Search

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 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.

In this study, a new large-scale triaxial testing apparatus was constructed and calibrated. To verify the new testing system, synthetic specimens were prepared and tested by three different testing methods: tests using the newly constructed large-scale triaxial testing apparatus, resonant column/torsional shear tests, and free–free resonant column tests. Young’s moduli of the synthetic specimens measured by the three methods were compared, providing comparable results. In addition, cyclic triaxial tests were performed under various experimental conditions on coarse grain materials. Two series of parallel graded samples were prepared by mixing crushed rock. The samples were tested using the new triaxial testing apparatus under various testing conditions. The influence of grain size, loading pattern, loading frequency, and fine contents were analyzed and discussed.