Bender elements are used to measure the low-strain shear-wave velocity of soils in many geotechnical laboratories worldwide. Despite their popularity, the method remains without a standard, mainly owing to the difficulties in controlling the actual behaviour of the bender element inside the specimen. Previous experimental and analytical studies on the behaviour of bender elements have found evidence that the actual transmitter movement is not equal in shape or frequency content of the electrical signal used as the input excitation. Furthermore, the size and geometry of the specimen influence the response of these piezoelectric transducers. In this study, miniature accelerometers are used to measure the particle vibration inside specimens during bender elements tests, which were performed in a resonant column device. The comparison between shear wave velocities obtained by resonant column and miniature accelerometers tests allow a better interpretation of bender element testing. The sine pulse bender element test results showed good agreement with the resonant column measurements. The results show the importance of the resonant frequencies of the bender element and their influence on frequency domain analysis of bender element testing.
The 2nd International Conference on Site Characterization (ISC'2), held in 2004, included a seminar for prediction of pile capacity involving three 6 m embedment length test piles, one 350 mm square driven concrete pile, and two 600 mm diameter, strain-gage instrumented, bored piles. Invited predictors were provided with results of in situ, laboratory tests and dynamic tests. Test layout, soil information, and pile data are presented with calculations of pile capacity and load distribution, submitted predictions, and results of the static loading tests. The CPT-calculated capacities show considerable scatter-total values ranged from 500 to 1400 kN for the driven pile and from 1000 to 1900 kN for the bored piles. The static loading test on the driven pile showed an offset limit load of 1200 kN and a plunging capacity of 1500 kN. Despite pile movements of 100 mm for 1200 kN of applied load, neither of the bored piles showed signs of having reached an ultimate resistance value. Effective stress analysis of strain measurements for the bored piles showed the data to correlate to a β coefficient of 1.0 and a toe coefficient of 16. Most submitted predictions underestimated the capacity of the driven pile and overestimated the capacities of the bored piles.
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