Heejung Youn

Microbially Induced Calcite Precipitation Employing Environmental Isolates

In this study, five microbes were employed to precipitate calcite in cohesionless soils. Four microbes were selected from calcite-precipitating microbes isolated from calcareous sand and limestone cave soils, with Sporosarcina pasteurii ATCC 11859 (standard strain) used as a control. Urease activities of the four microbes were higher than that of S. pasteurii. The microbes and urea–CaCl2 medium were injected at least four times into cohesionless soils of two different relative densities (60% and 80%), and the amount of calcite precipitation was measured. It was found that the relative density of cohesionless soils significantly affects the amount of calcite precipitation and that there is a weak correlation between urease activity and calcite precipitation.

Repairing cracks developed in mortar beams reinforced by cold-drawn NiTi or NiTiNb SMA fibers

In this study, mortar beams reinforced by shape memory alloy (SMA) fibers of NiTi and NiTiNb alloys were prepared to perform crack-repairing tests using three-point bending tests. The SMA fibers had a length of 30 mm, and their types were straight, dog-bone, and dog-bone with paper wrapping. For the bending tests, twelve types of mortar beams with the dimensions of 40 mm × 40 mm × 160 mm (B × H × L) were prepared. Half of them had a top steel reinforcement, and equal numbers of beams were assigned to the NiTi and NiTiNb fibers. Five SMA fibers were located at the bottom center of the beams along with an artificial crack of 10 mm depth and 1 mm thickness. Epoxy was used to fill the cracks to bond the cracked surfaces using injection, and a hot-gun was used to heat the SMA fibers in the cracks. The crack widths were measured before and after the cracks were repaired, and force–displacement curves were obtained to assess the flexural strength recovery ratio of the beams. It does not appear that the crack-closing capacity of SMA fibers is a crucial factor to recover the flexural strength in repaired beams. However, adequate application of epoxy is critical for repairing cracks, and the residual stress of SMA fibers seems to contribute to increase flexural strength of repaired beams. The residual stress of SMA fibers functions as prestress on mortar and delays the initiation of cracking.

Horizontal Pullout Capacity of Steel Chain Embedded in Sand

This paper presents the pullout capacity of steel chain embedded in pure sands using 1g lab-scale model test. The anchor chain made of steel is often embedded into sands connecting anchor to floating structures. Even though the lateral or vertical forces applied to the floating structures are resisted by both anchor and anchor chain, most researches have focused on the capacity from the anchor system not from the anchor chain. In the model test, a steel anchor chain was embedded into Jumunjin sands and horizontally pulled out to failure. The pullout capacity of anchor chain was divided into frictional resistance and passive resistance, and the necessary factors to estimate each component were proposed using test results. It was found that the maximum pullout capacity of anchor chains increases as the embedment depth increases. The contribution of passive resistance to the total pullout capacity was calculated to be as much as 81~93%.

Experimental Study on Sinkholes: Soil–Groundwater Behaviors Under Varied Hydrogeological Conditions

Sinkholes have been one of the major geohazards in karst terrain and have resulted in loss of human life, as well as significant civil engineering infrastructure. Approximately 20 % of the United States is underlain by karst terrain formed from the dissolution of soluble rocks, and is susceptible to a sinkhole hazard. Particularly, Texas, Florida, Tennessee, Alabama, Missouri, Kentucky, and Pennsylvania are known as sinkhole states. Surprisingly, the understanding of sinkholes is still poor in geotechnical engineering. This paper presents and discusses a preliminary study on the mechanism of Floridas sinkholes that are triggered by groundwater flow. A physical soil–groundwater model was devised, and multiple tests were conducted under different hydrogeological conditions (e.g., with/without aquitard and overburden soil thickness). Groundwater at multiple locations was monitored during the sinkhole-simulation process so that integrated soil–groundwater behaviors could be investigated. It is found that groundwater responds before the surface collapse (showing a cone of depression); thus, a pattern change of groundwater flow can be used as input to pre-detect a sinkhole. In addition, having a clayey sand layer within sand shows a significant influence that includes a rapid change of groundwater flow in the sinkhole process and a larger surface sinkhole.

Stiffness and Confinement Ratios of SMA Wire Jackets for Confining Concrete

This article discusses the effects of the stiffness and confinement ratios of shape memory alloy (SMA) wire jackets on the behavior of confined concrete. SMA wire jackets are an effective confining material to improve concrete behavior; for example, by increasing peak strength and failure strain. The stiffness and confinement ratios of fiber-reinforced polymer jackets have been extensively discussed and their effects are well known. However, assessment of the stiffness and confinement ratios of SMA wire jackets has not previously been conducted. In this study, we investigate the effects of the stiffness and confinement ratios of steel jackets, and then compare the results with those of SMA wire jackets. In general, the stiffness ratios of SMA wire jackets are relatively smaller than those of steel jackets, and most of them have lower stiffness ratios because the Young’s moduli of the SMAs are relatively small. The active confining pressure of the SMA wires does not improve the lower stiffness-ratio effect since the amount of active confining pressure is not sufficiently large.

Wear Resistance of Four Rocks Using Rotary Abrasion Test in Dry and Wet Conditions

This article investigates the wear resistance of four rock types using the rotary abrasion test. The tested rocks include argillaceous rocks (mudstone and shale) and igneous rocks (granite and andesite). The wear volume of the rock specimen was measured with various wear time, normal loads, specimen lengths, and water conditions. It was found that the wear volume of rock specimen linearly increased with wear time and normal load per unit length. The Wear Volume Index was proposed to represent the wear volume of the rocks, which was measured to be the greatest for mudstone, followed by shale, granite, and andesite for both wet and dry conditions. The presence of water significantly affects the Wear Volume Index of argillaceous rocks, which increased by 60 % for mudstone and 25 % for shale. On the other hand, the effect of water was found to be insignificant for igneous rocks such as granite and andesite.

Variation of Lateral Soil Resistance Due to Two-Way Cyclic Loading

In this study, the variation of lateral soil resistance of the monopile were investigated through centrifuge tests. The soil used in the experiment was Jumunjin standard sand, and the soil deposit was created with a relative density of 80%. The static load test was carried out to obtain the static lateral capacity, and the magnitudes of cyclic load were determined at 30, 50, 80 and 120% of the static capacity. A hundred cycles were applied to the pile head with a frequency of 0.125 Hz. The lateral soil resistance and pile displacement of the monopile subjected to cyclic loading were calculated through experimental data. At 2 and 5 m depth, the soil resistance increased with increasing number of cyclic loading, while the resistance decreased at 7 and 9 m depth.

An Optimized Data Interpretation for Marshall Flow and Stability Test

Due to its simple and quick manner in testing, Marshall Stability and Flow test method is still widely used over the world. Testing method provides useful information such as strength, deformation, and stiffness represented by Stability, Flow, and Martial Quotient (MQ) respectively. However, an initial error in load-displacement curves can generate significant misleading in those mechanical properties. This paper presents an optimized data processing method for data interpretation. Previous researches and specifications related to MQ are reviewed to identify limitations of the exiting data interpretation methods. Laboratory experiments were performed to verify the limitation of the current methods. This study illustrates that a non-resistive section of specimen causes an “initial delay” in the load-deformation curve, resulting in misleading of the mixture properties. Lastly, the automated algorithm to offset the initial delay and determine the best MQ was devised.

Numerical Analysis of Laterally Loaded Piles Affected by Bedrock Depth

This study investigates the lateral behavior of pile foundations socketed into bedrocks using 3D finite difference analysis. The lateral load-displacement curve, pile deflection, and bending moment distribution were obtained for different bedrock depths between 3 and 20 m. It was discovered that bedrocks that have a depth of 7 m (7D) or less influence the lateral behavior of the pile. The p-y curves were collected at depths of 2.0–4.5 m, and the effect of the bedrock on the curves was evaluated. It was observed that the p-y curves were significantly affected by the material properties of the bedrock if the rock is located in close proximity (within 3D), but the effect is diminished if the p-y curves were 3.5 m (3.5D) or farther from the bedrock.

Static pullout resistance of anchor chains in cohesionless soil—laboratory testing

ABSTRACT This paper investigates the static pullout resistance of anchor chains embedded into cohesionless soil. The anchor chains, which are made of steel, were buried into Jumunjin sand whose relative density was set to approximately 60%. The anchor chains were horizontally pulled out through a displacement of 70 mm in laboratory model tests. Three different embedment depths and seven different numbers of chain links were adopted. The pullout resistance of the anchor chains was found to increase with increasing embedment depth and the number of anchor chain links. The measured resistance was significantly higher than the calculated frictional resistance, implying that the passive resistance at the front of the anchor chain significantly contributes to the pullout resistance. The contribution of the passive resistance tends to decrease with increasing number of chain links.