Zheng-Bo Sun's research while affiliated with Zhejiang University and other places

Stone column is one of the prevailing liquefaction mitigation techniques to densify the surrounding soil, expedite the drainage and bring the possible shear reinforcement effect. The mitigation mechanisms of liquefaction triggering and the associated deformation of the improved ground have not been well explored yet, which hinders the development of robust design in engineering practices. Three centrifuge model tests, including a loose sand model, a dense sand model and a dense sand model with stone columns were conducted to explore the individual effect of densification and drainage caused by stone columns on seismic responses of a level silty sand ground, including the excess pore water pressure, acceleration and ground settlement, etc. The densification effect increases the dilatancy of the surrounding soil and hinders the generation of excess pore water pressure during shaking, thus significantly reduces the liquefaction potential and post-shaking settlement as well. However, it amplifies the upward propagating ground motion considerably. The drainage effect in a densified ground with SCs mainly takes place after shaking, and it accelerates the dissipation rate about 5–10 times that of a densified ground without SCs, which helps to quickly restore the ground stiffness and reduce the deformation to some extent. The contributions from the two effects to the post-shaking ground settlement were also observed and discussed. The present study provides insights on liquefaction mitigation mechanisms of the stone column-improved ground and valuable benchmark tests for further analysis.

LEAP (Liquefaction Experiments and Analysis Projects) is an effort to formalize the process and provide data needed for validation of numerical models designed to predict liquefaction phenomena. For LEAP-GWU-2015, one project within LEAP, an experiment was repeated at 6 centrifuge facilities (Cambridge University, Kyoto University, University of California Davis, National Central University, Rensselaer Polytechnic Institute, and Zhejiang University) and the results were shared and archived for the purposes of validation of numerical models. This paper describes the specifications for the LEAP-GWU-2015 experiment and compares the experimental results from the six facilities. The specified experiment was for uniform medium dense sand with a 5 degree slope in a rigid container subject to a ramped, 1 Hz sine wave base motion. The experiment was meant to be relatively simple to enable different facilities to produce comparable experiments. Although it cannot be claimed that identical experiments were precisely replicated on different centrifuges, it is argued that the results are similar enough that each experiment lends veracity to the set of results. A benefit of variability between experiments is that the variety enables a more general validation. Important lessons with regard to specification of future experiments for validation of numerical models are summarized. LEAP-GWU-2015 has demonstrated an approach that is a useful reference for future validation studies.

In-flight measurement of shear-wave velocity (Vs) in centrifuge model test is important for real-time characterization and has multiple engineering applications. Accurate measurement of Vs by bender elements (BE) in centrifuge models is still challenging, in part because of the curved raypath of shear-wave propagation. By focusing on the variable g-fields inside the models, this paper provides new equations with improved Vs-depth function to raypaths of shear waves for two typical centrifuge model setups. Parametric analyses including the centrifuge specifications, testing layout, and soil characteristics were carried out to study their effects on raypaths and Vs accuracy. The results show that testing layout has significant effect on Vs accuracy whereas soil characteristics have considerable effect. Variable g-fields will cause a further reduction of Vs accuracy, which is dominated by centrifuge radius. To secure an accurate Vs measurement in centrifuges, it is recommended that Di=L should be larger than 0.5 for cross hole measurement, and curved raypath inversion is necessary for tomography at shallow depth (Di=L < 0.5). Then two centrifuge model tests with BE testing corresponding to two cases of variable g-fields were performed to validate the proposed recommendations, where the apparent velocity is proved accurate enough to be used as the actual velocity when testing layouts meet the requirement.

A dynamic centrifuge model test of mildly sloping ground was conducted at Zhejiang University according to the LEAP-GWU-2015 experiment specifications. A 5-degree slope consisting of a saturated medium dense Ottawa F-65 sand was prepared by air pluviation method in rigid container, which simulated a prototype with a 20. m length and 4. m depth under 26. g centrifugal acceleration. The model was subjected to three smaller and two destructive motions. Test data and preliminary analyses of model responses during multiple shakings are presented. In-flight measurement of shear wave velocity by bender elements was used to characterize the evolution of soil state. Acceleration response spectra were presented and site amplification effects were observed. Shear stress-strain loops were calculated from acceleration records, and asymmetric dilatancy in the sloping ground was observed during liquefaction. Generation and dissipation of excess pore pressure were depicted. Large liquefaction-induced ground displacements were measured and analyzed. This study provides valuable data and delivers some important observations of liquefaction responses of a sloping ground.