5:15 PM - 7:15 PM
[HRE13-P03] An Examination of Collapse Factors in Limestone Caves: Examples from Akiyoshidai, Hiraodai, and Atetsudai.
Keywords:Limestone cave, Fault, Collapse, Akiyosi-dai plateau, Hirao-dai plateau, Atetsu-dai plateau
[Introduction] The Akiyoshi Limestone was formed on top of a seamount, and it is of higher purity compared to continental shelf limestones due to the lack of detrital particles of continental origin (Wakita, 2020). Additionally, the Akiyoshi limestone is notably dense and massive without stratification, and the development of karst landscapes is influenced by fracture systems developed within the limestone (Fujii et al., 1969). The expansion of limestone caves is also considered to be caused by the collapse of limestone, and faults are pointed out as a contributing factor to these collapses (Ota et al., 1980). Furthermore, Inui and Tsuji (2023) suggested that especially long and wide faults contribute to collapses. However, it has also become clear that in massive limestone without such discontinuities, collapses are unlikely to occur based on mechanical calculations. By examining and constraining the factors of collapse occurrence in homogeneous rock masses, it is thought that this can contribute to examining the factors of collapse in non-limestone rock masses. In Tsuji et al. (2024) and Inui et al. (2023), surveys were conducted in limestone caves in the Akiyoshi-dai plateau to investigate which faults contribute to collapses, the characteristics of the faults, the width of the caves, and the relationship between the faults and cavities. This study conducted a re-description of faults and sediments at collapse sites, as well as quantitative analysis of survey data in Akiyoshi-dai plateau (Kagekiyo-do and Taisho-do), Hirao-dai plateau (Ojika-do and Senbutsu-shonyudo), and Atetsu-dai Plateau (Maki-do and Ikura-do). The aim is to quantitatively and qualitatively examine the factors contributing to limestone collapse, with a speleological perspective.
[Results] There were many collapse sites in wide caves, whereas relatively fewer collapse sites but more areas with rock falls and spalling in narrow caves. Then in this study, the caves were categorized into wide caves (Kagekiyo-do, Taisho-do, Maki-do) and narrow caves (Ojika-do, Senbutsu-syonyudo , Ikura-do).
Characteristics of faults in wide caves included multiple intersecting faults, low-angle faults with crushing zones, multiple parallel faults, and low-angle faults developing in the ceiling. Collapse points featured collapse traces in the ceiling and debris heaps on the cave floor. Characteristics of faults in narrow caves included developing faults with crushing zones and intersecting high-angle faults. Collapse points showed small-scale rockfall and spalling, with scattered debris on the cave floor. Regarding sedimentary structures, fluvial deposits were observed above and below collapse gravels at collapse sites of a large cave. Additionally, dissolution (“notches”) were seen on the cave walls, but no dissolution (“scallops”) were observed on the fallen debris.
Examining correlations between quantitative data such as collapse or non-collapses cave size, gravel diameter, and fault density, revealed that in most wide caves, collapses were frequent where cave widths were large and faults developed densely.
[Discussion] The factors contributing to collapses inferred basedon qualitative and quantitative data are listed below:
(1) Presence of faults
1-a: Faults with fracture zone
1-b: Multiple intersecting/parallel faults (fault density)
1-c: Long faults
1-d: Low-angle faults developing in the ceiling (angle and relationship with space)
(2) Presence of spaces
2-a: Large cave width
2-b: Sediment discharge
2-c: Groundwater discharge
Regarding (1), it was found that faults are involved in all collapses. As for 2-b and 2-c, many collapses occurred after the groundwater levels and sediments have been discharged were interpreted as significant factors. It is thought that these factors combine to lead to collapse.
[Results] There were many collapse sites in wide caves, whereas relatively fewer collapse sites but more areas with rock falls and spalling in narrow caves. Then in this study, the caves were categorized into wide caves (Kagekiyo-do, Taisho-do, Maki-do) and narrow caves (Ojika-do, Senbutsu-syonyudo , Ikura-do).
Characteristics of faults in wide caves included multiple intersecting faults, low-angle faults with crushing zones, multiple parallel faults, and low-angle faults developing in the ceiling. Collapse points featured collapse traces in the ceiling and debris heaps on the cave floor. Characteristics of faults in narrow caves included developing faults with crushing zones and intersecting high-angle faults. Collapse points showed small-scale rockfall and spalling, with scattered debris on the cave floor. Regarding sedimentary structures, fluvial deposits were observed above and below collapse gravels at collapse sites of a large cave. Additionally, dissolution (“notches”) were seen on the cave walls, but no dissolution (“scallops”) were observed on the fallen debris.
Examining correlations between quantitative data such as collapse or non-collapses cave size, gravel diameter, and fault density, revealed that in most wide caves, collapses were frequent where cave widths were large and faults developed densely.
[Discussion] The factors contributing to collapses inferred basedon qualitative and quantitative data are listed below:
(1) Presence of faults
1-a: Faults with fracture zone
1-b: Multiple intersecting/parallel faults (fault density)
1-c: Long faults
1-d: Low-angle faults developing in the ceiling (angle and relationship with space)
(2) Presence of spaces
2-a: Large cave width
2-b: Sediment discharge
2-c: Groundwater discharge
Regarding (1), it was found that faults are involved in all collapses. As for 2-b and 2-c, many collapses occurred after the groundwater levels and sediments have been discharged were interpreted as significant factors. It is thought that these factors combine to lead to collapse.