Rock around an underground opening may be damaged with excavation. Recently, it was pointed out that damaged zone or fractured rock might be compacted and recovered in strength under a proper condition. Strength recovery will, however, not contribute to long-term stability of an underground structure if time-dependency of the strength-recovered zone is so large and deformation of the region is increasing with elapsed time.
As a first step to understand time-dependent behavior of a strength-recovered or compacted specimen, in this study, loading-rate dependency of the specimen in uniaxial compression was investigated. A rock specimen was, at first, completely fractured and then compacted in a steel cylinder. After pushed out of the cylinder, the compacted rock sample was loaded at alternating loading-rates to obtain loading-rate dependency accurately.
Strength recovery was remarkable for Tage tuff, Kimachi sandstone and Kawazu tuff, but poor for Sanjome andesite. In the alternating loading-rate test, stress fluctuated clearly corresponding to up and down of the loading-rate in both pre- and post-failure regions. For the four rocks, loading-rate dependency of uniaxial compressive strength (peak strength) was found to be similar to the ordinary rock specimens. The result indicated that damaged zone or fractured rock around an underground opening might show time-dependency similar to the surrounding rock mass.