Showing papers by "Dong-Soo Kim published in 2013"

A Newly Developed State-of-the-Art Geotechnical Centrifuge in Korea

Abstract: The first large scale geotechnical centrifuge in Korea has recently been developed at KAIST under the Korea Construction Engineering Development (KOCED) Collaboratory program. A 5 m platform radius, 240 g-tons state-of-the-art geotechnical centrifuge has been installed in a new facility. The centrifuge has the unique feature of an automatic balancing system and includes parts for general testing purposes such as fluid rotary joints, slip rings, a fiber optic rotary joint and an Ethernet network system. In addition, a four degree-of-freedom in-flight robot can be equipped to simulate complex construction or in-situ testing process during centrifuge flight. In order to simulate earthquake motion during operation, a self-balancing type biaxial shaking table has also been developed. Since the KOCED program promotes collaboration and remote use, tele-presence and tele-participation environments have been implemented in this facility.

Self-balanced earthquake simulator on centrifuge and dynamic performance verification

Abstract: This paper describes some details of a self-balanced earthquake simulator on the centrifuge in KAIST and results of a series of proof tests for verifying its dynamic performance and excitation capacity. The main feature of the earthquake simulator is the dynamic self-balancing technique adopted to eliminate a large portion of the undesired reaction forces and vibrations transmitted to the centrifuge main body. This feature is achieved by embarking counter-weight platform and two back-to-back hydraulic bearings. The maximum base shaking acceleration of the earthquake simulator is 20 g in horizontal direction under 40 g of centrifuge acceleration with a maximum payload of 700 kg, corresponding to 0.5 g of horizontal shaking acceleration in the prototype scale. The loading frequency ranges from 40 Hz to 200 Hz (300 Hz) for sinusoidal (real earthquake) inputs. The dimension of slip table is 670 mm × 670 m in length and width. The proof test results show that the earthquake simulator can reproduce mono-frequency sinusoidal inputs in a wide band of frequencies as well as multi-frequency real earthquake inputs at the bottom of soil models with satisfactory fidelity, and the dynamic self-balancing contributes to the safety of the centrifuge structure.

Settlement of Piled Rafts with Different Pile Arrangement Schemes via Centrifuge Tests

Abstract: In the design of piled raft foundations, the control of total and differential settlements is crucial. On the basis of centrifuge tests, this study presents the feasibility of a fairly optimal pile arrangement scheme for reducing total and differential settlements. Two models of piled raft foundations having the same flexible raft and number of piles, the uniform pile arrangement model and the concentrated pile arrangement model, and one model having a rigid raft with a concentrated pile arrangement were designed for centrifuge tests. The settlements of three rafts of the piled raft models and the induced bending moments of two flexible rafts were measured during the load application process. The measured results were compared between the tests to illustrate the ability of the concentrated pile arrangement scheme in reducing total and differential settlements, as well as induced bending moment. Moreover, three piled rafts were simulated by the Plaxis 3D Foundation software package to calculate the...

Seepage behavior of drainage zoning in a concrete faced gravel-fill dam via centrifuge and numerical modeling

Abstract: Sandy gravel materials have recently been utilized in place of crushed rock materials as the main rockfill materials in Concrete Faced Rockfill Dams (CRFD) to address geological and environmental problems. In this paper, an experimental scheme for centrifuge modeling was developed to simulate a Concrete Faced Gravel-fill Dam (CFGD). The dam considered in this study was designed to implement a drainage zone of high permeability in the main gravel-fill zone to enhance safety against accidental water infiltration into the dam. Two centrifuge tests were performed and compared to investigate the performance of the drainage zone. The first test was done with the drainage zone and the second without the drainage zone. In the centrifuge tests, water infiltration was simulated by raising the water table over pre-implemented cracks on the model face slab. The infiltration behaviors were monitored by pore water pressure transducers. The centrifuge tests showed that the drainage zone of the CFGD effectively drains infiltrating water out of the dam body in a short time. Numerical modeling was also performed to help understand the process of seepage through cracks.

Development of a New Asymmetric Anchor Plate for Prefabricated Vertical Drain Installation via Centrifuge Model Tests

Abstract: This study investigates the kinematic behavior of a clay-mandrel-anchor system during the driving-retrieval process and its influence on the installation of prefabricated vertical drains and clay disturbance. In addition, a new anchor plate with an asymmetric shape is proposed as an alternative for improved anchoring performance and clay disturbance mitigation. To investigate the behavior of the clay-mandrel-anchor system, two-dimensional centrifuge model tests were developed and successfully performed to visualize anchoring kinematics and deformation of the clay layer. The tests demonstrated that the asymmetric anchor plate effectively provides an anchoring mechanism and reduces the degree of disturbance. The conventional anchor has potential risk of delayed anchoring and thus unsuccessful installation. While the amount and extent of clay deformation are obviously affected by the protruding length of the anchor plates into the clay, the protrusion length has less influence on the range of the sme...

Seismic risk assessment of architectural heritages in Gyeongju considering local site effects

Abstract: . A seismic risk assessment is conducted for cultural heritage sites in Gyeongju, the capital of Korea's ancient Silla Kingdom. Gyeongju, home to UNESCO World Heritage sites, contains remarkable artifacts of Korean Buddhist art. An extensive geotechnical survey including a series of in situ tests is presented, providing pertinent soil profiles for site response analyses on thirty cultural heritage sites. After the shear wave velocity profiles and dynamic material properties were obtained, site response analyses were carried out at each historical site and the amplification characteristics, site period, and response spectrum of the site were determined for the earthquake levels of 2400 yr and 1000 yr return periods based on the Korean seismic hazard map. Response spectrum and corresponding site coefficients obtained from site response analyses considering geologic conditions differ significantly from the current Korean seismic code. This study confirms the importance of site-specific ground response analyses considering local geological conditions. Results are given in the form of the spatial distribution of bedrock depth, site period, and site amplification coefficients, which are particularly valuable in the context of a seismic vulnerability study. This study presents the potential amplification of hazard maps and provides primary data on the seismic risk assessment of each cultural heritage.

Numerical Analysis of Cluster and Monopod Suction Bucket Foundation

Abstract: Numerical analyses were performed to study the cluster suction bucket foundation performance in comparison with the monopod suction bucket foundation. The possibility of improving the resistance performance of suction bucket foundations by using cluster configuration was studied. Clustered foundations are formed by arranging multiple bucket foundations in relatively short distances among them. A model cluster foundation composed of three individual buckets was configured such that it has the same cross sectional area, length and total bucket weight to a monopod foundation. Results of numerical modeling are presented in the form of load-displacement curves. Vertical, horizontal and horizontal combined with moment load conditions were considered. The results show that the cluster type configuration improved the resistance performance of the foundations under typical loading conditions.Copyright © 2013 by ASME

Capacity of Horizontally Loaded Suction Anchor Installed in Silty Sand

Abstract: A suction anchor is one of the most popular anchors for deepsea floating systems. An anchor used for catenary mooring is predominantly under a horizontal load. In this study, the behavior of a suction anchor installed in cohesionless soil was investigated when the anchor was mainly subjected to a horizontal load induced by a catenary line. In order to study the behavior of the suction anchor, 3D FEM analysis models were developed and analyzed. Depending on the location of the load (padeye), the ultimate horizontal load was monitored. The distributions of the reaction forces around the anchor induced by the seabed were analyzed using the circumferential stress to understand the behavior of the suction anchor under a horizontal load.

Bearing Capacity of a Monopod Bucket Foundation for Offshore Wind Towers - Centrifuge and Numerical Modeling

Abstract: In order to evaluate the bearing capacity behaviour of a monopod suction bucket foundation for an offshore wind tower at the western sea of Korea, a centrifuge load test and numerical analyses were performed. The monopod bucket foundation was designed to be installed in a silty sand layer. The model soil was prepared to simulate a target site by using soil samples having similar properties and controlling relative density. In-flight miniature cone penetration test and bender element array were used to confirm that the model soil had represented the target site conditions. The load - rotation curve of the centrifuge load test was analysed. A series of numerical analyses were performed to validate the experimental conditions. Self-weight of the model, distance to the boundary and elastic modulus of the soil layer were varied to study their effects on the load - rotation curves.

Development of Miniature Cone and Characteristics of Cone Tip Resistance in Centrifuge Model Tests

Abstract: The standard CPT(Cone Penetration Test), which can be easily performed to investigate in-situ soil engineering properties, has been widely used. CPT are also widely being utilized in centrifuge model tests. In this study, a miniature cone with 10mm diameter was developed and its applicability in the centrifuge was evaluated. The developed miniature cone was equipped with a four degree-of-freedom in-flight robot. A series of cone penetration tests was performed under four centrifuge acceleration levels. As results, the cone resistances measured at the same confining stress within shallow penetration depth were affected by the centrifugal accelerations. The critical depth was proportional to the cone diameter and relative density. Cone resistances results below the critical depth and soil parameters obtained from the laboratory tests were compared with those by previously proposed empirical relations.

Nickel-oxide film as an AR coating of Si window for IR sensor packaging

Abstract: An infrared (IR) transparent window is necessary for the IR sensor package. The most commonly used materials for IR transparent window are germanium (Ge) an d silicon (Si). Ge has excellent optical properties but also the disadvantage of expensive price. Si has merits such as inexpensive cost and CMOS process compatibility but it has lower transmittance in the range of LWIR region than Ge. Therefore, an altern ative anti-reflection (AR) coating is necessary to increase the transmittance of Si as an IR transp arent window in the LWIR region. A simple single layer antireflection coating was newly designed on the silicon window for th e infrared sensor package. Among the various materials, nickel oxide (NiO) was selected as an AR coating material due to its suitable optical properties and simple process. NiO film was deposited onto the double sided polished Si wafer by reactive rf sputtering with Ni target in an environment of Ar and O 2 mixed gas. The thickness of the NiO film was determined by Essential Macleod simulation. FT-IR was used to measure the transmittance of the samples in the LWIR region. After the nickel oxide film was sputtered onto the double sides of the silic on wafer, the measured transmittance of the Si wafer was increased over 20% in the LWIR region co mpared with that of uncoated Si wafer. Additionally, annealing effect on the transmittance of NiO coated Si wafer was studied. By incr easing the annealing temperature from 300 ƒ to 700ƒ , an ad ditional increase of transmittance was achieved. Keywords: AR coating, nickel oxide, window, IR sensor packaging

Verification of Nonlinear Numerical Analysis for Seismic Response of Single Degree of Freedom Structure with Shallow Foundation

Abstract: Seismic response of single degree of freedom system supported by shallow foundation was analyzed by using nonlinear explicit finite difference element code. Numerical analysis results were verified with dynamic centrifuge test results of the same soil profile and structural dimensions with the numerical analysis model at a centrifugal acceleration of 20 g. Differences between the analysis and the test results induced by the boundary conditions of control points can be reduced by adding additional local damping to the natural born cyclic hysteretic damping of the soil strata. The analysis results show good agreement with the test results in terms of both time histories and response spectra. Thus, it can be concluded that the nonlinear explicit finite difference element code will be a useful technique for estimating seismic residual displacement, earthpressure etc. which are difficult to measure during laboratory tests and real earthquake.

A Case Study of Evaluating Inertial Effects for Inverted T-shape Retaining Wall via Dynamic Centrifuge Test

Abstract: Mononobe-Okabe (M-O) theory is widely used for evaluating seismic earth pressure of retaining wall. It was originally developed for gravity walls, which have rigid behavior, retaining cohesionless backfill materials. However, it is used for cantilever retaining wall on the various foundation conditions. Considering only inertial force of the soil wedge as a dynamic force in the M-O method, inertial force of the wall does not take into account the effect on the dynamic earth pressure. This paper presents the theoretical background for the calculation of the dynamic earth pressure of retaining wall during earthquakes, and the current research trends are organized. Besides, the discrepancies between real seismic behavior and M-O method for inverted T-shape retaining wall with 5.4m height subjected to earthquake motions were evaluated using dynamic centrifuge test. From previous studies, it was found that application point, distribution of dynamic earth pressure and M-O method are needed to be re-examined. Test results show that real behavior of retaining wall during an earthquake has a different phase between dynamic earth pressure and inertial force of retaining wall. Moreover, when bending moments of retaining wall reach maximum values, the measured earth pressures are lower than static earth pressures and it is considered due to inertial effects of retaining wall.