Heon-Joon Park

Evaluation of K o in Centrifuge Model Using Shear Wave Velocity

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.

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

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.

Soil-Rounding Effect on Embedded Rocking Foundation via Horizontal Slow Cyclic Tests

AbstractThe rocking behavior of a shallow foundation of a bridge pier has emerged as an effective mechanism for reducing the seismic load on the superstructure during a strong earthquake. However, ...

Comparison between cyclic and dynamic rocking behavior for embedded shallow foundation using centrifuge tests

Designs allowing the rocking behavior of the foundation during earthquake have been introduced to reduce the seismic load on the superstructure and the ductility demand on the structural column. In addition, several studies have been conducted on rocking foundation based on the slow cyclic and dynamic tests by assuming the structure as a rigid oscillator. However, when structural bending is included, the rocking behaviors of the foundation for the slow cyclic and dynamic tests are different. Therefore, a clear description of each method and how each behavior is different should be investigated by considering structural bending motion. To fill the gap between cyclic and dynamic rocking behaviors, embedded foundation models with various slenderness ratios of the systems were investigated using horizontal slow cyclic tests and dynamic tests in a centrifuge. Test results show that the rocking foundation was affected by structural bending. The overturning moment in the dynamic test determined by the conventional method was different compared with results obtained from the slow cyclic test due to the structural bending motion. Finally, the overturning moment was re-evaluated by considering structural net displacement, and the re-evaluated dynamic overturning moment matched the results from the slow cyclic tests.

Evaluation of Soil State Variation using Bender Elements Within Centrifuge Models

In centrifuge modelling, in-flight soil conditions can vary horizontally depending on the location, i.e., from the center of model to a point close to the rigid wall. Moreover, loading history and earthquake shaking events affecting the particle contact force can influence the in-flight soil conditions. For reliable centrifuge modelling, therefore, it is important to evaluate the in-flight soil properties in a soil model at specific locations before target experiments are performed. A series of centrifuge model tests were undertaken on a uniform dry sand to investigate the in-flight soil parameters by measuring shear wave velocity (Vs) via bender elements (BE), as the Vs of the soil is directly related with soil properties. Three plausible conditions likely to influence the soil parameters were discussed in this paper: (i) boundary condition (location from the container boundary), (ii) load history (sequence of spinning up and down in the centrifuge), and (iii) earthquake shaking events. The Vs measured in the centrifuge was verified through comparison with resonant column (RC) test results. The results showed that the Vs is different in horizontal plane with locations within the model as well as with different loading events. The results also confirmed that the evaluation of in-flight soil properties before performing the test is important for interpreting the soil behavior accurately.