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[SCG45-P11] Change in the physical properties of sedimentary soft rocks with heat duration
Keywords:sedimentary soft rock, heating duration, consolidation, tensile strength, P-wave velocity
Temperature of underground is higher than that of the surface due to thermal gradient. Recently, carbon neutrality is focused due to global warming all over the world and the importance of the facilities at great depth has been increasing. With the increase in these interests in great depth, the importance of studies about the behavior of rocks under high temperatures has simultaneously been increasing. The tensile strength of granite under a high temperature or heated at a high temperature decreases with the temperature increase though that of andesite increases slightly or does not change with temperature increase. It is obvious that the relationship between tensile strength and temperature varies depending on the kind of rocks. However, there are a limited number of previous studies focusing on relationships between temperature and physical properties of sedimentary soft rocks and it is unclear how the physical properties of sedimentary soft rocks vary with temperature increasing and/or heating duration at a high temperature. In this study, we focused on the change in the physical properties of siltstone with heating duration. We performed consolidation tests to examine the effect of heating duration on the consolidation properties such as consolidation yield stress. Additionally, P-wave velocity measurements and Brazilian tests were performed to examine quantitatively the change in the physical properties with heating duration.
The samples used in this study are siltstones collected from the Kazusa Group located in the eastern part of the Boso Peninsula in Chiba prefecture, central Japan. Cylindrical specimens were drilled from fresh and intact rock blocks and perpendicular to the bedding plane. After cutting these cores to appropriate lengths, the specimens were placed in heat-resistant containers with water and heated at around 90°C in the oven. We set three conditions of the heating durations in this study, i.e., unheated (room temperature), heated at 90°C for 1 day and heated at 90°C for 1 month. Consolidation tests, Brazilian tests and P-wave measurements were conducted at room temperature. The consolidation yield stress and the compressive index were derived from consolidation curves (e-logp) of consolidation tests by the Mikasa’s graphical method. Tensile strength was derived from maximum load. The method to measure P-wave velocity was transmission measurement and P-wave velocity was derived from travel time of the first arrival.
The consolidation yield stress and the average of tensile strength decreased after heating for 1 day and, after heating for 1 month, became almost the same as those of unheated specimens. The average of P-wave velocity increased after heating for 1 day and, after heating for 1 month, nearly reached that without heating. Comparing porosity before and after heating, though it decreased after heating for 1 day, it increased after heating for 1 month. New water-induced cracks caused the decrease in the consolidation yield stress of the specimens heated for 1 day because they can be generated due to heating. The decrease in porosity and the increase in P-wave velocity might be caused by the contraction of pore due to thermal expansion of particles. Tensile strength of the specimens heated for 1 month increased, which indicates that interparticle bonding is stronger due to the long-time heating.
The samples used in this study are siltstones collected from the Kazusa Group located in the eastern part of the Boso Peninsula in Chiba prefecture, central Japan. Cylindrical specimens were drilled from fresh and intact rock blocks and perpendicular to the bedding plane. After cutting these cores to appropriate lengths, the specimens were placed in heat-resistant containers with water and heated at around 90°C in the oven. We set three conditions of the heating durations in this study, i.e., unheated (room temperature), heated at 90°C for 1 day and heated at 90°C for 1 month. Consolidation tests, Brazilian tests and P-wave measurements were conducted at room temperature. The consolidation yield stress and the compressive index were derived from consolidation curves (e-logp) of consolidation tests by the Mikasa’s graphical method. Tensile strength was derived from maximum load. The method to measure P-wave velocity was transmission measurement and P-wave velocity was derived from travel time of the first arrival.
The consolidation yield stress and the average of tensile strength decreased after heating for 1 day and, after heating for 1 month, became almost the same as those of unheated specimens. The average of P-wave velocity increased after heating for 1 day and, after heating for 1 month, nearly reached that without heating. Comparing porosity before and after heating, though it decreased after heating for 1 day, it increased after heating for 1 month. New water-induced cracks caused the decrease in the consolidation yield stress of the specimens heated for 1 day because they can be generated due to heating. The decrease in porosity and the increase in P-wave velocity might be caused by the contraction of pore due to thermal expansion of particles. Tensile strength of the specimens heated for 1 month increased, which indicates that interparticle bonding is stronger due to the long-time heating.