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Econ. Environ. Geol. 2022; 55(5): 497-510

Published online October 31, 2022

https://doi.org/10.9719/EEG.2022.55.5.497

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Cause of Rockfall at Natural Monument Pohang Daljeon-ri Columnar Joint

Jae Hwan Kim1, Dal-Yong Kong2,*

1Natural Heritage Center, National Research Institute of Cultural Heritage, Daejeon 35204, Korea
2International Cooperation Division, Cultural Heritage Administration, Daejeon 35208, Korea

Received: October 9, 2022; Revised: October 18, 2022; Accepted: October 18, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Monthly monitoring, 3D scan survey, and electrical resistivity survey were conducted from January 2018 to August 2022 to identify the cause of rockfall occurring in Daljeon-ri Columnar Joint (Natural Monument No. 415), Pohang. A total of 3,231 rocks fell from the columnar joint over the past 5 years, and 1,521 (47%) of the falling rocks were below 20 cm in length, 978 (30.3%) of 20-30 cm, and 732 (22.7%) of rocks over 30 cm. While the number of rockfalls by year has decreased since 2018, the frequency of rockfalls bigger than 30 cm tends to increase. Large-scale rockfalls occurred mainly during the thawing season (March-April) and the rainy season (June-July), and the analysis of the relationship between cumulative rainfall and rockfall occurrence showed that cumulative rainfall for 3 to 4 days is also closely related to the occurrence of rockfall. Smectite and illite, which are expansible clay minerals, were observed in XRD analysis of the slope material (filling minerals) in the columnar joint, and the presence of a fault fracture zone was confirmed in the electrical resistivity survey. In addition, the confirmed fault fracture zone and the maximum erosion point analyzed through 3D precision measurement coincided with the main rockfall occurrence point observed by the BTC-6PXD camera. Therefore, the main cause of rockfall at Daljeon-ri columnar joint in Pohang is a combination of internal factors (development of fault fracture zones and joints, weathering of rocks, presence of expansive clay minerals) and external factors (precipitation, rapid thawing phenomenon), resulting in large-scale rockfall. Meanwhile, it was also confirmed that the Pohang-Gyeongju earthquake, which was continuously raised, was not the main cause.

Keywords natural monument, Pohang Daljeon-ri Columnar Joint, monitoring, rockfall, earthquake

천연기념물 포항 달전리 주상절리의 낙석 발생원인

김재환1 · 공달용2,*

1국립문화재연구원 자연문화재연구실
2문화재청 국제협력과

요 약

포항 달전리 주상절리(천연기념물 제415호)에서 발생하고 있는 낙석의 원인을 파악하기 위해 2018년 1월부터 2022년 8월까지 월별 모니터링과 3차원 스캔 측량, 전기 비저항탐사를 실시하였다. 약 5년간 주상절리대에서 떨어져 나온 낙석은 총 3,231개이며, 낙석의 크기(길이)는 20cm 이하가 1,521개(47%)로 가장 많고, 20~30cm는 978(30.3%), 30cm 이상은 732개(22.7%)가 발생하였다. 2018년부터 연도별 낙석 발생 개수는 감소하는 반면, 30cm 이상의 낙석 발생 빈도는 증가하는 경향을 보인다. 대규모의 낙석은 해빙기(3월~4월)와 장마기(6월~7월)에 주로 발생하였으며, 누적강우량과 낙석 발생 관계 분석에서 3~4일 간의 누적강우량도 낙석 발생에 밀접한 관계가 있는 것으로 나타났다. 주상절리 내의 사면물질(충진물)을 대상으로 한 XRD 분석에서 팽창성 점토광물인 스멕타이트와 일라이트가 관찰되었으며 전기비저항탐사에서 단층파쇄대가 존재하는 것이 확인되었다. 또한, 확인된 단층파쇄대와 3D 정밀측정을 통해 분석된 최대 침식지점이 무인카메라에서 관찰된 주 낙석 발생 지점과 일치하였다. 따라서 포항 달전리 주상절리의 낙석 발생 주요 원인은 내부요인(단층파쇄대와 절리의 발달, 암석의 풍화, 팽창성 점토광물의 존재)과 외부요인(강우량, 해빙기 급속한 융해현상)이 복합적으로 작용하여 대규모 낙석을 발생하는 것으로 판단된다. 반면, 일부에서 지속적으로 제기 되었던 포항-경주 지진은 주요 원인이 아님이 확인되었다.

주요어 천연기념물, 포항 달전리 주상절리, 모니터링, 낙석, 지진

Article

Research Paper

Econ. Environ. Geol. 2022; 55(5): 497-510

Published online October 31, 2022 https://doi.org/10.9719/EEG.2022.55.5.497

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Cause of Rockfall at Natural Monument Pohang Daljeon-ri Columnar Joint

Jae Hwan Kim1, Dal-Yong Kong2,*

1Natural Heritage Center, National Research Institute of Cultural Heritage, Daejeon 35204, Korea
2International Cooperation Division, Cultural Heritage Administration, Daejeon 35208, Korea

Received: October 9, 2022; Revised: October 18, 2022; Accepted: October 18, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

Monthly monitoring, 3D scan survey, and electrical resistivity survey were conducted from January 2018 to August 2022 to identify the cause of rockfall occurring in Daljeon-ri Columnar Joint (Natural Monument No. 415), Pohang. A total of 3,231 rocks fell from the columnar joint over the past 5 years, and 1,521 (47%) of the falling rocks were below 20 cm in length, 978 (30.3%) of 20-30 cm, and 732 (22.7%) of rocks over 30 cm. While the number of rockfalls by year has decreased since 2018, the frequency of rockfalls bigger than 30 cm tends to increase. Large-scale rockfalls occurred mainly during the thawing season (March-April) and the rainy season (June-July), and the analysis of the relationship between cumulative rainfall and rockfall occurrence showed that cumulative rainfall for 3 to 4 days is also closely related to the occurrence of rockfall. Smectite and illite, which are expansible clay minerals, were observed in XRD analysis of the slope material (filling minerals) in the columnar joint, and the presence of a fault fracture zone was confirmed in the electrical resistivity survey. In addition, the confirmed fault fracture zone and the maximum erosion point analyzed through 3D precision measurement coincided with the main rockfall occurrence point observed by the BTC-6PXD camera. Therefore, the main cause of rockfall at Daljeon-ri columnar joint in Pohang is a combination of internal factors (development of fault fracture zones and joints, weathering of rocks, presence of expansive clay minerals) and external factors (precipitation, rapid thawing phenomenon), resulting in large-scale rockfall. Meanwhile, it was also confirmed that the Pohang-Gyeongju earthquake, which was continuously raised, was not the main cause.

Keywords natural monument, Pohang Daljeon-ri Columnar Joint, monitoring, rockfall, earthquake

천연기념물 포항 달전리 주상절리의 낙석 발생원인

김재환1 · 공달용2,*

1국립문화재연구원 자연문화재연구실
2문화재청 국제협력과

Received: October 9, 2022; Revised: October 18, 2022; Accepted: October 18, 2022

요 약

포항 달전리 주상절리(천연기념물 제415호)에서 발생하고 있는 낙석의 원인을 파악하기 위해 2018년 1월부터 2022년 8월까지 월별 모니터링과 3차원 스캔 측량, 전기 비저항탐사를 실시하였다. 약 5년간 주상절리대에서 떨어져 나온 낙석은 총 3,231개이며, 낙석의 크기(길이)는 20cm 이하가 1,521개(47%)로 가장 많고, 20~30cm는 978(30.3%), 30cm 이상은 732개(22.7%)가 발생하였다. 2018년부터 연도별 낙석 발생 개수는 감소하는 반면, 30cm 이상의 낙석 발생 빈도는 증가하는 경향을 보인다. 대규모의 낙석은 해빙기(3월~4월)와 장마기(6월~7월)에 주로 발생하였으며, 누적강우량과 낙석 발생 관계 분석에서 3~4일 간의 누적강우량도 낙석 발생에 밀접한 관계가 있는 것으로 나타났다. 주상절리 내의 사면물질(충진물)을 대상으로 한 XRD 분석에서 팽창성 점토광물인 스멕타이트와 일라이트가 관찰되었으며 전기비저항탐사에서 단층파쇄대가 존재하는 것이 확인되었다. 또한, 확인된 단층파쇄대와 3D 정밀측정을 통해 분석된 최대 침식지점이 무인카메라에서 관찰된 주 낙석 발생 지점과 일치하였다. 따라서 포항 달전리 주상절리의 낙석 발생 주요 원인은 내부요인(단층파쇄대와 절리의 발달, 암석의 풍화, 팽창성 점토광물의 존재)과 외부요인(강우량, 해빙기 급속한 융해현상)이 복합적으로 작용하여 대규모 낙석을 발생하는 것으로 판단된다. 반면, 일부에서 지속적으로 제기 되었던 포항-경주 지진은 주요 원인이 아님이 확인되었다.

주요어 천연기념물, 포항 달전리 주상절리, 모니터링, 낙석, 지진

    Fig 1.

    Figure 1.Geographical location (a) and numerical elevation model (b: angle of inclination, c: direction of inclination) of Daljeon-ri columnar joints in Pohang.
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 2.

    Figure 2.Pohang Daljeon-ri columnar joint (a: panoramic view, b: columnar joint in the southwest, c: columnar joint in northeast view, d: collected rockfalls).
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 3.

    Figure 3.Zonation of Pohang Daljeon-ri columnar joint area (a) and slope stability evaluation results by each zone (b).
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 4.

    Figure 4.Rockfall occurrence at Pohang Daljeon-ri columnar joint (a-b: location and image of large-scale rockfall captured by BTC-6PXD camera, c-d: debris flow moved from the northeast boundary slope, e-f: pillars of columnar joints that have recently fallen.
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 5.

    Figure 5.Rockfall occurrence status by size (length) of rockfall, Pohang Daljeon-ri columnar joint.
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 6.

    Figure 6.Volume change due to erosion of Daljeon-ri columnar joint (a) and volume change due to sedimentation (b).
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 7.

    Figure 7.Two-dimensional resistivity diagram (a) and result of X-ray diffraction analysis of the slope material (b) of the columnar joint in Daljeon-ri.
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 8.

    Figure 8.Earthquakes and rockfalls frequency by year in the Pohang-Gyeongju area (a: 2018, b; 2019, c; 2020, d: 2021, e: 2022).
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 9.

    Figure 9.Monthly rockfall occurrence and rainfall at Pohang Daljeon-ri columnar joint.
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 10.

    Figure 10.Trend of rockfall compared to monthly rainfall at columnar joint in Daljeon-ri (A, B: trend line of amount of rockfall according to rainfall).
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Fig 11.

    Figure 11.Hourly rainfall and accumulated rainfall from June 1 to July 30, 2018 at columnar joint in Daljeon-ri, Pohang.
    Economic and Environmental Geology 2022; 55: 497-510https://doi.org/10.9719/EEG.2022.55.5.497

    Table 1 . Number of falling rocks by year at Daljeon-ri columnat joint, Pohang.

    Jan.Feb.Mar.Apr.MayJun.Jul.Aug.Sep.Oct.Nov.Dec.Total
    2018Preparation8423711320190141732319291,127
    201913177154135261561161921711709
    20205325737261221541711531414605
    20212026346133931Environmental arrangement214
    2022Environmental arrangement93742413594156----576

    KSEEG
    Apr 30, 2024 Vol.57 No.2, pp. 107~280

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