Graduate Thesis Or Dissertation
 

Use of Controlled Blasting to Quantify the Dynamic, In-Situ, Nonlinear Inelastic Response of Soils

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  • The seismic response of deep deposits of liquefiable and cyclic-softening susceptible soils has presented the geotechnical profession with significant engineering challenges. Strong ground motions may serve to soften soils under cyclic shear, trigger liquefaction, and produce damaging displacements as a consequence. This study improves the understanding of the in-situ, nonlinear, inelastic and coupled fluid-mechanical response of soils by leveraging previously-developed instrumentation techaniques and controlled blasting as a source of seismic waves to develop the in-situ dynamic properties of soil, including liquefiable, medium dense sands and cyclic softening-susceptible silts. The seismic energy resulting from the detonation of explosives facilitates the observation of the degradation of shear stiffness and generation of excess pore pressure with the imposed shear strains. First, the controlled blasting technique was implemented in deep medium stiff silt and medium dense sand deposits at depths as great as 25 m below ground surface. The ground motion characteristics associated with controlled blasting were quantified, indicating that compression waves operate at frequencies too high to generate significant particle displacements and corresponding strains. The shear waves generated due to near- and far-field unloading of the initial compression wave were found to control the soil response, and were associated with frequencies common in earthquake ground motions. Laboratory investigations were conducted on intact silt specimens and on reconstituted sand specimens and are compared to the measured, in-situ dynamic response, including the threshold shear strain to trigger generation of excess pore pressure,tp the variation of excess pore pressure with shear strain, and the degradation of shear modulus with shear strain. Three blast experiments provide the basis for the in-situ observation of constitutive soil properties including the threshold shear strains to trigger soil nonlinearity, te and residual excess pore pressure, ue,r, as well as changes in constitutive responses as a result of alterations in the soil fabric and geostatic stress state. The te of the natural sand deposit ranged from 0.001% to 0.002% and tp, ranged from 0.008% to 0.01% for intact, natural deposit. Reduction in normalized of shear modulus, G of approximately 0.70Gmax +/- 0.08Gmax was necessary to trigger ue,r within the intact, natural sand deposit. The te and tp of the natural silt deposit ranged from 0.002 to 0.003% and 0.008 to 0.012%, respectively. Field drainage during the experiments was observed to increase the large-strain G relative to, and its effects distinguishes the in-situ response from those observed in, dynamic, fully-undrained or constant-volume laboratory experiments. Then, a shallow, instrumented silt deposit was dynamically loaded using a large vibroseis mobile shaker, T-Rex, and controlled blasting. The side-by-side comparison of the dynamic soil properties derived from these two different field-testing techniques indicated excellent agreement and validated the use of controlled blasting for use in developing in-situ dynamic soil properties and responses. This study demonstrates that the novel, in-situ experimental protocols such as those described herein can be used to determine fundamental dynamic properties of any kind of soil, are particularly well-suited for soils that are not easily sampled, and can be conducted at depths of engineering interest, providing a new avenue for obtaining vital dynamic characteristics of geological materials.
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  • Pending Publication
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  • 2021-05-27 to 2023-06-28

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