Shear Wave Velocity-Based Characterization Of Soil Structure And Its Effects On Dynamic Behavior

The seismic response and failure mechanism of soil deposits during earthquakes areresponsible for the failure of many geotechnical structures on it. The damage evolution ofsoil structure plays a very important role in analyzing geotechnical seismic hazards. Naturalsoils are always highly structured, with mechanical properties different from each otheraccordingly. The comprehensive understanding of structure effects on dynamic behaviorwill shed light on the failure mechanism of soil deposits, and contribute to seismic designof earth and earth-retaining structures.Shear wave velocity is a readily determined parameter accounting for the small strainshear stiffness of soils, which characterizes soil structure accurately. This researchencompasses a comprehensive investigation of piezoelectric bender element testing, thecharacterization of soil structure by shear wave velocity, and the application of thisenhanced understanding to evaluate the effects of soil structure on dynamic behavior viaoedometer test, cyclic triaxial test and dynamic geotechnical centrifuge tests with benderelement testing. In detail, this study includes topics as follows:1. For the electro-mechanical response analyses of piezoelectric bender elements, anefficient and accurate analytical beam model was established based on improved ftrst-ordershear deformation theory (FSDT). This analytical model successfully explains the actuator(transmitter) and sensor (receiver) function of bender elements, and reveals that the seriesand parallel .types of bender elements are essentially the same and differ only in energyconversion capability. Therefore, the use of parallel-type bender elements as source and theseries-type bender elements as receiver is recommend.2. Experimental and theoretical approaches are implemented to explore variousaspects of bender element testing, and the appropriate method of identifying the arrival ofshear wave is summarized for technique standardization. Portable bender element system was invented for measuring surface shear wave velocity, for possible applications insampling quality check and soil improvement evaluation. The performance of test data inthe "International parallel test on the measurement of G_max using bender elements byTC-29" verified the efficiency and reliability of bender element systems used in this study.3. The influence of seismic cyclic loading history on small strain shear modulus ofsaturated sands was studied via undrained cyclic triaxial tests with bender element testing.And the G_max values of samples without such influences were investigated for comparison.The tests results indicate that G_max of sand subjected to high amplitude cyclic loading ismoderately lower by 6-9% than the corresponding value without cyclic loading effects atthe same effective stress, which could be attributed to the soil structure degradation causedby seismic loadings. Hence it is necessary to reinvestigate the determination of G_max inseismic response analysis carefully to predict the ground responses more reasonably.4. Theoretical considerations and experimental studies based on cyclic triaxial testswith V_s measured by bender elements reveals that CRR will vary proportionally with (V_s1⁴.Then laboratory CRR-V_s correlation based liquefaction assessment method is established.Detailed comparisons with V_s-based site-specific investigations show that the presentCRR-V_s1 curve is a reliable prediction especially for sites with higher CSR or V_s1.5. Dynamic centrifuge model tests were performed on Sifica sand No. 8 to investigatethe reliability of laboratory CRR-V_s correlation, where the soil structure of model groundwas evaluated by shear wave velocity measured by bender elements. The test resultsconfirm the reliability of soil type-dependent CRR-V_s correlation.6. The destructuration process of natural clay was monitored by shear wave velocity inoedometer tests. And the effects of progressive destructuration on the correspondingdynamic behavior were studied in cyclic triaxial tests. Further study of post-earthquakesettlement of soft soil foundation using dynamic centrifuge model tests shows that,deformation behavior is closely associated with soil structure evolution, which can becharacterized by shear wave velocity accurately.All the abovementioned studies illustrated the validity of bender element testing forshear wave velocity measurement, and proved the efficiency and reliability of shear wavevelocity-based characterization of soil structure. These findings present the fundamentals and parameters for seismic liquefaction and deformation analyses of soil deposits in theperspective of soil structure, and will contribute much to soil dynamics and geotechnicalearthquake engineering from theory to practice.