Risk Control Analysis of Metro Construction Based on Complex System Brittleness Theory

Nie TingTing* Postgraduate Institute of architecture and environment, Sichuan University, Chengdu 610500,China;e-mail:[email protected]

Xing HuiGe Associate Professor Supported by the National Natural Science Youth Fund (51209156) Institute of architecture and environment, Sichuan University, Chengdu 610500, China; e-mail: [email protected]

ABSTRACT Taking the characteristics of the complex system brittleness theory and metro construction into account, the metro construction system was divided into four subsystems: design, technology, environment and management subsystems, so as to analyze the corresponding brittleness risk. This paper made an analysis on the collapse accident which was the typical ground subsidence during the metro construction, and calculated the brittleness degree of various brittleness by means of quantitative analysis on the brittleness in different security level, thus having determined the scope of each grade’s brittle correlation in the safety pre-evaluation for the metro construction. In addition, taking a certain metro project as an example, this paper had verified the complex system brittleness theory by determining the risk level and presenting the evaluation results, and specific measures had also been proposed. Finally, the risk control model for metro construction system had been established, and the risk control strategies for the four subsystems, i.e. design, technology, environment and management subsystems, had also been listed out, which could improve the risk control system of metro construction to a certain extent. KEYWORDS: Construction risk; Metro; Brittleness risk;Complex system; Brittle safety.

FOREWORD As one green facility, the metro is very popular because of its diversified advantages. For instance, it is convenient, saving floor space and energy (according to statistics of Minnesota of the United States, the energy in refrigeration and heating can be saved up to 50%+), reducing the pollution, and conducive to the ecological balance of the city [1]. However, in the construction, due to the geological conditions and the unforeseeable risks, the existing building structures around, management technology and other factors, in recent ten years, the domestic construction accidents occur frequently, causing many serious pavement settlements and collapses.

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The traditional metro construction risk control is mainly based on the qualitative analysis, such as identifying the risk factors by means of Delphi method, check table, reason-result analysis, etc., thus making construction personnel understand the dangers they face. However, as the metro belongs to the underground project, the construction is influenced by various factors, such as hydro-geological conditions, weather conditions and mechanical equipment, etc.. Therefore, the qualitative analysis could not show the loss value and the occurrence of loss when the risk is in face [2]-[5]. Targeting at this problem, the brittleness theory of complex system is employed to analyze the brittleness risk in underground construction as well as the accident factors leading to brittle settlement of pavement combined with the analysis on pavement settlement accidents. Furthermore, the metro construction risk control model has been established according to the brittleness factors for construction and the four subsystems, i.e. the design, technology, environment and management, thus obtaining a valid threshold for the brittle link and determining the safety level of metro construction. In the end, the risk control strategies have been made for the four subsystems, namely, design, technology, environment and management, reducing the brittle connection degree of the risk in metro construction and providing guidance for metro construction so as to reduce the brittleness events, thus then being able to prevent the risk or reduce the risk degree. At present, complex system brittleness theory has been applied in many fields, such as coal mine production [6], power grid system [7], project combination [8], and building construction [9], etc..

COMPLEX SYSTEM BRITTLENESS ANALYSIS Metro is a complex engineering consisting of design, technology, environment and management systems. And the inherent characteristic of complex system is brittleness. Once a system crashes, other subsystems will be interfered, thus leading to the collapse of the whole system. The association between each system is named as the brittleness; when a brittle source is triggered, one or more brittle recipients will be resulted in, the process of which is called the brittle process. The first collapsed subsystem is called the brittle source, for instance, because of the improper supporting structure in the working well, the pit collapse and landslide will result in surface subsidence, during which, the brittle source is the improper supporting structure in working well. In the case of the collapse of one subsystem leading other subsystems to collapse, the other subsystems are called brittle recipients. For instance, the improper ground treatment will result in water or mortar leakage when the shield is lead to tunnel, which may cause sudden inflow of water and sand, resulting into large scale surface subsidence, and even the collapse of the working well, burying the shield machine. During this process, the brittle recipient is the shield. The brittle source corresponds to one or more brittle recipients, the relationship of which is shown in the following Fig.1.

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Brittle recipient1

Brittle recipient2

…… Brittle source Brittle recipient Brittle source

Brittle recipient n

One to one relationship One to many relationship

Figure 1: The relationship between the brittle source and brittle recipients

Complex system brittleness theory Complex system brittleness theory is mainly to study the mechanism of the collapse of complex system, and to prevent the occurrence of brittleness risk. Brittleness risk can be divided into internal and external factors, the former refers to the brittleness factors interaction between the subsystems due to mutual influence, and the latter refers to the brittle factors and brittle events imposed upon the system by the external environment. This current paper mainly analyzes the internal factors of complex system. The brittle structure model of complex system, shown in Fig.2, shows the hierarchy and brittleness relationship in the complex system. The bottom is the brittleness factor, which leads to the security event. Under the influence of the brittle connection, the security event causes the collapse of subsystem, and the whole subsystems make up of the entire brittle system. S

uper Complex system

structure

Subsystem 1 Subsystem m …

Security event 1 B Security event 2 … Security event m ottom structure

Brittle factor1 Brittle factor2 … Brittle factor m

Figure 2: Complex system brittleness structure model Vol. 21 [2016], Bund. 25 9800

Brittle correlation function Risk control of construction projects can be carried out through the evaluation of the safety level from a macro point of view, determining the extent of the brittleness in the important indicators of brittleness, thus providing the basis and ideas for risk control. Brittle correlation H (S) refers to the degree of association between the brittleness indices when the system is in normal operation (without the interference of external factors). The greater degree of brittle relation, the higher the risk of complex system; and the smaller the degree, the more stable the complex system. The brittle correlation function is shown as:

H (S) = f (x , x , x ,x ) 1 2 3 4

= (1− x )[w (1− x ) + w (1− x ) + w (1− x )] 1 2 2 3 3 4 4 (1)

In the formula (1), H(S) [0,1] ,x respectively indicates the quantitative value of management, w design, technology, environment,∈ i=1,2,3,4; And j means the weights of people, machine and environment in the internal brittleness correlation degree of the complex system, j=2,3,4 and w + w + w =1 2 3 4 During the construction of the metro system, because of its inherent defects such as tough construction environment and complicated construction technology, the brittleness is an inherent property of complex systems, which might make one small trigger of brittle factor result into the collapse of the subsystem, or even domino effect among various brittle factors, security events, and subsystems, thus making the whole complex system collapse. We should first collect specific information about a metro project and determine the unstable brittle factors, which are then detected and strictly controlled. By means of limiting the inspiration of the brittleness factors, effective measures should be adopted to control brittle connection degree or to reduce the influence of the brittleness risk, which plays an important role in controlling metro construction risk.

ESTABLISHMENT OF RISK INDEX SYSTEM FOR METRO CONSTRUCTION

Project overview The Chengdu Metro Line 4 initiates to integrate the Guanghua Park Station, which is located on the west side of the green belt in the intersection of Guanghua Avenue and Tiaobao Street, and laid out on both sides of the Guanghua Avenue. The station is the underground island type platform station with double deck 11 meters, the total length of which is 481.8m, and the width of standard section 19.9m, the height of standard station 13.13m. The overall terrain of Guanghua Park Station is relatively flat, with intensive housing distribution. The landform belongs to Ⅰ-degree terrace of alluvial fan plain of Minjiang River, which is the erosion accumulation landform, with open and flat terrain. Vol. 21 [2016], Bund. 25 9801

Establishing risk index system Since the collapse accidents accounts for the largest proportion in the risk accidents of metro construction [10], this paper takes the collapse accident as an example for the brittleness analysis. Based on the major domestic and foreign land subsidence collapses in recent years, the risk cause, as well as the main influencing factors in the actual metro construction, this paper employs the risk identification method and failure tree method, and lists the brittle factors of 4 subsystems, i.e. the design, technology, environment and management subsystem, which is shown in table 1. According to The Safety Assessment Standard of Metro Engineering Construction[11], the 12 brittleness indexes of construction risk factors are determined as in the following table 1, among which, I, II, III and IV indicate low risk, moderate risk, higher risk and highest risk respectively. Designing subsystem. Design Changes: according to the size of the change, design changes can be divided into three categories, namely general changes, important changes and major changes. In the actual construction, the design changes are due to the uncertainty of the metro project itself; Structure Form: adopting the symmetrical structure, especially in the earthquake prone metro station, will be helpful to improve the seismic performance. Settlement seam could be employed if the symmetrical structure is not used; Construction Organization Design: the project is guided by the specific and reasonable construction organization design, so as to ensure the quality of construction. The construction personnel possess the strong consciousness of safe production and civilized construction. Technology subsystem. Technical Proficiency: the technical proficiency of first line staff directly affect the construction safety, especially special post personnel must be trained before beginning to work. Construction Method: the construction method of the metro is influenced by the nature and the scale of the project, the hydro-geological conditions, underground pipelines and the surrounding buildings and other factors. Safety and Protective Measures: before the work, the safety technical aspects shall be disclosed. For those projects with high risk, special construction plan should be formulated or the experts should be invited to evaluate the project. Environment subsystem. Geological Conditions: according to the geological survey report, the geological risk list and response plan should be prepared, preventing leakage and water inrush. Underground Pipelines: the demolition, relocation, suspension and recovery of the underground pipeline caused by the construction should be verified whether they are in line with the relevant provisions .In addition, it is necessary to decide whether the impact of the underground pipeline settlement or the joint deformation is within the scope of deformation. Surrounding Buildings: the construction of metro projects may lead to the settlement or the tilt of surrounding buildings, damaging its overall structure and load-bearing components. Management subsystem. Management Personnel Qualifications: whether the personnel engaged in important management, such as project manager and safety management personnel, have passed through the pre-assessment, training and work experience evaluation, will directly impact on the construction. Safety Production System: the core idea is “safety and prevention are top priorities”. The safety system should be established and improved, standardizing various processes and strengthening the supervision and inspection mechanism. The Safety Management of Construction Site: the following should be attached importance to: hazard identification, safety technical disclosure, safety hazards investigation, construction site personnel management, equipment management, the mechanical and electrical products management, explosive and chemical hazards management, and lightning protection management. Vol. 21 [2016], Bund. 25 9802

Table 1: Brittleness Index of Construction Risk Subsystem Brittleness factor I Ⅱ Ⅲ Ⅳ Design change X1 (relative index) 9 7 5 3 design Structure form X2 8 6 4 2 Construction organization designX3 90 75 60 40 Technical proficiency X4 9 7 5 3 technology Construction method X5 90 75 60 40 Safety protection measures X6 20 18 15 12 Geological conditions X7 (relative 10 8 6 2 environment index) Underground pipeline X8 60 45 30 15 Surrounding buildings X9 60 52 44 36 Management personnel qualification ≥10 ≥10 ≥8 ≥5 management X10 (working experience) Safety production system X11 90 75 60 40 Safety management of construction 90 75 60 40 site X12

Brittleness Analysis of Metro Construction Risk In the index system of metro construction risk, when the index value is small, indicating more brittleness, the following formula (2) could be used to calculate and analyze the brittleness of the brittle factors in Grade I, II, III and IV, among which, xi means the actual value or relative value of the safety evaluation index in construction, and xi max means the actual or relative maximum value of the index. 1 1

H l x ln x H = i = i i i 1 1 (2)

H max xmax ln xl max Based on the above formula, the brittleness of each brittle factor in GradeⅠ,Ⅱ,Ⅲ and Ⅳ could be calculated. Then the weight value of each subsystem is evaluated according to The Safety Evaluation Standard of Metro Construction :

α = 0.45，α = 0.2，α = 0.1，α = 0.25 1 2 3 4

The weight value of each brittle factor:

ω =（0.135，0.135，0.18，0.04，0.12，0.04，0.02，0.03，0.05，0.075，0.075，0.1） The brittle correlation degree of the risk assessment results in different grades could be calculated with the formula as (3):

n l l H = ∑ωi Hi (3) i=1 Vol. 21 [2016], Bund. 25 9803

The brittle correlation of the project construction in Grade I, II, III and IV is shown in Table 2 as follows:

Table 2: Brittle correlation Grade Ⅰ Ⅱ Ⅲ Ⅳ Brittle correlation 0.28535 0.36215 0.50655 1 Then the scope of the brittleness degree is determined by means of the mean value method, and the formula for calculation is as (4): H n+1 − H n H n = H m + ,n =1,2,3,4 (4) max 2 The risk index of the Guanghua Park Station in Chengdu Metro Line 4 is shown in Table 3, and the brittle correlation is as follows:

12 H = ∑ωi Hi = 0.3252 i=1

Table 3: The Risk Index of Guanghua Park Station Index X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 Value 9 8 80 8 90 18 9 35 50 10 20 80 Therefore, the brittle correlation of this metro project is within Grade Ⅱ , which belongs to medium low risk level. The excavation depth of this metro project is about 17m, which is large enough to be listed as Grade I foundation. On the basis of the construction experience in deep foundation pit in Chengdu area, the retaining structure of the foundation pit should adopt the bored pile or the underground continuous wall. And buried depth shall be determined after the evaluation project, such as the subsoil stability, anti overturning, and the piping checking, etc. The supporting system should employ the steel or reinforced concrete as the horizontal support. The surface is for road traffic and there are intensive buildings around. In addition, there are many underground pipelines. Therefore, the horizontal displacement caused by foundation pit excavation should be strictly controlled: for the foundation pit with intensive buildings around and dense pipelines underground, the supporting methods such as increasing the number of support and the depth of retaining wall should be adopted. The rock stratum in the field is mainly composed of gravel soil, which has poor self-stability, strong permeability and high underground water level. In order to ensure the smooth construction of foundation pit excavation, underground water level must be reduced to not less than 0.5m below the bottom surface of the foundation. According to the geological conditions of the site, the well dewatering measures shall be prefered.

RISK CONTROL MODEL FOR METRO CONSTRUCTION The specific information in the four subsystems of metro project, i.e. design, technology, environment and management, should be collected. Based on the major brittle factor monitored, the brittleness of brittle factors should be dynamically updated. If the brittleness is more than security level, the group scheme should be immediately developed and adjusted accordingly, thus implementing the scheduled resource allocation. According to this idea, the following risk control model for metro construction is constructed as shown in Figure 3.1: Vol. 21 [2016], Bund. 25 9804

According to the risk control model for metro construction, the control strategies for the four subsystems, namely, design, technology, environment and management subsystems, are put forward. (1) the control of design factors. Familiarity with the surrounding environment is an important guarantee to improve the quality and efficiency of design. In the design process, multiple plans should be selected and comprehensively analyzed. The first-hand information in the field test and experience could help more overcome the deficiencies in design or the design wastes. (2) the control of technical factors. During the construction process, specific personnel should be assigned to strengthen the observation, checking the cable pull force and ensuring the safety. The electrical components of the power line and equipment should be regularly checked to ensure the safety of electricity. The horizontal displacement caused by foundation pit excavation should be strictly controlled. When the excavation depth is large, the construction technology of the key points should be strictly employed. When applying shield method in and out of the tunnel, the well dewatering measures for the portal construction should be adopted. In reinforcement design, full consideration should be given to the engineering performance of shield machine.

design subsystem Brittle technology informa normal monitoring exceed the No subsystem tion system threshold Monitor and of

collection environment control the brittleness subsystem risk

Yes management

subsystem

Strategy control the size of the formulation and Implementation risk

Figure 3 risk control model for metro construction (3) the control of environmental factors. The project location should be determined according to the project’s multifaceted factors, such as engineering geological and hydro-geological conditions, environmental conditions, line position, buried depth and excavation width, and economic factor, etc.. For the various types of pipelines in construction, the concrete inspection should precede the beginning of work. All types of underground pipelines should be properly protected, thus ensuring the urban public facilities. (4) the control of management factors. The safety management system should be put into practice, and the risk management rules should also be implemented. Before construction, adequate emergency supplies and equipment should be equipped with. Duty officer should be on duty, who immediately reports to the emergency leading group on the construction of the site, the pipeline deformation, materials, equipment and the number of rescue personnel. Vol. 21 [2016], Bund. 25 9805

CONCLUSION With the metro as the research subject, according to the current situation of the risk management in the metro project in China, as well as comparing the risk research theories and methods in metro construction in our country, this paper has firstly applied the theory of complex system to the metro project. And following the principles such as the theory with practice, and quantitative analysis and qualitative analysis, this paper has initially established brittleness risk model in metro construction. The main conclusions are as follows: (1) The metro construction system is a typical complex system, which can be divided into four subsystems, namely, design, technology, environment and management subsystems. With the synergistic effect as well as the coupling effect, the brittleness factors within each subsystem will cause brittleness risk, even the system collapse. (2) Taking the ground subsidence collapse as an example, this paper made a quantitative analysis of the brittle factors’ effect in different security levels, and obtained the brittle degree of brittleness factors, determining the scope of each grade’s brittle correlation in the safety pre-evaluation for metro construction. (3) Through the research on the construction risk of the metro tunnel, the preferable construction risk management system for the metro tunnel has been established, which can effectively promote the implementation of the construction risk management, thus being able to help achieve the expected goal.

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Editor’s note. This paper may be referred to, in other articles, as: Nie TingTing and Xing HuiGe: “Risk Control Analysis of Metro Construction Based on Complex System Brittleness Theory” Electronic Journal of Geotechnical Engineering, 2016 (21.25), pp 9797-9806. Available at ejge.com.