In Situ Determination of Soil Stiffness and Damping

2006

Unsaturated Soils 2006

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Geophysical down-hole surveys can be used to measure the small strain dynamic properties of soils by the effects these properties have on wave propagation. The relevant effects include amplitude decay (corrected for beam divergence) and velocity dispersion. In this paper, down-hole data collected during the GeoInstitute's Denver 2000 field day are presented and analyzed as a Kelvin-Voigt solid. Findings for these unsaturated soils include viscous damping and stiffness which differ significantly for shear and compressional waves. A strong viscous damping is observed in compression, but weak damping is presented in shear. Lumped parameter constitutive models are discussed which mathematically represent the soil dynamics.It appears that, in the case of unsaturated soils, the relatively low level of viscous damping in shear may be explained by the low mass of the air in the pores. That is, it is difficult for inertial decoupling to occur between the soil frame and the pore fluid when the pore fluid (air) is of such low density. Thus, a pore fluid in coupled motion with the frame can not produce significant viscous drag. On the other hand, large viscous damping is observed for compressional waves. This larger damping may be due to the larger relative motions between air and frame which can be forced by compression of the frame matrix. These observations may be relevant in areas such as the design of driven piles and the estimation of potential for damages from vibrations due to construction.

Section: Viscoelastic Analysismentioning

confidence: 71%

“…The slope of that linear solution is the determined decay value for that frequency. A least squares inversion jointly solves for the two coefficients of the 1-D inelastic wave equation (Michaels, 1998). The governing wave equation is…”

Section: Viscoelastic Analysismentioning

confidence: 99%

Comparison of Viscous Damping in Unsaturated Soils, Compression and Shear

2006

Unsaturated Soils 2006

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“…Shear and surface waves may be more useful than P-waves. They relate well not only to the soil frame but also to the interaction between pore fluids and the frame via the complex modulus (Stoll, 1985;Michaels, 1998).…”

Section: Discussionmentioning

confidence: 96%

Identification of subsonicP‐waves

2002

GEOPHYSICS

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A field trial was conducted to test the existence of subsonic (V p < 331 m/s) P-waves previously reported in the literature. A 1-m-long reverse profile was acquired with three-component (3C) geophones on a sandy silt (unified classification ML). The silt had a porosity of 54%, a degree of water saturation of 63%, and a plasticity index of 10. No subsonic P-waves were observed, although highfrequency (up to 1200 Hz) Rayleigh waves were identified by hodogram analysis. These surface waves were observed with horizontal velocities that varied from 40 to 200 m/s. Hodogram observations and theory suggest that a portion of the data were also in the near-field.

“…Coordinates and dimensions are to a local metric datum. The source was an inclined hammer as described in Michaels (1998). A surface reference 3-component Oyo geophone monitored any variations in the source waveform or trigger timing.…”

Section: Bore Hole Geophysicsmentioning

confidence: 99%

Uncertainty in a Geophysical Survey for a Bridge Foundation Design

2017

Geo-Risk 2017

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In the preparation stage of a bridge replacement located in Boise Idaho, the Idaho Transportation Department (ITD) commissioned a geophysical survey to address uncertainties in the soil profile across the river. Differing site conditions can impact the final design and cost of a project. To address the geologic uncertainties under the planned bridge replacement, the refraction method was selected. Selection of a geophysical method bears its own risk. Refractions are used to map the top of the competent geologic formation below those soils incapable of supporting the structure. The refraction method requires that P-wave velocity increase at the boundary to be mapped. That requirement was not met in this project. Unexpected pore pressures in the formation to be mapped reduced the P-wave velocity by 30%, causing the plan to fail. Fortunately, surface wave analysis provided an viable alternative to work with the data, but with less depth of penetration due to the lack of low frequency content in the air gun source.