ABSTRACT:

Based on the field observation of the abutment pressure, the cracking process of coal mass is stimulated on the real triaxial compress machine, and the whole stress-strain curve is obtained. The theory of fractal dimension and damage mechanics is applied to describe the evolution of fissure system quantificationally, and the relation between the compression strength and the fractal dimension is obtained. The results have guidance meaning towards engineering, and new progress of cracking mechanism of fissured coal mass is made.

I.
INSTRUCTION

As is known to all that self-similarity is the important foundation of fractal dimension theory. So it is rational that the fracture evolution law of large coal sample described by fractal dimension may be generalized to the cracking study of fissured coal mass by the abutment pressure. Based on fractal dimension and damage mechanics, the constitutive equation is deduced.

2.
DEFINITION OF FRACTAL DIMENSION

The length-crack number fractal dimension (Jin, 1996) and the length-square number fractal dimension (Jin, 1994) are defined. The increase of fractal dimension value reflects the growing in number and denseness of the fractures, because the quantity of the same long fractures will not decrease. Moreover, the value decreasing of fractal dimension shows the extension of the fractures and the growing in number of the certain long fissures. Therefore, the fractal dimension value expresses the tendency towards denseness or extension of the fissure system during the pressing cracking process of tile sample.

3.
THE DESIGN OF EXPERIMENT

The coal sample's size is 300mm X 300mm X 300mm and the sampIe is collected from the long wall face whose depth is 300m. The two-dimensional load is used to simulate the state of stress under the action of abutment pressure, and the free end looks like the long wall face. Based on the field observation, the axial stress (б1), that simulates the abutment pressure, is increased from the value of the primitive three dimensional stress (yH) to the peak value (KyH) observed, then, it is decreased to zero gradually. The one side-pressure (б 2 = 0.25 ~ 0.3 б '1), which simulates horizontal stress parallel to the working face, is gradually reduced to zero; the other side-pressure (б3 = U) is looked upon as the work face is being close to. During the experiment, the line drawing of fissure is made continuously to calculate the dimension fractal, and the deformation is measured also.

4.
ANALYSIS OJ EXPERIMENT RESULTS
4.1
Analysis of deformation characteristic

The typical stress-strain curve of the coal sample is shown in Fig, I, which consists of four parts. In the concave part of AB, the strain rate is higher than the stress rate, for the protogenic fissures in the coal are compacted. The approximately straight-line part of BC indicates the elastic deformation of coal sample. In the stress-hardening stage of CD, the strain rate increases quickly, new fissures grow, extend and link up, and the volume of sample expanses. D is the strength limitation.

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