Shen Jian, Chen Sumeng
Shenzhen Municipal Government Investment Project Evaluation Center AECOM Design & Consulting (Shenzhen) Co., Ltd
Abstract: The principle analysis of the index system method recommended in the "Guidelines for Safety Risk Assessment of Highway Cutting High Slope Engineering" is carried out, and the conclusion that "the improvement direction is to expand the evaluation index and eliminate the subjective bias of weight" is obtained. From the perspective of eliminating the subjective bias of weights, this paper innovatively applies the ranking test method to the safety risk assessment of the construction of high cutting slope engineering, and ranks the importance of the indicators based on the collected 30 engineering examples, and calculates the weights. The calculation results are applied to the risk assessment of the slope of a new expressway, and according to the follow-up investigation, it can be seen that the probability of slope deformation is low, and the main reason for the deformation is that the protection after excavation is not timely, which verifies the operability and superiority of the ranking test method in the construction risk assessment of the high cutting slope.
Keywords: high slope of road cuts; construction risk assessment; Ranking test method; weight calculation; indicator system method;
About author:SHEN Jian (1991—), male, engineer, engaged in research on geological disasters such as landslides and debris flows.
0 Introduction
The mountainous landform, fragile geological environment [1,2], and the planning of a huge highway network [3] have caused frequent deformation and damage accidents of high cutting slopes during the construction period, which not only delays the construction period, but also causes casualties and economic losses [4,5]. Based on this, the Ministry of Transport organized the compilation of the "Guidelines for the Construction Safety Risk Assessment of Highway Cutting High Slope Engineering" [6] (hereinafter referred to as the "Guide"), which requires that the construction safety risk assessment of high cutting slope should be carried out in the implementation stage of the project, so as to effectively control the construction safety risk, scientifically avoid the occurrence of construction safety accidents, and ensure the construction safety of the high cutting slope.
The Guidelines recommend the use of the index system method for the overall risk assessment of the construction safety of high cutting slopes, which is simple in principle and highly operable, and has been welcomed by many engineers [7].
Through the analysis of the principle of the indicator system method, it can be seen that the improvement direction of the method is to expand the evaluation index and eliminate the subjective bias of the weight. From the perspective of eliminating the subjective bias of weights, the ranking test method is innovatively introduced to calculate the index weights, and the calculation results are applied to the actual project, so as to carry out follow-up investigation of the project. By analyzing the survey results, the operability and superiority of the ranking test method in the construction risk assessment of high cutting slope are verified.
1. Research on the improvement of evaluation methods
1.1 Principles of the indicator system method
The principles of the index system method recommended by the Guidelines are briefly described as follows:
(1) Select the appropriate evaluation index from the 11 indicators of slope height, slope shape and slope ratio, stratum lithology, slope structure, groundwater, construction season, impact of natural disasters, type of engineering measures, surrounding environment, geological data, and design documents, and carefully check the design data on the basis of geological survey according to the grading criteria given in the guide, and score FI for a single index, which is the score of the ith index.
(2) The weight value is determined according to the importance of the index, and the calculation method is as follows:
In equation (1), n is the number of indicators, m is the importance ranking number, and ri is the weight coefficient of the ith index.
The Guidelines do not provide a method for determining the ranking of the importance of each indicator, and in actual engineering, it is mostly determined by the experience of engineers or experts [8].
(3) The weighted sum of each index is obtained to obtain the risk assessment value R, see equation (2):
According to the calculation results, the construction risk level of the cutting high slope is determined according to the grading standard of the guideline.
1.2 Improve direction analysis
The principle of the index system method is analyzed, and the improvement direction of the method is as follows:
(1) The index is limited, and the factors affecting the construction risk of the cutting slope cannot be fully considered, such as the level of the construction team;
(2) The weight determination method relies on the subjective judgment of engineers, which can easily lead to large deviations in the evaluation results between different individuals.
In summary, the improvement of the existing evaluation direction is to expand the evaluation index and eliminate the subjective bias of weight. Due to the limited work done, this paper only improves the indicator system method from the direction of eliminating the subjective bias of weights.
2 Research on the ranking test
The ranking test is a subjective test method in which multiple samples are sorted out according to their characteristics or overall impression [9], and this method is mostly used in the food field to reduce the bias of individual subjective judgment by increasing the number of samples to obtain an accurate and applicable index importance ranking [10,11]. The principle is briefly described as follows:
(1)确定指标集U={u1,u2,... ,un}(ui为第i个指标)。
(2) Investigate m evaluators, sort the index ui, and obtain the evaluation set Vi={v1,v2,...,vm} (vj is the ranking of the ith indicator by the jth evaluator).
(3) then the sum of the order of each index can be obtained as equation (3):
则可得秩序集为W={w1,w2,... ,wn}(wi为第i个指标的秩序和)。
(4) The significant difference and the degree of difference are tested on the order set to determine the importance of the index.
Through the analysis of the principle of the ranking test method, it can be seen that the method is not complex and has strong operability. In this paper, the ranking test method is intended to be used to determine the weight of safety risk assessment of highway cutting high slope engineering construction.
3. The weight of the risk assessment index of slope construction is determined
3.1 Collation of evaluation results
In this paper, 30 engineering project examples are collected, and the evaluation results of the index importance of different engineers are collated, and the results are shown in Table 1.
Table 1 Sorting result statistics Download the original image
Download the original table
As can be seen from Table 1, different evaluation engineers have different levels of awareness of the importance of indicators, and the evaluation results of different engineers under the existing methods are quite different.
3.2 Result Testing
Based on the number of evaluators of 30 and the number of sensory indicators of 11, the following cut-off values were obtained by checking the ranking test table [12], as shown in Table 2.
Table 2 Significant level (a=1%) Download the original image
Comprehensive analysis of Table 1 and Table 2 shows that:
(1) The upper section is 127~233, the order and minimum values are 90<127, and the order and maximum values are 288>233, indicating that there are significant differences in the importance of evaluation indicators.
(2) according to the following paragraph, (90, 99, 105, 135), < 140< (174, 180, 192, 213), < 220< (249, 255, 288). That is, at the 1% significant level, slope height, slope shape and slope ratio, stratum lithology and slope structure are the most important indicators, followed by groundwater, type of engineering measures, geological data and design documents, and the importance of construction season, impact of natural disasters and surrounding environment is the lowest.
(3) The importance of slope height, slope shape and slope ratio, stratum lithology and slope structure is small. There are little differences in the importance of groundwater, types of engineering measures, geological data, and design documents; There is little difference in the construction season, the impact of natural disasters, and the importance of the surrounding environment.
To sum up, the importance of the 11 indicators is ranked as follows: stratum lithology, slope structure, slope height, slope shape and slope ratio, geological data, groundwater, type of engineering measures, design documents, construction season, surrounding environment, and the impact of natural disasters.
3.3 Weight Calculation
According to the ranking results of importance, the weights of each index are calculated by combining the indicator system method, and the results are shown in Table 3.
Table 3 Metric weight calculation results Download the original image
4. Application of engineering examples and results verification
4.1 Evaluation Results
Taking 134 cutting slopes of a new expressway in Guangdong Province as the research object, the above weight calculation results were used to carry out risk assessment, and the results are shown in Figure 1.
Figure 1 Evaluation results Download the original image
From the assessment results, it can be seen that:
(1) There are 74 grade II construction safety risk slopes, accounting for 55% of the total, and risk prevention and control measures need to be taken for such slopes, strict daily safety production management, and on-site inspections should be strengthened;
(2) There are 60 grade III construction safety risk slopes, accounting for 45% of the total, and measures must be taken to reduce the risk, and the cost of reducing the risk cannot be higher than the loss after the risk occurs;
(3) There are no grade I. and IV construction safety risks in the results of this assessment.
4.2 Verification of Results
After the construction safety risk assessment is completed, corresponding risk control measures are taken according to the assessment results. A three-year dynamic design follow-up survey was carried out on the highway slope project. During the tracking period, 10 of the 134 cutting high slopes had deformation and failure accidents during construction, among them, the number of multi-level deformation and failure slopes was 1, and the remaining 8 slopes were single-stage local landslides during slope construction. Some of the survey results are shown in Figure 2 and Figure 3.
Fig.2. Slope with multi-stage deformation and failure Download the original drawing
Fig.3. Slope with single-stage deformation and failure Download the original drawing
Analysis of the survey results shows that:
(1) The probability of deformation during the construction of the high slope of the highway cutting is 7.46%, and the probability of large deformation is 1.49%;
(2) The main reasons for the deformation are the untimely protection of the slope after excavation and the influence of the landslide.
In summary, after the corresponding risk control measures were taken according to the risk assessment results, the probability of slope deformation was low, and the main reason for the deformation was that the protection after excavation was not timely. The results show that the weights calculated by the index ranking method are reliable and can be applied to the safety risk assessment of highway cutting high slope engineering.
5 Conclusion
(1) The improvement direction of the evaluation method recommended by the Guidelines is to expand the evaluation indicators and eliminate the subjective bias of weights.
(2) The 11 indicators provided by the "Guide" were sorted and tested, and the order of importance was obtained: stratum lithology, slope structure, slope height, slope shape and slope ratio, geological data, groundwater, type of engineering measures, design documents, construction season, surrounding environment, and the impact of natural disasters.
(3) The ranking test method is applied to the safety risk assessment of the construction of a highway cutting high slope project, and according to the follow-up investigation, it can be seen that the probability of slope deformation is low, and the main reason for the deformation is that the protection after excavation is not timely. The method is reliable.
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