Study on Strength Recovery of Loosen Zone

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Rock around an underground opening such as tunnel or gateway may fail. It is usually considered that loosen or damaged zone is gradually extending with elapsed time and stability is gradually decreasing with time going.Recently, it was pointed out that loosen zone might be recovered in strength under a proper condition. For example, if a tunnel is supported with a material of enough strength and stiffness, the loosen zone between supports and surrounding rock is compacted and consolidated. Finally, the stability will be gradually increasing.Very few studies on strength recovery of loosen zone or failed rock have been done up to now. In this study, strength recovery of Dotan (siltstone or mudstone), Tage tuff, Kimachi sandstone and Sanjome andesite is investigated by the following experimental procedures :1) A sample rock (a cylindrical specimen) is inserted into a small pipe and then broken by a servo-controlled testing machine.2) Load is increased to the predetermined value and then the broken rock is compacted in the pipe.3) The compacted rock is pulled out from the pipe and uniaxial compression or indirect tension test is carried out. The experimental results indicate that :1) Strength recovery is verified in both uniaxial compression and indirect tension tests.2) A degree of strength recovery varies from rock to rock. In Dotan, strength recovery is most remarkable among the 4 rocks. On the other hand, Sanjome andesite shows only poor strength recovery.3) Strength recovery is much higher under air-dried condition than water-saturated condition.