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.
In this study, we evaluated the slope stability of the Pohang Daljeon-ri columnar joint (Natural Monuments # 415) and calculated the maximum energy, jumping height and moving distance of rockfalls using a simulation. Based on the results, we established the range of rockfall risk. The slopes of the Pohang Daljeon-ri columnar joint have dip directions of 93.79°, 131.99°, 165.54° and 259.84° from left (SW) to right (NE). Furthermore, they have a fan-like shape. The Pohang Daljeon-ri columnar joints are divided into four sections depending on the dip direction. The measurement results of the discontinuous face show that zone 1 is 125, zone 2 is 261, zone 3 is 262, zone 4 is 43. The results of slope stability analyses for each section using a stereographic projection method correspond to the range of planar and toppling failure. Although it is difficult to diagnose the type of failure, risk evaluation of currently falling rocks requires further focus. The maximum movement distance of a rockfall in the simulation was approximately 66 m and the rockfall risk range was the entire area under slope. In addition, it is difficult to forecast where a rock will fall as it rolls in various directions due to topographic factors. Thus, the installation of measures to prevent falling is suggested to secure the stability based on the results of the rockfall simulations and its probabilistic analysis.
A comparative petrological study was conducted on the columnar joints of Guryongpo, Pohang, Yangnam, Gyeongju, and Gangdong-hwaam, Ulsan in the Cenozoic Miocene sedimentary basin. Horizontal columnar joints, in these columnar joints, are characteristically developed, and the cross-sectional shape of each columnar joint is predominantly quadrilateral and pentagonal in Guryongpo and Yangnam columnar joints, and pentagonal to hexagonal in Gangdong-hwaam columnar joint. As a result of XRF analysis, it was confirmed that Guryongpo columnar joint is andesite, Yangnam columnar joint is basalt, and Gangdong-hwaam columnar joint is basaltic andesite, all of which are sub-alkaline. They are easily distinguished because Guryongpo columnar joint contains small-sized plagioclase (1∼2 mm), while rather large plagioclase phenocrysts (several mm∼1 cm) are uniformly distributed both in Yangnam and Gangdong-hwaam columnar joints. The value of whole rock magnetic susceptibility (WRMS) are also well classified according to the rock composition. In addition, in the tectonic discriminant diagram, these columnar joints correspond to the orogeny zone and island-arc tholeiite regions of the orogeny, but the AFM diagram and WRMS analysis show that each columnar joint was formed from the different magmas.
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