During the construction of subway tunnels, safety issues should not be ignored, so it is necessary to prevent and resolve the risk in time and accurately. However, there are some shortcomings in the research of risk assessment, such as the subjectivity of initial data or the lack of scientific evaluation model, in order to solve the problem, this paper relies on the Changping section of the Guanhui Intercity Metro, in order to establish a dynamic risk-warning model for the construction process of subway tunnel with the CD-Bench composite method. First, a monitoring plan was equationted according to the specification requirements and the actual situation of the project, and based on this, an evaluation index system was established from five aspects: geological and support conditions, crown settlement, clearance convergence, and ground settlement and building settlement. Secondly, according to the established risk evaluation standards, the risk level of the index is determined by introducing extension theory and determining the combined weight of the index based on the distance function. Finally, feedback the corresponding risk-warning signals and take control measures. Through the application analysis shows that the model can alarm the risks during the tunnel construction process directly and timely, so the model is feasible and practical, and it is worthy of popularization and application in similar projects.
As a very complex system engineering, subway has the characteristics of large investment, large number of project participants, complex technology, long period, strong concealment and various geological conditions and so on, which brings great risks to construction. Therefore, scholars at home and abroad have carried out a lot of research on tunnel construction safety. There are many factors affecting construction safety and their contribution to risk value is different, so, in order to evaluate the construction safety scientifically and systematically, scholars like Wang et al. [
In summary, in the aspect of risk research, on the one hand, some scholars can comprehensively identify the risk evaluation index and use scientific mathematical model to calculate risk values. However, the qualitative index is mostly and difficult to quantify, and their value are subjective only based on expert scoring. On the other hand, some scholars also use monitoring technology to carry out risk warning according to deformation value, but do not consider the different contribution of each index deformation to the overall risk. In this paper combines these two methods, it will rely on the Changping section tunnel of Guanhui Intercity Metro, and establish a dynamic risk-warning model for the construction process of subway tunnel with the CD-Bench composite method. Not only does the each risk evaluation index adopt a combination weighting method to give different weights, and uses scientific mathematical models to calculate their risk value, but also greatly reduces the difficulty of quantifying indicators.
The rest of this paper is organized as follows. In
The Guanhui Intercity Rail Transit runs from Hongmei Station in Dongguan City to Huizhou Railway Station, This article mainly focuses on the Changping section of the tunnel as the research object. This part of the tunnel has such characteristics: the surrounding environment is complex, close to factory buildings, residential areas, etc., and it needs to pass through the artificial lake of the railway park, the HanViRiver, and cross the broken zone; and its ground elevation of this section is 7.31∼14.32 m, and its buried depth is about 22–36 m. The engineering geological conditions are poor, mainly quaternary strata and broken zones; and need to pass through the artificial lake and HanViRiver in the railway park, so the surface water is abundant, and the groundwater is mainly bedrock fissure water and pour water, which has a greater impact on construction and is locally corrosive. In order to control tunnel deformation and ensure the safety of the construction, it is particularly important to establish a risk warning model.
In order to effectively prevent surface subsidence and large deformation of surrounding rock, the Changping section of the Guanhui intercity adopts the CD-Bench composite construction method, which means that the tunnel section is divided into two parts for excavation: The upper part is constructed by the CD method and the lower part is constructed by the bench method. This makes full use of the respective advantages of these two construction methods: the upper part is provided with vertical support by the partition wall, which effectively controls the settlement of the surrounding rock vault; and the lower part adopts the bench cut method for construction and facilitates the access of large machinery. In addition, due to the addition of temporary inverted arches, closed steel arches and other measures, the tunnel is less deformed during construction than the traditional single construction method, so has higher safety. At the same time, the construction period is shortened due to staggered construction. Although the construction cost of this method is high, compared with other treatment measures, the “performance-to-price ratio” is still the highest. The on-site construction drawing is shown in
The surface of this section of subway tunnel is roads with high traffic volume, HanViRiver Bridge, residential houses and levees, etc., and it needs to pass through the broken zone. Therefore, accurate and timely monitoring and measurement play an important role in the protection of ground buildings (structures) and the safety of tunnel construction. According to the Technical Specification for Monitoring and Measurement of Railway Tunnels (Q/CR 9218-2015) [
Serial number | Monitoring measurement Project | Common instruments | |
---|---|---|---|
Important monitoring items | 1 | Observation inside and outside | Domainobservation, compass, digital camera |
2 | Vault crown sinking | Total station, steel gauge, level | |
3 | Clear height variations | Convergence or total station | |
4 | Grand subsidence | Level, total station, indium steel ruler | |
Selection monitoring items | 1 | Surrounding rock pressure | Pressure cell |
2 | Steel frame pressure | Steel gauge, strain gauge | |
3 | Internal forces of secondary lining | Concrete steel gauge, strain gauge | |
4 | Internal displacement of surrounding rock | Multi-point extensometer | |
5 | Tunnel floor heave | Total station, indium steel ruler and level | |
6 | Axial force of anchor bolt | Steel bar meter | |
7 | Blasting vibration | Vibration recorder, sensor |
To sum up, with reference to the scale, characteristics and design requirements of this project, the monitoring content and frequency are shown in
Crown settlement | Clearance convergence | Ground settlement | Building settlement | |
---|---|---|---|---|
General | 1/1d | 1/2d | 1/1d | 1/1d |
Big deformation | 2/1d | 1/1d | 2/1d | 2/1d |
(1)The measurement points are arranged in accordance with the construction monitoring measurement plan. If the actual terrain does not allow, the measuring points can be set near the design points, but the surface points, the convergence points and the settlement points of the vault should corresponding to the mileage of the same section, which should be stable, obvious and simple, which can reflect the actual state and development trend of the monitoring object; (2)The measuring points should be arranged a certain time in advance, and the initial value should be read early; (3)Arrange the points for verifying the design parameters at the most unfavorable position of the section and the design. the point is set up for guiding the construction to be arranged at the first construction position under the same working condition, which purpose is to feedback information in time to facilitate the modification of the design and guide the construction; (4)In the course of construction, the measuring points should avoid the influence of the construction. Once damaged, the measuring point should be added as soon as possible in the original position or as close as possible to the original position to ensure the continuity of the monitoring data of the measuring point; (5)Three points should be taken into consideration for the location of the measuring points of surface deformation and building settlement: firstly, it can fully reflect the deformation characteristics of the target object; secondly, it is easy to use instruments to monitor and measure; thirdly, it is necessary to ensure that the measuring points are not easily damaged as much as possible, the measuring points of surrounding buildings are mainly arranged at the corners; (6)Select representative crack monitoring, with at least 2 measuring points for each crack.
According to the principle of measuring point arrangement, the arrangement of measuring points for crown settlement and clearance convergence of this project is shown in
Embedment of surface settlement measurement points: Before the tunnel excavation, the surface settlement observation points are buried in time. The specific measurement point arrangement rules are: the ground surface is set up a section every 20 meters along the longitudinal direction, each section is arranged for 10 measurement points, which are set on the tunnel axis and both sides and can be properly encrypted according to the needs. When burying, 20∼50 cm deep hole is drilled on the surface by using the core-drilling machine, and the steel bar Φ 22∼25 is vertically placed, and cement mortar can be filled between the steel bar and the hole wall, and the steel bars should be about 1 cm above the ground. As shown in
For the setting of the alarm value, according to the “Technical Regulations for Monitoring and Measurement of Railway Tunnels” (QCR 9218-2015) and the research results of Zhang et al. [
Surrounding rock grade | III | IV | V-VI |
---|---|---|---|
Cumulative deformation/mm | 60 | 80 | 100 |
Deformation rate/(mm/d) | 5 | 5 | 10 |
Organizational preparation. Organizational preparation refers to the organizational safeguard activities for the implementation of risk early warning activities, which includes not only aquatinting emergency countermeasures for construction risk accidents, but also establishing and improving corresponding safety rules, regulations and standards to provide a secure organizational environment for the entire early warning management. Daily monitoring. The management activity of special third-party personnel monitoring the indicator system determined by the early warning analysis activity, which is the core of the early warning activities. These indicators are dangerous. Once a safety risk accident is induced, other disasters associated with it are likely to quickly cause uncontrollable dangerous situations, causing huge casualties and property losses. In addition, In addition, in daily monitoring, it is necessary to predict the possible outcome of the crisis and the severity of the accident after the spread of the accident as much as possible, so as to prevent problems before they occur. Emergency plan. The emergency plan mainly involves the emergency leading group, emergency plan, special rescue plan and emergency response measures, etc., which are the key to early warning activities. Once a risk accident occurs during tunnel construction and can be restored to a controllable state as soon as possible, the task of emergency management plan will be declared to be accomplished.
Based on the Technical Specification for Monitoring and Measurement of Railway Tunnels, and combined with the construction monitoring and measurement design plan of this project, the construction risk evaluation index system of CD-Bench composite method based on the monitoring method rely on the Changping section tunnel of Guanhui Intercity Subwayrisk warning indicator system for the CD-bench composite construction method in the Changping section of the Guanhui intercity is equationted, it contains the following five first-level indicators: geological and supporting condition, crown settlement, clearance convergence, ground settlement and building settlement, The complete index system is shown in The geological and supporting conditions will seriously affect the construction safety. By monitoring the risk factors such as groundwater, initial branch cracks, surrounding rock characteristics, state of seepage and fault fracture zone, and their quantitative values are determined according to the quantitative rules of qualitative indicators, in order to the risk level of geological and supporting condition can be evaluated. Crown settlement, refers to the absolute settlement (quantity) of the tunnel crown measuring point, is a key indicator that characterizes the level of risk during the construction phase. Through the measurement of the absolute settlement and settlement rate of the vault measurement points, it will pave the way for the overall risk assessment. The change of the relative position between two points in the tunnel is characterized by the clearance convergence value. Therefore, by measuring the absolute convergence value and convergence rate of this indicator, which also is a key indicator to evaluate the risk status. As urban subway tunnel construction generally causes surface settlement, the measurement of the absolute settlement and settlement rate of the surface measurement points, which can reflect the level of surrounding rock, the quality of the construction level, and the magnitude of the risk to a certain extent. During underground construction, it is inevitable that the displacement and settlement of the building (construction) within a certain range, which may cause danger to existing buildings (structures), so by measuring their absolute settlement and settlement rate can also reflect the risks in the construction phase.
Researcher Cai Wen, a Chinese scholar, created a new subject—Extension, which assumes that the name of the thing is denoted as
According to the extension theory, classical domain is expressed as in
where:
The controlled domain is expressed as in
For things T to be evaluated, the collected data is expressed in the form of objects, that is, the matter element to be evaluated R, which is expressed as:
The correlation degree of the i-th single evaluation index to the j-th category level of the matter element to be evaluated can be expressed as:
The scientific level of weight directly affects the objectivity of the calculation results of the whole evaluation model. At present, the method of subjective and objective combination weighting is channeled into solving the weight problem, which leads to the favorable results have been achieved in practical applications. Therefore, this paper uses entropy method and improved analytic hierarchy process to determine the subjective and objective weight separately, and then introduce the distance function to combine the subjective and objective weights, and then determine the weight value of each evaluation index.
Set
In order to facilitate the comparison of indicators, this paper selects the improved three-scale analytic hierarchical process [ Establishment of a cluster judgment matrix
Based on the construction risk-warning index system of CD-Bench composite method which has been established, First, according to the “0”, “1”, and “2” three-level scale method, the relevant experts are asked to compare the importance of the same level indicators in pairs, and the feedback situation is handled by the cluster method; Secondly, To report back the processed results to the experts and ask the experts to re-mark. Finally, we re-feedback the index importance scale to the experts. After three rounds of consultation, if the opinions of the experts tend to be unified, we will take the last round group judgment matrix as the final result of the evaluation index system and enter the next analysis process.
Where, the cluster judgement matrix is to process the scores of the experts on the same index according to the following mathematical treatment:
Finally, after mathematical processing, the group judgment matrix A can be obtained: Converting comparison matrix A into judgement matrix D
Calculate the arithmetic mean of all elements in each row of the judgment matrix D and normalize to obtain the weight
Consistency testing Calculate the consistency index
where: (
Find the corresponding average random consistency index RI (
Calculation consistency ratio
n | 1 | 2 | 3 | 4 | 5 | 6 |
---|---|---|---|---|---|---|
RI | 0 | 0 | 0.52 | 0.89 | 1.12 | 1.26 |
When
Suppose the subjective weight is
By multiplying the comprehensive weight vector
Same as solving the first-level index risk membership matrix, the membership matrix of the target layer is shown as below:
If
According to the principle of setting the alarm level and the construction risk characteristics of this type of weak surrounding rock section, the construction risk of this project is divided into five levels, which are shown in
Risk grade | Risk status | Treatment measures |
---|---|---|
5 | High risk | Stop construction and take special control measures |
4 | Higher risk | Stop construction, reinforced support and measurement |
3 | Medium risk | Support should be reinforced |
2 | Low risk | Normal construction, pay more attention |
1 | Security | Normal construction |
The five-color method is applied to set the alarm signal [
Risk-warning grade | Definition | Indicator control |
---|---|---|
Red pre-warning | Especially urgent, it is suitable for large-scale roof collapse or mud burst, which will lead to a fatal safety accident and bring consequences of disaster | Both the rate and the cumulative absolute values of the measured deformation have reached the limit. And one of the following two situations occurs: the measured rate of settlement (or displacement) has increased rapidly, or cracks have appeared on the surface of the concrete of initial support, and started to seep water |
Orange pre-warning | Emergency, suitable for local roof collapse or mud burst, which will cause major safety accidents and affect tunnel construction seriously | Both the rate and the accumulative absolute value of the measured deformation reach 85% ∼100% of the limit value. Or one of the dual control indicators has reached the limit value and the other has not; Or the dual control indicators both reach the limit value, but the whole project has yet to show no signs of insecurity |
Yellow pre-warning | More urgent, it is suitable for rock falling, water bursting, mud bursting or partial collapse. Although it will not cause major safety accidents, it will still affect the tunnel construction | Both the rate and the accumulative absolute value of the measured deformation reach 70% ∼85% of the limit value, Or one of the dual control indicators reaches between 85%∼ 100% of the limit value, and the other index has not yet reached |
Blue pre-warning | General emergency, suitable for areas with a small amount of water flow or large deformation risk, which can induce collapse, but generally will not cause safety accidents and affect the tunnel construction | One of the rate and the accumulative absolute value of the measured deformation exceeds 70% of the control value, but the curve has no unstable trend yet |
Green pre-warning | No need for measures | Security |
Let the value range of the dynamic risk index and risk-warning signal be the Ux and Uy respectively, f(x) represents a mapping from Ux to Uy, recorded as Y = f (x), and the mapping relationship between dynamic risk index and risk-warning signal is shown in
Refer to the research results of Zhang et al. [
Indicators | Construction risk evaluation grade | |||||||
---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | ||||
A1 | B1 | Description | waterless | Leakage | Dripping water | Gushing water | Flowing water | |
Rating values | 0∼0.15 | 0.15∼0.35 | 0.35∼0.55 | 0.55∼0.75 | 0.75∼1 | 0.32 | ||
B2 | Description | None | Microstructure | Weaknesses | Strong | Drama | ||
Rating values | 0∼0.15 | 0.15∼0.35 | 0.35∼0.55 | 0.55∼0.75 | 0.75∼1 | 0.34 | ||
B3 | Description | Stability | basically stable | Poor local stability | Instability | Extremely unstable | ||
Rating values | 0∼0.15 | 0.15∼0.35 | 0.35∼0.55 | 0.55∼0.75 | 0.75∼1 | 0.17 | ||
B4 | Description | waterless | mild | Medium | Severe | overall water gushing | ||
Rating values | 0∼0.15 | 0.15∼0.35 | 0.35∼0.55 | 0.55∼0.75 | 0.75∼1 | 0.30 | ||
B5 | Description | None | Tensional | Less | More | Broken | ||
Rating values | 0∼0.15 | 0.15∼0.35 | 0.35∼0.55 | 0.55∼0.75 | 0.75∼1 | 0.14 | ||
A2 | B6 (%) | Measured values | 0∼60 | 60∼70 | 70∼80 | 80∼90 | 90∼100 | |
Dimensionless processing | 0∼0.60 | 0.60∼0.70 | 0.70∼0.80 | 0.80∼0.90 | 0.90∼1 | 0.78 | ||
B7 (mm/d) | Measured values | 0∼1 | 1∼2 | 2∼3 | 3∼5 | 5∼10 | ||
Dimensionless processing | 0∼0.10 | 0.10∼0.20 | 0.20∼0.30 | 0.30∼0.50 | 0.50∼1 | 0.31 | ||
A3 | B8 (%) | Measured values | 0∼60 | 60∼70 | 70∼80 | 80∼90 | 90∼100 | |
Dimensionless processing | 0∼0.60 | 0.60∼0.70 | 0.70∼0.80 | 0.80∼0.90 | 0.90∼1 | 0.81 | ||
B9 (mm/d) | Measured values | 0∼1 | 1∼2 | 2∼3 | 3∼5 | 5∼10 | ||
Dimensionless processing | 0∼0.10 | 0.10∼0.20 | 0.20∼0.30 | 0.30∼0.50 | 0.50∼1 | 0.25 | ||
A4 | B10 (%) | Measured values | 0∼60 | 60∼70 | 70∼80 | 80∼90 | 90∼100 | |
Dimensionless processing | 0∼0.60 | 0.60∼0.70 | 0.70∼0.80 | 0.80∼0.90 | 0.90∼1 | 0.85 | ||
B11 (mm/d) | Measured values | 0∼1 | 1∼2 | 2∼3 | 3∼5 | 5∼10 | ||
Dimensionless processing | 0∼0.10 | 0.10∼0.20 | 0.20∼0.30 | 0.30∼0.50 | 0.50∼1 | 0.22 | ||
A5 | B12 (%) | Measured values | 0∼60 | 60∼70 | 70∼80 | 80∼90 | 90∼100 | |
Dimensionless processing | 0∼0.60 | 0.60∼0.70 | 0.70∼0.80 | 0.80∼0.90 | 0.90∼1 | 0.77 | ||
B13 (mm/d) | Measured values | 0∼1 | 1∼2 | 2∼3 | 3∼5 | 5∼10 | ||
Dimensionless processing | 0∼0.10 | 0.10∼0.20 | 0.20∼0.30 | 0.30∼0.50 | 0.50∼1 | 0.13 |
Taking the secondary index of the Determine the degree of risk association
According to
It can be obtain according to
Then the objective weight of B1–B5 is calculated as:
Index | A1 | A2 | A3 | A4 | A5 | B1 | B2 | B3 | B4 |
---|---|---|---|---|---|---|---|---|---|
Subjective weight | 0.49 | 0.14 | 0.26 | 0.05 | 0.06 | 0.20 | 0.21 | 0.24 | 0.15 |
Objective weight | 0.42 | 0.20 | 0.21 | 0.07 | 0.10 | 0.20 | 0.22 | 0.20 | 0.20 |
Combination weight | 0.46 | 0.17 | 0.24 | 0.06 | 0.08 | 0.20 | 0.21 | 0.22 | 0.17 |
Index | B5 | B6 | B7 | B8 | B9 | B10 | B11 | B12 | B13 |
Subjective weight | 0.20 | 0.25 | 0.75 | 0.25 | 0.75 | 0.25 | 0.75 | 0.25 | 0.75 |
Objective weight | 0.19 | 0.40 | 0.60 | 0.40 | 0.60 | 0.40 | 0.60 | 0.40 | 0.60 |
Combination weight | 0.20 | 0.32 | 0.68 | 0.32 | 0.68 | 0.32 | 0.68 | 0.32 | 0.68 |
Take the first-level indicator layer as an example, according to
Find out
The obtained objective weight and subjective weight are brought into
It can be observed from
According to
The membership degree of other first-level indicators can be obtained in the same method.
According to
Get: K1(T) = −0.3175; K2(T) = −0.0688; K3(T) = −0.1064; K4(T) = −0.3085; K5(T) = −0.5513.
It can be get accord to
The risk grade characteristic value (
By substituting the daily measurement data into the construction risk-warning model established in this paper, it can be getting that risk grade characteristic value which reflects the construction risks at this stage was 2.50. Thus the risk-warning signal of this evaluation is “yellow”, namely the construction risk is level 3, indicating that the phenomenon of local collapse and water and mud inrush have occurred at this time, which will affect the tunnel construction, but will not cause major accidents. Among the crown settlement rate, clearance convergence accumulation and ground subsidence accumulation are the higher risk sources, which are classified grade 4. And the vault settlement accumulation, clearance convergence rate, ground subsidence rate and building settlement accumulation are medium risks, which are classified grade 3. Therefore, the construction party should take corresponding emergency measures and safety schemes based on the above judgment.
According to the risk-warning results, the relevant units negotiated to change the original design and construction plan, as follows: Long anchors are added to the crown. Due to the relatively complex construction force at the crown, and the phenomenon of deformation exceeding the alarm value is particularly frequent. In order to control the further development of the risk unit at this location, it is recommended to add long anchors to the top of the tunnel arch in time. Increase the amount of deformation reserved. On the one hand, it can offset the large displacement generated by the initial support, release the in-situ stress to a large extent, and effectively prevent the problem of the exceeding the limit of initial support after the large deformation. On the other hand, it can reduce the load acting on the secondary lining, which is beneficial to the safety and stability of the tunnel structure. Therefore, it is recommended to adjust the reserved deformation of individual dangerous areas to 30–40 cm. Improve the rigidity of the supporting structure. According to the characteristics of deeper plastic zone and large damage range of soft rock tunnel, Measures should be taken to enlarge the steel frame, such as: the enlarged steel frame is adopted, the I-beam is adjusted from I20b to H175, longitudinal spacing is adjusted from the current 0.6 m to 0.5 m, and the large stiffness support system is sprayed with 30 cm-thick C25 concrete.
This paper relies on the Changping section tunnel of Guanhui Intercity Metro, a dynamic risk-warning model is established for the construction process of subway tunnel with the CD-Bench composite method. The following conclusions can be drawn: Due to the particularity and complexity of subway tunnel construction, most of the index systems established in previous studies are qualitative indexes and difficult to quantify. In the actual construction process, the deformation value can reflect the construction risk simply and directly. Therefore, based on this feature, this article equationtes a monitoring plan based on the monitoring specifications and the project characteristics, and establishes a risk evaluation index system on this basis, avoiding the difficulty of quantifying qualitative indicators. In addition, assigning different combination weights to each indicator and the mathematical model is used to calculate the risk value, which improves the one-sidedness and irrationality of evaluating construction risks based on deformation alone, and promotes scientific research on dynamic risk control. Among the four indexes of crown settlement, clearance convergence, ground settlement, and building settlement, clearance convergence has the greatest impact on construction safety risks. For their two secondary indicators, the rate of change is much greater than the cumulative change value contributing to the risk value of the upper indicator. Therefore, in actual construction, it is necessary to appropriately increase the frequency of monitoring for poor geological conditions or abnormalities in monitoring data, and focus on the settlement value of clearance convergence. Through analysis and calculation, the initial construction risk characteristic value of this stage is 2.50, and the early warning signal is yellow, indicating that a major accident may occur at present, and control or rectification measures must be taken. For this reason, the construction unit recommends measures such as long anchors are added to the vault, increase the amount of deformation reserved and the stiffness of supporting structures and so on. After the calculation is performed again, the warning signal turns green, showing that the construction risk status at this stage is safe after taking these measures, indicating that these technologies can reduce the construction risk value and provide technical reference for similar projects.
In the future, we will continue to explore the applicability of dynamic risk-warning; the comparative analysis will also be conducted on different theoretical methods to achieve higher prediction accuracy.