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Abstract The mechanical response and unloading mechanism of rock are the key issues to evaluate and control stability for rock mass engineering excavated under high geostress. Based on the unloading confining pressure tests with triaxial high stress under conditions of different unloading rates and different initial confining pressures，combining the fractal theory and energy principle，the distribution rule of rupture fragmentation of marble specimens and its correlation with strain energy transformation are investigated. The characteristics of fragment fractal of marble specimens fractured is obvious locality under unloading confining pressure with triaxial high stress；and only in the range of sizes which is smaller than a certain features scale(fractal feature sizes threshold)，it presents good fractal property. The fractal dimensions of all fragmentation are greater than 2 in the test. The fractal dimension monotonously decreases with unloading rate increasing；and the influence of initial confining pressure on it closely relates to the unloading rate. Comparing with conventional triaxial compression test specimens，the dissipation and storage strain energies of rock specimens under high confining pressure near peak point are much smaller；but the rates of strain energy transformation before and after peak point under high confining pressure are much larger；especially the rates of elastic strain energy release and strain energy dissipation by confining deformation. The unloading rate is faster and the initial confining pressure is higher，the time of damage before peak and whole crack rupturing after peak are shorter；the dissipation strain energy and storage elastic energy are larger；the transformation rate of energy before and after peak is faster；the threshold of characteristics size of fractal for broken rock sample is bigger；the fractal dimension is smaller and tensile fracture is more obvious.
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2012, Vol. 31 
