Showing papers by "Yun Wook Choo published in 2014"

Investigation of Monotonic and Cyclic Behavior of Tripod Suction Bucket Foundations for Offshore Wind Towers Using Centrifuge Modeling

Abstract: This paper performed centrifuge load tests of a tripod bucket foundation preliminarily designed as a supporting system of wind turbines and compared the results to those obtained from a test of a monopod bucket foundation. The tripod foundation prototype studied in this study has three bucket caissons, each of which is 6.5 m in diameter and 8.0 m in length. The center-to-center distance between the buckets was 26.9 m. The site is composed of an 11-m thick layer of dense silty sand overlying a thick sandy silt layer. The horizontal load was applied at a height of 33 m from the seabed floor according to the design load condition, and the vertical load was simulated by the self-weight of the model. The moment-rotation angle curves for the tripod foundations were compared with that of the monopod bucket foundation. The moment-rotation curve of the tripod was nearly bilinear, whereas that of the monopod showed a gradual decrease in slope. The yield moment for the tripod foundation was half that of the monopod, but the rotation angle for the yield moment was only 20% that of the monopod. The behavior of the tripod foundation under a cyclic load with respect to the accumulated plastic deformation has also been examined in this study. When the resultant moment of cyclic loading was smaller than the monotonic yield moment, negligible accumulated plastic deformation was observed for both one-way and two-way loading. However, when the resultant moment was higher than the monotonic yield moment, significant cumulated deformation resulted.

Seismic behavior of an inverted T-shape flexible retaining wall via dynamic centrifuge tests

Abstract: In the design procedure for a retaining wall, the pseudo-static method has been widely used and dynamic earth pressure is calculated by the Mononobe–Okabe method, which is an extension of Coulomb’s earth pressure theory computed by force equilibrium However, there is no clear empirical basis for treating the seismic force as a static force, and recent experimental research has shown that the Mononobe–Okabe method is quite conservative, and there exists a discrepancy between the assumed conditions and real seismic behavior during an earthquake Two dynamic centrifuge tests were designed and conducted to reexamine the Mononobe–Okabe method and to evaluate the seismic lateral earth pressure on an inverted T-shape flexible retaining wall with a dry medium sand backfill Results from two sets of dynamic centrifuge experiments show that inertial force has a significant impact on the seismic behavior on the flexible retaining wall The dynamic earth pressure at the time of maximum moment during the earthquake was not synchronized and almost zero The relationship between the back-calculated dynamic earth pressure coefficient at the time of maximum dynamic wall moment and the peak ground acceleration obtained from the wall base peak ground acceleration indicates that the seismic earth pressure on flexible cantilever retaining walls can be neglected at accelerations below 04 g These results suggest that a wall designed with a static factor of safety should be able to resist seismic loads up to 03–04 g

Lateral Response of Large-Diameter Monopiles for Offshore Wind Turbines from Centrifuge Model Tests

Abstract: Centrifuge tests were carried out to investigate the lateral response of 6-m-diameter monopiles used for offshore wind turbine foundations. Well-instrumented large-diameter model monopiles were tested in dense sands under different end bearing layer conditions and different pile rigidities. A lateral load was applied to monopiles at a level of 33 m above the seabed on a prototype scale to simulate the combination of a high lateral load and significant overturning moment acting on the piles at the seabed level. The test results showed that the measured lateral displacements and moments along the shaft for a given lateral load are much greater than those predicted from p–y analyses using the typically used models of the American Petroleum Institute (API) (1993, “Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design,” API Recommended Practice 2A-WSD, 20th ed., API, Washington, DC) and Reese et al. (1974, “Analysis of Laterally Loaded Piles in Sand,” Proceedings of the 6th Offshore Technology Conference, Houston, TX, Paper No. OTC2080, pp. 473–483. In addition, p–y analyses using these two methods underestimated the effect of vertical load on the lateral response of the monopiles, whereas the effect was more pronounced in the centrifuge test results. Monopiles generally support large vertical loads transferred from the self-weight of the tower shaft and wind turbine compartment. Therefore, more studies are required to develop new p–y curves for large-diameter monopiles to replace the existing API and Reese et al. p–y curves, which were developed for small- or medium-diameter driven piles.

Evaluation of K o in Centrifuge Model Using Shear Wave Velocity

Abstract: The coefficient of earth pressure at rest (Ko) is an important parameter in the analysis and design of geotechnical structures. Methods based on the shear wave velocity (Vs) can be utilized to estimate Ko using the directionality of a shear wave and its dependence on effective stresses. For centrifuge modeling, it is important to understand the in-flight stress states at various locations during loading-unloading-reloading cycles and to develop a direct method for measuring Ko in flight. In this study, previously proposed methods based on Vs (Vs–Ko relationships) were assessed to determine Ko in a centrifuge model using the results of bender element tests performed in a cross-hole configuration. The suitability of the Vs–Ko relationship for the centrifuge test and variation in Ko with respect to location in the centrifuge model were investigated. Through changes in centrifugal acceleration, the loading and unloading stress conditions were simulated, and the Ko value calculated based on the Vs–Ko relationship was compared to values obtained from an earth pressure transducer and via empirical equations. It was found that the method using a single horizontally propagating Vs and one pressure measurement was adequate to evaluate Ko in a centrifuge model. In addition, Ko in the centrifuge model was higher at the center than near the boundary for the unloading stage because of the arching effect.

Centrifuge modeling of differential settlement and levee stability due to staged construction of enlarged embankment

Abstract: Many old levees along major rivers in South Korea have been reinforced by enlargement construction. Conventionally, the design settlement is simply estimated without considering variations in the material properties of the foundation layer underneath existing and enlarged embankments. A case study on an actual enlarged embankment located near Buyeo along the Kum River was performed using centrifuge tests. First, construction of the existing part on a soft foundation layer of kaolin clay was simulated. Second, enlargement of the existing embankment was simulated. The deformation pattern and differential settlement were then observed. After the enlargement simulation, block samples were extracted from the foundation layer for consolidation tests. The deformation pattern of the embankment was analyzed using GeoPIV. Clear differential settlement occurred and caused a tension crack in the enlarged embankment simulation test. The conventional method estimated a design settlement that was different from the measured settlement, and the differential settlement between foundations under the existing and enlarged embankments could not be estimated. Material properties from the consolidation tests differed depending on the location, and the foundation settlement differed according to the acquiring method. Based on these findings, design settlement should be estimated by considering the differential conditions of an actual field.

Profile and Frictional Capacity of a Mooring Line Embedded in Sand via Centrifuge Model Testing

Abstract: The authors present a method for calculating a mooring line’s tension and angle of inclination at the anchor pad eye to analyze the behavior of a mooring line embedded in sand. They adopted the governing equation from a previous study for clay and reanalyzed the frictional and bearing resistances. The authors performed a number of centrifuge tests at two anchor depths while maintaining relative densities of 76 or 51% in the cylindrical test box to validate the proposed analytical method. The analytical solutions agreed well with the test results and the results of the simplified method. The authors conducted a parametric study to evaluate the effects of various factors on the behavior of an embedded mooring line in sand, including the depth of the attachment point, the internal friction angle of sand, the submerged unit weight of sand, the angle of the mooring line on the seabed, and the self-weight and nominal diameter of the mooring line. The results demonstrated that the proposed analytical met...

Evaluation of the seismic response of stone pagodas using centrifuge model tests

Abstract: This study attempts to propose dynamic centrifuge model tests as a method of seismic risk assessment in order to discover how stone architectural heritages with masonry structures have endured seismic load, and whether there is any possibility of future earthquake damage. Dynamic centrifuge tests have been conducted for one fifteenth scale models of Seok-ga-tap and the five-storey stone pagoda of Jeongnimsa temple site, which are Korean representative stone pagodas. In order to make input motions of the earthquake simulator, site investigation and site-specific response analysis have been performed. The models of two stone pagodas, which have the same number of pieces with the real structures, have been produced and the dynamic centrifuge tests have been conducted for the model pagodas. Accelerometers were attached at different heights of the pagoda. The measured acceleration records and frequency responses were analysed during dynamic centrifuge test. Two real earthquake records, Hachinohe and Ofunato earthquakes and a sweeping signal with ranged frequency were utilised for input motions of dynamic centrifuge tests to evaluate the behaviour of the stone pagodas. For Seok-ga-tap models, it was observed that acceleration tends to be amplified with height. The third floor body shows at most 2.5 amplification of acceleration in comparison to the surface ground. The amplification was at a frequency of 3.83 Hz and it was considered as the natural frequency of the pagoda. For the five-storey stone pagoda, the seismic wave energy significantly reduced while it passed the first body floor, and then the peak acceleration was gradually amplified upwards. It was found that the pagodas did not collapse when the peak acceleration of ground surface was raised to 0.4 g. Given that the maximum design seismic acceleration specified in Korean seismic design guide is 0.22 g and the amplification ratio of peak acceleration in the supporting ground of the pagodas ranges from 1.45 to 1.74, it can be shown that the two pagodas are stable against 2400-year return period earthquake level, and have excellent seismic performance.

Numerical Analysis of Group Suction Anchor of Parallel Arrangement Installed in Sand Subjected to Pullout Load

Abstract: In this study, the performance of group suction anchors installed in sand and subjected to pullout loading was investigated by numerical analysis. The group suction anchors consist of two or three units rigidly connected to each other in parallel array and the pullout resistances were compared with that of a single anchor. Parametric study was performed using numerical models to study the effect of the physical conditions of the group anchor. The parameters include the skirt length to diameter ratio of a unit suction anchor, the pad-eye location, inclination of loading and the spacing between unit suction anchors. The analysis shows that the ratios of the pullout capacity of double suction anchor and triple suction anchor to that of single anchor are 1.7 and 2.4, respectively. The ratio increases with the increase in the spacing between the unit anchors. The other parameters such as the skirt length to the diameter ratio, the location of the pad-eye and the loading inclination have negligible effect on the ratio of pullout resistances of the group anchor to the single anchor.

Centrifuge test of a clustered bucket foundation for offshore wind towers

Abstract: This paper reports the results from a series of centrifuge model tests undertaken to provide insight into the behaviour of a clustered suction bucket foundation. Of particular interest was to compare the performance with that of a monopod foundation. The clustered foundation consisted of three buckets, spacing at 1.5 diameters centre to centre, and was fabricated based on the area equivalent principle i.e. the total plan area of the three buckets was approximately equal to the area of the corresponding monopod foundation. Horizontal load, combined with overturning moment load, was applied to simulate a loading condition that offshore wind towers typically withstand. The resisting performances of the clustered and monopod bucket foundations were evaluated and compared. The use of the clustered bucket foundation improved the resistance as much as 58%, with the efficiency decreasing with increasing rotation angle. et al., 2005; Gourvenec, 2007; LeBlanc et al., 2009; Zhu et al., 2011; Hung and Kim, 2012). Various factors such as water depth, soil conditions, fabrication facilities, installation equipment, and transport vessels should be taken into account in order to conceive a suitable foundation type. At some sites, bucket foundations may prove more economical. By comparison with traditional foundation systems, such as piles or massive concrete bases, large savings can be made on installation time and materials. The foundation may be monopod or tripod/ quadruped (i.e. may consist of 1∼4 individual buckets; Houslby and Byrne, 2000; Byrne and Houslby, 2003). For the former, the overturning load is applied directly to the single large foundation. In this case the bucket may be embedded solely in the sand and the foundation response to an overturning moment will be critical. For the latter, the overturning loads applied by the wind and waves are resisted predominantly by a ‘push-pull’ action, involving equal and opposite vertical loads at foundation level. In this design, the foundations are likely to be embedded in sand, and it will be the response of the foundation to vertical loads that is critical (Byrne and Houslby, 2003). This

Numerical Studies on Combined VH Loading and Inclination Factor of Circular Footings on Sand

Abstract: For circular rigid footings with a rough base on sand, combined vertical - horizontal loading capacity was studied by three-dimensional numerical modelling. A numerical model was implemented to simulate the swipe loading and the probe loading methods and an interpretation procedure was devised in order to eliminate the numerical error from the restricted mesh density. Using the Mohr-Coulomb plasticity model, the effect of friction angle was studied under the associated flow-rule condition. The swipe loading method, which is efficient in that the interaction diagram can be drawn with smaller number of analyses, was confirmed to give similar results with the probe loading method, which follows closely the load-paths applied to real structures. For circular footings with a rough base, the interaction diagram for combined vertical (V) - horizontal (H) loading and the inclination factor were barely affected by the friction angle. It was found that the inclination factors for strip and rectangular footings are applicable to circular footings. For high H/V ratios, the results by numerical modelling of this study were smaller than the results of previous studies. Discussions are made on the factors affecting the numerical results and the areas for further researches.

Seepage Characteristics of a Buttressed Embankment with a Low Permeable Foundation: Centrifuge and Numerical Studies

Abstract: . The importance of levee stability is increasingly being recognized due to critical failures of river embankments and levees worldwide. On the basis of centrifuge tests and numerical analysis, this study presents seepage characteristics of embankments with buttressed slopes on low permeable, thick clay foundations. Two models of embankments having reinforcement placed on opposite slopes and constructed on a low permeable foundation were designed for centrifuge tests. Seepage centrifuge tests were performed at a high water level and beyond a high water level. Seepage characteristics of the embankment and foundation were analyzed by means of embedded pore water transducers. Moreover, numerical analysis results under various conditions simulated by SEEP/W were compared with the test results. Safety factors simulated by SLOPE/W were also compared with both models under the same water levels. The results show that the low permeable foundation above groundwater level presented a high degree of saturation before submerging, and the river-side buttressed slope embankment is safer than the land-side buttressed slope embankment. The conditions of the foundation and the reinforcement direction of the embankment should be considered when the seepage behavior and stability of an embankment are determined.