不飽和含水状態にある岩石の封圧下における繰り返し疲労特性

Abstract

In recent years, span of bridges has become longer and machine generating vibration has become larger as in Honshu-Shikoku bridge and dynamos in power station. Moreover, so-called soft rock mass must frequently be selected as foundations of such as bridges and power stations. In these cases, rock mass is subjected to severe cyclic loading in comparison to its strength.F or the safe design of these rock foundations, it is prerequisite to clarify the cyclic fatigue charactericstics of unsaturated rocks including soft rocks under confining pressure.In this work, cyclic fatigue tests using 1 Hz sinusoidal wave of load were conducted under confining pressure for two kinds of unsaturated rocks; Kimachi sandstone and Hirono sandy mudstone. Their uniaxial compressive strengths are 420 and 40kg/cm2, respectively.From these tests, following results were mainly obtained.(1) Endurance limit was found only for sandstone, but not for sandy mudstone within the maximum cycle of 106 in this study as shown in Fig. 3. Endurance limit of sandstone increases with confining pressure.(2) Number of cycles to failure increases with confining pressure for both rocks. However, for sandy mudstone, rate of increase in number of cycles to failure with confining pressure decreases with confining pressure.(3) Main failure mechanism is so different between two rocks in respect to volumetric strain in cyclic fatigue process. Namely, fatigue process proceeds with dilatancy in sandstone, but with contractancy in sandy mudstone especially under larger confining pressure as shown in Figs. 6 and 8. This difference could explain the facts described in (1) and (2). As confining pressure suppresses dilatancy in sandstone, endurance limit of sandstone increases with confining pressure. On the other hand, number of cycles to failure of sandy mudstone does not increase with confining pressure as compared to sandstone because contractancy causes excess pore pressure which lowers the effective confining pressure.