Yanguo Zhou

Verification of the Soil-Type Specific Correlation between Liquefaction Resistance and Shear-Wave Velocity of Sand by Dynamic Centrifuge Test

Liquefaction of granular soil deposits is one of the major causes of loss resulting from earthquakes. The accuracy of the liquefaction potential assessment at a site affects the safety and economy of an engineering project. Although shear-wave velocity ( Vs ) -based methods have become prevailing, very few works have addressed the problem of the reliability of various relationships between liquefaction resistance (CRR) and Vs used in practices. In this paper, both cyclic triaxial and dynamic centrifuge model tests were performed on saturated Silica sand No. 8 with Vs measurements using bender elements to investigate the reliability of the CRR- Vs1 correlation previously proposed by the authors. The test results show that the semiempirical CRR- Vs1 curve derived from laboratory liquefaction test of Silica sand No. 8 can accurately classify the ( CRR, Vs1 ) database produced by dynamic centrifuge test of the same sand, while other existing correlations based on various sandy soils will significantly under o...

Analytical Modeling of Sandwich Beam for Piezoelectric Bender Elements

Piezoelectric bender elements are widely used as electromechanical sensors and actuators. An analytical sandwich beam model for piezoelectric bender elements was developed based on the first-order shear deformation theory (FSDT), which assumes a single rotation angle for the whole cross-section and a quadratic distribution function for coupled electric potential in piezoelectric layers, and corrects the effect of transverse shear strain on the electric displacement integration. Free vibration analysis of simply-supported bender elements was carried out and the numerical results showed that, solutions of the present model for various thickness-to-length ratios are compared well with the exact two-dimensional solutions, which presents an efficient and accurate model for analyzing dynamic electromechanical responses of bender elements.

Curved Raypaths of Shear Waves and Measurement Accuracy of Bender Elements in Centrifuge Model Tests

AbstractIn-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 lar...

Shear wave velocity-based evaluation and design of stone column improved ground for liquefaction mitigation

The evaluation and design of stone column improvement ground for liquefaction mitigation is a challenging issue for the state of practice. In this paper, a shear wave velocity-based approach is proposed based on the well-defined correlations of liquefaction resistance (CRR)-shear wave velocity (Vs)-void ratio (e) of sandy soils, and the values of parameters in this approach are recommended for preliminary design purpose when site specific values are not available. The detailed procedures of pre- and post-improvement liquefaction evaluations and stone column design are given. According to this approach, the required level of ground improvement will be met once the target Vs of soil is raised high enough (i.e., no less than the critical velocity) to resist the given earthquake loading according to the CRR-Vs relationship, and then this requirement is transferred to the control of target void ratio (i.e., the critical e) according to the Vs-e relationship. As this approach relies on the densification of the surrounding soil instead of the whole improved ground and is conservative by nature, specific considerations of the densification mechanism and effect are given, and the effects of drainage and reinforcement of stone columns are also discussed. A case study of a thermal power plant in Indonesia is introduced, where the effectiveness of stone column improved ground was evaluated by the proposed Vs-based method and compared with the SPT-based evaluation. This improved ground performed well and experienced no liquefaction during subsequent strong earthquakes.

Evaluation of Ground Improvement for Liquefiable Deposits Using Shear Wave Velocity

Quality control of ground improvement is a difficult but very important aspect of liquefaction mitigation construction. In this paper, an approach to evaluate the improvement level in liquefiable soils treated by stone columns was developed based on the liquefaction resistance-shear wave velocity-void ratio correlations of sandy soils. According to this method, the required level of ground improvement is supposed to be obtained once the target velocity is reached for a given earthquake magnitude, and this requirement will transfer to the void ratio control during stone column installation. A case study using vibro-stone column is introduced, where field tests including seismic testing (SASW) and SPT were performed before and after ground treatment, and the effectiveness of stone columns for liquefaction mitigation was properly evaluated by shear wave velocity. The high consistency between Vs- based and SPT-N value-based evaluations indicates that shear wave velocity could be used to develop criteria for ground improvement needed to mitigate liquefaction.

Threshold seismic energy and liquefaction distance limit during the 2008 Wenchuan earthquake

Estimating the possible region of liquefaction occurrence during a strong earthquake is highly valuable for economy loss estimation, reconnaissance efforts and site investigations after the event. This study identified and compiled a large amount of liquefaction case histories from the 2008 Wenchuan earthquake, China, to investigate the relationship between the attenuation of seismic wave energy and liquefaction distance limit during this earthquake. Firstly, we introduced the concept of energy absorption ratio, which is defined as the absorbed energy of soil divided by the imparted energy of seismic waves at a given site, and the relationship between the energy absorption ratio and the material damping ratio was established based on shear stress–strain loop of soil element and the seismic wave propagation process from the source to the site. Secondly, the threshold imparted seismic energy of liquefaction was obtained based on existing researches of absorbed energy required to trigger liquefaction of sandy soils and the ground motion attenuation characteristics of the 2008 Wenchuan earthquake, and the liquefaction distance limit of this earthquake was estimated according to the proposed magnitude–energy–distance relationship. Finally, the field liquefaction database of 209 sites of the 2008 Wenchuan earthquake was used to validate such an estimation, and the field observed threshold imparted seismic energy to cause liquefaction in recent major earthquakes worldwide was back-analyzed to check the predictability of the present method, and several possible mechanisms were discussed to explain the discrepancy between the field observations and the theoretical predictions. This study indicates that seismic energy attenuation and liquefaction distance limit are regional specific and earthquake dependent, and 382xa0J/m3 is the average level of threshold imparted seismic energy to cause liquefaction for loose saturated sandy soils, and the corresponding liquefaction distance limit is approximately 87.4xa0km in fault distance for a Mwu2009=u20097.9 event in the Chengdu Plain. The proposed regional energy attenuation model and threshold imparted seismic energy may be considered as an approximate tool in evaluating the liquefaction hazard during potential earthquakes in this area.

Geotechnical physical modeling and high gravity technology

Summary To investigate geotechnical systems, high gravity is needed due to the dominance of material self-weight. A centrifuge is a common technique to generate high gravity, which causes centrifugal acceleration. In geotechnical physical modeling, geotechnical centrifuge can generate the same gravity stress as the prototype, guarantees the consistency of model deformation and failure mechanism between the centrifugal model and real objects and solve complex problems in geotechnical engineering, hydraulic engineering and environmental engineering etc. In view of the scientific values of centrifuge and shaking table, Zhejiang University develops one of the largest geotechnical centrifuges in China. The centrifuge is a beam of double platform type with a payload capacity of 400gCton and an effective arm radius of 4.5 m. The maximum centrifugal acceleration is 150 g for static tests and 100 g for dynamic tests. The centrifuge platforms have overall dimensions of 1.5m (L) A1.2m (W) A-1.5m (H). Meanwhile, an in-flight uni-axial electro-hydraulic shaker is made for the centrifuge to simulate seismic excitation. Within the payload of 500kg, the maximum lateral displacement and acceleration is 0.6cm and 40 g. This report presents the basis principle and similitude of physical modeling using high gravity and some details about the specifications and components of the ZJU400 centrifuge. Several model tests are presented to show the powerful capability in solving practical problems.