Risk Factor Identification and Evaluation on SPB Shield Underneath Existing Railroad Settlement

Regarding the situation of slurry pressure shield tunneling underneath existing operating railway, the traditional risk assessment method takes the settlement risk of shield tunneling underneath the railroad as the evaluation objective cannot determine the risk development trend. Therefore, a shield tunneling underneath railroad risk evaluation model based on set pair analysis was proposed. Firstly, according to the Slurry Pressure Balance shield construction mechanism, the risk source identification of the shield construction process and railroad operation was carried out, and the index factor system that leads to the settlement risk was formed. The importance of each evaluation index in the model was optimized by rough set, and then a five-element connection number model with objectively improved weights was obtained. Finally, assessed the risk level of railroad settlement caused by the shield tunneling. This model is suitable for the actual construction of a subway shield tunnel, which is consistent with the site survey results and has reference significance for the risk assessment of similar projects.


Introduction
With the construction of urban rail transit entering a new era of rapid development, many newly-built subway tunnels crossed underneath existing buildings (structures), which inevitably led to the status quo that many section tunnels pass under the existing railway projects. Shield construction disturbs the surrounding formation easily, causing subgrade settlement and ground collapse, which causes track irregularities and affects existing railroad operations in turn. The safety problem of shield tunneling cannot be ignored. Thus, carrying out shield tunneling underneath railroad risk evaluation; proposing scientific and effective measures have become problems that need to be solved urgently.
Many scholars at home and abroad have gained previously relevant established knowledge. For example, the construction risk of tunnel boring machines was evaluated using an improved fuzzy comprehensive evaluation method (Liang et al. 2019). An indepth discussion and analysis using the finite element software MIDAS/GTS NX on shield tunneling underneath operating railroad were conducted (Liu et al. 2020). Another example is that the safety risks of full-section tunnel construction with unfavorable geological conditions based on the fuzzy analytic hierarchy process were analyzed (Hamidi et al. 2010). The performance of three tunnel construction methods in stiff clay was compared (Ieronymaki et al. 2018). The shield machine type selection and shield tunneling were researched using the analytic hierarchy process and fault tree (Hyun et al. 2015). The influence of subway tunnel excavation on pile foundation settlement of overpass is studied through field measurement and computer simulation (Xiang et al. 2008). There were many pieces of research on the risk analysis of subway tunnel construction and railway shield tunneling underneath existing structures, mostly numerical simulation analysis of its influence; however, there were few risk evaluation models with shield tunneling underneath existing operating railways as evaluation objectives. Among the existing studies, the method adopted, such as the analytic hierarchy process, is subjective in determining the weight, while fuzzy mathematics reflects the fuzziness in the evaluation, does not show advantages in the weight calculation, and is unable to predict the trend of security risks.
In this thesis, a city subway under the existing railway section project was relied on. Firstly, risk analysis was carried out, and 14 secondary evaluation indexes were identified, which constitute the index system of the existing railway under shield tunneling. Then, the promoted theory of set pair analysis based on the five-element link number was introduced into the risk assessment system to build the risk evaluation model based on the five-element connection, and it was applied to an engineering instance of a subway shield tunnel-crossing railroad.
This model adopts the decision table system of rough set theory to calculate the weight of evaluation indexes objectively, which reduces the subjectivity of evaluation effectively and makes up for the deficiency of traditional evaluation methods. The Rough Set and Set Pair Analysis (SPA-RS) method can accurately evaluate the risk level of shield tunneling accurately (Pawlak 1982), and predict its evolution trend, which can provide reference for similar engineering practices (He et al. 2018).

Project Backgrounds
In the bidding section of a city's subway project, the settlement control of the shield tunnel underneath the constructions (structures) is the key point of the construction control of this section. The normal operation of the railway must be ensured. The requirements for both shield machine settlement and surface settlement control are relatively high. To be specific, shield tunneling needs to go underneath essential facilities such as railroads, high-rise residential buildings, bridge piles, navigation channels, etc. With complex surrounding conditions, many risk sources along the route, great difficulty in construction control, and high demands for shield functions. In order to reduce the fluctuation of the slurry pressure, this section adopts the indirect controlled slurry pressure balance (SPB) shield with pneumatic compound mode, which has a good slurry pressure adjustment function. The interval adopts the construction scheme of single-hole double-line with double layer lining.
The buried depth of the interval tunnel is 17.2 m, and the length of the railway crossing is 67 m. The main risks of shield tunneling process are as follows: the shield tunneling machine underpasses 12 tracks of the existing railroad line, and the interval tunnel underpasses them. In the process of under-passing, the ground settled or raised due to the mining parameters and other related reasons.
It is the key to ensuring that the shield machine can pass through buildings and other structures safely and successfully to control the surface settlement within the specified range strictly. To ensure that the subgrade settlement is controlled within the specified scope, not to affect the safety of the above railroad, several major settlement risk assessment results were taken as the risk assessment results of the shield tunneling underneath the existing railroad in the whole process of shield construction. The time-history curve of ground settlement of monitoring points is shown in Fig. 1. Among the surface settlement monitoring data, the cumulative variation volume of the monitoring point DBC-05-01 is the largest, which is -9.46 mm. The standards of construction monitoring settlement control are shown in Table 1.

Identification of Shield Tunneling Risk Factor
The construction project that shield tunneling underneath the existing railroad of a subway section was taken as an example. To prevent the ground surface settlement, in addition to considering the risks of the existing operation railroad itself, identify the influencing factors for the settlement risk of the shield tunneling process and list the parameters risks of the shield tunneling is also necessary. Due to the use of SPB shield, the influence of slurry pressure control and slurry index control was considered in the risk factors of shield tunneling control.

Slurry Pressure control
The slurry pressure is the key to establish the slurry balance, and the appropriate slurry quality and pressure are of great importance to the stability of the excavation surface, and the instability of which may lead to further settlement of the ground. The pressure setting too large or too small can cause the soil in front of the notch to be uplifted or settled. As the starting end of the shield was strengthened, the setting of water pressure of the cutting edge should not be too high. The initial set value was 1.0 kg/cm 2 , which shall be adjusted appropriately during the actual construction. During the propulsion of each cutting edge, the slurry pressure of the cutting edge should be strictly controlled to reduce its fluctuation and keep the soil on the front of the notch stable to reduce the disturbance to the soil. With the propulsion of the construction heading face, the slurry pressure of the notch was gradually increased to achieve the dynamic balance of the slurry.

Propulsion Speed Control
The strict control of propulsion speed is the key to ensure shield tunneling to avoid overcut and reduce formation loss. The propulsion speed in the testdriving section should be kept relatively stable and gradually increased to the maximum mining speed. Under normal driving conditions, the propulsion speed should be set as 20-40 mm/min. During the normal propulsion of each edge, the propulsion speed should be kept as constant as possible to reduce fluctuation and avoid excessive disturbance to the soil around the tunnel due to improper speed selection. The synchronous grouting speed should match the propulsion speed. Adjust the grouting amount according to the propulsion speed so that the synchronous grouting amount can adapt to the mining speed.

Digging and Cutting Amount Control
The amount of digging and cutting is another parameter to ensure the stability of the excavation surface. With the ongoing excavation of shield, the stress state  The ground surface settlement (mm) Warning value 20 ? 10 * -30 of soil in the notch and its adjacent area also changes constantly. Due to the different stress states of soil, the displacement amount and direction of soil in front of the shield notch were different. Under different overload conditions, the soil in front of the notch may uplift or settle.

Slurry Index Control
After the performance index of slurry water was selected, it must be adjusted in time according to the monitoring results of surface uplift or settlement. The poor slurry performance cannot guarantee the stability of the soil in the excavation surface, which would result in excessive surface deformation. Besides, the monitoring of slurry is a time-varying process, and the criteria to test whether the proportion of mud and water is reasonable are the stability of excavation face, the state of fluid transportation, and the amount of ground settlement.

Synchronous Grouting Control
Synchronous grouting technique is an important measure to control strata deformation, land settlement. Untimely grouting work and unreasonable grouting pressure would induce the surface settlement; thus, the construction gap between the outer diameter of the shield shell and the outer diameter of the pipe segment must be timely and fully filled with the propulsion of the shield. Also, unreasonable synchronous grouting slurry parameters and proportioning would cause the shield tunnel to have excessive uplift. The synchronous grouting amount and grouting pressure should be adjusted according to the mining speed and the ballast amount.

Shield Posture Control
Improper shield posture control would lead to excessive axis deviation and tunnel uplift. The axis deviation that exceeds a specific limit would lead to the tunnel lining invasion and the gap of shield tail smaller; worsen the local stress of the pipe segment; increase the formation loss and surface settlement. Avoid the ''snake'' shape of the shield machine as much as possible to reduce the disturbance to the formation. Paid close attention to the uneven friction resistance between the shield surface and the tunnel, as well as the different resistance when the cutting edge cuts the formation to prevent the deviation. Slow down the tunneling speed, made the upper and lower parts of the cutter head bear the same force as much as possible at the moment of tunneling, and reduced the shield machine's elevation phenomenon.

Cutter Head Torque Control
In the process of mining, the cutter head and cutter should be well protected to prevent abnormal wear of the cutter. Abnormal damage to the cutter head and cutter of shield tunneling machine would cause excessive disturbance to the soil around the tunnel. The torque change of the cutter head should be paid close attention to before receiving the hole. When the torque of the cutter head increases significantly, it indicates that the interlocking pile has been jacked up, and the mining should be stopped immediately.

The Total Thrust Control
The total thrust of shield was strictly controlled to prevent excessive thrust and water inlet accidents caused by segments crack. Uplift or settlement occurred when the total thrust became too large or too be small.

Secondary Grouting Control
Secondary grouting is also a measure to prevent formation loss caused by shield tunneling. After synchronous grouting, the annular gap behind the pipe was filled, and the formation deformation and settlement in most sections were controlled. As for partial sections, there may be local unevenness, slurry solidification shrinkage, and slurry dilution loss in the synchronous slurry solidification process, resulting in the surface settlement, which requires secondary reinforcement grouting.

Speed Control of Cutter Head
In order to protect the cutter head, the rotation speed of the cutter head was controlled at 1.8-2.0 r/min when tunneling in the soft and hard formation. Before the shield machine entered the hole, gradually reduced the speed of the cutter head to minimize the disturbance of the cutter head to the formation. In the second ring before stopping, controlled the speed of the cutter head not to exceed 0.75 rpm.

Subgrade Settlement Deformation
The disturbance of shield tunnel construction causes soil overlay and the settlement deformation of railroad foundation structure. The greater the settlement was, the higher the risk would be. In serious cases, the track would be suspended, and the derailment accident would be induced.

Surface Settlement Deformation in the Station Yard
In strict accordance with the standards put forward by relevant railroad departments, the maximum cumulative ground surface uplift shall not exceed 10 mm, and the maximum cumulative ground surface settlement shall not exceed 30 mm.

Uneven Settlement and Deformation of Track and Switch
Due to the disturbance of shield tunneling, the soil under the track would have differential settlement; the uneven settlement between the track and the switch would lead to the rail's abrasion; those would affect the train running and railroad operation. According to the relevant requirements in Railway Line Maintenance Rules, the allowable deviation management value of the static geometric dimension of the track shall not exceed 10 mm.

Settlement and Deformation of the Overhead Line System Bar
During the construction process of shield tunneling underneath the existing railroad, shield tunneling would affect the auxiliary facilities of the railroad inevitably, such as signals, overhead lines, and pipelines; the settlement and tilt of the catenary bar are the main monitoring items of facilities and equipment. During the construction, the monitoring data should be used to understand the monitoring situation of the settlement and tilt of the overhead catenary columns that affect the railroad safety in time, to grasp the daily safety operation status of the railroad in real-time. Once the settlement of the catenary bar occurred, it would lead to power supply failure, traffic interruption, causing significant economic losses.

Construction of Evaluation Index System
In order to assess the risk degree of shield tunneling through railroad accurately, an evaluation index system of shield tunneling underneath existing railroad was constructed from two aspects, namely, the risk of shield tunneling parameters and the risk of existing railroad operation, as shown in Fig. 2.

The Five-Element Connection Number Model
To achieve the goal of assessing risk level accurately, a set of set pair (R, U) was determined, and the shield underneath the existing railroad risk was divided into five levels. The grade evaluation standard set R = {grade I (extremely low risk), grade II (low risk), grade III (medium risk), grade IV (high risk), grade V (extremely high risk)}, referring to Risk Management Guidelines for Subway and Underground Construction and Guidelines for construction safety risk Assessment of Highway Bridges and tunnels. According to the principle of SPA, the standard relation degree formula was generalized, and the differences were specifically subdivided into partial homology difference, partial centrality difference, and partial inverse difference (He et al. 2018): Usually written as: In practical application, it was convenient to evaluate the security status of object U by corresponding to the five-element connection number and the level of security state respectively, that is, a, b, c, d, and e all represent the status of safety, relative safety, relative danger, danger and extreme danger (Ma et al. 2015), See Table 2 for details.

The Calculation Formula of the Five-Element Connection Number
Supposed U = {U1, U2,…, Un} to be the sample set of risk assessment factors. According to the evaluation criteria listed (Ma et al. 2015) (see Table 2), a total of 14 risk factor indexes were formed from two aspects of railroad operation risk and shield tunneling parameter risk. The division basis of factor evaluation grade is shown in Table 3 to check and evaluate the safety status of each point. The calculation formula of occurrence degree of each point evaluation index was u Ã ij ¼ u ij N (N as the total number of check item points, u ij as the number of item points when the evaluation index i is at risk level j).
The risk state of the evaluation object U was analyzed, and the comprehensive connection number of the evaluation object U was calculated.
Assume u* r1 , u* r2 , u* r3 , u* r4 , u* r5 (1 B r B n) represent, in turn, the standardized value of the safety, relative safety, relative danger, danger and extreme danger item points of the r index, and the weight of each evaluation index was w 1 ,w 2 ,…, w n , then the connection number model for risk assessment of the evaluation object U was (Ma et al. 2015): The matrix expression was: In the formula, W was the weight coefficient vector. b was the matrix of item points number of each risk evaluation index. Each row should be normalized. E was the matrix of the association degree coefficient. a ¼ P n r¼1 w r u Ã r1 was the same degree component, determined item, and matches the ''extremely low risk'' correspondingly. The value reflects the proportion of the item points in the evaluation system conforming to ''safety'' in the total inspection item points comprehensively.
b ¼ P n r¼1 w r u Ã r2 was the partial same component of difference degree, uncertain item, matching ''low risk'', its size reflecting ''relative safety'' proportion.
c ¼ P n r¼1 w r u Ã r3 was the middle component of the difference degree, uncertain item, and matches ''medium risk'', the size of which reflects the proportion of ''relative danger.'' d ¼ P n r¼1 w r u Ã r4 was the inverse component of the difference degree, uncertainty item, and matches ''high risk'', the size of which reflects the proportion of the ''danger'' state. e ¼ P n r¼1 w r u Ã r5 was the oppositional component, determined item, which matches ''extremely high risk'', and its value size reflects the proportion of ''extremely dangerous''.

Determination of Index Factor Weight
There are two categories of weighting indexes: subjective and objective. The subjective weighting method includes the analytic hierarchy process, binomial coefficient method, Delphi method, etc. The disadvantage was that the decision evaluation result has strong subjective arbitrariness, which greatly influences the fluctuation of index weight; moreover, the evaluation result may be completely opposite, which only increased the influence of the bias of the decision analyst and had a great limitation in the application. Therefore, the objective weighting method was more accurate and scientific. In this thesis, the decision table of rough set theory was used to calculate the index weight to determine.

Determination of Index System and Its Weight Value
The decision table system of shield tunneling parameter control factors was given as an example. There is much non-compliance with safety standards, which have a slight impact on the safety target function of the upper body. Appropriate measures must be taken to reduce the risk 4 Danger Many points do not meet the safety standards and significantly impact the safety target function of the upper main body, which may cause casualties, property losses, and other accidents. Appropriate measures must be taken to reduce the risk 5 Extreme danger The failure to meet the safety standards and has a serious impact on the safety target function of the upper main body, easily causing casualties, property losses, and other accidents, and appropriate measures must be taken to reduce the risk A sample information table was established based on the actual observation statistic data of similar shield tunneling under crossing projects in a certain province. The risk monitoring data of 23 samples with relatively complete data were selected as the information object. The equal frequency binning discretization function of ROSETTA software was used to fill the missing data with the mean value. And the data were pre-processed to obtain {1, 2, 3, 4, 5} corresponding to the security level {safety, relative safety, relative danger, danger, extreme danger (Laplante et al. 2005), as shown in Table 4. In turn, the dependence of the decision attribute relative to each conditional attribute and the importance of the decision attribute relative to each attribute were calculated, and the results are shown in Table 5.
Similarly, the inspection results of the second-grade index items of the existing railroad operation risks were obtained, as shown in Table 6.

Determination of Connection Number
Took the shield crossing the existing railroad construction of a specific subway section. The connection number for slurry pressure control(U 1-1 ) was determined. According to the site monitoring report, a total of 30 items were checked. Among them, there were 26 safety items (completely conform to the safety standards requirements). And 4 relative safety items (During the process of each edge propulsion, the standard cannot be fully met when the fluctuation range of incision slurry pressure was strictly controlled). No item points were extremely dangerous, dangerous, or relative dangerous safety status was found, as shown in Table 6. The connection number expression (u1-1) of the second-grade index ''slurry pressure control U 1-1 '' was: By analogy, the remaining indexes can be solved, as shown in Table 6.

Connection number analysis model of shield
tunneling under passing existing railroad risk