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The actual "Code for Design of Road Tunnel" (JTG D70-2004) divides rock mass into six grades,however, gradeⅢ,Ⅳ,Ⅴof rock spans a little long, it’s not convenient to apply in practice. So many design company divide gradeⅢ,Ⅳ,Ⅴinto sub class based on practice. Nevertheless, these company have great difference in how to rate sub class of rock mass and the index system being used, because their purpose are different. So it leads to confusion, and lacks contrast.The actual code lacks method to rate rock mass in construction moment. The method in common use bases on observation of working plane. But most of these index gained by observation are qualitative, so the method has great subjectivity. Moreover, in constructing practice, when rock mass grade needs to adjust half grade based on practical geologic condition, engineers have to adjust one grade because the actual code does not explain how to rate sub class. This result in great waste,but if engineers want to adjust half grade,they have to redesign, and it’s not convenient in working place.So, we need to establish a suit of utility metood to rate sub class of rock mass in construction moment. This can contribute to rate rock mass impersonally.This paper discusses the application of the theory of qualification to rate the sub classes of rock mass classification when rock road tunnel is in construction moment.Firstly, research the index being used by kinds of rock mass classification method, and also research these 32 highway tunnel which are under consreuction, so establish the index system being used to rate sub class of rock tunnel,these index are: rock properties, degree of rock mass integrity and status of underground water. After the index system is established, the writer establish an mathematics model to quantitatively evaluate rock mass classification by bring to bear the theory of qualification,, figure out the modulus to rate the sub classes of rock mass classification, and establish the sub classes determinant equation.Considering the practice of engineering, one works out relevant software system using Visual C++ as workspace. This software system can transfers information of rock mass into 0～1 index quickly and effectively, accordingly , figure out which class does rock mass belong to.
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