Vibration Testing of Full-Scale Pile Group

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*** 9617613 Rollins The lateral load capacity of foundation piles and pile groups is a key consideration in earthquake hazard mitigation. Because of the cost and logistical difficulties of full-scale testing, only three pile group tests are currently available to guide engineers regarding pile group behavior, especially under dynamic loading. Due to the dearth of experimental data, computer programs for analyzing pile groups have not been thoroughly validated, and empirical methods are extremely restricted in their application. This is particularly true when it comes to dynamic loading such as earthquakes. As a result, designers are forced to be conservative and this leads to increased costs for retrofits as well as new construction. Brigham Young University is performing a series of lateral load tests on a full-scale pile group in soft clay at the Salt Lake International Airport. This testing involves both static and dynamic testing at large strain levels. The objective of this action is to support additional low-strain vibration testing of the pile group, thereby providing data for calibrating and validating computer models. Since the pile group is already in place, and soil exploration is completed, the incremental cost of performing these additional tests is small in comparison to the information to be gained. Due to time constraints on the original test schedule, it is necessary to expedite the schedule for the low strain testing: once the large-strain testing is completed, it is impossible to obtain meaningful small strain results. The low-strain vibration testing is conducted with an eccentric mass shaker which is mounted on top of a pile cap. The testing makes it possible to obtain the natural frequency of the pile-soil system, as well as the stiffness and damping at low strain levels. These parameters, in combination with the large-strain parameters, provide an excellent case history which researchers can use to validate their computer codes for both static and dyna mic conditions.***