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 is a payload to National Science Foundation award 0530478, "NEESR-GC: Seismic Risk Mitigation for Port Systems," and will utilize the tests being conducted by award 0530478 in the NEES geotechnical centrifuge at the University of California, Davis. This project is led by the University of Vermont and includes a subaward to 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 loss and its eventual recovery evolve 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 is envisioned that the shear strength of liquefying sand can be measurable in-flight in the NEES geotechnical centrifuge model using a thin coupon (plate, about 25 millimeters by 25 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 will be 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 models will provide realistic field-scale stresses and boundary conditions, and the dense array of instrumentation will facilitate observations to be made on the strength changes in the liquefying sand from beginning to end of simulated earthquakes. The results would also be used to validate companion ring shear and modified cyclic triaxial testing. The combined results of a series of centrifuge and small-scale laboratory experiments will provide guidance on how to simulate the large-scale tests in smaller laboratory apparatus, thus making it easier to study the behavior of other soil types, such as silty and clayey sands, during liquefaction both for general studies and for specific engineering design purposes. A simple yet rational model for predicting the rate-dependent evolution of shearing strength of granular soils as pore pressures build up and the soil mass deforms will be developed. This will permit more accurate simulation of such problems as estimating the forces exerted by liquefied soil on obstacles like pile-supported structures, and the prediction of flow slide behavior in general. These results are expected to give designers enhanced understanding of how to choose residual strength values for remediation of earth structures. Equipment required to conduct the payload tests will be 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 companion ring shear and modified cyclic triaxial tests will be carried out by a civil engineering graduate student at the University of New Hampshire. Data from this project will be archived in the NEES data repository (www.nees.org).
