Yong-Hoon Byun

Integrity evaluation of grouting in umbrella arch methods by using guided ultrasonic waves

Umbrella arch method (UAM) used for improving the stability of the tunnel ground condition has been widely applied in the tunnel construction projects due to the advantage of obtaining both reinforcement and waterproof. The purpose of this study is to develop the evaluation technique of the integrity of bore-hole in UAM by using a non-destructive test and to evaluate the possibility of being applied to the field. In order to investigate the variations of frequency depending on grouted length, the specimens with different grouted ratios are made in the two constraint conditions (free boundary condition and embedded condition). The hammer impact reflection method in which excitation and reception occur simultaneously at the head of pipe was used. The guided waves generated by hitting a pipe with a hammer were reflected at the tip and returned to the head, and the signals were received by an acoustic emission (AE) sensor installed at the head. For the laboratory experiments, the specimens were prepared with different grouted ratios (25 %, 50 %, 75 %, 100 %). In addition, field tests were performed for the application of the evaluation technique. Fast Fourier transform and wavelet transform were applied to analyze the measured waves. The experimental studies show that grouted ratio has little effects on the velocities of guided waves. Main frequencies of reflected waves tend to decrease with an increase in the grouted length in the time-frequency domain. This study suggests that the non-destructive tests using guided ultrasonic waves be effective to evaluate the bore-hole integrity of the UAM in the field.

Scour Evaluation of Offshore Foundations by using Ultrasonic Reflection Images and Natural Frequency Variation

Abstract Local loss of geomaterials, which reduces the stability of the structures, around foundation of offshore structure occurs due tothe interaction between water and ground. The objective of this study is to evaluate scouring in offshore foundation structures byusing ultrasonic reflection method and natural frequency analyses. Sand-cement mixture is prepared to model the ground, which isencountered the scour depth and the scour shape. Ultrasonic wave and natural frequency are measured according to the scourdepth. Ultrasonic transducers are used to measure the ultrasonic wave, and strain guages are used to measure the natural fre-quency. Ultrasonic transducers are fixed on horizontal movement system and rotational movement system for the change of loca-tion. Strain guages are attached on upper part of model monopile. Ultrasonic reflection image and strain responses are monitoredin laboratory experiment. Maximum scour depth is estimated by natural frequency, and the characteristics of the scour shape areestimated by ultrasonic reflection image. This study suggests that the ultrasonic scanning and natural frequency analyses may beeffectively used to monitor the characteristics of the scour depth and shape. Key words : Monitoring, Monopile, Natural frequency, Ultrasonic imaging, Scour

Behavior Evaluation of Monopile Foundation Subjected to Impact Lateral Loads

Lateral loads, including wind, ocean wave and tide affect on the stability of the monopile in the offshore wind farm. The goal of this study is to characterize the behavior of the monopile under impact lateral loading by using strain gauges. A laboratory-scale monopile system, which consists of a monopile and a support plate, is fixed into the support plate for modelling the monopile embeded in rock mass. The length of the monopile is adjusted by a rotation of the screw thread carved on the monopile for the investigation of the natural frequency and moment. The impact lateral load is applied by the pendulum hammer, which falls at the fixed pendulum length and angle. For the analyses of the monopile behavior due to the impact loading, strain guages are symmetrically installed on the surface of the monopile in the axial direction. Experimental results show that the maximum moment increases with the increase of natural frequency of the monopile. In addition, the predicted natural frequencies of the model monopile are similar to the measured natural frequencies. This study demonstrates that the model monopile suggested in this study may be effectively used for the characterization of behavior of the monopile installed in the field.