value range corresponding to the jth risk grade, that is, the classical domain.The joint domain matter element matrix of the construction safety risk assessment can be expressed as(11)Rp=Np,Ci,Vpi=Npc1ap1,bp1c2ap2,bp2⋮⋮cnapn,bpnwhere Rp is the joint domain matter element; (api,bpi) is the quantized value range of the feature ci; p is the overall level of risk grade.


## 3.2.4. Calculating the Correlation Function and Correlation Degree to Determine the Risk Grade of Each Third-Level Index

The index correlation functionK(x) of construction safety risk assessment is defined as(12)Kxi=-ρX,XoXo,X∈XoρX,XoρX,Xp-ρX,Xo,X∉Xo(13)Where:ρX,Xo=X-12ao+bo-12bo-ao(14)ρX,Xp=X-12ap+bp-12bp-ap(15)Xo=bo-aowhere ρ(X,Xo) is the distance between point X and finite interval Xo=ao,bo; ρ(X,Xp) is the distance between point X and finite interval Xp=ap,bp; X is the quantized value of matter element to be assessed; Xo is the quantized value range of the classical domain matter element; XP is the quantized value range of the joint domain matter element.


## 3.2.5. Calculating the Comprehensive Correlation Degree and Determining the Risk Grade of the Project

The comprehensive correlation degreeKj(Nx) of Nx with respect to risk grade j is as follows:(16)KjNx=∑i=1nωiKjXiwhere Kj(Nx) is the comprehensive correlation degree; Kj(Xj) is the single correlation degree j=(1,2,⋯,m); ωi is the weight of each index. If(17)Kji=max⁡KjXi,j=1,2,⋯,nthen the ith index belongs to the jth grade of construction safety risk assessment. If(18)Kjx=max⁡KjNx,j=1,2,⋯,nthen Nx belongs to the jth grade of construction safety risk assessment.When0<Kj(Nx)<1, it indicates that the unit to be assessed meets the requirements of the assessment standard; when -1<Kj(Nx)<0, it indicates that the unit to be assessed does not meet the requirements of a certain grade of assessment standard, but the unit to be assessed has the conditions to convert to the assessment standard, and the smaller the value is, the easier it is to convert; when KjNx<-1, it indicates that the unit to be assessed does not meet the requirements of a certain grade of assessment standard and does not have the conditions to convert to the assessment standard.


## 4. Case Study

### 4.1. Project Profile

Dongdalu Transfer Station of Chengdu Rail Transit Line 8 is located at Huiyuan Road with a road width of 30m. The transfer station is arranged in the east-west direction, using a 13.5 m island platform. The station is a three-story, three-span box-type underground frame structure. The main body of the station is constructed by open-cut method, and the underground excavation method is used at the transfer node of the station and Chengdu Rail Transit Line 2. The total length of the transfer station is 306.2m, the width of the standard section is 23m, the covering soil is 2.6m, and the buried depth of the station baseplate is 24.2m. The station transfers with the operating Line 2 in the form of T-type node transfer, as shown in Figure4. At this T-type node, the existing two-story underground station is on Line 2, the new three-story underground station is on Line 8, and the new station on Line 8 is close-attached undercrossing the existing operating station on Line 2.Figure 4
3D model diagram of Dongdalu transfer station.


### 4.2. Construction Safety Risk Assessment

#### 4.2.1. Calculating Index Weight

As discussed above, 30 experts were consulted in the selecting of key risk factors. These 30 experts are invited to grade and judge each index of the risk assessment index system in Table5, and the judgment matrix of each index at all levels is established. Then the judgment matrix is input into the Matlab program based on AHP to determine whether it passes the consistency test and get the weight of each index.For example, the judgment matrix of the first-level index “existing subway station (A)” is established, as is shown in Table6.Table 6
Judgment matrix of the first-level index “Existing subway station (A).”
A1 A2 A3 A4 A5 A1 Station structure 1 1/3 1/2 3 2 A2 Track structure 3 1 2 5 4 A3 Power supply system 2 1/2 1 4 3 A4 Water supply and drainage system 1/3 1/5 1/4 1 1/2 A5 Ventilation and signal system 1/2 1/4 1/3 2 1CI = 0.0170, CR = 0.0152 are obtained by AHP. CR < 0.1 means that the consistency test has been passed, and the calculated weights of each second-level index under the first-level index “existing subway station (A)” are 0.1599, 0.4185, 0.2625, 0.0618, and 0.0973, respectively.Similarly, the weight of other second-level and third-level indexes can be determined. The calculation results are shown in Table7.Table 7
Index weight of construction safety risk assessment at all levels.
First-level index(Weight) Second-level index Weight of second-level index Third-level index Weight of third-level index A Existing subway station (0.5) A1 Station structure 0.1599 A12 0.0630 A13 0.1098 A15 0.3005 A16 0.5267 A2 Track structure 0.4185 A22 0.2000 A24 0.2000 A25 0.6000 A3 Power supply system 0.2625 A31 0.1667 A32 0.8333 A4 Water supply and drainage system 0.0618 A41 0.2000 A43 0.8000 A5 Ventilation and signal system 0.0973 A51 0.2389 A53 0.5495 A56 0.0650 A57 0.1466  B New subway station (0.5) B1 Construction monitoring of existing station 0.3543 B11 0.2295 B12 0.3305 B14 0.1040 B16 0.0444 B17 0.0444 B18 0.1557 B19 0.0684 B110 0.0231 B2 New space enclosing structure 0.1587 B21 0.1667 B23 0.8333 B3 Engineering dewatering 0.0676 B31 0.2000 B33 0.8000 B4 Foundation pit excavation 0.0448 B42 0.6667 B44 0.3333 B5 Surrounding rock and soil mass and surrounding environment 0.1036 B51 0.1556 B52 0.0623 B53 0.0623 B54 0.0997 B55 0.2375 B58 0.3523 B59 0.0304 B6 Construction technology management of main structure 0.2399 B61 0.3777 B62 0.0916 B63 0.0916 B65 0.1528 B66 0.2449 B67 0.0413 B7 Green construction 0.0312 B72 0.0533 B74 0.1351 B75 0.3371 B76 0.0829 B77 0.2205 B78 0.1351 B79 0.0362


#### 4.2.2. Determining Matter Element to Be Assessed

In the comprehensive assessment, the assessed unit is an n-dimensional matter element, and its eigenvalues vary according to its influence. The 30 experts consulted during the selecting of key risk factors are invited to estimate the specific status of the indexes in the index system in Table5. For example, experts estimate that the absolute settlement value of the station structure (A12) of this project may be 4 mm based on the practical experience of similar projects, and the evaluation value of this index is 4. Similarly, the evaluation values of other indexes can be obtained, and the evaluation values are averaged arithmetically to get Rx as shown below.(19)
R
x
=
N
x
A
12
3.5
A
13
2.2
A
15
0.8
A
16
0.16
A
22
1.8
A
24
1.9
A
25
4.2
A
31
0.3
A
32
0.6
A
41
0.9
A
43
1.1
A
51
97
A
53
12
A
56
2.6
A
57
5.7
B
11
6.2
B
12
87.5
B
14
15
B
16
6
B
17
7
B
18
2.5
B
19
4
B
110
5
B
21
12
B
23
7.4
B
31
2
∗
10
-
5
B
33
0.8
B
42
8.8
B
44
26
B
51
8.4
B
52
7.3
B
53
7.5
B
54
8.1
B
55
24.2
B
58
7.1
B
59
0.9
B
61
9.1
B
62
0.81
B
63
8.2
B
65
8.1
B
66
7.2
B
67
8.3
B
72
5
.
1
B
74
67
29
B
75
293
B
76
7
B
77
2.1
B
78
12
B
79
15.8


#### 4.2.3. Determining Matter Element Matrices of Classical Domain and Joint Domain

The determination of the classical domain is the basis of Fuzzy Matter Element Method. According to the extensibility of construction safety risk assessment of new subway station close-attached undercrossing the existing subway station, the risk grade of each index is divided into three grades, namely,No1, No2, and No3, which are qualitatively described as high risk, medium risk, and low risk. The classic domain matter element matrices Ro1, Ro2, and Ro3 and the joint domain matter element matrix Rp established based on Table 5 are shown in Table 8.Table 8
Assessment criteria of construction safety risk assessment.
Index Unit High risk (Ro1) Medium risk (Ro2) Low risk (Ro3) Joint domain (Rp) A12 mm 7~10 3~7 0~3 0~10 A13 mm 5~10 3~5 0~3 0~10 A15 mm 2~5 1~2 0~1 0~5 A16 mm 0.2~0.5 0.1~0.2 0~0.1 0~0.5 A22 mm 4~10 2~4 0~2 0~10 A24 mm 4~10 2~4 0~2 0~10 A25 mm 5~10 3~5 0~3 0~10 A31 h/times 2~24 0.5~2 0~0.5 0~24 A32 times/month 3~31 1~3 0~1 0~31 A41 h/times 5~24 2~5 0~2 0~24 A43 times/month 3~31 1~3 0~1 0~31 A51 % 0~60 60~90 90~100 0~100 A53 times/month 30~50 15~30 0~15 0~50 A56 % 7~20 3~7 0~3 0~10 A57 % 10~100 5~10 0~5 0~100 B11 times/month 10~31 5~10 0~5 0~31 B12 % 0~90 90~95 95~100 0~100 B14 m 20~50 10~20 0~10 0~50 B16 pieces 0~3 3~5 5~10 0~10 B17 pieces 0~3 3~5 5~10 0~10 B18 m 8~20 3~8 0~3 0~20 B19 m 5~10 3~5 0~3 0~10 B110 pieces 0~4 4~6 6~10 0~10 B21 m 15~30 10~15 0~10 0~30 B23 mm 50~80 25~50 0~25 0~80 B31 cm/s 10−4~10−2 10−6~10−4 10−8~10−6 10−4~10−2 B33 10000−1 1~2 1/2~1 0~1/2 0~2 B42 ∖ 0~6 6~8 8~10 0~10 B44 m 20~35 10~20 0~10 0~30 B51 ∖ 0~6 6~8 8~10 0~10 B52 ∖ 0~6 6~8 8~10 0~10 B53 ∖ 0~6 6~8 8~10 0~10 B54 ∖ 0~6 6~8 8~10 0~10 B55 m 16~30 10~16 0~10 0~30 B58 ∖ 0~6 6~8 8~10 0~10 B59 ∖ 0~0.7 0.7~1 1~2 0~2 B61 ∖ 0~6 6~8 8~10 0~10 B62 N/mm2 0~0.6 0.6~0.8 0.8~1 0~1 B63 ∖ 0~6 6~8 8~10 0~10 B65 ∖ 0~6 6~8 8~10 0~10 B66 ∖ 0~6 6~8 8~10 0~10 B67 ∖ 0~6 6~8 8~10 0~10 B72 thousand vehicles/h 5~10 3~5 0~3 0~10 B74 dB daytime:70~120 60~70 0~60 0~120 nighttime:55~100 45~55 0~45 0~100 B75 mg/L 400~600 250~400 0~250 0~600 B76 W/m2 11~20 10~11 0~10 0~20 B77 m 0~2 2~2.5 2.5~3.5 0~3.5 B78 % 0~20 20~30 30~100 0~100 B79 ten thousand person-time/month 0~10 10~20 20~30 0~30


#### 4.2.4. Calculating the Correlation Degree to Determine the Risk Grade of Each Third-Level Index

The values of the matter elementRx are input into Excel program which has been compiled based on the FMEM, and the corresponding calculation results are shown in Table 9.Table 9
Correlation degree and risk grade of each third-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A12) -0.5 0.125 -0.125 Medium risk K j (A13) -0.56 -0.267 0.267 Low risk K j (A15) -0.6 -0.2 0.2 Low risk K j (A16) -0.2 0.4 -0.273 Medium risk K j (A22) -0.55 -0.1 0.1 Low risk K j (A24) -0.525 -0.05 0.05 Low risk K j (A25) -0.16 0.4 -0.222 Medium risk K j (A31) -0.85 -0.133 2 Low risk K j (A32) -0.8 -0.2 2 Low risk K j (A41) -0.82 -0.55 0.45 Low risk K j (A43) -0.633 0.05 -0.083 Medium risk K j (A51) -0.925 -0.7 0.3 Low risk K j (A53) -0.6 -0.2 0.2 Low risk K j (A56) -0.629 -0.133 0.133 Low risk K j (A57) -0.43 0.14 -0.109 Medium risk K j (B11) -0.38 0.24 -0.162 Medium risk K j (B12) 0.028 -0.167 -0.375 High risk K j (B14) -0.25 0.5 -0.25 Medium risk K j (B16) -0.429 -0.2 0.2 Low risk K j (B17) -0.571 -0.4 0.4 Low risk K j (B18) -0.688 -0.167 0.167 Low risk K j (B19) -0.2 0.5 -0.2 Medium risk K j (B110) -0.167 0.5 -0.167 Medium risk K j (B21) -0.2 0.4 -0.143 Medium risk K j (B23) -0.63 -0.26 0.26 Low risk K j (B31) -0.8 0.192 -0.487 Medium risk K j (B33) -0.2 0.4 -0.273 Medium risk K j (B42) -0.375 0.25 -0.167 Medium risk K j (B44) 0.4 -0.4 -0.64 High risk K j (B51) -0.6 -0.2 0.2 Low risk K j (B52) -0.325 0.35 -0.206 Medium risk K j (B53) -0.375 0.25 -0.167 Medium risk K j (B54) -0.525 -0.05 0.05 Low risk K j (B55) 0.414 -0.586 -0.71 High risk K j (B58) -0.275 0.45 -0.237 Medium risk K j (B59) -0.182 0.333 -0.1 Medium risk K j (B61) -0.775 -0.55 0.45 Low risk K j (B62) -0.525 -0.05 0.05 Low risk K j (B63) -0.55 -0.1 0.1 Low risk K j (B65) -0.525 -0.05 0.05 Low risk K j (B66) -0.3 0.4 -0.222 Medium risk K j (B67) -0.575 -0.15 0.15 Low risk K j (B72) 0.02 -0.02 -0.3 High risk K j (B74) -0.527 -0.056 0.239 Low risk K j (B75) -0.268 0.287 -0.128 Medium risk K j (B76) -0.364 -0.3 0.3 Low risk K j (B77) -0.067 0.2 -0.222 Medium risk K j (B78) 0.4 -0.4 -0.6 High risk K j (B79) -0.29 0.42 -0.228 Medium risk


#### 4.2.5. Calculating the Comprehensive Correlation Degree and Determining the Risk Grade of the Project

The comprehensive correlation degree is calculated according to FMEM, and the comprehensive correlation degree and risk grade of each second-level index can be obtained as shown in Table10.Table 10
Comprehensive correlation degree and risk grade of each second-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A1 Station structure) -0.061 0.021 -0.010 Medium risk K j (A2 Track structure) -0.130 0.130 -0.088 Medium risk K j (A3 Power supply system) -0.188 -0.031 0.031 Low risk K j (A4 Water supply and drainage system) -0.041 -0.004 0.001 Low risk K j (A5 Ventilation and signal system) -0.064 -0.026 0.017 Low risk K j (B1 Construction monitoring of existing station) -0.097 0.016 -0.054 Medium risk K j (B2 New space enclosing structure) -0.089 -0.024 0.031 Low risk K j (B3 Engineering dewatering) -0.022 0.024 -0.021 Medium risk K j (B4 Foundation pit excavation) -0.005 0.001 -0.015 Medium risk K j (B5 Surrounding rock and soil mass and surrounding environment) -0.020 0.003 -0.025 Medium risk K j (B6 Construction technology management of main structure) -0.136 -0.033 0.034 Low risk K j (B7 Green construction) -0.005 0.002 -0.004 Medium riskCombining the correlation degree of each index and the corresponding index weight in Table7, the comprehensive correlation degree of each first-level index is obtained, as shown in Table 11.Table 11
Comprehensive correlation degree and risk grade of each first-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A New subway station) -0.484 0.048 -0.004 Medium risk K j (B Existing subway station) -0.374 -0.010 -0.054 Medium riskThe correlation degree of each index and the corresponding index weight in Table7 are input into formula (18) to obtain the comprehensive correlation degree of the matter element to be assessed, as shown in Table 12.Table 12
Comprehensive correlation degree of the matter element to be assessed.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (P0) -0.429 0.019 -0.029 Medium risk


### 4.3. Result Analysis

#### 4.3.1. Result Analysis of Existing Subway Station

The risk grade of the first-level index “existing subway station” is medium risk, which indicates that the close-attached undercrossing project is easy to affect the existing station. According to Table10, in the second-level indexes, the risk grade of “station structure” and “track structure” is medium risk, while the risk grade of “power supply system,” “water supply and drainage system,” and “ventilation and signal system” is low risk.Table9 shows that, in the third-level indexes, the risk grade of “absolute settlement value of station structure,” “crack width of station structure,” and “void amount of track-bed” is medium risk. To guard against these risk factors, it is necessary to have strict control of each working procedure that may cause settlement in the construction process. As the structural settlement and deformation caused by construction are the main factors affecting the deformation of station structure and track structure, good construction measures to control the value of station settlement within the allowable range can ensure the normal operation of existing operating station.The results of risk assessment are compared with the actual engineering data. The risk assessment results of the qualitative indexes in the third-level indexes are consistent with the actual engineering measurement results. The risk assessment results of quantitative indexes and the actual engineering measurement results are compared and analyzed as shown in Table13. There are 15 quantitative indexes in Table 13. Among them, the risk grade corresponding to the risk assessment results of 13 indexes is consistent with the risk grade corresponding to the actual engineering measurement results, and 2 indexes are inconsistent. For instance, the risk grade corresponding to the expert assessment results of “pipeline damage” and “equipment pending repair rate” is medium risk, while the risk grade corresponding to the actual engineering measurement results is low risk, which indicates that the construction unit has strengthened the management of pipelines and equipment on site according to the result of risk assessment and has taken effective control measures to reduce the risk grade of these indexes.Table 13
Comparison of the third-level index data of “existing subway station.”
Third-level index Risk assessment result Corresponding risk grade of risk assessment Actual engineering measurement result Corresponding risk grade of actual measurement Consistency A 1 2 Absolute settlement value of station structure (mm) 3.5 Medium risk 5.5 Medium risk Yes A 1 3 Horizontal displacement of the station structure (mm) 2.2 Low risk 2.9 Low risk Yes A 1 5 Differential settlement of station structure deformation joint (mm) 0.8 Low risk 0.7 Low risk Yes A 1 6 Crack width of station structure (mm) 0.16 Medium risk 0.1 Medium risk Yes A 22 Orbital height difference (Vector value) (mm) 1.8 Low risk 0.5 Low risk Yes A 24 Transverse elevation difference of track (mm) 1.9 Low risk 1.7 Low risk Yes A 25 Void amount of track-bed (mm) 4.2 Medium risk 3.0 Medium risk Yes A 31 Area power outage renovation (h/times) 0.3 Low risk 0.2 Low risk Yes A 32 Circuit failure (times/month) 0.6 Low risk 0.5 Low risk Yes A 41 System blocking (h/times) 0.9 Low risk 0.6 Low risk Yes A 43 Pipeline damage (times/month) 1.1 Medium risk 0.7 Low risk No A 51 Equipment sampling pass rate (%) 97 Low risk 96 Low risk Yes A 53 Equipment failures (times/month) 12 Low risk 10 Low risk Yes A 56 Equipment renewal rate (%) 2.6 Low risk 2.1 Low risk Yes A 57 Equipment pending repair rate (%) 5.7 Medium risk 3.6 Low risk No


#### 4.3.2. Result Analysis of New Subway Station

The risk grade of first-level index “new subway station” is medium risk, which indicates that the operation of existing subway station will have a great impact on the construction of new subway station. From Table10, it can be concluded that, in the second-level indexes, the risk grade of “construction monitoring of existing station,” “engineering dewatering,” “foundation pit excavation,” “surrounding rock and soil mass and surrounding environment,” and “green construction” is medium risk, while the risk grade of “new space enclosing structure” and “construction technology management of main structure” is low risk.Table9 shows that, in the third-level indexes, the risk grade of “accuracy rate of feedback real-time data,” “depth of foundation pit,” “buried depth,” “ground traffic control,” and “construction trash control” is high risk. In order to prevent the occurrence of these risk factors, it is necessary to strengthen the protective work and monitoring of the station structure and line of the existing station and analyze the monitoring data in time. For the alarm data issued by the monitoring device in the construction process, the operator should respond in time, report to the superior department immediately, and take effective control measures. Then, it is necessary to pay attention to the monitoring of various indicators in the construction process of new subway station and to timely analysis of monitoring data to ensure the accuracy rate of data feedback. At the same time, the construction unit should reasonably guide the ground transportation, and the trash generated in the construction process should be transported to the designated location in time, and protective measures should be taken in the transportation process to prevent dust, soil pollution, and other problems.The results of risk assessment are compared with the actual engineering data. The risk assessment results of the qualitative indexes in the third-level indexes are consistent with the actual engineering measurement results. The comparative analysis of the risk assessment results of the quantitative indexes and the actual engineering measurement results is shown in Table14. There are 23 quantitative indexes in Table 14. Among them, the risk grade corresponding to the risk assessment results of 19 indexes is consistent with the risk grade corresponding to the actual engineering measurement results, and 4 indexes are inconsistent.Table 14
Comparison of the third-level index data of “new subway station.”
Third-level index Risk assessment result Corresponding risk grade of risk assessment Actual engineering measurement result Corresponding risk grade of actual measurement Consistency B 1 1 Information monitoring equipment failures (times/month) 6.2 Medium risk 5.3 Medium risk Yes B 1 2 Accuracy rate of feedback real-time data (%) 87.5 High risk 93 Medium risk No B 1 4 Horizontal and vertical displacement of retaining pile (wall) and top of slop (m) 15 Medium risk 12 Medium risk Yes B 1 6 Axial force of struts (pieces) 6 Low risk 8 Low risk Yes B 1 7 Anchor tension (pieces) 7 Low risk 8 Low risk Yes B 1 8 Ground settlement (m) 2.5 Low risk 2 Low risk Yes B 1 9 Clearance convergence of shaft support structure (m) 4 Medium risk 5 Medium risk Yes B 1 10 Monitoring hole of groundwater level (pieces) 5 Medium risk 6 Low risk No B 2 1 Survey spot arrangement spacing (m) 12 Medium risk 9.5 Low risk No B 23 Lateral displacement of space enclosing structure (mm) 7.4 Low risk 4.9 Low risk Yes B 3 1 Stratum filtering flow (permeability coefficient cm/s) 2×105 Medium risk 3×105 Medium risk Yes B 33 Sand content control (10000−1) 0.8 Medium risk 0.9 Medium risk Yes B 44 Depth of foundation pit (m) 26 High risk 25.5 High risk Yes B 55 Buried depth (m) 24.2 High risk 24.2 High risk Yes B 59 Impact of surrounding buildings on construction 0.9 Medium risk 0.9 Medium risk Yes B 62 Stiffness of initial support (N/mm2) 0.81 Low risk 0.92 Low risk Yes B 72 Ground traffic control (thousand vehicles/h) 5.1 High risk 6.2 High risk Yes B 74 Noise control (dB) daytime67 Low risk daytime59 Low risk Yes night29 night 30 B 75 Sewage control (mg/L) 293 Medium risk 262 Medium risk Yes B 76 Light pollution control (W/m2) 7 Low risk 6.2 Low risk Yes B 77 Dust control (m) 2.1 Medium risk 2.5 Low risk No B 78 Construction trash control (%) 12 High risk 18 High risk Yes B 79 Impact on business climate (ten thousand person-time/month) 15.8 Medium risk 12 Medium risk YesFor instance, the risk grade corresponding to risk assessment result of “accuracy rate of feedback real-time data” is high risk, while the risk grade corresponding to the actual measurement result is medium risk. The risk grade corresponding to risk assessment result of “monitoring hole of groundwater level,” “survey spot arrangement spacing,” and “dust control” is medium risk, while the risk grade corresponding to the actual measurement result is low risk. It shows that the results of the risk assessment model have a certain warning effect on the construction unit. The construction unit has strengthened the feedback of real-time data on site, strengthened the layout of groundwater level monitoring holes, the spacing control of survey spot arrangement, and dust control, and adopted effective prevention and control measures to reduce the risk grade of these indexes.In summary, the risk grade of the close-attached undercrossing project is medium risk. The assessment result is basically consistent with the actual engineering situation, which indicates that the construction safety risk assessment model has certain practicability.


## 4.1. Project Profile

Dongdalu Transfer Station of Chengdu Rail Transit Line 8 is located at Huiyuan Road with a road width of 30m. The transfer station is arranged in the east-west direction, using a 13.5 m island platform. The station is a three-story, three-span box-type underground frame structure. The main body of the station is constructed by open-cut method, and the underground excavation method is used at the transfer node of the station and Chengdu Rail Transit Line 2. The total length of the transfer station is 306.2m, the width of the standard section is 23m, the covering soil is 2.6m, and the buried depth of the station baseplate is 24.2m. The station transfers with the operating Line 2 in the form of T-type node transfer, as shown in Figure4. At this T-type node, the existing two-story underground station is on Line 2, the new three-story underground station is on Line 8, and the new station on Line 8 is close-attached undercrossing the existing operating station on Line 2.Figure 4
3D model diagram of Dongdalu transfer station.


## 4.2. Construction Safety Risk Assessment

### 4.2.1. Calculating Index Weight

As discussed above, 30 experts were consulted in the selecting of key risk factors. These 30 experts are invited to grade and judge each index of the risk assessment index system in Table5, and the judgment matrix of each index at all levels is established. Then the judgment matrix is input into the Matlab program based on AHP to determine whether it passes the consistency test and get the weight of each index.For example, the judgment matrix of the first-level index “existing subway station (A)” is established, as is shown in Table6.Table 6
Judgment matrix of the first-level index “Existing subway station (A).”
A1 A2 A3 A4 A5 A1 Station structure 1 1/3 1/2 3 2 A2 Track structure 3 1 2 5 4 A3 Power supply system 2 1/2 1 4 3 A4 Water supply and drainage system 1/3 1/5 1/4 1 1/2 A5 Ventilation and signal system 1/2 1/4 1/3 2 1CI = 0.0170, CR = 0.0152 are obtained by AHP. CR < 0.1 means that the consistency test has been passed, and the calculated weights of each second-level index under the first-level index “existing subway station (A)” are 0.1599, 0.4185, 0.2625, 0.0618, and 0.0973, respectively.Similarly, the weight of other second-level and third-level indexes can be determined. The calculation results are shown in Table7.Table 7
Index weight of construction safety risk assessment at all levels.
First-level index(Weight) Second-level index Weight of second-level index Third-level index Weight of third-level index A Existing subway station (0.5) A1 Station structure 0.1599 A12 0.0630 A13 0.1098 A15 0.3005 A16 0.5267 A2 Track structure 0.4185 A22 0.2000 A24 0.2000 A25 0.6000 A3 Power supply system 0.2625 A31 0.1667 A32 0.8333 A4 Water supply and drainage system 0.0618 A41 0.2000 A43 0.8000 A5 Ventilation and signal system 0.0973 A51 0.2389 A53 0.5495 A56 0.0650 A57 0.1466  B New subway station (0.5) B1 Construction monitoring of existing station 0.3543 B11 0.2295 B12 0.3305 B14 0.1040 B16 0.0444 B17 0.0444 B18 0.1557 B19 0.0684 B110 0.0231 B2 New space enclosing structure 0.1587 B21 0.1667 B23 0.8333 B3 Engineering dewatering 0.0676 B31 0.2000 B33 0.8000 B4 Foundation pit excavation 0.0448 B42 0.6667 B44 0.3333 B5 Surrounding rock and soil mass and surrounding environment 0.1036 B51 0.1556 B52 0.0623 B53 0.0623 B54 0.0997 B55 0.2375 B58 0.3523 B59 0.0304 B6 Construction technology management of main structure 0.2399 B61 0.3777 B62 0.0916 B63 0.0916 B65 0.1528 B66 0.2449 B67 0.0413 B7 Green construction 0.0312 B72 0.0533 B74 0.1351 B75 0.3371 B76 0.0829 B77 0.2205 B78 0.1351 B79 0.0362


### 4.2.2. Determining Matter Element to Be Assessed

In the comprehensive assessment, the assessed unit is an n-dimensional matter element, and its eigenvalues vary according to its influence. The 30 experts consulted during the selecting of key risk factors are invited to estimate the specific status of the indexes in the index system in Table5. For example, experts estimate that the absolute settlement value of the station structure (A12) of this project may be 4 mm based on the practical experience of similar projects, and the evaluation value of this index is 4. Similarly, the evaluation values of other indexes can be obtained, and the evaluation values are averaged arithmetically to get Rx as shown below.(19)
R
x
=
N
x
A
12
3.5
A
13
2.2
A
15
0.8
A
16
0.16
A
22
1.8
A
24
1.9
A
25
4.2
A
31
0.3
A
32
0.6
A
41
0.9
A
43
1.1
A
51
97
A
53
12
A
56
2.6
A
57
5.7
B
11
6.2
B
12
87.5
B
14
15
B
16
6
B
17
7
B
18
2.5
B
19
4
B
110
5
B
21
12
B
23
7.4
B
31
2
∗
10
-
5
B
33
0.8
B
42
8.8
B
44
26
B
51
8.4
B
52
7.3
B
53
7.5
B
54
8.1
B
55
24.2
B
58
7.1
B
59
0.9
B
61
9.1
B
62
0.81
B
63
8.2
B
65
8.1
B
66
7.2
B
67
8.3
B
72
5
.
1
B
74
67
29
B
75
293
B
76
7
B
77
2.1
B
78
12
B
79
15.8


### 4.2.3. Determining Matter Element Matrices of Classical Domain and Joint Domain

The determination of the classical domain is the basis of Fuzzy Matter Element Method. According to the extensibility of construction safety risk assessment of new subway station close-attached undercrossing the existing subway station, the risk grade of each index is divided into three grades, namely,No1, No2, and No3, which are qualitatively described as high risk, medium risk, and low risk. The classic domain matter element matrices Ro1, Ro2, and Ro3 and the joint domain matter element matrix Rp established based on Table 5 are shown in Table 8.Table 8
Assessment criteria of construction safety risk assessment.
Index Unit High risk (Ro1) Medium risk (Ro2) Low risk (Ro3) Joint domain (Rp) A12 mm 7~10 3~7 0~3 0~10 A13 mm 5~10 3~5 0~3 0~10 A15 mm 2~5 1~2 0~1 0~5 A16 mm 0.2~0.5 0.1~0.2 0~0.1 0~0.5 A22 mm 4~10 2~4 0~2 0~10 A24 mm 4~10 2~4 0~2 0~10 A25 mm 5~10 3~5 0~3 0~10 A31 h/times 2~24 0.5~2 0~0.5 0~24 A32 times/month 3~31 1~3 0~1 0~31 A41 h/times 5~24 2~5 0~2 0~24 A43 times/month 3~31 1~3 0~1 0~31 A51 % 0~60 60~90 90~100 0~100 A53 times/month 30~50 15~30 0~15 0~50 A56 % 7~20 3~7 0~3 0~10 A57 % 10~100 5~10 0~5 0~100 B11 times/month 10~31 5~10 0~5 0~31 B12 % 0~90 90~95 95~100 0~100 B14 m 20~50 10~20 0~10 0~50 B16 pieces 0~3 3~5 5~10 0~10 B17 pieces 0~3 3~5 5~10 0~10 B18 m 8~20 3~8 0~3 0~20 B19 m 5~10 3~5 0~3 0~10 B110 pieces 0~4 4~6 6~10 0~10 B21 m 15~30 10~15 0~10 0~30 B23 mm 50~80 25~50 0~25 0~80 B31 cm/s 10−4~10−2 10−6~10−4 10−8~10−6 10−4~10−2 B33 10000−1 1~2 1/2~1 0~1/2 0~2 B42 ∖ 0~6 6~8 8~10 0~10 B44 m 20~35 10~20 0~10 0~30 B51 ∖ 0~6 6~8 8~10 0~10 B52 ∖ 0~6 6~8 8~10 0~10 B53 ∖ 0~6 6~8 8~10 0~10 B54 ∖ 0~6 6~8 8~10 0~10 B55 m 16~30 10~16 0~10 0~30 B58 ∖ 0~6 6~8 8~10 0~10 B59 ∖ 0~0.7 0.7~1 1~2 0~2 B61 ∖ 0~6 6~8 8~10 0~10 B62 N/mm2 0~0.6 0.6~0.8 0.8~1 0~1 B63 ∖ 0~6 6~8 8~10 0~10 B65 ∖ 0~6 6~8 8~10 0~10 B66 ∖ 0~6 6~8 8~10 0~10 B67 ∖ 0~6 6~8 8~10 0~10 B72 thousand vehicles/h 5~10 3~5 0~3 0~10 B74 dB daytime:70~120 60~70 0~60 0~120 nighttime:55~100 45~55 0~45 0~100 B75 mg/L 400~600 250~400 0~250 0~600 B76 W/m2 11~20 10~11 0~10 0~20 B77 m 0~2 2~2.5 2.5~3.5 0~3.5 B78 % 0~20 20~30 30~100 0~100 B79 ten thousand person-time/month 0~10 10~20 20~30 0~30


### 4.2.4. Calculating the Correlation Degree to Determine the Risk Grade of Each Third-Level Index

The values of the matter elementRx are input into Excel program which has been compiled based on the FMEM, and the corresponding calculation results are shown in Table 9.Table 9
Correlation degree and risk grade of each third-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A12) -0.5 0.125 -0.125 Medium risk K j (A13) -0.56 -0.267 0.267 Low risk K j (A15) -0.6 -0.2 0.2 Low risk K j (A16) -0.2 0.4 -0.273 Medium risk K j (A22) -0.55 -0.1 0.1 Low risk K j (A24) -0.525 -0.05 0.05 Low risk K j (A25) -0.16 0.4 -0.222 Medium risk K j (A31) -0.85 -0.133 2 Low risk K j (A32) -0.8 -0.2 2 Low risk K j (A41) -0.82 -0.55 0.45 Low risk K j (A43) -0.633 0.05 -0.083 Medium risk K j (A51) -0.925 -0.7 0.3 Low risk K j (A53) -0.6 -0.2 0.2 Low risk K j (A56) -0.629 -0.133 0.133 Low risk K j (A57) -0.43 0.14 -0.109 Medium risk K j (B11) -0.38 0.24 -0.162 Medium risk K j (B12) 0.028 -0.167 -0.375 High risk K j (B14) -0.25 0.5 -0.25 Medium risk K j (B16) -0.429 -0.2 0.2 Low risk K j (B17) -0.571 -0.4 0.4 Low risk K j (B18) -0.688 -0.167 0.167 Low risk K j (B19) -0.2 0.5 -0.2 Medium risk K j (B110) -0.167 0.5 -0.167 Medium risk K j (B21) -0.2 0.4 -0.143 Medium risk K j (B23) -0.63 -0.26 0.26 Low risk K j (B31) -0.8 0.192 -0.487 Medium risk K j (B33) -0.2 0.4 -0.273 Medium risk K j (B42) -0.375 0.25 -0.167 Medium risk K j (B44) 0.4 -0.4 -0.64 High risk K j (B51) -0.6 -0.2 0.2 Low risk K j (B52) -0.325 0.35 -0.206 Medium risk K j (B53) -0.375 0.25 -0.167 Medium risk K j (B54) -0.525 -0.05 0.05 Low risk K j (B55) 0.414 -0.586 -0.71 High risk K j (B58) -0.275 0.45 -0.237 Medium risk K j (B59) -0.182 0.333 -0.1 Medium risk K j (B61) -0.775 -0.55 0.45 Low risk K j (B62) -0.525 -0.05 0.05 Low risk K j (B63) -0.55 -0.1 0.1 Low risk K j (B65) -0.525 -0.05 0.05 Low risk K j (B66) -0.3 0.4 -0.222 Medium risk K j (B67) -0.575 -0.15 0.15 Low risk K j (B72) 0.02 -0.02 -0.3 High risk K j (B74) -0.527 -0.056 0.239 Low risk K j (B75) -0.268 0.287 -0.128 Medium risk K j (B76) -0.364 -0.3 0.3 Low risk K j (B77) -0.067 0.2 -0.222 Medium risk K j (B78) 0.4 -0.4 -0.6 High risk K j (B79) -0.29 0.42 -0.228 Medium risk


### 4.2.5. Calculating the Comprehensive Correlation Degree and Determining the Risk Grade of the Project

The comprehensive correlation degree is calculated according to FMEM, and the comprehensive correlation degree and risk grade of each second-level index can be obtained as shown in Table10.Table 10
Comprehensive correlation degree and risk grade of each second-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A1 Station structure) -0.061 0.021 -0.010 Medium risk K j (A2 Track structure) -0.130 0.130 -0.088 Medium risk K j (A3 Power supply system) -0.188 -0.031 0.031 Low risk K j (A4 Water supply and drainage system) -0.041 -0.004 0.001 Low risk K j (A5 Ventilation and signal system) -0.064 -0.026 0.017 Low risk K j (B1 Construction monitoring of existing station) -0.097 0.016 -0.054 Medium risk K j (B2 New space enclosing structure) -0.089 -0.024 0.031 Low risk K j (B3 Engineering dewatering) -0.022 0.024 -0.021 Medium risk K j (B4 Foundation pit excavation) -0.005 0.001 -0.015 Medium risk K j (B5 Surrounding rock and soil mass and surrounding environment) -0.020 0.003 -0.025 Medium risk K j (B6 Construction technology management of main structure) -0.136 -0.033 0.034 Low risk K j (B7 Green construction) -0.005 0.002 -0.004 Medium riskCombining the correlation degree of each index and the corresponding index weight in Table7, the comprehensive correlation degree of each first-level index is obtained, as shown in Table 11.Table 11
Comprehensive correlation degree and risk grade of each first-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A New subway station) -0.484 0.048 -0.004 Medium risk K j (B Existing subway station) -0.374 -0.010 -0.054 Medium riskThe correlation degree of each index and the corresponding index weight in Table7 are input into formula (18) to obtain the comprehensive correlation degree of the matter element to be assessed, as shown in Table 12.Table 12
Comprehensive correlation degree of the matter element to be assessed.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (P0) -0.429 0.019 -0.029 Medium risk


## 4.2.1. Calculating Index Weight

As discussed above, 30 experts were consulted in the selecting of key risk factors. These 30 experts are invited to grade and judge each index of the risk assessment index system in Table5, and the judgment matrix of each index at all levels is established. Then the judgment matrix is input into the Matlab program based on AHP to determine whether it passes the consistency test and get the weight of each index.For example, the judgment matrix of the first-level index “existing subway station (A)” is established, as is shown in Table6.Table 6
Judgment matrix of the first-level index “Existing subway station (A).”
A1 A2 A3 A4 A5 A1 Station structure 1 1/3 1/2 3 2 A2 Track structure 3 1 2 5 4 A3 Power supply system 2 1/2 1 4 3 A4 Water supply and drainage system 1/3 1/5 1/4 1 1/2 A5 Ventilation and signal system 1/2 1/4 1/3 2 1CI = 0.0170, CR = 0.0152 are obtained by AHP. CR < 0.1 means that the consistency test has been passed, and the calculated weights of each second-level index under the first-level index “existing subway station (A)” are 0.1599, 0.4185, 0.2625, 0.0618, and 0.0973, respectively.Similarly, the weight of other second-level and third-level indexes can be determined. The calculation results are shown in Table7.Table 7
Index weight of construction safety risk assessment at all levels.
First-level index(Weight) Second-level index Weight of second-level index Third-level index Weight of third-level index A Existing subway station (0.5) A1 Station structure 0.1599 A12 0.0630 A13 0.1098 A15 0.3005 A16 0.5267 A2 Track structure 0.4185 A22 0.2000 A24 0.2000 A25 0.6000 A3 Power supply system 0.2625 A31 0.1667 A32 0.8333 A4 Water supply and drainage system 0.0618 A41 0.2000 A43 0.8000 A5 Ventilation and signal system 0.0973 A51 0.2389 A53 0.5495 A56 0.0650 A57 0.1466  B New subway station (0.5) B1 Construction monitoring of existing station 0.3543 B11 0.2295 B12 0.3305 B14 0.1040 B16 0.0444 B17 0.0444 B18 0.1557 B19 0.0684 B110 0.0231 B2 New space enclosing structure 0.1587 B21 0.1667 B23 0.8333 B3 Engineering dewatering 0.0676 B31 0.2000 B33 0.8000 B4 Foundation pit excavation 0.0448 B42 0.6667 B44 0.3333 B5 Surrounding rock and soil mass and surrounding environment 0.1036 B51 0.1556 B52 0.0623 B53 0.0623 B54 0.0997 B55 0.2375 B58 0.3523 B59 0.0304 B6 Construction technology management of main structure 0.2399 B61 0.3777 B62 0.0916 B63 0.0916 B65 0.1528 B66 0.2449 B67 0.0413 B7 Green construction 0.0312 B72 0.0533 B74 0.1351 B75 0.3371 B76 0.0829 B77 0.2205 B78 0.1351 B79 0.0362


## 4.2.2. Determining Matter Element to Be Assessed

In the comprehensive assessment, the assessed unit is an n-dimensional matter element, and its eigenvalues vary according to its influence. The 30 experts consulted during the selecting of key risk factors are invited to estimate the specific status of the indexes in the index system in Table5. For example, experts estimate that the absolute settlement value of the station structure (A12) of this project may be 4 mm based on the practical experience of similar projects, and the evaluation value of this index is 4. Similarly, the evaluation values of other indexes can be obtained, and the evaluation values are averaged arithmetically to get Rx as shown below.(19)
R
x
=
N
x
A
12
3.5
A
13
2.2
A
15
0.8
A
16
0.16
A
22
1.8
A
24
1.9
A
25
4.2
A
31
0.3
A
32
0.6
A
41
0.9
A
43
1.1
A
51
97
A
53
12
A
56
2.6
A
57
5.7
B
11
6.2
B
12
87.5
B
14
15
B
16
6
B
17
7
B
18
2.5
B
19
4
B
110
5
B
21
12
B
23
7.4
B
31
2
∗
10
-
5
B
33
0.8
B
42
8.8
B
44
26
B
51
8.4
B
52
7.3
B
53
7.5
B
54
8.1
B
55
24.2
B
58
7.1
B
59
0.9
B
61
9.1
B
62
0.81
B
63
8.2
B
65
8.1
B
66
7.2
B
67
8.3
B
72
5
.
1
B
74
67
29
B
75
293
B
76
7
B
77
2.1
B
78
12
B
79
15.8


## 4.2.3. Determining Matter Element Matrices of Classical Domain and Joint Domain

The determination of the classical domain is the basis of Fuzzy Matter Element Method. According to the extensibility of construction safety risk assessment of new subway station close-attached undercrossing the existing subway station, the risk grade of each index is divided into three grades, namely,No1, No2, and No3, which are qualitatively described as high risk, medium risk, and low risk. The classic domain matter element matrices Ro1, Ro2, and Ro3 and the joint domain matter element matrix Rp established based on Table 5 are shown in Table 8.Table 8
Assessment criteria of construction safety risk assessment.
Index Unit High risk (Ro1) Medium risk (Ro2) Low risk (Ro3) Joint domain (Rp) A12 mm 7~10 3~7 0~3 0~10 A13 mm 5~10 3~5 0~3 0~10 A15 mm 2~5 1~2 0~1 0~5 A16 mm 0.2~0.5 0.1~0.2 0~0.1 0~0.5 A22 mm 4~10 2~4 0~2 0~10 A24 mm 4~10 2~4 0~2 0~10 A25 mm 5~10 3~5 0~3 0~10 A31 h/times 2~24 0.5~2 0~0.5 0~24 A32 times/month 3~31 1~3 0~1 0~31 A41 h/times 5~24 2~5 0~2 0~24 A43 times/month 3~31 1~3 0~1 0~31 A51 % 0~60 60~90 90~100 0~100 A53 times/month 30~50 15~30 0~15 0~50 A56 % 7~20 3~7 0~3 0~10 A57 % 10~100 5~10 0~5 0~100 B11 times/month 10~31 5~10 0~5 0~31 B12 % 0~90 90~95 95~100 0~100 B14 m 20~50 10~20 0~10 0~50 B16 pieces 0~3 3~5 5~10 0~10 B17 pieces 0~3 3~5 5~10 0~10 B18 m 8~20 3~8 0~3 0~20 B19 m 5~10 3~5 0~3 0~10 B110 pieces 0~4 4~6 6~10 0~10 B21 m 15~30 10~15 0~10 0~30 B23 mm 50~80 25~50 0~25 0~80 B31 cm/s 10−4~10−2 10−6~10−4 10−8~10−6 10−4~10−2 B33 10000−1 1~2 1/2~1 0~1/2 0~2 B42 ∖ 0~6 6~8 8~10 0~10 B44 m 20~35 10~20 0~10 0~30 B51 ∖ 0~6 6~8 8~10 0~10 B52 ∖ 0~6 6~8 8~10 0~10 B53 ∖ 0~6 6~8 8~10 0~10 B54 ∖ 0~6 6~8 8~10 0~10 B55 m 16~30 10~16 0~10 0~30 B58 ∖ 0~6 6~8 8~10 0~10 B59 ∖ 0~0.7 0.7~1 1~2 0~2 B61 ∖ 0~6 6~8 8~10 0~10 B62 N/mm2 0~0.6 0.6~0.8 0.8~1 0~1 B63 ∖ 0~6 6~8 8~10 0~10 B65 ∖ 0~6 6~8 8~10 0~10 B66 ∖ 0~6 6~8 8~10 0~10 B67 ∖ 0~6 6~8 8~10 0~10 B72 thousand vehicles/h 5~10 3~5 0~3 0~10 B74 dB daytime:70~120 60~70 0~60 0~120 nighttime:55~100 45~55 0~45 0~100 B75 mg/L 400~600 250~400 0~250 0~600 B76 W/m2 11~20 10~11 0~10 0~20 B77 m 0~2 2~2.5 2.5~3.5 0~3.5 B78 % 0~20 20~30 30~100 0~100 B79 ten thousand person-time/month 0~10 10~20 20~30 0~30


## 4.2.4. Calculating the Correlation Degree to Determine the Risk Grade of Each Third-Level Index

The values of the matter elementRx are input into Excel program which has been compiled based on the FMEM, and the corresponding calculation results are shown in Table 9.Table 9
Correlation degree and risk grade of each third-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A12) -0.5 0.125 -0.125 Medium risk K j (A13) -0.56 -0.267 0.267 Low risk K j (A15) -0.6 -0.2 0.2 Low risk K j (A16) -0.2 0.4 -0.273 Medium risk K j (A22) -0.55 -0.1 0.1 Low risk K j (A24) -0.525 -0.05 0.05 Low risk K j (A25) -0.16 0.4 -0.222 Medium risk K j (A31) -0.85 -0.133 2 Low risk K j (A32) -0.8 -0.2 2 Low risk K j (A41) -0.82 -0.55 0.45 Low risk K j (A43) -0.633 0.05 -0.083 Medium risk K j (A51) -0.925 -0.7 0.3 Low risk K j (A53) -0.6 -0.2 0.2 Low risk K j (A56) -0.629 -0.133 0.133 Low risk K j (A57) -0.43 0.14 -0.109 Medium risk K j (B11) -0.38 0.24 -0.162 Medium risk K j (B12) 0.028 -0.167 -0.375 High risk K j (B14) -0.25 0.5 -0.25 Medium risk K j (B16) -0.429 -0.2 0.2 Low risk K j (B17) -0.571 -0.4 0.4 Low risk K j (B18) -0.688 -0.167 0.167 Low risk K j (B19) -0.2 0.5 -0.2 Medium risk K j (B110) -0.167 0.5 -0.167 Medium risk K j (B21) -0.2 0.4 -0.143 Medium risk K j (B23) -0.63 -0.26 0.26 Low risk K j (B31) -0.8 0.192 -0.487 Medium risk K j (B33) -0.2 0.4 -0.273 Medium risk K j (B42) -0.375 0.25 -0.167 Medium risk K j (B44) 0.4 -0.4 -0.64 High risk K j (B51) -0.6 -0.2 0.2 Low risk K j (B52) -0.325 0.35 -0.206 Medium risk K j (B53) -0.375 0.25 -0.167 Medium risk K j (B54) -0.525 -0.05 0.05 Low risk K j (B55) 0.414 -0.586 -0.71 High risk K j (B58) -0.275 0.45 -0.237 Medium risk K j (B59) -0.182 0.333 -0.1 Medium risk K j (B61) -0.775 -0.55 0.45 Low risk K j (B62) -0.525 -0.05 0.05 Low risk K j (B63) -0.55 -0.1 0.1 Low risk K j (B65) -0.525 -0.05 0.05 Low risk K j (B66) -0.3 0.4 -0.222 Medium risk K j (B67) -0.575 -0.15 0.15 Low risk K j (B72) 0.02 -0.02 -0.3 High risk K j (B74) -0.527 -0.056 0.239 Low risk K j (B75) -0.268 0.287 -0.128 Medium risk K j (B76) -0.364 -0.3 0.3 Low risk K j (B77) -0.067 0.2 -0.222 Medium risk K j (B78) 0.4 -0.4 -0.6 High risk K j (B79) -0.29 0.42 -0.228 Medium risk


## 4.2.5. Calculating the Comprehensive Correlation Degree and Determining the Risk Grade of the Project

The comprehensive correlation degree is calculated according to FMEM, and the comprehensive correlation degree and risk grade of each second-level index can be obtained as shown in Table10.Table 10
Comprehensive correlation degree and risk grade of each second-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A1 Station structure) -0.061 0.021 -0.010 Medium risk K j (A2 Track structure) -0.130 0.130 -0.088 Medium risk K j (A3 Power supply system) -0.188 -0.031 0.031 Low risk K j (A4 Water supply and drainage system) -0.041 -0.004 0.001 Low risk K j (A5 Ventilation and signal system) -0.064 -0.026 0.017 Low risk K j (B1 Construction monitoring of existing station) -0.097 0.016 -0.054 Medium risk K j (B2 New space enclosing structure) -0.089 -0.024 0.031 Low risk K j (B3 Engineering dewatering) -0.022 0.024 -0.021 Medium risk K j (B4 Foundation pit excavation) -0.005 0.001 -0.015 Medium risk K j (B5 Surrounding rock and soil mass and surrounding environment) -0.020 0.003 -0.025 Medium risk K j (B6 Construction technology management of main structure) -0.136 -0.033 0.034 Low risk K j (B7 Green construction) -0.005 0.002 -0.004 Medium riskCombining the correlation degree of each index and the corresponding index weight in Table7, the comprehensive correlation degree of each first-level index is obtained, as shown in Table 11.Table 11
Comprehensive correlation degree and risk grade of each first-level index.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (A New subway station) -0.484 0.048 -0.004 Medium risk K j (B Existing subway station) -0.374 -0.010 -0.054 Medium riskThe correlation degree of each index and the corresponding index weight in Table7 are input into formula (18) to obtain the comprehensive correlation degree of the matter element to be assessed, as shown in Table 12.Table 12
Comprehensive correlation degree of the matter element to be assessed.
Correlation degree R o 1 R o 2 R o 3 Risk grade K j (P0) -0.429 0.019 -0.029 Medium risk


## 4.3. Result Analysis

### 4.3.1. Result Analysis of Existing Subway Station

The risk grade of the first-level index “existing subway station” is medium risk, which indicates that the close-attached undercrossing project is easy to affect the existing station. According to Table10, in the second-level indexes, the risk grade of “station structure” and “track structure” is medium risk, while the risk grade of “power supply system,” “water supply and drainage system,” and “ventilation and signal system” is low risk.Table9 shows that, in the third-level indexes, the risk grade of “absolute settlement value of station structure,” “crack width of station structure,” and “void amount of track-bed” is medium risk. To guard against these risk factors, it is necessary to have strict control of each working procedure that may cause settlement in the construction process. As the structural settlement and deformation caused by construction are the main factors affecting the deformation of station structure and track structure, good construction measures to control the value of station settlement within the allowable range can ensure the normal operation of existing operating station.The results of risk assessment are compared with the actual engineering data. The risk assessment results of the qualitative indexes in the third-level indexes are consistent with the actual engineering measurement results. The risk assessment results of quantitative indexes and the actual engineering measurement results are compared and analyzed as shown in Table13. There are 15 quantitative indexes in Table 13. Among them, the risk grade corresponding to the risk assessment results of 13 indexes is consistent with the risk grade corresponding to the actual engineering measurement results, and 2 indexes are inconsistent. For instance, the risk grade corresponding to the expert assessment results of “pipeline damage” and “equipment pending repair rate” is medium risk, while the risk grade corresponding to the actual engineering measurement results is low risk, which indicates that the construction unit has strengthened the management of pipelines and equipment on site according to the result of risk assessment and has taken effective control measures to reduce the risk grade of these indexes.Table 13
Comparison of the third-level index data of “existing subway station.”
Third-level index Risk assessment result Corresponding risk grade of risk assessment Actual engineering measurement result Corresponding risk grade of actual measurement Consistency A 1 2 Absolute settlement value of station structure (mm) 3.5 Medium risk 5.5 Medium risk Yes A 1 3 Horizontal displacement of the station structure (mm) 2.2 Low risk 2.9 Low risk Yes A 1 5 Differential settlement of station structure deformation joint (mm) 0.8 Low risk 0.7 Low risk Yes A 1 6 Crack width of station structure (mm) 0.16 Medium risk 0.1 Medium risk Yes A 22 Orbital height difference (Vector value) (mm) 1.8 Low risk 0.5 Low risk Yes A 24 Transverse elevation difference of track (mm) 1.9 Low risk 1.7 Low risk Yes A 25 Void amount of track-bed (mm) 4.2 Medium risk 3.0 Medium risk Yes A 31 Area power outage renovation (h/times) 0.3 Low risk 0.2 Low risk Yes A 32 Circuit failure (times/month) 0.6 Low risk 0.5 Low risk Yes A 41 System blocking (h/times) 0.9 Low risk 0.6 Low risk Yes A 43 Pipeline damage (times/month) 1.1 Medium risk 0.7 Low risk No A 51 Equipment sampling pass rate (%) 97 Low risk 96 Low risk Yes A 53 Equipment failures (times/month) 12 Low risk 10 Low risk Yes A 56 Equipment renewal rate (%) 2.6 Low risk 2.1 Low risk Yes A 57 Equipment pending repair rate (%) 5.7 Medium risk 3.6 Low risk No


### 4.3.2. Result Analysis of New Subway Station

The risk grade of first-level index “new subway station” is medium risk, which indicates that the operation of existing subway station will have a great impact on the construction of new subway station. From Table10, it can be concluded that, in the second-level indexes, the risk grade of “construction monitoring of existing station,” “engineering dewatering,” “foundation pit excavation,” “surrounding rock and soil mass and surrounding environment,” and “green construction” is medium risk, while the risk grade of “new space enclosing structure” and “construction technology management of main structure” is low risk.Table9 shows that, in the third-level indexes, the risk grade of “accuracy rate of feedback real-time data,” “depth of foundation pit,” “buried depth,” “ground traffic control,” and “construction trash control” is high risk. In order to prevent the occurrence of these risk factors, it is necessary to strengthen the protective work and monitoring of the station structure and line of the existing station and analyze the monitoring data in time. For the alarm data issued by the monitoring device in the construction process, the operator should respond in time, report to the superior department immediately, and take effective control measures. Then, it is necessary to pay attention to the monitoring of various indicators in the construction process of new subway station and to timely analysis of monitoring data to ensure the accuracy rate of data feedback. At the same time, the construction unit should reasonably guide the ground transportation, and the trash generated in the construction process should be transported to the designated location in time, and protective measures should be taken in the transportation process to prevent dust, soil pollution, and other problems.The results of risk assessment are compared with the actual engineering data. The risk assessment results of the qualitative indexes in the third-level indexes are consistent with the actual engineering measurement results. The comparative analysis of the risk assessment results of the quantitative indexes and the actual engineering measurement results is shown in Table14. There are 23 quantitative indexes in Table 14. Among them, the risk grade corresponding to the risk assessment results of 19 indexes is consistent with the risk grade corresponding to the actual engineering measurement results, and 4 indexes are inconsistent.Table 14
Comparison of the third-level index data of “new subway station.”
Third-level index Risk assessment result Corresponding risk grade of risk assessment Actual engineering measurement result Corresponding risk grade of actual measurement Consistency B 1 1 Information monitoring equipment failures (times/month) 6.2 Medium risk 5.3 Medium risk Yes B 1 2 Accuracy rate of feedback real-time data (%) 87.5 High risk 93 Medium risk No B 1 4 Horizontal and vertical displacement of retaining pile (wall) and top of slop (m) 15 Medium risk 12 Medium risk Yes B 1 6 Axial force of struts (pieces) 6 Low risk 8 Low risk Yes B 1 7 Anchor tension (pieces) 7 Low risk 8 Low risk Yes B 1 8 Ground settlement (m) 2.5 Low risk 2 Low risk Yes B 1 9 Clearance convergence of shaft support structure (m) 4 Medium risk 5 Medium risk Yes B 1 10 Monitoring hole of groundwater level (pieces) 5 Medium risk 6 Low risk No B 2 1 Survey spot arrangement spacing (m) 12 Medium risk 9.5 Low risk No B 23 Lateral displacement of space enclosing structure (mm) 7.4 Low risk 4.9 Low risk Yes B 3 1 Stratum filtering flow (permeability coefficient cm/s) 2×105 Medium risk 3×105 Medium risk Yes B 33 Sand content control (10000−1) 0.8 Medium risk 0.9 Medium risk Yes B 44 Depth of foundation pit (m) 26 High risk 25.5 High risk Yes B 55 Buried depth (m) 24.2 High risk 24.2 High risk Yes B 59 Impact of surrounding buildings on construction 0.9 Medium risk 0.9 Medium risk Yes B 62 Stiffness of initial support (N/mm2) 0.81 Low risk 0.92 Low risk Yes B 72 Ground traffic control (thousand vehicles/h) 5.1 High risk 6.2 High risk Yes B 74 Noise control (dB) daytime67 Low risk daytime59 Low risk Yes night29 night 30 B 75 Sewage control (mg/L) 293 Medium risk 262 Medium risk Yes B 76 Light pollution control (W/m2) 7 Low risk 6.2 Low risk Yes B 77 Dust control (m) 2.1 Medium risk 2.5 Low risk No B 78 Construction trash control (%) 12 High risk 18 High risk Yes B 79 Impact on business climate (ten thousand person-time/month) 15.8 Medium risk 12 Medium risk YesFor instance, the risk grade corresponding to risk assessment result of “accuracy rate of feedback real-time data” is high risk, while the risk grade corresponding to the actual measurement result is medium risk. The risk grade corresponding to risk assessment result of “monitoring hole of groundwater level,” “survey spot arrangement spacing,” and “dust control” is medium risk, while the risk grade corresponding to the actual measurement result is low risk. It shows that the results of the risk assessment model have a certain warning effect on the construction unit. The construction unit has strengthened the feedback of real-time data on site, strengthened the layout of groundwater level monitoring holes, the spacing control of survey spot arrangement, and dust control, and adopted effective prevention and control measures to reduce the risk grade of these indexes.In summary, the risk grade of the close-attached undercrossing project is medium risk. The assessment result is basically consistent with the actual engineering situation, which indicates that the construction safety risk assessment model has certain practicability.


## 4.3.1. Result Analysis of Existing Subway Station

The risk grade of the first-level index “existing subway station” is medium risk, which indicates that the close-attached undercrossing project is easy to affect the existing station. According to Table10, in the second-level indexes, the risk grade of “station structure” and “track structure” is medium risk, while the risk grade of “power supply system,” “water supply and drainage system,” and “ventilation and signal system” is low risk.Table9 shows that, in the third-level indexes, the risk grade of “absolute settlement value of station structure,” “crack width of station structure,” and “void amount of track-bed” is medium risk. To guard against these risk factors, it is necessary to have strict control of each working procedure that may cause settlement in the construction process. As the structural settlement and deformation caused by construction are the main factors affecting the deformation of station structure and track structure, good construction measures to control the value of station settlement within the allowable range can ensure the normal operation of existing operating station.The results of risk assessment are compared with the actual engineering data. The risk assessment results of the qualitative indexes in the third-level indexes are consistent with the actual engineering measurement results. The risk assessment results of quantitative indexes and the actual engineering measurement results are compared and analyzed as shown in Table13. There are 15 quantitative indexes in Table 13. Among them, the risk grade corresponding to the risk assessment results of 13 indexes is consistent with the risk grade corresponding to the actual engineering measurement results, and 2 indexes are inconsistent. For instance, the risk grade corresponding to the expert assessment results of “pipeline damage” and “equipment pending repair rate” is medium risk, while the risk grade corresponding to the actual engineering measurement results is low risk, which indicates that the construction unit has strengthened the management of pipelines and equipment on site according to the result of risk assessment and has taken effective control measures to reduce the risk grade of these indexes.Table 13
Comparison of the third-level index data of “existing subway station.”
Third-level index Risk assessment result Corresponding risk grade of risk assessment Actual engineering measurement result Corresponding risk grade of actual measurement Consistency A 1 2 Absolute settlement value of station structure (mm) 3.5 Medium risk 5.5 Medium risk Yes A 1 3 Horizontal displacement of the station structure (mm) 2.2 Low risk 2.9 Low risk Yes A 1 5 Differential settlement of station structure deformation joint (mm) 0.8 Low risk 0.7 Low risk Yes A 1 6 Crack width of station structure (mm) 0.16 Medium risk 0.1 Medium risk Yes A 22 Orbital height difference (Vector value) (mm) 1.8 Low risk 0.5 Low risk Yes A 24 Transverse elevation difference of track (mm) 1.9 Low risk 1.7 Low risk Yes A 25 Void amount of track-bed (mm) 4.2 Medium risk 3.0 Medium risk Yes A 31 Area power outage renovation (h/times) 0.3 Low risk 0.2 Low risk Yes A 32 Circuit failure (times/month) 0.6 Low risk 0.5 Low risk Yes A 41 System blocking (h/times) 0.9 Low risk 0.6 Low risk Yes A 43 Pipeline damage (times/month) 1.1 Medium risk 0.7 Low risk No A 51 Equipment sampling pass rate (%) 97 Low risk 96 Low risk Yes A 53 Equipment failures (times/month) 12 Low risk 10 Low risk Yes A 56 Equipment renewal rate (%) 2.6 Low risk 2.1 Low risk Yes A 57 Equipment pending repair rate (%) 5.7 Medium risk 3.6 Low risk No


## 4.3.2. Result Analysis of New Subway Station

The risk grade of first-level index “new subway station” is medium risk, which indicates that the operation of existing subway station will have a great impact on the construction of new subway station. From Table10, it can be concluded that, in the second-level indexes, the risk grade of “construction monitoring of existing station,” “engineering dewatering,” “foundation pit excavation,” “surrounding rock and soil mass and surrounding environment,” and “green construction” is medium risk, while the risk grade of “new space enclosing structure” and “construction technology management of main structure” is low risk.Table9 shows that, in the third-level indexes, the risk grade of “accuracy rate of feedback real-time data,” “depth of foundation pit,” “buried depth,” “ground traffic control,” and “construction trash control” is high risk. In order to prevent the occurrence of these risk factors, it is necessary to strengthen the protective work and monitoring of the station structure and line of the existing station and analyze the monitoring data in time. For the alarm data issued by the monitoring device in the construction process, the operator should respond in time, report to the superior department immediately, and take effective control measures. Then, it is necessary to pay attention to the monitoring of various indicators in the construction process of new subway station and to timely analysis of monitoring data to ensure the accuracy rate of data feedback. At the same time, the construction unit should reasonably guide the ground transportation, and the trash generated in the construction process should be transported to the designated location in time, and protective measures should be taken in the transportation process to prevent dust, soil pollution, and other problems.The results of risk assessment are compared with the actual engineering data. The risk assessment results of the qualitative indexes in the third-level indexes are consistent with the actual engineering measurement results. The comparative analysis of the risk assessment results of the quantitative indexes and the actual engineering measurement results is shown in Table14. There are 23 quantitative indexes in Table 14. Among them, the risk grade corresponding to the risk assessment results of 19 indexes is consistent with the risk grade corresponding to the actual engineering measurement results, and 4 indexes are inconsistent.Table 14
Comparison of the third-level index data of “new subway station.”
Third-level index Risk assessment result Corresponding risk grade of risk assessment Actual engineering measurement result Corresponding risk grade of actual measurement Consistency B 1 1 Information monitoring equipment failures (times/month) 6.2 Medium risk 5.3 Medium risk Yes B 1 2 Accuracy rate of feedback real-time data (%) 87.5 High risk 93 Medium risk No B 1 4 Horizontal and vertical displacement of retaining pile (wall) and top of slop (m) 15 Medium risk 12 Medium risk Yes B 1 6 Axial force of struts (pieces) 6 Low risk 8 Low risk Yes B 1 7 Anchor tension (pieces) 7 Low risk 8 Low risk Yes B 1 8 Ground settlement (m) 2.5 Low risk 2 Low risk Yes B 1 9 Clearance convergence of shaft support structure (m) 4 Medium risk 5 Medium risk Yes B 1 10 Monitoring hole of groundwater level (pieces) 5 Medium risk 6 Low risk No B 2 1 Survey spot arrangement spacing (m) 12 Medium risk 9.5 Low risk No B 23 Lateral displacement of space enclosing structure (mm) 7.4 Low risk 4.9 Low risk Yes B 3 1 Stratum filtering flow (permeability coefficient cm/s) 2×105 Medium risk 3×105 Medium risk Yes B 33 Sand content control (10000−1) 0.8 Medium risk 0.9 Medium risk Yes B 44 Depth of foundation pit (m) 26 High risk 25.5 High risk Yes B 55 Buried depth (m) 24.2 High risk 24.2 High risk Yes B 59 Impact of surrounding buildings on construction 0.9 Medium risk 0.9 Medium risk Yes B 62 Stiffness of initial support (N/mm2) 0.81 Low risk 0.92 Low risk Yes B 72 Ground traffic control (thousand vehicles/h) 5.1 High risk 6.2 High risk Yes B 74 Noise control (dB) daytime67 Low risk daytime59 Low risk Yes night29 night 30 B 75 Sewage control (mg/L) 293 Medium risk 262 Medium risk Yes B 76 Light pollution control (W/m2) 7 Low risk 6.2 Low risk Yes B 77 Dust control (m) 2.1 Medium risk 2.5 Low risk No B 78 Construction trash control (%) 12 High risk 18 High risk Yes B 79 Impact on business climate (ten thousand person-time/month) 15.8 Medium risk 12 Medium risk YesFor instance, the risk grade corresponding to risk assessment result of “accuracy rate of feedback real-time data” is high risk, while the risk grade corresponding to the actual measurement result is medium risk. The risk grade corresponding to risk assessment result of “monitoring hole of groundwater level,” “survey spot arrangement spacing,” and “dust control” is medium risk, while the risk grade corresponding to the actual measurement result is low risk. It shows that the results of the risk assessment model have a certain warning effect on the construction unit. The construction unit has strengthened the feedback of real-time data on site, strengthened the layout of groundwater level monitoring holes, the spacing control of survey spot arrangement, and dust control, and adopted effective prevention and control measures to reduce the risk grade of these indexes.In summary, the risk grade of the close-attached undercrossing project is medium risk. The assessment result is basically consistent with the actual engineering situation, which indicates that the construction safety risk assessment model has certain practicability.


## 5. Conclusion

In order to solve the problem of safety risk assessment in the construction of new subway station close-attached undercrossing the existing station, this paper considers the two objectives of ensuring the smooth operation of existing subway station and the safe construction of new subway station and uses Delphi Method and Entropy Weight Method to select the list of key risk factors from the list of preliminary risk factors. According to the list of key risk factors, the construction safety risk assessment index system is established. Based on the assessment index system, a construction safety risk assessment model is established by using AHP and FMEM. The assessment result of the model is basically in line with the actual situation through case analysis, which shows that the model has good practicability.In this paper, Delphi Method and Entropy Weight Method are used to select key risk factors, AHP and FMEM are applied to assess construction safety risk, and a new construction safety risk assessment model for subway crossing projects is proposed, which provides a theoretical basis for the construction safety risk assessment of new subway station close-attached undercrossing the existing operating station.



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*Source: 3215219-2019-07-18.xml*