Edge-oxidized graphene oxide (EOGO) is a nano-sized material that is chemically stable and easily mixed with water due to its hydrophilic properties; thus, it has been used in various engineering fields, particularly for the reinforcement of building and construction materials. In this study, the effect of EOGO in soil reinforcement was investigated. When mixed with soil, it affects the mechanical properties of the soil–GO mixture. Various amounts of the GO (0%, 0.02%, 0.06%, 0.1%) were added into the sand–clay mixture, and their geotechnical properties were evaluated via multiple laboratories testing methods, including a standard Proctor test, direct shear test, compressibility test, and contact angle measurement. The experimental results show that with the addition of EOGO in soil of up to 0.06% EOGO, the compressibility decreases, the shear strength increases, and the maximum dry density (after compaction) increases.

The rapid growth of tourism on Jeju Island has led to an increase in traffic volume, causing accelerated ground subsidence at the intersections between natural caves and roads. Natural phenomena are not caused by a single specific factor, but rather occur through interactions among multiple factors. Therefore, to analyze natural phenomena, it is necessary to consider not only the main effects, but also the interactions among various factors. In this study, the design of experiments was utilized to analyze the vibration-generating mechanism according to the geometrical characteristics of lava tubes, a kind of natural cave. Analysis of variance was used as the main analysis method, and multiple linear regression analysis and decision trees were additionally used. To detect all the interaction effects, multi-way factorial design, which can estimate the effect of any combination of levels, was utilized. The joint orientation, aspect ratio, and cover depth were selected as factors representing the geometrical characteristic of the lava tubes, and vehicle speed was selected as an external influence factor. As a result of the analysis, all the main effects were found to be significant, and the results of the additional two analysis methods were consistent. The two-factor interactions that showed significant results were aspect ratio $\times$ cover depth, joint orientation$\times$ vehicle speed, and joint orientation$\times$ shape ratio. However, it was difficult to analyze the interaction effect using multiple regression analysis and decision tree analysis. Among the two-factor interactions, aspect ratio $\times$ cover depth was found to be the most significant, while the main effect of the two interacting factors was the least significant. The most significant effect at a specific level of aspect ratio and cover depth could be analyzed through an interaction plot. These findings will be useful for evaluating the stability of lava tubes using only geometrical characteristics.

Most geotechnical engineers focus on ground strength and stability. However, when determining stability by analyzing the exact strength of the ground, the failure surface is essential. Nevertheless, limited studies have been conducted on the methods to determine the exact failure surface of the ground. This study shows that the failure surface and plastic area can be analyzed using the slip line method based on the slip line and lower bound analysis. To improve existing studies limited to single ground, we analyzed the failure area of heterogeneous ground conditions. The results of the slip line method were compared and verified through FELM, a finite element analysis method to which a lower limit analysis was applied. As a result, the failure area and maximum internal stress according to the ground properties and the slope angle are presented. In addition, the points of the slip line method and the finite element limit analysis are summarized. Finally, we propose limitations and solutions when applying the slip line method to the slope.

In this study, edge-oxidized graphene oxide (EOGO) was used as an additive in fly ash (FA) geopolymer paste. The effect of EOGO on the properties of the fly ash geopolymer was investigated. EOGO was added to the FA geopolymer at four different percentages (0%, 0.1%, 0.5% and 1%), and the mixture was cured under two different conditions: room curing (~20 °C) and heat curing (~60 °C). To characterize the FA-EOGO geopolymer, multiple laboratory tests were employed, including compressive strength, Free-Free Resonance Column (FFRC), density, water absorption, and setting tests. The FFRC test was used to evaluate the stiffness at small strain (Young’s modulus) via the resonance of the specimen. The mechanical test results showed that the strength and elastic modulus were high during heat curing, and the highest compressive strength and elastic modulus were achieved at 0.1% EOGO. In the physical test, 0.1% EOGO had the highest density and the lowest porosity and water absorption. As a result of the setting time test, as the EOGO content increased, the setting time was shortened. It is concluded that the optimum proportion of EOGO is 0.1% in FA geopolymer paste.