10:45 AM - 12:15 PM
[HRE12-P02] Weathering and alteration driven by deformation in rocks: Importance in the engineering geology dealing with bedrocks
Keywords:landslide, coefficient of internal friction, deformation bands, weathering, smectite, permeability reduction by clay minerals
Engineering geology deals with a variety of subjects, among which the survey and analyses of bedrocks have been an essential part of it. Further, due to disasters caused by a heavy rain fall such as landslides and debris flow, which have been occurring more often than before due to the global warming, the demand and importance of such engineering geology dealing with bedrocks has been increasing. In addition, the problem of radioactive waste disposal, which must be urgently resolved, the strength and permeability of bedrocks is very important. In the present talk, I will stress that weathering and alteration in bedrocks plays a very important role in various processes in them, and they could be controlled by deformation, in relation to the two problems mentioned above which engineering geology takes care of.
Construction consulting companies have worked on survey and analysis of landslides and construction method to prevent it. The main cause of landslides is the decrease of internal coefficient of friction, and increase of pore fluid pressure during a heavy rain fall, which further reduces the resistance force to landslides. Landslides tend to occur in such formations consisting of mudstones and tuffs, where a lot of clay minerals and mica with a low internal coefficient of friction are included. While, in fresh rocks, where these minerals are less abundant, landslides tend not to occur, in weathered or altered rocks, where abundant clay minerals, in particular, smectite grew, landslides tend to occur (e.g., Regmi et al., 2013). It has been also known that landslides frequently occurred in fractured zones along major faults. This is also due to the decrease of rocks strength by not only fracturing, but also the formation of many clay minerals which grew assisted by the percolation of hot waters along fractures in association with the intrusion of volcanic rocks (e.g., Tamura et al., 2007).
We have recently found the initiation of landslides located in Sashiushi, Shiranuka-town, in the west of Kushiro-city, Hokkaido, could be strongly related to the formation of flexure in bedrocks (Kaji and Takeshita, 2022). These strata consist of forearc sediments called the Urahoro Group, which deposited in the Eocene and mainly consists of arkosic sandstone and mudstone. The landslide occurred on the plane parallel to the bedding, which dips SE at c. 20 degrees. Based on the geological survey, it has been found that plural large-scale landslides occurred along the NE-SW trending flexure, implying that the occurrence of landslides was controlled by it.
Furthermore, deformation bands and fault gouge developed at the axial part of flexure, which conforms to the model of stress concentration at this part (Ballas et al., 2014). An important point is that smectite grew in the pore spaces created by not only fracturing of detrital grains in deformation bands, but also cleavage-fracturing associated with kinking in biotite detrital grains which constitute phyllosilicate bands. Furthermore, based on SEM-EDS and EPMA analyses, biotite detrital grains became altered to vermiculite, and further to kaolinite, which is the final product of alteration. Therefore, it would be clear that the formation of fluid pathway by fracturing facilitated the alteration and weathering in these rocks, and ultimately drove the formation of these clay minerals leading to landslides.
For the radioactive waste disposal in bedrocks, the basic strategy is basically to prevent the leakage and migration of radioactive elements from the repository using the dual barriers: synthetic and natural barriers. Here, in bedrocks which are supposed to work as natural barriers, the problem is the concerns about to what extent the water containing radioactive elements can migrate in fracture filling water and percolate into the surrounding rocks. Considering this problem, not only the permeability of rocks containing fractures matters, but also what chemical reactions which could affect the changes in permeability with time during the migration of water in fractures (reactive transport, MacQuarrie and Mayer, 2005) occur must be taken into consideration.
Construction consulting companies have worked on survey and analysis of landslides and construction method to prevent it. The main cause of landslides is the decrease of internal coefficient of friction, and increase of pore fluid pressure during a heavy rain fall, which further reduces the resistance force to landslides. Landslides tend to occur in such formations consisting of mudstones and tuffs, where a lot of clay minerals and mica with a low internal coefficient of friction are included. While, in fresh rocks, where these minerals are less abundant, landslides tend not to occur, in weathered or altered rocks, where abundant clay minerals, in particular, smectite grew, landslides tend to occur (e.g., Regmi et al., 2013). It has been also known that landslides frequently occurred in fractured zones along major faults. This is also due to the decrease of rocks strength by not only fracturing, but also the formation of many clay minerals which grew assisted by the percolation of hot waters along fractures in association with the intrusion of volcanic rocks (e.g., Tamura et al., 2007).
We have recently found the initiation of landslides located in Sashiushi, Shiranuka-town, in the west of Kushiro-city, Hokkaido, could be strongly related to the formation of flexure in bedrocks (Kaji and Takeshita, 2022). These strata consist of forearc sediments called the Urahoro Group, which deposited in the Eocene and mainly consists of arkosic sandstone and mudstone. The landslide occurred on the plane parallel to the bedding, which dips SE at c. 20 degrees. Based on the geological survey, it has been found that plural large-scale landslides occurred along the NE-SW trending flexure, implying that the occurrence of landslides was controlled by it.
Furthermore, deformation bands and fault gouge developed at the axial part of flexure, which conforms to the model of stress concentration at this part (Ballas et al., 2014). An important point is that smectite grew in the pore spaces created by not only fracturing of detrital grains in deformation bands, but also cleavage-fracturing associated with kinking in biotite detrital grains which constitute phyllosilicate bands. Furthermore, based on SEM-EDS and EPMA analyses, biotite detrital grains became altered to vermiculite, and further to kaolinite, which is the final product of alteration. Therefore, it would be clear that the formation of fluid pathway by fracturing facilitated the alteration and weathering in these rocks, and ultimately drove the formation of these clay minerals leading to landslides.
For the radioactive waste disposal in bedrocks, the basic strategy is basically to prevent the leakage and migration of radioactive elements from the repository using the dual barriers: synthetic and natural barriers. Here, in bedrocks which are supposed to work as natural barriers, the problem is the concerns about to what extent the water containing radioactive elements can migrate in fracture filling water and percolate into the surrounding rocks. Considering this problem, not only the permeability of rocks containing fractures matters, but also what chemical reactions which could affect the changes in permeability with time during the migration of water in fractures (reactive transport, MacQuarrie and Mayer, 2005) occur must be taken into consideration.