Experimental Investigation on Permeability Evolution of Limestone Caprock under Coupled THM Processes Abstract To investigate the gas sealing capacity of limestone as cap rock for gas reservoir at different burial depths, triaxial compression coupled with permeability tests of limestone subjected to different confining pressures and temperatures were carried out using the improved MTS 815 test machine. During the tests, acoustic emission (AE) monitoring system was also employed to estimate the rock damage. Test results demonstrated a degradation of triaxial compression strength with the increase in temperature and decrease of confining pressure. An empirical equation for the peak stress was established considering confining pressure and temperature. Three different permeability evolution patterns of limestone samples can be proposed based on the experimental results under different coupled thermo-hydro-mechanical (THM) conditions. According to the spatial-temporal characteristics of AE signals, permeability evolution was highly related to the fracture growth and connection in limestone. Ultimately, the experimental results revealed that the permeability of limestone gradually decreased with the increasing confining pressure and temperature. In addition, an exponential relationship between initial permeability and confining pressure was established.

Remote Sensing-based Agricultural Drought Monitoring using Hydrometeorological Variables Abstract A new drought index, the agricultural dry condition index (ADCI), was developed to combine various hydrometeorological variables associated with agricultural droughts. It was calculated by applying weights to the soil moisture, vegetation activity, and land surface temperature data, which are used to monitor agricultural droughts. The vegetation health index (VHI) and microwave integrated drought index (MIDI) are also used to monitor agricultural droughts; these were calculated using satellite image data collected between 2001 and 2015 in South Korea and their spatiotemporal variations were analyzed. In order to compare the ADCI with actual agricultural drought conditions, land in South Korea was divided into two classes (rice paddies and croplands) and the ADCI values were compared to the corresponding crop yields (rice from rice paddies, potatoes and soybeans from croplands). There was no significant correlation between the ADCI and crop yield for the rice paddies because the water supply is controlled by irrigation. However, in the croplands there was a high degree of correlation with correlation coefficients of 0.83 and 0.80 for potatoes and soybeans, respectively. In order to confirm agreement with the actual affected areas, a receiver operating characteristic analysis was conducted for 2001 and 2015 when there was severe drought. This analysis found that the ADCI peaked at 0.68 in 2001 (June) 0.64 in 2015 (June). The ADCI was found to be highly applicable to the assessment of agricultural drought conditions. The VHI responded positively to land surface temperature while the MIDI responded to rainfall. However, the ADCI showed the best results because it is a weighted index of the input data, such as the land surface temperature, soil moisture, and vegetation activity, and their combination. The results confirmed that soil moisture, vegetation activity, and land surface temperature are the most important variables associated with droughts and that the ADCI can be effectively used to monitor agricultural droughts.

Failure Envelopes of Bucket Foundations for Offshore Wind Turbines under Combined Loading Including Torsion Abstract The loading condition of bucket foundations for offshore wind turbines is very complicated and often under combined loading, but the torsion is always ignored. So, it is necessary to investigate the failure envelopes including torsion of bucket foundations. Under the premise of validating the reliability of the numerical model by comparing with other published data, failure envelopes in each loading space are obtained and the applicability of swipe test for the determination of H-T failure envelope and M-T failure envelope of bucket foundations is explored. The impacts of torsion and aspect ratio on the combined bearing capacities are studied. While revealing the law of impacts, equations are proposed to determine the respective failure envelopes and evaluate the combined bearing capacities. The V-H-M-T failure envelope can reveal the torsion effect on envelope under traditional V-H-M loading, the results show that the normalized V-H-M failure envelopes under different torsions almost coincide, torsion effect can be easily considered according to this characteristic.

Sediment Yield for Ungauged Watersheds in South Korea Abstract Specific degradation (SD) is defined as the ratio of the sediment yield divided by the watershed area to compare sediment yield at the basin scale. The SD from 35 watersheds was calculated from field measurements of discharge and sediment concentrations. The watershed characteristics for each watershed were analyzed using GIS tools. All sediment gauging stations are located in alluvial river reaches and the estimated specific degradation typically ranges between 100 and 1,000 tons/km2·yr. Six regression models based on the watershed characteristics are proposed to estimate the mean annual sediment yield. The most useful relationship is function of the drainage area and mean annual precipitation. The proposed models were tested and validated with 15 additional river stations. The root mean square errors (RMSE) of the predictions are approximately 100 tons/km2·yr which is found to be satisfactory. The proposed models should be useful to estimate the sediment yield from ungauged watershed in South Korea.

Experimental Study on the Shear Behaviors of PolypropyleneFiber-Reinforced Sand Abstract With excellent water permeability and high shear strength S, fiber-reinforced sand (FRS) is an ideal material for subgrades and has promising application prospects. In this study, the FRS were formed by adding the randomly distributed fibers to the sand specimens. Based on the triaxial consolidated drained tests, the effects of factors such as fiber content ζ, fiber length lf, relative density Dr and confining pressure σ3 on the S of FRS were systematically investigated. The test results showed that the S of the FRS increased as ζ, lf, Dr and σ3 increased. Under axial loads, the stress-strain curve of the FRS exhibited a hardening trend. Under specific Dr and σ3, the S increment of the FRS had a strong linear relationship with ζ and lf. The Mohr’s stress circle results showed that the FRS possessed not only frictional strength but also high cohesive strength. Based on the triaxial consolidated drained test results, the S of the FRS was calculated using the Zornberg model and Michalowski model. The calculation results showed that the S predicated by the Zornberg model differed significantly from the measured values. However, the S predicted by the Michalowski model were in good agreement with the measured values.

Feasible Ranges of Runoff Curve Numbers for Korean Watersheds Based on the Interior Point Optimization Algorithm Abstract Rainfall runoff is a complex phenomenon in nature. It differs from place to place due to different topographical features and rainfall patterns. The Natural Resources Conservation Service - Curve Number (NRCS-CN) is a well-adopted model to account for direct runoff volume from storm events. There are several studies on determining the initial abstraction and the CN from observed rainfall-runoff data; however, few studies demonstrate their statistical characteristics. The major aim of this study is to determine the feasible range and the confidence intervals of the CN. We examined 660 rainfall-runoff events collected from six medium sized watersheds in South Korea. The interior point optimization algorithm was adopted to ascertain the optimum value of CN and the initial abstraction coefficient (λ). The obtained results show that the CN value ranged from 45 to 90 and the average λ = 0.12 was best suited for Korean watersheds. The estimated confidence intervals were highly significant and strongly recommended for Korean watersheds.

Arching Effect between the Pipes of a Pipe Umbrella Support System in a Shallow-Buried Tunnel Abstract The pipe umbrella is one of the most commonly used prereinforcement techniques in tunnel construction. Current theoretical analyses of the pipe umbrella support mechanism are primarily based on the elastic foundation beam principle, whereby only a single pipe is concerned, and the interactions among pipes are ignored. In this study, a double-parameter elastic foundation beam theory-based model was established with consideration of the arching effect between two pipes. The model can simulate the support mechanism of the pipe umbrella system in shallow-buried tunnels. The support mechanisms of a pipe umbrella system installed in a rectangular tunnel and an arched tunnel were investigated. In the two cases, the arching effect and a single pipe were considered. The results indicate that the arching effect leads to larger pipe deflection and internal forces than considering a single pipe. The influences of the diameter of a single pipe, the distance between two pipes and the shear parameters of the surrounding rock on the pipe deflection and the arching effect are discussed. In addition, the proposed model was validated by the in situ monitoring results.

Proposing an Elliptic Equation for the Symmetrical Sag Vertical Curvesbased on Sight Distance in Highway Abstract The sight distance at night is one of the most important driving safety factors. Sag vertical curves are one of the important elements of roads, which require a sufficient sight distance at night. The headlight sight distance is used for determining the length of the sag vertical curves. Presently, the parabolic curve is mainly used in symmetrical sag vertical curves. The present study aimed to propose the elliptic curve as a new alternative for the parabolic curve. An elliptic equation is developed for use in vertical curve and sight distance of the sag elliptic vertical curve in nighttime is calculated based on the characteristics of the road geometry. Results indicate that contrary to the parabolic curve, the sight distance of the elliptic curve varies depending on the vehicle location and increases as the vehicle moves forward on the curve; therefore, driver sees the end of curve in elliptic curves much earlier than parabolic curves. The results also indicate that the sight distance of the parabolic curve is more than that of the elliptic curve in the shortest length after the curve beginning (up to 8% of the curve length) while the sight distance of the elliptic curve is more than that of the parabolic curve at the rest of the curve (at least 92% of the curve length). Based on the obtained results, for S > L, the stopping sight distances of the elliptic and parabolic vertical curves are equal.

Influence of Soil Type on Single and Competitive Retention Behavior of Inorganic Macro Cations in Binary as well as Ternary Solution Abstract Inorganic macro cations (Na+, K+ and Ca+) co-exist in agricultural and municipal landfill sites. The retention behavior of these ions in different soils and combinations (binary and ternary system) has not been fully understood by previous studies. This study seeks to probe into the retention behavior of macro cations (Na+, K+, Ca+2) in six different soils under multiple combinations of ions solution. The retention of common ions was quantified by using Freundlich and Langmuir isotherms. Na+ (single) pollutant-soil interaction was desorbed in most of the soils. The retention of K+ and Ca+2 was much greater than that of Na+. There is no definite trend for retention of Na+ in the presence of K+. For all soils, the retention results of K+ in the presence of Na+ decreased. Affinity for K+ was greater than that for Ca+2 in most of the soils. This study also demonstrated the affinity sequence of the ion retention for each soil and the percentage reduction of ions in competition with respect to single ions. Anomalous trends of the isotherm parameter fitting suggested the limitation of the mathematical models in predicting the experimental data. These results can be helpful in improving accuracy of fate prediction of pollutant fate and thus, design of waste containment facilities for various wastes that contains inorganic ions.

A Pore Size Distribution-based Microscopic Model for Evaluating the Permeability of Clay Abstract This work aimed to propose a semi-empirical model that predicts the permeability of saturated clay using the data of mercury intrusion porosimetry (MIP). First, the pore size distribution (PSD) curve obtained from an MIP test was regarded as a discrete probability function of pore diameters; thus, its shape could be characterized by probability parameters (e.g., the expected value and the standard deviation). Subsequently, these probability parameters, combined with the microporosity calculated from the volume of intruded mercury, were correlated with the permeability of clay based on Hagen-Poiseuille's equation. Next, the performance of the proposed permeability model was verified using data reported in the literature. Thereafter, the model was applied to estimate the permeability of normally consolidated (NC) and overconsolidated (OC) kaolin subjected to various triaxial loading. The results highlighted that the proposed model is capable of characterizing the sensitive variation of kaolin permeability under different overconsolidation ratios (OCRs), stress paths, and stress levels.