Myoung-Mo Kim

Dynamic P-Y Curves for Dry Sand from Centrifuge Tests

In this article, the simplified dynamic p-y backbone curve was suggested through a series of dynamic centrifuge tests. The centrifuge tests were carried out for a single pile in dry sand, while changing the conditions such as pile diameters, relative densities, input acceleration amplitudes and frequencies. Based on the results, the dynamic p-y backbone curve was formed as a hyperbolic function by connecting the peak points of the resulting experimental dynamic p-y curves, which corresponded to the maximum soil resistances. In order to represent the p-y backbone curve numerically, the initial stiffness and the ultimate subgrade reaction of soil, which are required to formulate the p-y backbone curve, were proposed as a function of the properties of a pile and soil. The dynamic p-y backbone curve was compared with the p-y curves that are currently used in practice. Also, the applicability of the dynamic p-y backbone curve was verified using psuedo-static analysis, and the analysis result using the backbone curve more successfully simulated the other centrifuge test results than that using the existing p-y curve.

3D Numerical Simulation of a Soil-Pile System Under Dynamic Loading

In this research, a new modeling method for simulation of the dynamic behavior of a pile, which can be applied to the conventional finite difference analysis program FLAC 3D, was developed to reduce the calculation time. The soil domain in this method is divided into a near-field region and a far-field region, the latter of which is not influenced by the soil-pile dynamic interaction. The ground motion of the far-field is then applied to the boundaries of the near-field instead of modeling the far-field region with finite meshes. In addition, the non-linear soil behavior is modeled by using the hysteretic damping model, which determines the soil tangent modulus as a function of shear strain and the interface element was applied to simulate the separation and slip between the soil and pile. The proposed method reduced the calculation time by as much as two thirds compared to the usual modeling method, while maintaining the accuracy of the calculated results. The calculated results by the proposed method showed good agreement with the prototype pile behavior, which was obtained by applying a similitude law to the 1-g shaking table test results.

Failure Case Study of Tieback Wall in Urban Area, Korea

In this study, numerical analysis was performed to reproduce the sequential behavior of an anchored retaining structure in an urban area. The numerical analysis was verified through comparisons between the prediction and a field failure case. Emphasis was placed on the wall behavior and the location of the sliding surface based on elasto-plastic method and shear strength reduction FEM method. Through the comparison study, it was found that coupled analysis using shear strength reduction method can be effectively used to perform back calculation analysis to find a critical surface in the anchored wall structures, whereas uncoupled analysis by elasto-plastic method can be applicable to the preliminary design of a retaining wall with a suitable safety factor.

Reliable Evaluation of Dynamic Ground Properties from Cross-hole Seismic Test using Spying-loaded Lateral Impact Source

Soil and rock dynamic properties such as shear wave velocity , compressional wave velocity and corresponding Poisson`s ratio (v) are very important geotechnical parameters in predicting deformational behavior of structures as well as practicing seismic design and performance evaluation. In an effort to measure the parameter efficiently and accurately, various bore-hole seismic testing techniques have been, thus, developed and used during past several decades. In this study, cross-hole seismic testing technique which is known as the most reliable seismic method was adopted for obtaining geotechnical dynamic properties. To perform successfully the cross-hole test for rock as well as soil layers regardless of the ground water level, spring-loaded source which impact laterally a subsurface ground in vertical bore-hole was developed and applied at three study areas, which contain four sites composed of two existing port sites and two new LNG storage facility sites. The geotechnical dynamic properties such as and v with depth from the soil surface to the engineering and seismic bedrock were efficiently determined from the laterally impacted cross-hole seismic tests at study sites, and were provided as the fundamental parameters for the seismic performance evaluation of the existing ports and the seismic design of the LNG storage facilities.

Evaluation of 1-G Similitude Law in Predicting Behavior of a Pile-Soil Model

A series of 1-g shaking table tests was performed using a pile-soil model to verify the existing similitude law used in 1-g shaking table tests. Modeling of the model technique was used for three different sizes of the model, manufactured according to Iais similitude law, and tests were carried out while varying input parameters, such as input frequency and input ground acceleration. Evaluation of the accuracy of Iais scaling factor of a frequency showed that the maximum error in the converted frequency could be within 17%, 35% and 55% when the scaling factor is 2, 5 and 20, respectively. Combining the error occurring in the estimation of frequency, with the possible error occurring in the test results, the maximum error was found to be less than 9%, 21% and 59% when the scaling factor was 2, 5, and 20, respectively, when the frequency ratios in the model tests were smaller than 0.6. Therefore, it can be concluded that the 1-g shaking model test, based on Iais similitude law, can be used on a quantitative basis to predict the dynamic behavior of a pile foundation.

Determination of Shaft and Base Resistance Factors for Drilled Shaft Embedded in Rock in Korea

The Load Resistance Factor Design(LRFD) method was introduced about a decade ago and is being used increasingly in Korea in the geotechnical engineering field. The resistance factors suggested by AASHTO for drilled shafts also were introduced in Korea. However, since most bedrock conditions in Korea differ from those of the US in terms of weathering and strength, direct application of the AASHTO resistance factors for geotechnical design in Korea is thought to be inappropriate. Thus, appropriate resistance factors for the design of drilled shafts socketed into weathered rock in Korea were pursued in this study. Towards this aim, 22 cases of data for shaft resistance and 11 cases for base resistance were collected from the results of pile load tests performed at 13 different sites, and analyzed using a reliability-based approach wherein the target reliability index was set to 2.0. Obtained resistance factors ranged over 0.48~0.56 for the shaft resistance and 0.34~1.93 for the base resistance. When the target reliability was considered as 3.0, the resistance factors were evaluated as approximately one half of the shaft resistance and two thirds of the base resistance when the target reliability index was 2.0.