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【正文】 ., within 40 m. The range of the vertical stress is clearly smaller after the working face advances 30 m. According to the relationship of the variation between vertical and horizontal stress, the multiplication of the variation of vertical stress and its corresponding coefficient of horizontal pressure (λ) is equal to the increment of horizontal stress at the point M[1]. Then the increment of horizontal stress and the horizontal stress at the point M continues to be superposed, which is inversed analysis when the working face advances 30 m. The results of the variation in stress show that the vertical stress is larger than the horizontal stress when the working face is at its original position: the maximum principal stress is the vertical stress。 the minimum principal stress is horizontal stress. Because the rate of decrease of the vertical stress is faster than the horizontal stress, the horizontal stress is larger than the vertical stress within 42 m when the working face advances 30 m (for details, see Fig. 4). Considering the effect of the variation in vertical stress, the horizontal stress is much larger than the vertical stress. The maximum principal stress is the horizontal stress and the minimum principal stress is the vertical stress. It agrees with the partial reasons of the mechanical principle of floor heave[1214]. Fig. 3 also shows that the variation is almost steady on the section x= when the working face advances 30 m. Therefore, the relationship of variation in stress with depth is calculated when the working face advances from 0 to 30 m. The details are shown in Table 1. Table 1 Data of rock characteristics and correlative stress of the floor on 2702 working face in Yangcun colliery (MPa) 巖層 深度 (m) Δ λ λΔ x=0 m x=30 m x=30 m x=30 m λΔ 泥巖 0 5 砂巖 10 15 21 石灰?guī)r 25 28 From the analysis of the related data, the stresses + λΔ in Table 1 can be regarded as the stress values ， obtained from mechanical rock tests. So the variations of the principal stress from theoretical calculations and the results from the servocontrolled tests can be contrasted. Given these contrasts it is seen that, the largest stress value of mudstone is MPa and the largest stress value of sandstone MPa. When bining Fig. 5 with Table 1 it is seen that, the largest calculated principal stress is less than the peak value of the principal stress in Fig. 5, and the calculated section is at an elastic deformation section of Fig. 5, where permeability is relatively weak. So there is still a certain ability of water resistance. It can be shown that the obvious destruction is not produced in the mudstone and sandstone when the working face advances 30 m. This is essentially consistent with the conclusions of the survey report. 4 Conclusions 1) Based on the mechanical model of the floor, the analysis of stress distribution is obtained on the relatively fixed floor position with an advancing of working face. Owing to heterogeneity and discontinuity of the rock mass of the coal floor, there is a certain divergence between the ideal model and actual conditions. But from analyses and calculations, the basic variation law of stress distribution is discovered on the relatively fixed floor position with an advancing of working face when specific parameters are given for the working face. 2) The decreasing rate of the vertical stress is faster than that of the horizontal stress up to a certain depth and the direction of the maximum principal stress is changed from vertical at the original position to horizontal with an advancing of the working face. The horizontal st
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