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Case Study on Slope Stability Changes Caused by Earthquakes—Focusing on Gyeongju 5.8 ML EQ

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Case Study on Slope Stability Changes Caused by Earthquakes—Focusing on Gyeongju 5.8 ML EQ sustainability Article Case Study on Slope Stability Changes Caused by Earthquakes—Focusing on Gyeongju 5.8 ML EQ Sangki Park , Wooseok Kim *, Jonghyun Lee and Yong Baek Korea Institute of Civil Engineering and Building Technology, 283, Goyang-daero, Ilsanseo-gu, Goyang-si 10223, Gyeonggi-do, Korea; [email protected] (S.P.); [email protected] (J.L.); [email protected] (Y.B.) * Correspondence: [email protected]; Tel.: +82-31-910-0519 Received: 16 July 2018; Accepted: 16 September 2018; Published: 27 September 2018 Abstract: Slope failure is a natural hazard occurring around the world and can lead to severe damage of properties and loss of lives. Even in stabilized slopes, changes in external loads, such as those from earthquakes, may cause slope failure and collapse, generating social impacts and, eventually causing loss of lives. In this research, the slope stability changes caused by the Gyeongju earthquake, which occurred on 12 September 2016, are numerically analyzed in a slope located in the Gyeongju area, South Korea. Slope property data, collected through an on-site survey, was used in the analysis. Additionally, slope stability changes with and without the earthquake were analyzed and compared. The analysis was performed within a peak ground acceleration (PGA) range of 0.0 (g)–2.0 (g) to identify the correlation between the slope safety factor and peak ground acceleration. The correlation between the slope safety factor and peak ground acceleration could be used as a reference for performing on-site slope stability evaluations. It also provides a reference for design and earthquake stability improvements in the slopes of road and tunnel construction projects, thus supporting the attainment of slope stability in South Korea. Keywords: seismic wave; slope stability analysis; slope safety factor; finite element method; nonlinear time history analysis; Bishop simplified method 1. Introduction Based on topographic analyses and records of the past, South Korea has long been considered as relatively safer with respect to earthquakes than neighboring countries, such as China and Japan. However, on 12 September 2016, a magnitude-5.8 earthquake (hereinafter referred to as the Gyeongju earthquake) occurred about 11.6 km south-southwest of Gyeongju City, located in the southeastern part of the Korean peninsula. This, as the largest earthquake ever recorded since the beginning of seismological observation in 1905, caused more than 600 aftershocks (until April 2017) [1–3]. The Gyeongju earthquake had a depth of approximately 13 km, and many high-frequency components of 13.77 Hz were included in the seismic waves recorded at near stations. Further, it had a duration of approximately 5–7 s, which is shorter than Japan’s most recent earthquakes (the 1995 Great Hanshin earthquake, the 2011 Tohoku¯ earthquake and tsunami, and the 2016 Kumamoto earthquakes). Thus, it caused relatively lower damage than it should have, considering its significant depth, high vibration, and short duration characteristics [4–6]. Geologically, Mesozoic and Cenozoic layers are widely distributed in the southeastern part of the Korean peninsula, which is where the Gyeongju earthquake took place, and there are large-scale faults, such as the Yangsan and Ulsan faults. Particularly, most Mesozoic Cretaceous strata are distributed along sediments, which have southeastward slopes, and igneous rocks composed of granite and andesite. Moreover, the tertiary base of the Cenozoic layer is formed in the shape of a basin in which sedimentary and volcanic layers are alternately distributed. The boundary between the Mesozoic layer Sustainability 2018, 10, 3441; doi:10.3390/su10103441 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 3441 2 of 15 and the tertiary layer is mostly formed by boundary faults. Further, owing to the boundary between the southwest-northeastward Yangsan fault and the southeast-northwestward Ulsan fault, many of the active faults observed in the Korean peninsula are distributed in this area. Although most faults formed by previous earthquakes are located to the east of the Yangsan fault, it was reported that the 2016 Gyeongju earthquake occurred because of the strike-slip fault located to the west of the Yangsan fault (see Figure1). Figure 1. Geological map of the southeastern Korean Peninsula: (a) Regional satellite image of the Cenozoic rocks area, southeast Korean Peninsula (Blue quad angle shows the study point); and (b) regional structural map and distribution area of the Neogene rocks, southeast Korean Peninsula [7] (Gray area). Generally, the term, earthquake, refers to ground vibrations that propagate outwards when the stress accumulated in faults within the Earth’s crust exceeds its limit, causing the fault plane to break along the fault line. Earthquakes may cause significant loss of lives and property damage, as the ground motions generated by earthquakes cause relatively wide areas to vibrate during a certain period of time. They primarily produce huge damage to facilities and slopes located in the target area, and, secondarily, cause collapses [8,9]. Owing to changes in external loads (such as those from earthquakes) and groundwater levels, shear stress is generated within slopes previously stabilized, naturally or artificially. Once the shear strength of the ground is exceeded, slope failure occurs, leading to slope collapse. Various methods have been applied so far in slope stability analyses. The typical method mainly used in practice is the limit state equilibrium method. However, owing to the advances in the finite element method (FEM), active research has been conducted using it in slope analysis and safety factor calculation. Instead of the limit state equilibrium method, which is widely used in practice, Griffiths and Lane [10], and Fredlund and Scoular [11] employed the FEM for slope safety analysis in their research. The advantages of the FEM include the lack of requirement of destruction of the target slope or assumptions about the cutting plane. Moreover, if there is experimental data on the actual slope, it can be used directly in the analysis. Al-Homoud and Tahtamoni [12] performed research on 1 Sustainability 2018, 10, 3441 3 of 15 the analysis and alleviation of the degree of risk of slopes regarding seismic loads. They developed a method and program to analyze three-dimensionally the safety of slopes under seismic loading, considering displacement standards for slope failure and safety factors. The method and program were applied to draw a slope risk map for earthquakes, which could be used for disaster relief, including landslide situations. Li [13] performed in parallel an FEM-based slope safety analysis using the power law for non-linear shear-stress reference values and an analysis based on the limit state equilibrium method, and compared their results. This comparison revealed that the FEM-based analysis provided results that are more accurate. Jibson [14] performed a survey on various methods used for slope safety analysis regarding earthquakes, which included the pseudo-static analysis, limit state equilibrium, and Newmark’s sliding block methods. Furthermore, it was found that many studies are currently being performed on stress-displacement analyses using FEM [15–23]. In this research, the slope safety of a soil slope located in the vicinity of Yangbuk-myeon where the Gyeongju earthquake occurred was numerically analyzed. Prior to this, the soil properties of the target slope were examined through an on-site survey, and the Gyeongju earthquake data was used to numerically analyze the slope safety regarding the earthquake’s impact. Additionally, a slope safety analysis was performed by increasing the scale of the earthquake to examine the correlation between the slope safety factor and earthquake scale. This correlation can be used as a reference value for slope safety evaluation in sites, such as roads and tunnels, and as a reference value for slope design and safety improvement against earthquakes. Thus, it is possible to contribute to securing slope stability through continuous contrast study against the earthquake. 2. Theoretical Background and Mathematical Formulation Theoretical Background for Slope Stability The term, slope stability, can be defined to assess the degree of resistance to the collapse of the slope based on its factor of safety. The factor of safety is a value that results from dividing the shear strength by the shear stress of the soil of a given slip surface. In other words, it can be defined as the ratio of the forces that block the activity of the slope to the forces that trigger it. Therefore, theoretically, if the calculated factor of safety is above 1.0, the target slope can be judged as safe. Nevertheless, when analyzing the actual behavior of a slope, its soil properties are examined through an on-site survey. However, even after a significant on-site survey, uncertainties about properties that cannot be considered (such as strength parameters), external loads that trigger the activity of the slope, and failure models are still present. Owing to such uncertainties, the concept of the allowable factor of safety was introduced [24], and the factor of safety specified in design standards is the allowable factor of safety. Therefore, in practice and for a slope to be deemed safe, its calculated factor of safety must be not above 1.0, but above the allowable safety factor specified in the design standards. The analysis of the safety of a slope against earthquakes can be divided into two stages. First, the slope response analysis stage, in which the forces that act on the slope when seismic waves are applied, along with the slope acceleration, speed, and displacement, are analyzed. Second, the safety analysis stage, in which the slope safety is analyzed by calculating the factor of safety of the slope where deformation has occurred. The slope response analysis regarding earthquakes can be divided into the pseudo-static analysis (or equivalent static analysis), which considers seismic loads as static, displacement analysis, which uses the Newmark’s sliding block, and dynamic analysis, which uses the finite difference method and the FEM.
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