Improvements to field liquefaction testing with large mobile shakes




Zhang, Benchen

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Earthquake-induced soil liquefaction can cause devastating damage to buildings and infrastructure such as bridges, embankment dams, and retaining structures. For the past 15 years, large, mobile shakers operated by researchers at the University of Texas at Austin have been used to investigate the liquefaction susceptibility of granular and fine-grained soils directly in the field. The original test procedure involved dynamically loading the surface of a natural soil deposit horizontally with one shaker (T-Rex) in stages of increasing load levels. In each loading stage, simultaneous measurements of dynamic motions and pore-water pressures at multiple depths in the soil were performed with a sensor array embedded in the soil below the loading baseplate of T-Rex. The objectives of the field shaking test were to determine excess pore-water pressure generation and nonlinear shear moduli as functions of number of loading cycles over a wide range of induced cyclic shear strains. During recent field liquefaction studies, three new improvements have been developed and incorporated into the existing test procedure to improve the capability, success, and accuracy of the field shaking tests. The first improvement to the field shaking test is focused on increasing the maximum, shaking-induced strain level and is composed of two new techniques. The first technique involves shaking at or near the resonant frequency, f [subscript r], of the shaker-and-site system. The second technique is to combine a second shaker (Rattler) with the first shaker (T-Rex). The two shakers are parked side by side and synchronized to apply dynamic loading simultaneously so that a larger range of shear strains can be induced. The second improvement to the field shaking test is a new test method named liquefaction screening test. The liquefaction screening test is designed to quickly identify the existence and extent of potentially liquefiable soil at the site. A liquefaction screening sensor is pushed to a series of increasing depths and, at each depth, measurements of the horizontal particle velocity (used to estimate shear strain) and excess pore-water pressure generation at one or more force levels of shaking are conducted. This approach greatly enhances proper placement of liquefaction instrumentation array. The third improvement to the field shaking test aims to better characterize the linear and nonlinear shear moduli during the shaking tests. Two mobile shakers, Rattler and Thumper, are used to excite the instrumented soils at different loading frequencies simultaneously, which permits measurement of the small-strain shear modulus to be performed many times during each larger strain cycle. These new approaches have been developed and validated in recent field liquefaction projects at multiple locations in the United States. The results of the improved field shaking tests are presented and discussed in this dissertation.


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