Risk Assessment on Constructors during Over-water Riprap Based on Entropy Weight and FAHP

Being aimed at waterway regulation engineering, there exist risks of over-water riprap for constructors which keeps uncertainty and complexity. For the purpose of evaluating the possibility and consequence, this paper utilizes fuzzy analytic hierarchy process with abbreviation of FAHP to do empowerment on the related risk indicators, constructs FAHP under entropy weight and establishes relevant evaluation factor set and evaluation language for constructors during over-water riprap construction process. Through doing risk probability estimation and risk consequence size evaluation on the factor of constructors, this paper introduces this model into risk analysis on constructors during over-water riprap of Ching River waterway regulation project. Results show that evaluation of this method is so credible that it could be utilized in practical engineering.


Introduction
Under the strategic guidance of Yangtze River economic zone, comprehensive transportation network has been constructed around Yangtze River which has already achieved initial success through doing integration on waterway, highway, railway and airline. However, much potential in Yangtze River shipping still remains to be fulfilled. It is particularly reflected in obstruction of upper and middle reaches of channel, which seriously hinders promotion of effective gathering and transferring system and improvement of proportion of high-level channel [1]. "Intestinal obstruction" of Jingjiang River channel is so serious that development of Yangtze River economic zone is greatly restricted. Therefore our government invested 4.3 billion yuan to dredge the "nine-bend and eighteen-curve" waterway of Jingjiang River. Meanwhile flood control is also taken into consideration whose main construction technologies include paving, rip-rapping, permeable frame flipping and slope protection with gabion mesh, etc. However, its construction quality may be influenced by multiple factors which keep various uncertainties. Therefore, it seems particularly important to make rational analysis on typical qualitative indexes.
Analytic hierarchy process is adopted for quantitative analysis on qualitative elements. However, the analysis needs frequent adjustment and consistency examination which increases calculation workload. Then FAHP is established to do improvement. Li Fawen[2] takes rural drinking water system as example and utilizes FAHP to do calculation on weights of dominant factors in brittle hierarchy structure thus evaluating safety of rural drinking water system from brittleness angle. Aiming at one tunnel collapse, Chen Jiejin [3]  construction site, whose conditions are identical. Jin Lianghai [4] constructs the comprehensive assessment model FAHP to do comprehensive evaluation and analysis on all involved indexes, which exempts deficiency of much adjustment and consistency examination of AHP and improves computational efficiency. The model is introduced in XiaoWan hydro-power project of Yunnan Province, whose reliability acquires verification. However, subjective factors of experts during its analysis process greatly influence the final results thus decreasing the credibility of analyzing results. For the purpose of overcoming the subjectivity and uncertainty in the determination of risk assessment index proportion, entropy coefficient is introduced into fuzzy comprehensive evaluation to implement abundant research. For example, Gu Yujiong [5] introduces entropy weight, combines it with AHP and does scientific analysis on equipment maintenance mode of power station through considering objectification of experts' opinions. Researchers like Wu Dingyong [6] adopt matterelement entropy coefficient model to do safety evaluation on navigational environment of five shipping lanes. In a practical engineering project, researchers like Zhang Lingwan [7] compared the entropy coefficient fuzzy comprehensive evaluation with fuzzy comprehensive safety evaluation which verifies the rationality of model. In the assessment on flood risk degree of Chongqing-Guizhou expressway, researchers like Chen Hongkai adopted the method and got an accurate assessment. Luo Jungang [9] constructed entropy coefficient fuzzy comprehensive evaluation model and did evaluation analysis on risk of water shortage. Yang Yuzhong [10] adopted entropy coefficient fuzzy comprehensive evaluation method to assess and analyze safety of coal transportation.
Focusing on the complicity and uncertainty of risks for constructors in rip-raping of Jingjiang River Waterway regulation project, this paper adopts entropy coefficient method to analyze risk indexes of constructors in rip-raping of waterway regulation project, constructs FAHP under entropy coefficient and establishes relevant evaluation factor set and evaluation markers on constructors in riprapping to assess and analyze the risk level which accurately reflects risk level of constructors, makes assessment results keep reliability and has certain application value in physical engineering.

Fuzzy analytic hierarchy process
Analytic Hierarchy Process (AHP) sets out all factors which are involved in analyses and decisions and divides them into levels of objective, principle, scheme, etc. On the basis of it, "qualitative analysis and quantitative analysis" could be realized. Decisions or assessment process can be quantified according to ideal and psychological rules. Since the method is especially pragmatic and effective for complicated assessment problems, it is widely used in risk assessment in various fields.
The key to AHP is the construction of assessment matrix. For sake of meeting requirements of adjustment and consistency examination, the matrix should be adjusted repeatedly, which is quite time-wasting and inefficient. If analysis keeps many layers and high complicity, it would be rather difficult to modify the judgment matrix. Therefore the idea of introduction of fuzzy math is utilized to determine assessment results during construction of judgment matrix. Generally speaking, there are reciprocal judgment matrix, complementary judgment matrix and mixed judgment matrix [11]. This paper adopts fuzzy complementary judgment matrix. That is, if matrix a a a , it is a fuzzy complementary judgment matrix.

Entropy and entropy coefficient
In information science, there exists information entropy which presents the uncertainty or the probability an object may occur. It is a measurement of the disorder degree of a system. From the practical use of a piece of information, the size of entropy stands for the amount of information. If the information amount is very large, then the event will be of great certainty and the entropy value will

Construction of Fuzzy Analytic Hierarchy Process Assessment Model-Entropy Coefficient
For sake of lowering the risks, hydro geological conditions, raw materials, shipping equipment and construction site during rip-raping process of waterway realignment are all strictly controlled. However, it is complicated and difficult to control worker element in the process. As it could not be ignored, it would be necessary to make risk assessment for constructors. Therefore, AHP is adopted and fuzzy math is introduced to "obfuscate" the judgment matrix. After that a fuzzy comprehensive judgment matrix is constructed and entropy coefficient is utilized to make the result be more objective.

Index factor set of risk assessment for constructors in rip-raping
According to relevant materials and field research, risk carriers in rip-raping can be divided into six types including raw materials, shipping equipment, hydro geological conditions, constructors, environment inside and outside the construction site. This paper focuses on the risk assessment on constructors in rip-raping and makes a risk factor set U , which analyses the probability and consequences of risks for constructors in the construction. It is shown in figure 1.

Risk assessment markers
Risk assessment on workers in rip-raping is aimed at highlighting constructors. The probability and consequences of the risks are put into five grades. For the probability, there are extremely low, low, medium, high and extremely high. The formula is  

Calculation and judgment on overall weight
In the entropy coefficient comprehensive assessment model, there are subjective weight in FAHP and objective entropy coefficient in fuzzy comprehensive assessment as well as entropy coefficient process.

Overall assessment result.
For single-level result of overall assessment, fuzzy comprehensive evaluation is adopted based on formula: .So forth, the fuzzy comprehensive evaluation result of the top layer is B . This study focuses on the importance of a factor and processes the fuzzy evaluation result with the maximum subordination principle. That is:

Determination of Assessment indexes
In order to issue management document of strong pertinence to prevent risks and guide workers, factors influencing the safety of workers must be collected and analyzed through entropy coefficient-FAHP. All risk factor indexes can be seen in figure 1 whose assessment markers are in 2.