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[HDS10-P03] Analysis of Atami debris flow using borehole cores of embankment
Keywords:debris flow, embankment
On July 3, 2021, a debris flow caused by a collapse of embankment occurred along the Aizome River in the Izusan area of Atami City, Shizuoka Prefecture, killed 28 people and destroyed 64 houses. Based on the lessons learned from the disaster, the Ministry of Land, Infrastructure, Transport and Tourism conducted a nationwide survey of embankments and reported approximately 36,000 locations that needed to be inspected. Furthermore, the government promulgated the "Embankment Regulation Law" in May 2022, which requires that the topographical and geological conditions of the area where the embankment is to be constructed must be investigated in advance. But the government does not provide specific standards. The only embankment that collapsed due to heavy rainfall in July 2021 in the Izu area was the embankment at the head of the Aizome River. This indicates that this embankment had the greatest disaster risk, and the investigation of the cause of its collapse will provide essential information for the "Embankment Regulation Law". Therefore, the authors report the results of a borehole core sample of an uncollapsed embankment at the head of the river.
Shizuoka Prefecture drilled borehole cores at five locations in and around the unconsolidated embankment in late August 2021, and three of the cores examined in this study. Sedimentary facies were described, sediments were sampled, and grain size analysis was performed.
Core No. 3 consists mainly of gravelly muddy sand, with four well-sorted sand layers. The mean grain size, standard deviation, and mud content of the sand layers range from 0.37-1.68φ, 1.34-2.25φ, and 3.0-11.4%, indicating a unimodal pattern grain distributions. Compared to the sediments immediately below, the mean grain size is slightly larger, the standard deviation is 0.2-0.5φ smaller, and the mud content is 38-58% lower.
Core No. 4 consists mainly of gravelly muddy sand, with two well-sorted alternating layers of sand and gravel. The mean grain size, standard deviation, and mud content of the sand layer range from 0.6-1.9φ, 1.2-2.8φ, and 1.8-8.5%, respectively, and the grain-size composition shows unimodal.
Core No. 5 consists of massive gravelly muddy sand to sandy mud layers, with mean grain size, standard deviation, and mud content ranging from 2.5-3.5φ, 2.4-3.3φ, and 33.8-54.6%, respectively, with no well-sorted sand layers.
Since Core No. 4 is located 9.6 m seaward of core No. 3, the well-sorted sand layers may be 9.6 m continuous in the horizontal direction. The gravel layer associated with the sand layer in core No. 4 may indicate coarser grained toward the seaward side.
Kitamura et al. (2022; Geosci Rep Shizuoka Univ, 49, 61–72) reported that a medium-sized subrounded gravel layer (0.4 m thick) was identified in an uncollapsed embankment at the lowest end of the embankment, and that the sandy sediment between the gravels had low mud contents of ca.10%.
According to Shindo (1993; The Quaternary Research, 32, 315–322), the concentration of slope groundwater at the boundary of strata with different permeability and the drainage of slope materials lead to the development of large pore spaces, such as pipes. Therefore, it is quite possible that large pore spaces, such as pipes, are partially formed by groundwater on the slope. In 2019, Shizuoka Prefecture reported a "small collapses at the lower end of the embankment", which may well be due to voids in the highly permeable layers caused by ground water.
Kitamura (2023; Environment and Measuring Technology, 50, 33-42) estimated that high permeable layers and voids affected the occurrence of Atami debris flow. Therefore, the "small collapse of the embankment" reported by Shizuoka Prefecture is a precursor to the collapse of the entire embankment, and the examination of presence or absence of a highly permeable layer within the embankment is an indicator for evaluating the safety of the existing embankment.
Shizuoka Prefecture drilled borehole cores at five locations in and around the unconsolidated embankment in late August 2021, and three of the cores examined in this study. Sedimentary facies were described, sediments were sampled, and grain size analysis was performed.
Core No. 3 consists mainly of gravelly muddy sand, with four well-sorted sand layers. The mean grain size, standard deviation, and mud content of the sand layers range from 0.37-1.68φ, 1.34-2.25φ, and 3.0-11.4%, indicating a unimodal pattern grain distributions. Compared to the sediments immediately below, the mean grain size is slightly larger, the standard deviation is 0.2-0.5φ smaller, and the mud content is 38-58% lower.
Core No. 4 consists mainly of gravelly muddy sand, with two well-sorted alternating layers of sand and gravel. The mean grain size, standard deviation, and mud content of the sand layer range from 0.6-1.9φ, 1.2-2.8φ, and 1.8-8.5%, respectively, and the grain-size composition shows unimodal.
Core No. 5 consists of massive gravelly muddy sand to sandy mud layers, with mean grain size, standard deviation, and mud content ranging from 2.5-3.5φ, 2.4-3.3φ, and 33.8-54.6%, respectively, with no well-sorted sand layers.
Since Core No. 4 is located 9.6 m seaward of core No. 3, the well-sorted sand layers may be 9.6 m continuous in the horizontal direction. The gravel layer associated with the sand layer in core No. 4 may indicate coarser grained toward the seaward side.
Kitamura et al. (2022; Geosci Rep Shizuoka Univ, 49, 61–72) reported that a medium-sized subrounded gravel layer (0.4 m thick) was identified in an uncollapsed embankment at the lowest end of the embankment, and that the sandy sediment between the gravels had low mud contents of ca.10%.
According to Shindo (1993; The Quaternary Research, 32, 315–322), the concentration of slope groundwater at the boundary of strata with different permeability and the drainage of slope materials lead to the development of large pore spaces, such as pipes. Therefore, it is quite possible that large pore spaces, such as pipes, are partially formed by groundwater on the slope. In 2019, Shizuoka Prefecture reported a "small collapses at the lower end of the embankment", which may well be due to voids in the highly permeable layers caused by ground water.
Kitamura (2023; Environment and Measuring Technology, 50, 33-42) estimated that high permeable layers and voids affected the occurrence of Atami debris flow. Therefore, the "small collapse of the embankment" reported by Shizuoka Prefecture is a precursor to the collapse of the entire embankment, and the examination of presence or absence of a highly permeable layer within the embankment is an indicator for evaluating the safety of the existing embankment.