Dynamic Centrifuge Study On Shear Wave Velocity-based Stone Column Techniques For Liquefaction Mitigation

The earthquake, as a sudden natural disaster, has caused serious harm and damage to social society in recenctly 30 years. Earthquake-induced damge of sites and buildings is closely related to geotechnical engineering issue, and especially the liquefaction-induced damage cause serve damage. China locates in the junction of two earthquake zones (Circum-Pacific seismic belt and Eurasia seismic belt), and the high seismic intensity above 7 degrees covers more than half of the land, including most cities with population larger than one million. Thus, accurate liquefaction evaluation and proper liquefaction mitigation techniques are greatly needed to reduce the earthquake-induced disaster. At present, the simplified procedure for liquefaction evaluation is developed from empirical evaluations of field observations and field tests, and there is lack of a complete set of design method of stone column for liquefaction mitigation. In consequence, the liquefaction evaluation and design of stone column are more empirical and simplified. Therefore, a precise method of liquefaction evaluation, and a complete set of design procedure for stone column for liquefaction mitigation are in great need, and possess important theoretical and practical significance.Theoretical analysis and centrifuge tests were carried out to study the shear wave velocity accuracy in centrifuge test, and conducted the basic analysis problems of seismic liquefaction. Based on these, the paper proposed a complete set of shear wave velocity-based procedure of stone column for liquefaction mitigation, and the corresponding validation by centrifuge model tests. The main research works and conclusions are as follows:(1) Considering the demand of in-flight measurement of shear-wave velocity in centrifuge model tests, this paper proposed the test method and techniques. The transducers were designed to reduce the environment vibration and high noisy. 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. At last, bending disk used for saturation degree monitoring in centrifuge test was studied preliminary. The results show that 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 the normalized depth of bender element should be larger than 0.5 for cross-hole measurement, and and curved raypath inversion is necessary for tomography at shallow depth. The P-wave velocity measured by bending disk characterizes the saturation degree well.(2) By summarizing the design method at home and abroad, and assessing the three reinforcement effect of stone column, this paper proposed a complete method of shear wave velocity-based ground liquefaction evaluation, stone column design and ground assess after improvement. In this method, only densification effect is considered, while the shear stress re-distribution effect and drainage benefit are served as safety stock. The procedure of the shear wave velocity-based liquefaction evaluation and stone column design is proposed. The suggested "CRR-Vs" characterization for liquefaction asseement method and the "Vs-e" correlation were given. The proposed method is easy for engineering application.(3) Three centrifuge model tests were conducted to validate the proposed design method, including loose liquefiable ground model (Model 1), densified ground model (Model 2) and stone column improved ground model (Model 3). Bender elements and CPTu were combined to characterize the evoluation of soil structure and liquefaction evaluation. Through analyzing the loose liquefiable ground model, the ground seismic responses such as site amplification were obtained. By comparing loose liquefiable ground model and densified ground model, the densification effect was estimated, and the effect of densification on ground seismic response was analyzed. Through the comparison of densified ground model and stone column improved ground model, the effect of shear stress re-distribution on cyclic shear ratio and the effect of drainage on ground seismic response were analyzed. The long term performance of the drainage effect was evaluated. The results show that, the ZJU-400 shaker can produce the target motion at the aid of transfer function. The centrifuge can reproduce the liquefaction phenomena, and the seismic response of the ground can be analyzed by the transducers. Tthe evaluation of soil structure characterized by shear wave velocity and CPT tip resistance was almost the same. The liquefaction evaluation by shear wave velocity was more accurate than that by CPT tip resistance.In Model 2, the densification effect was represented by the coefficient of earth pressure at rest, the shear wave velocity and the CPT tip resistance. After densification, the amplification was large, which induced greater CSR, but the increase in CRR can ensure the liquefaction mitigation. The effect of shear stress re-distribution was not obvious, and the existence of stone columns increased the CSR. The drainage of stone columns could reduce the rate of excess pore pressure build upand speed the rate of excess pore pressure dissipation. However, the maximum of the excess pore pressure were almost the same between Model 2 and Model 3. The excavation show than the clogging phenomenon was obvious at shallow depth, and it would reduce the liquefaction resistance. In a world, the stone column provides a safety stock.(4) Based on the stone column mitigation on level ground, we analyzed the the basic liquefiable issues of the sloping ground according to the LEAP project. The results show that, the seismic response of sloping ground is different with the level ground, especially that the liquefaction will cause large lateral spreading in the sloping ground. Ignoring the initial shear stress, the shear wave velocity based liquefaction evaluation is available in the sloping ground, and the validity of the corresponding stone column design needs to be verified.(5) Two power plants were used to validate the proposed shear wave velocity-based method. The results show that, the ground was improved according to the proposed method. After improved, the ground was estimated not to liquefy.In this research, a complete set of shear wave velocity-based method for liquefaction mitigation was proposed through theoretical analysis and centrifuge tests. The studies provide a better understanding of the issues lay a theoretical basis for the design of stone column for liquefaction mitigation.