5:15 PM - 7:15 PM
[U11-P02] Effects of physical and mechanical properties on slaking of pyroclastic rock and siliceous mudstone in the Noto Peninsula
Keywords:Slaking, Pyroclastic rock, Ciliceous mudstone, Noto Peninsula, Weathering
The 2024 Noto Peninsula Earthquake caused coastal uplift mainly in the northern part of the peninsula. The uplifted rocky coastline is expected to be subject to wave erosion and platform formation, depending on the physical and mechanical properties of the rocks that consist of the coastline because weathering of the rocks has already been observed by the authors. Therefore, the uplifted coastline associated with the earthquake allows us to investigate the formation process of the rocky coastline from the early stage of the uplift. Many of the rocks along the coastline also consist of the entire Noto Peninsula. Thus, the knowledge of the vulnerability of the rocks will help us understand the geomorphic process of the Noto Peninsula. This study measured physical and mechanical properties of the rocks to elucidate the mechanisms of weathering and weakening.
We collected the tuff of the Nawamata Formation from the Paleogene Oligocene, dacite pyroclastic rock of the Matsunagi Formation from the Neogene Miocene, the tuff of the Touge Formation, siliceous mudstone of the Hojuji Formation, siliceous mudstone of the Iizuka Formation. The dry density and porosity of the siliceous mudstones are as follows: dry density and porosity ranged from 1.55-1.98 Mg/m3 to 25.3-37.6% for the pyroclastic rocks and from 1.28-2.01 Mg/m3 to 24.0-50.3% for the siliceous mudstones.
The presence of swelling clay minerals is a possible cause of rock slaking. X-ray diffraction (XRD) analysis was performed to identify the minerals contained in the collected rocks. The pore size distribution of the rocks was measured by the mercury injection method.
The rocks on the coast repeatedly dry and wet with changing tidal levels. To evaluate the characteristics of wet and dry weathering (slaking), the weight of the largest piece of the residual sample was measured at the end of one cycle of wetting and drying (at the end of drying). In this study, wetting refers to the state in which the sample is immersed in a container of pure water, and drying refers to the state in which the sample is removed from the container and allowed to dry naturally. The duration of wetting and drying per cycle was varied as follows: 1:2 (8 hours wetting, 16 hours drying), 1:8 (8 hours wetting, 64 hours drying), and 1:20 (8 hours wetting, 160 hours drying).
XRD analysis revealed that almost all rocks contain smectite, a swelling clay mineral. One type of siliceous mudstone of the Hojuji Formation showed no clear peaks indicating smectite. The diffraction intensity of smectite was 2-3 times higher in pyroclastic rocks and tuff than in siliceous mudstones. Pore size distribution measured by the mercury injection method showed a bimodal distribution in both pyroclastic rocks and siliceous mudstones. In the pyroclastic rocks, the largest peak was observed in the range of 10 to several 10 nm, and the second peak was observed in the range of several 100 to 1000 nm. In siliceous mudstone, the maximum peak was at several 100-1000 nm, and the second peak was at several 10 nm. 300 nm was the maximum peak in the siliceous mudstone of the Hojuji Formation, where no clear peak of smectite was observed in the XRD analysis, and there was no bimodality.
The results of the slaking test showed that the rocks with the largest weight loss at the end of the three cycles were, in descending order from largest to smallest, Nawamata Formation tuff, Touge Formation tuff, and Matsunagi Formation dacite pyroclastic rocks, which are clearly more susceptible to slaking than siliceous mudstones. In these pyroclastic rocks, fragmentation began at the same time as water immersion. Cracks formed in the Nawamata and Touge tuffs during drying. Regardless of the rock type, the rate of weight loss increased with increasing drying time.
We collected the tuff of the Nawamata Formation from the Paleogene Oligocene, dacite pyroclastic rock of the Matsunagi Formation from the Neogene Miocene, the tuff of the Touge Formation, siliceous mudstone of the Hojuji Formation, siliceous mudstone of the Iizuka Formation. The dry density and porosity of the siliceous mudstones are as follows: dry density and porosity ranged from 1.55-1.98 Mg/m3 to 25.3-37.6% for the pyroclastic rocks and from 1.28-2.01 Mg/m3 to 24.0-50.3% for the siliceous mudstones.
The presence of swelling clay minerals is a possible cause of rock slaking. X-ray diffraction (XRD) analysis was performed to identify the minerals contained in the collected rocks. The pore size distribution of the rocks was measured by the mercury injection method.
The rocks on the coast repeatedly dry and wet with changing tidal levels. To evaluate the characteristics of wet and dry weathering (slaking), the weight of the largest piece of the residual sample was measured at the end of one cycle of wetting and drying (at the end of drying). In this study, wetting refers to the state in which the sample is immersed in a container of pure water, and drying refers to the state in which the sample is removed from the container and allowed to dry naturally. The duration of wetting and drying per cycle was varied as follows: 1:2 (8 hours wetting, 16 hours drying), 1:8 (8 hours wetting, 64 hours drying), and 1:20 (8 hours wetting, 160 hours drying).
XRD analysis revealed that almost all rocks contain smectite, a swelling clay mineral. One type of siliceous mudstone of the Hojuji Formation showed no clear peaks indicating smectite. The diffraction intensity of smectite was 2-3 times higher in pyroclastic rocks and tuff than in siliceous mudstones. Pore size distribution measured by the mercury injection method showed a bimodal distribution in both pyroclastic rocks and siliceous mudstones. In the pyroclastic rocks, the largest peak was observed in the range of 10 to several 10 nm, and the second peak was observed in the range of several 100 to 1000 nm. In siliceous mudstone, the maximum peak was at several 100-1000 nm, and the second peak was at several 10 nm. 300 nm was the maximum peak in the siliceous mudstone of the Hojuji Formation, where no clear peak of smectite was observed in the XRD analysis, and there was no bimodality.
The results of the slaking test showed that the rocks with the largest weight loss at the end of the three cycles were, in descending order from largest to smallest, Nawamata Formation tuff, Touge Formation tuff, and Matsunagi Formation dacite pyroclastic rocks, which are clearly more susceptible to slaking than siliceous mudstones. In these pyroclastic rocks, fragmentation began at the same time as water immersion. Cracks formed in the Nawamata and Touge tuffs during drying. Regardless of the rock type, the rate of weight loss increased with increasing drying time.