NSF NEESR-Payload Project

This research is an outcome of the National Science Foundation 07-506 program solicitation "George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research" competition. This project was awarded as a NEESR-payload. This project was led by the University of Vermont and included a subaward to Prof. Pedro deAlba of the University of New Hampshire.

It has long been observed that saturated sands subjected to shock or earthquake loading experience drastic loss of strength and behave as heavy fluids, gradually regaining strength as internal water pressures dissipate. As long as the liquefied state persists, the soil will flow down slopes, producing destructive landslides and large drag forces on obstacles such as piled foundations. Modeling this behavior for risk studies and engineering design, however, requires adequate measurements of how shearing strength is lost and is eventually recovered as internal water pressures build up and subsequently dissipate. There are currently no full-scale field measurements of these strength changes to guide development of such models; existing field case histories are limited to observing the final damage produced by the liquefaction process. Controlled laboratory measurements would be desirable, but the onset of liquefaction is accompanied by such large strains that soil samples in conventional laboratory tests become so drastically deformed that reliable strength measurements can no longer be made. As a first step in measuring the evolving behavior of liquefied sands, it was envisioned that the shear strength of liquefying sand can be measurable in-flight in a seismic geotechnical centrifuge model using a thin coupon (plate, about 25 millimeters by 75 millimeters by ~1.5 millimeters) pulled horizontally through the soil model, with its major dimensions parallel to the base of the model. The large strains and strain rates associated with liquefaction flow failures would thus be simulated by moving the coupon relative to the sand, through and after the shaking until the excess pore pressures dissipate. By measuring the drag force on the coupon, it was possible to observe the evolution of the soil shear strength as it decreases to a minimum (residual strength) and subsequently increases as pore pressures dissipate. The centrifuge results were compared to companion ring shear tests.

Equipment required to conduct the payload tests was designed and built by a group of undergraduate mechanical and electrical engineering students at the University of Vermont as their senior capstone design project, and calibrated before installation by a civil engineering undergraduate student. The UVM civil engineering undergraduate student and UNH graduate student conducted centrifuge tests at the University of Colorado at Boulder. The companion ring shear tests were conducted at UNH. Data from this project is archived in the NEES data repository.

Publications from this work to date: