Collaborative Study on Dynamic Response of Transitional Soils

NHERI@UTexas team conducted field seismic testing to evaluate the response of transitional silty soils to seismic loading. Test results from two field sites are presented in the project. The first site, named Port of Longview site, is located in Longview, WA. The second site, named Troutdale site, is located in Portland, OR. This field study is part of the collaborative research project with Prof. Armin W. Stuedlein and his research group at Oregon State University (OSU).
The field experiments were conducted at six test panels during three visits to the site in 2016 and 2019. The soil specimens being tested at four of the six test panels consist primarily of silty soils with Plasticity Index (PI) ranging from 11 to 20. The ground water table at the test site varies between 1.2 m to 1.9 m. At each test panel, the dynamic field testing include (1) direct-push crosshole (DPCH) testing to measure the small-strain VS and VP, (2) liquefaction screening test to locate the potentially liquefiable soils, and (3) staged-loading nonlinear shaking tests to obtain the to determine the nonlinear shear modulus, G, and excess pore-water pressure ratio, ru, of the soil deposits as functions of number of cycles of loading, N, over a range of induced cyclic shear strains, γ.
The newly developed liquefaction screening test has been utilized in this field project. The screening test aims to quickly locate the liquefiable soil deposits below the ground surface. The staged-loading, field shaking tests have been improved to increase the maximum shear strain that can be induced in the instrumented soil zone so that excess pore water pressure generation of soils can be evaluated over a larger strain range. As a result of the improvements to the field shaking test procedure, the maximum shear strain level in the shaking tests has been increased to better characterize the excess pore water pressure vs strain relationships of silty soils with some plasticity. Results show that the threshold shear strain at which excess pore water pressure begins to be generated increases as the plasticity index of the silty soils increases. The normalized shear modulus vs strain relationships are also determined for the tested soils using the staged-loading, field shaking tests and laboratory resonant column and torsional shear testing. Despite the various level of the plasticity in the sandy and silty soils, the normalized shear modulus vs strain relationships determined at all the six test panels fall in a narrow range.