SUZUKI Takasuke Chuo Univ., Inst.Geosci., Professor, 理工学部, 教授 (60055168)
MATSUOKA Norikazu Inst.Geosci., University of Tsukuba Assistant Professor, 地球科学系, 講師 (10209512)
SUNAMURA Tsuguo Inst.Geosci., University of Tsukuba Professor, 地球科学系, 教授 (00011164)
|Budget Amount *help
¥7,100,000 (Direct Cost: ¥7,100,000)
Fiscal Year 1994: ¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 1993: ¥5,200,000 (Direct Cost: ¥5,200,000)
To examine the mechanism of rock-strength reduction due to weathering, temporal changes in microstructure of porous rhyolite were investigated selecting Kozu-shima, a small island in the Pacific, as the study site. In this island four lava domes formed successively with eruption of rhyolite at the time of 1,100,2,600,20,000, and 40,000 y BP.These four rhyolites are mainly composed of glassy groundmass, with flow structure, which is called here "glass columns".
Deep and uniform weathering in each lava dome indicates that the present-day rock properties can be assumed to represent the degree of weathering in the elapsed time from each eruption. Using rock block samples taken from the outcrop in each dome, strengths and pore size distribution (PSD) were measured, and microstructure of rocks were observed by the scanning electron microscope (SEM).
Main results of the measurements are as follows : (1) Both uniaxial compressive strength and tensile strength reduce rapidly in the initial stage
of weathering (for the first 20,000 years) ; (2) Rapid increasing in volume of large pores (about 10 mm in diameter) also occurs in this stage ; (3) A little increase in volume of medium pores (about 1 mm in diameter) occur in this stage, and the rate of the increase accelerated in the later stage (2,000-40,000 years). The SEM observations show that microcracks with a width of 1 mm, which seems to correspond to medium pores in PSD,begins to form on the surface of the glass columns in the initial stage. These evidences suggest that the rapid reduction of rock strength in the initial stage is resulted from increasing in volume of large pores and formation of microcracks.
Increasing in large pores is explained to be caused by the combination of contiguous closed pores into open pores due to "dissolution" of the glassy columns. On the other hand, formation of the microcracks on the surface of the glass columns is explained to be caused by swelling pressure generated in "hydration" process. Above discussion leads to the conclusion that the changes in microstructure due to chemical weathering strongly influence on the reduction of rock strength. Less