Search

The Liquefaction Experiments and Analysis Projects (LEAP) is an international effort to produce a set of high quality test data and to then use it in a validation exercise of existing computational models and simulation procedures for soil liquefaction analysis. A validation effort (LEAP-GWU 2015) was undertaken using a benchmark centrifuge model of a sloping deposit tested in a rigid-wall container. This article presents and discusses the shear stress-strain responses and effective stress paths of the LEAP-GW 2015 centrifuge tests and numerical predictions (including an assessment of the effects of the rigid boundaries).

LEAP (Liquefaction Experiments and Analysis Projects) is an effort to formalize the process and provide data needed for validation of numerical models designed to predict liquefaction phenomena. For LEAP-GWU-2015, one project within LEAP, an experiment was repeated at 6 centrifuge facilities (Cambridge University, Kyoto University, University of California Davis, National Central University, Rensselaer Polytechnic Institute, and Zhejiang University) and the results were shared and archived for the purposes of validation of numerical models. This paper describes the specifications for the LEAP-GWU-2015 experiment and compares the experimental results from the six facilities. The specified experiment was for uniform medium dense sand with a 5 degree slope in a rigid container subject to a ramped, 1 Hz sine wave base motion. The experiment was meant to be relatively simple to enable different facilities to produce comparable experiments. Although it cannot be claimed that identical experiments were precisely replicated on different centrifuges, it is argued that the results are similar enough that each experiment lends veracity to the set of results. A benefit of variability between experiments is that the variety enables a more general validation. Important lessons with regard to specification of future experiments for validation of numerical models are summarized. LEAP-GWU-2015 has demonstrated an approach that is a useful reference for future validation studies.

For the LEAP-GWU-2015 exercise, a relatively simple centrifuge test was conducted in parallel at 6 centrifuge facilities including the University of California Davis (UCD). The experiment consisted of a submerged medium dense clean sand with a 5° slope subjected to 1 Hz ramped sine wave base motion in a rigid container. This paper explains several details of the experiment at UCD including intended and unintended deviations from the specification and the implementation of new techniques for measurement of saturation of the centrifuge model. One unintended critical deviation was the use of pore fluid that was more viscous than specified; this had significant effect on the pore pressure dissipation time. The other important deviation from the specification was the incomplete ground motion sequence. While it is not ideal for purposes of determining the replicability of centrifuge tests, the differences between experiments diversifies the data available for validation of numerical models.

Many sensitive clay slopes exist that have a factor of safety greater than one based on the peak strength, and a factor of safety less than one based on the remolded strength. Obviously, it is possible for shaking from an earthquake to remold and soften a sensitive clay, but there is no criteria available to help engineers determine the extent of seismic loading required to induce sufficient strain softening to trigger instability. Slopes of varying sensitivity, produced by adding a small amount of Portland cement to clays of different plasticity were tested in a centrifuge to study the static and dynamic failure mechanism for sensitive clay slopes. The properties of the clay were determined by rudimentary laboratory tests. Results are interpreted in the context of existing literature to further our understanding of the factors controlling the potential for dynamic loading to trigger the instability of sensitive clay. The findings are based on a few key concepts. First, it must be recognized that structured sensitive clay is typically stiffer than remolded clay and hence, unless softening is triggered, sensitive clay may be expected to perform better than insensitive clay of the same residual strength. Second, softening behavior, which leads to localization on a thin shear band, is counteracted to some extent by strengthening associated with increase in strain rate. The rate effect favors thick shear bands, while softening leads to thin shear bands. Another factor affecting shear band behavior is that thick bands may be undrained during dynamic shear while thin shear bands are expected to be drained during static shear. Thirdly, the slope failure mechanisms for static and dynamic loading are quite different. During static (constant) loading, large deformations initiate precisely when one failure mechanism has a factor of safety FS

Soil–foundation–structure interaction (SFSI) and structure–soil–structure interaction (SSSI) influence the seismic response of a structure. Yet, consideration of nonlinear SFSI and SSSI in design practice is lacking. In this paper data from two centrifuge tests are examined. During each test, inelastic models of (1) a low-rise frame with shallowly embedded footings and (2) a mid-rise frame with a large basement are subjected to earthquake motions. In the first test, the structures are separated. In the second test, the structures are placed next to each other. Results show that the presence of the deep basement affects the moment–rotation behavior of the adjacent shallow footings, stiffening the response in the direction of loading towards the basement. This can be attributed to the additional restraint provided by the basement. Although the presence of the basement stiffens the response, it also limits the permanent displacements of the footing, which in turn limits physical damage to the superstructure. These results suggest that in addition to considering nonlinear SFSI effects, SSSI should be considered in the design of closely clustered structures.

This paper reports the experimental results from two geotechnical centrifuge tests examining seismic soil–foundation–structure interaction of frame structures. In the first test, two three-dimensional frame structures were placed apart, and in the second test, the same frame structures were located adjacent to each other. One of the frame structures was founded on shallowly embedded spread footings, and the other was founded on a deeper basement. During the second test, seismic structure–soil–structure interaction was also examined. In this paper, the experimental set-up is described in detail, some kinematic interaction observations are made, and seismic footing response results are reported. The results of the tests indicate that the seismic response of the shallow footings of a flexible frame structure is complex. This result is important because these complexities are not well incorporated in current soil–foundation–structure interaction analysis procedures. The adjacency of two frame structures, and thus structure–soil–structure interaction, further complicates analysis procedures. In the second test, the spread footings rock, slide, and settle less when they are directly adjacent to a deep basement. These results imply that SSSI can be beneficial or detrimental, depending on the earthquake motion and the structural system. In a companion paper, the seismic response of the frame superstructures and the footing force demands are also examined.

Shallow foundations supporting building structures might be loaded well into their nonlinear range during intense earthquake loading. The nonlinearity of the soil may act as an energy dissipation mechanism, potentially reducing shaking demands exerted on the building. This nonlinearity, however, may result in permanent deformations that also cause damage to the building. Five series of tests on a large centrifuge, including 40 models of shear wall footings, were performed to study the nonlinear load-deformation characteristics during cyclic and earthquake loading. Footing dimensions, depth of embedment, wall weight, initial static vertical factor of safety, soil density, and soil type (dry sand and saturated clay) were systematically varied. The moment capacity was not observed to degrade with cycling, but due to the deformed shape of the footing–soil interface and uplift associated with large rotations, stiffness degradation was observed. Permanent deformations beneath the footing continue to accumulate with the number of cycles of loading, though the rate of accumulation of settlement decreases as the footing embeds itself.