Abstract
With the in-depth advancement of underground transportation in China, the connecting aisle is used as a link between different underground spaces to provide sufficient security and connection. The basic structural units of underground and aboveground spaces play an extremely important role. The use of the mechanical connecting aisle construction of over mucky strata is difficult, and it is easy to have an impact on the main tunnel. To ensure the safety of connecting aisle construction, studies should be performed on the construction technology, equipment, scheme, and related risk analysis of mechanical connecting aisle construction. In this study the work breakdown structure-risk breakdown structure combined with the expert investigation method is used to comprehensively identify the construction process, construct the risk index system of mechanical connecting aisle construction, determine the initial risk value, and comprehensively use the fuzzy-analytic hierarchical process to establish a fuzzy complementary judgment matrix, obtain the risk-weighted correction value, and quantitatively evaluate the risk source level of the mechanical connecting aisle construction. Based on the assessment method, the risk assessment of the mechanical connecting aisle construction project in Daqing section of Qingdao Metro Line 8 was performed, 113 potential risk sources were identified, and the comprehensive assessment obtained 7 major risk sources such as “improperly designed construction organization,” “instability of excavation surface,” and “easy leakage of lower river crossing” that were greater than the total risk average of 2.053. Corresponding risk response measures were proposed, which provided a reference for the risk assessment of similar projects in the construction of similar projects on mechanical connecting aisle construction in the Bohai mudstone stratum with high water pressure.
1. Introduction
The use of mechanical connecting aisle construction of over mucky strata is difficult, which is easy to induce additional deformation and force in the adjacent existing tunnel. Moreover, it is easy to cause quicksand and water leakage in the tunnel, resulting in cavity or even ground collapse, which is of engineering significance for the safety assessment and control of mechanical connecting aisle construction risks of over mucky strata. Mechanical connecting aisle construction can be divided into two categories: mechanical and nonmechanical methods to overcome the many shortcomings of nonmechanical construction, such as long construction cycles, high costs, and large safety hazards. Scholars at home and abroad have performed exploration and practice in mechanical construction and obtained many results [1]. Ding et al. [2] studied the influence of seismic loads on the stress, displacement, and acceleration response of the main tunnel and the connecting aisle during mechanical connecting aisle construction using the finite element method. Bangliang et al. [3] addressed the problem at the joint. The acceleration and displacement response of the train load on the main tunnel and connecting aisle under different connection forms were studied by finite element simulation. Liu et al. [4] conducted two sets of parallel foot ruler tests to study the structural response of the shield main tunnel during the mechanical cutting process. According to the results, the hole-breaking process of the mechanical connecting aisle construction was the process of the joint force of the key segment and the internal support, and the structure was safe after opening holes and removing braces. Jia et al. [5] considered mechanical risks, manual operation risks, engineering geological risks, construction environment risks, and other factors and established a risk index system for shield tunnel construction throughout the Yellow River and advanced the risk of shield tunnels construction throughout the Yellow River under the Yingmentan-Matan. Huang et al. [6] proposed a comprehensive assessment model of shield tunnel construction risk based on cloud theory for the risk assessment of Shiziyang shield tunnel construction based on the ambiguity and randomness of the risk assessment of shield tunnel construction. Sun et al. [7] investigated and analyzed typical engineering cases of horizontal shield tunnel connecting aisle at home and abroad and summarized the key technologies of different cases into five aspects: construction method, main structure opening type, stratum reinforcement scheme, special annular pipe segment type of the main tunnel, and temporary support type of the main tunnel. Chen et al. [8] summarized the key technologies between different cases as construction methods, main structure opening type, formation reinforcement scheme, special ring key segment type of the main cave, and temporary support type of the main hole. Owing to the characteristics of water-rich round gravel strata in Nanning City, combined with the mechanical connecting aisle construction in the Sansu section of Nanning Metro Line 2, the reinforcement method of the connecting aisle of the shield tunnel adapted to this stratum is compared and analyzed; finally, the underground continuous wall plus paste grout grouting technology was used for prereinforcement treatment. Gao and Chen [9] paired the application of the pipe jacking method in the connecting aisle of shield tunnel Nanjing metro, and through the systematic study of the project, the reconstruction method is believed to have led to novel ideas for the selection of the construction scheme for the mechanical connecting aisle construction assuming a soft plastic land. Ding et al. [10] conducted field tests in the background of the connecting aisle project of the pipe jacking method of Wuxi Metro Line 3 and studied the main tunnel response of pipe jacking T connecting aisle in the construction process. Wang et al. [11] artificially solved the stratum instability and the gushing water and sand caused by pressure leakage when the mechanical connecting aisle boring machine broke the receiving tunnel door at the intersection between the connecting aisle and the main tunnel, and the construction method of the conventional connecting aisle was analyzed and studied, based on the tunneling principle of earth pressure balance boring machine. Wei et al. [12] studied the impact of the mechanical pipe jacking method T-connection construction on the surrounding environment, relying on the construction project of the connecting aisle in Wuxi Metro Line 3 to perform on-site testing. Simultaneously, combined with theoretical calculations, the law of stratigraphic uplift and sedimentation under the influence of construction was discussed. Zhu et al. [13] used the pipe jacking method to perform a foot-length test of the connecting aisle case in Wuxi Metro and studied the sinking structure of the main tunnel when the connecting aisle was constructed using mechanical methods. Based on the force performance in the entire construction process and through the analysis, the deformation of each ring is believed to be small, which is a relatively safe construction method.
In summary, in the exploration and practice of mechanical construction, scholars at home and abroad have made outstanding contributions and substantial research progress [14–17]; however, few studies on the construction risk assessment of over mucky strata presently exist. Therefore, the risk assessment technology system is still not perfect. The Qingdao Metro No. 8 Xian Daqing section project is the first application of mechanical construction in the Bohai mudstone stratum in China, and construction risk assessment should be performed. Based on on-site statistical analysis and expert investigation, the Daqing interval mechanical connecting aisle construction risk index system was established, and the work breakdown structure-risk breakdown structure (WBS-RBS) and fuzzy-analytic hierarchical process (FAHP) were used to identify and evaluate the project risks. The research results have a certain reference significance for the safety and risk control of related engineering constructions.
2. Risk Identification, Assessment Methods, and Principles
2.1. Risk Identification Theory Based on WBS-RBS and Expert Investigation Method
To comprehensively and accurately identify the risk sources of the entire construction process, the entire evaluation object project is decomposed into several work units based on the work, and the risks that run through the entire project are decomposed into several risk units layer by layer, based on which the WBS-RBS coupling matrix is established to form a mapping relationship between risk and operation [18, 19]. Subsequently, according to the expert scoring results, the position risk of each matrix element is judged whether there is a risk, the possible risk items and the cause of the risk, and according to the normalization of the expert survey data; we determined whether the risk source exists in the specific project operation package, and the existence is recorded as “1,” and if it does not exist or is extremely unlikely and the consequences are insignificant, it is “0” and follow-up risk analysis is conducted based on the results of risk identification. This is shown in Figure 1.
2.2. Fuzzy-Analytic Hierarchical Process (FAHP) Risk Analysis Theory
Based on the method of trigonometric fuzzy number complementary judgment matrix, a hierarchical system is conducted in this study, and a fuzzy complementary matrix is constructed for each layer to determine the weight coefficient of the risk evaluation index of the mechanical connecting aisle [20]. Assume the fuzzy set is on the domain , if is a membership function, the trigonometric fuzzy number is defined as follows [21]: where , and represent the upper and lower bounds of , respectively; is the median of fuzzy set . The larger the , the stronger the ambiguity. is the value taken when the membership of fuzzy set is 1. Each expert compares the elements of each indicator layer in twos and obtains the triangular fuzzy number judgment matrix of each indicator layer [22]. The matrix has the following properties: (1)(2)
2.3. Quantitative Assessment of Compound Risks Based on WBS-RBS and FAHP
The construction conditions of the mechanical connecting aisle are complex, and the type of construction risk is constantly changing with the construction process, operation location, and surrounding environment. To avoid the omission of risk sources and the low reliability of assessment results in conventional methods, a composite risk assessment method based on WBS-RBS and FAHP is proposed, and the modified risk value is used to characterize the risk severity. The risk level is judged based on the modified risk value and matrix [23]. A schematic of a risk assessment approach based on WBS-RBS and FAHP is shown in Figure 2. (1)Establishment of the hierarchical model of the mechanical connecting aisle
The construction conditions of the connecting aisle are more complex, many of these properties are difficult to quantitatively analyze and calculate, and only qualitative analysis calculation and logical judgment can be combined to solve these problems. Establishing a hierarchical model is the first step of the fuzzy analytic hierarchy method, and full attention should be paid to its scientific nature when analyzing the structure because the determination of the indicator system and establishment of the hierarchical model will directly influence the rationality of decision analysis [24]. Therefore, the choice of 4MIE as an important factor is considered, and a close connection exists between the aforementioned five criterion layers [25]. (2)Initial risk matrix calculation
Based on the WBS-RBS coupling matrix risk source identification results, the probability of occurrence, severity of consequences, and environmental impact of each risk source are quantified by expert investigation, event analysis, case investigation, data simulation, and other methods according to certain standards. The initial risk value is obtained by the product of the probability of occurrence of risk factors and the severity of consequences, which is mapped in the WBS-RBS coupling matrix; that is, the initial risk value of the matrix is obtained as: where represents the risk, is the rank value of the probability of the occurrence of the risk , and represents the possible consequences when the risk factor occurs. The consequences of risk loss are in accordance with the five categories , , , , and , and the calculation of the loss value of the first risk is shown as follows: where , , , , and denote the risk on the degree of impact of casualties , project quality , economic loss , environmental impact , and delays , respectively; weight coefficients are , , , , and , respectively, as the degree of influence of each aspect on the risk level differs in different risk assessment criteria. (3)Construction of a fuzzy complementary judgment matrix and consistency test
To quantify the decision evaluation, a moderate importance scale value is introduced to measure the relative weights between elements, and the 0.1 to 0.9 scale method shown in Table 1 is usually used to provide the quantitative scale. For fuzzy complementary judgment matrices , as shown in Equation (4) for any , matrix is a fuzzy complementary consensus judgment matrix [26].
If Equation (4) is satisfied, the fuzzy complementary judgment matrix is consistent; otherwise, consistency adjustment is required to ensure the weights calculated according to the fuzzy complementary judgment matrix are scientifically reasonable.
The adjustment of the fuzzy complementary judgment matrix is calculated as follows: for , using Equation (5) where . (4)The weights between the indicators at various levels are solved
According to the calculation method of the weight value of the trigonometric fuzzy number complementary judgment matrix in [27], the weight vector of the trigonometric fuzzy number of each subindex is obtained.
The trigonometric fuzzy number is defuzzied according to Equation (7) to obtain a definite value. (5)Risk assessment
The initial risk value of each risk source is multiplied by the corresponding risk weight, the revised indicator value is obtained, and the modified risk value matrix is established by combining all elements within the same evaluation criterion at this level.
The correction risk value represents the severity of the risk and the comprehensive ranking can obtain a sequence of specific risk items. When synthesizing different expert opinions, a weighted average should be used to calculate the risk value and determine different weights according to the experience level of different experts. The comprehensive weighted risk value for a single project is calculated as follows: where is the weight coefficient of the lth expert, is the weighted risk value calculated by the lth expert on the risk source of , and is the number of experts.
The risk value is more reasonable after correction. To further analyze the order of construction risks of each mechanical connecting aisle and the overall risk profile, the cumulative risk value of class I risk was obtained by the row-wise addition of . Similarly, the subitem risk value of operation can be obtained by column accumulation, as shown in Equations (10) and (11) where, and are the corresponding rows and columns of element , respectively; , , and are the modified risk value, occurrence probability value, and risk loss consequence value of matrix elements in , respectively; is the weight value of class risk.
Finally, the cumulative risk value and sizes are sorted by rows and columns, respectively, and the sequence of risk types and degrees of the operation process is obtained, to implement different preventive measures for different risk sources and work stages.
3. Engineering Examples
3.1. Project Overview
Qingdao Metro Line 8 starts from Jiaozhou North Railway Station and ends at May Fourth Square Station, and its cross-sea area (between Dayang Station and Qingdao North Railway Station) passes through the sea area of Jiaozhou Bay, with a length of 7.9 km and a width of 5.4 km. The section adopts the combined construction method of the “mine method + shield method” that the land section of the shield method adopts the earth pressure shield machine and the sea section, the muddy water shield machine. The shield section is designed with 6 connecting aisles (no. 6 to 11). To reduce the construction risk and construction period, three connecting aisles of 9#, 10#, and 11# will be constructed using mechanical technology, and the schematic of the interval is shown in Figure 3. The connecting aisle adopts 6 pieces (3 rings) of special steel-concrete key segments, with a ring width of 1500 mm, without wedges, and assembled by through-seam. The project is planned to be realized by China Railway Equipment and excavated with a diameter of 3290 mm of pipe jacking machine equipment of an earth pressure balanced connecting aisle.
The geological profile of a typical connecting aisle location is shown in Figure 4. Most of the sea areas across the section are shellfish farming areas, and the sea area section is a typical coastal sedimentary landform in Qingdao, running east-west through the sea area on the north side of Jiaozhou Bay, the surface is flow-plastic silt soil, the terrain is the trend of low in the middle and high on both sides, and the implementation conditions are complex.
The intersectional groundwater storage methods within the influenced area of the project are mainly three types: pore diving of the fourth series of loose rocks, pore water of weathered bedrock, and fracture water of tectonic bedrock. The crossing section is the northern area of Jiaozhou Bay, where the seawater is influenced by the tides, and the water depth changes significantly. The overall water depth increases from west to east and the highest tide water depth is 3 to 11 m. The interval main strata is shown in Table 2.
3.2. Engineering Weight, Difficulty, and Risk Analysis
(1)Safe construction risk control of high-pressure submarine tunnels
With the rapid development of China’s transportation and urban construction, the number of cross-river and cross-sea mechanical tunnel projects has increased significantly, and the scale of the project (the diameter and length of the tunnel) and the water pressure conditions are also increasing. This project is the first application of mechanical connecting aisle construction in the submarine tunnel, the three connecting aisles are deeply buried, the maximum burial depth is located in the sea section, which is 37.8 m, and the water depth is 41.76 m. The construction of the project is difficult, and it presents higher requirements for the deformation size and sealing of the tunnel boring machine (TBM) starting and receiving sleeves. (2)Excavation of the Bohai mudstone stratum
Shield machines balance pressure and maintain the stability of the excavation surface and have been widely used in the construction of urban subways in soft soil layers that cannot stabilize themselves. Presently, some cities also have rock formations in subway tunnels (such as Chongqing, Qingdao, and Shenzhen) that use shield machine construction; however, cutterhead and cutting tools wear significantly, and pipeline slagging is difficult. Most of the tunnels through the tunnel of the project are rock formations, which can easily damage the cutterhead and cutter tools; the rationality of the propulsion system setting also has a significant impact on the engineering excavation. (3)Main tunnel space limitations
The construction period of Qingdao Metro Line 8 is tight, the space of the main tunnel is narrow, and the construction space is limited. In the construction process, space conflicts and friction phenomena often exist, which reduces the construction efficiency and influences the construction progress, which may cause safety accidents. The pipe jacking method can adapt to the length of the conventional connecting aisle 1D (approximately 6-10 m) of the construction of a straight passage that is characterized by low equipment cost, fast efficiency, and low cost.
3.3. Risk Identification
According to the actual operation process and related studies [28] of mechanical connecting aisle construction, its process steps are summarized, and mechanical connecting aisle construction is decomposed into three main stages: the initiation and arrival of the pipe jacking machine, normal boring of the pipe jacking machine, and the dissolution and lifting of the pipe jacking machine; the construction of WBS is shown in Figure 5.
The risk of the mechanical subway connecting aisle construction can be decomposed using various methods, which can be decomposed from the endogenous and exogenous aspects of the project and can also be considered from the perspective of all parties involved in the project. According to the five aspects of “people,” “materials,” “machines,” “laws,” and “rings” comprehensively managed by 4M1E, the mechanical subway connecting aisle construction risks is decomposed [29], and the RBS is constructed as shown in Figure 6.
With WBS as the row and RBS as the column, the coupling forms a WBS-RBS matrix, which constitutes the correlation between the construction risk and the operation of the mechanical connecting aisle. The elements of the WBS-RBS matrix are the potential risk sources, 113 potential risk sources are obtained, and the results are shown in Table 3.
The risk sources in the WBS-RBS coupling matrix are listed separately, and the severity of the consequences of each risk source is calculated according to Equation (3) based on expert experience, the actual situation in the field, and relevant studies. In the equation, , , , , and are 0.5, 0.2, 0.1, 0.1, and 0.1, respectively; thus, the risk level of each risk is obtained according to Equation (2). The risk source is divided into 5 levels according to the probability of occurrence and the severity of the consequences and assigned a score of 1-5, as shown in Table 4. The corresponding positions of WBS-RBS for each risk value of the mechanical connecting aisle construction are filled in to obtain the initial risk matrix . The risk survey selected 20 relevant experts (construction and supervision unit), and each expert has been working for at least five years. The actual situation of the mechanical connecting aisle project in Daqing section of Qingdao Metro Line 8 was graded. To obtain more objective and real results, different weight values were assigned to the scoring results by integrating the factors of expert positions, professional titles, and engineering experience.
3.4. Risk Analysis
According to the RBS results, a hierarchical model was constructed: the topmost mechanical connecting aisle construction risk () as the target layer , middle layer personnel management risk (), construction material risk (), machinery and equipment risk (), construction technology risk (), environmental impact risk () as criterion layer , and the bottom 21 secondary indicators as indicator layer . According to the construction method of the trigonometric fuzzy complementary judgment matrix, the judgment matrix of each index is constructed, as shown in Table 5.
The median value of the two-by-two comparison of each factor is extracted from the - comprehensive fuzzy judgment matrix and the fuzzy complementary judgment matrix :
satisfies the equation of straight testing (5) and does not need to be adjusted. The weights of the trigonometric fuzzy number are calculated according to Equation (6): . The weight after defuzzification is constructed as shown in Equation (7).
Similarly, according to the fuzzy scale of Table 1, the judgment matrix of the two comparisons of each indicator is constructed, as shown in Tables 6–10.
As shown in Equation (7), the weights of each secondary index are obtained after defuzzing: , , , , and .
The initial risk matrix is multiplied by the corresponding weights of each risk source to obtain the corrected risk matrix . According to Equations (10) and (11), the matrix will be accumulated by rows and columns, respectively, and a comprehensive sorting algorithm will be introduced to obtain a sort of risk-type size and operation risk size, the result of which is shown in Table 11.
The score of the improperly designed risk decomposition structure in the construction organization is the highest and can be determined as a mechanical connecting aisle construction process of the most attention to the risk source; in addition, the excavation surface instability, construction experience, and the ability are insufficient. The tunnel has a large displacement and uneven settlement, complex geological conditions, monitoring and measurement risks, and lower river crossing easy to leak. All these exceed the average of the overall risk and prevention, and control should be the main focus. In the work decomposition structure, the scores of muck transportation and pipe jacking mud, soil silo construction pressure, lining assembly, cave entrance construction and soil reinforcement, and soil reinforcement at the entrance ranked top 5 that can be identified as the work process that is worth attending to, in the mechanical connecting aisle construction process [30].
4. Risk Control Countermeasures
Through the assessment of construction risk levels of mechanical connecting aisle project of Daqing section of Qingdao Metro Line 8, 113 risk sources were covered by the project including 7 major risk sources such as “improperly designed construction organization,” “instability of the excavation surface,” and “easy leakage of the lower river crossing,” and some representative risks sources and their specific controls are as follows.
The following control measures can be taken for the risk of improper construction organization design [31]: (1) implement the project manager responsibility system, strengthen project management, and use economic and administrative means to amend the construction organization design. (2) Optimize the design of the construction network and reasonably divide the construction section of the project. (3) Adhere to the system of leading cadres following the shift, and address problems in a timely manner. (4) Dynamically adjust the construction parameters.
The following control measures can be taken for the risk of excavation surface instability: (1) consider the changes in vertical, longitudinal, and lateral pressure of the soil in the process of instability, thereby improving the rationality and accuracy of the stability analysis of the excavation surface. (2) Enhance the support of the tunnel excavation surface. (3) Repair the blockage of the muddy water pipeline caused by the collapse and reestablish the balance of mud and water. (4) Excavation work is directed and supervised by a dedicated person to ensure a safe distance between personnel and excavation machinery.
For the risk of easy leakage of the lower river crossing, the following control measures can be taken [32]: (1) reduce the pressure of the bubble chamber, thereby reducing the pressure of mud and water. (2) Perform shield tail grease and synchronous grouting injection, and employ efforts to block the leakage channel. (3) Inject a certain amount of polyurethane into the key segment behind the shield tail to block the leakage of the shield tail. (4) Shield tail grease injection amount and pressure double control, blocking the leakage channel. (5) Synchronous grouting amount and pressure dual control to block the leakage channel.
5. Conclusion
(1)Based on the combination of WBS-RBS, FAHP, and comprehensive weighted risk ranking, the theory and method of assessing mechanical connecting aisle construction are proposed. After calculating the initial risk decomposition matrix combined with the weighted index, the risk matrix was corrected, and the final risk assessment matrix was obtained by quantitative analysis, to determine the risk sources that deserved attention during mechanical connecting aisle construction(2)By combining the WBS-RBS-coupled work decomposition and risk decomposition method with the expert investigation method, more objective and real results were obtained. In the case of complex geological conditions and construction processes, for mechanical connecting aisle construction, safety risk factors in each construction stage were systematically and comprehensively identified(3)Based on the FAHP comprehensive assessment method, the mechanical connecting aisle construction risk was evaluated, and the weighted calculation obtained 7 major risk sources such as “improperly designed construction organization,” “instability of the excavation surface,” and “easy leakage of the lower river crossing”; compared with the construction of the onshore connecting aisle, the environment of the underwater connecting aisle construction is more complex, and the security issues faced are more complex. Enormous risk factors, comprehensive consideration of geological conditions, construction process, water pressure conditions, and other factors reduce the impact of subjective factors on the assessment results, and the risk assessment results obtained are more in line with engineering practice(4)An effective, practical, and relatively complete safety risk evaluation system of underwater mechanical connecting aisle construction was established. By analyzing the risk factors of mechanical connecting aisle construction in Daqing section of Qingdao Metro Line 8, the risk level of pipe jacking machine start-up, excavation, pipe jacking machine disintegration, and hoisting stages was determined, to propose the corresponding risk control measures in a targeted manner, verify the feasibility of this risk evaluation system in the risk assessment of the connecting aisle, and provide a technical reference for the risk assessment of similar projects of mechanical connecting aisle construction in the Bohai mudstone stratum with high water pressure
At present, there are relatively few studies on the construction risk assessment of mechanical connecting aisle through silty strata, which rely to a certain extent on the engineering experience and technical level of expert personnel, and the risk assessment technical system is still not developed. The next research focuses on how to identify more potential construction risk factors based on a large number of engineering cases, combined with automated monitoring and manual remeasurement data, to provide support for construction safety, risk analysis, and risk control of similar projects.
Data Availability
All the data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Acknowledgments
The research is financially supported by the Natural Science Project of Henan University of Technology (2017BS012); the Basic Scientific Research Special Funds of Provincial Colleges and Universities (2017QNJH06); and Collaborative Innovation Center of Coastal Urban Rail Transit, Ningbo University (XT2021014).