Advancements in direct-push seismic testing

Date

2018-05-04

Authors

Stolte, Andrew Christopher

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Abstract

Invasive seismic testing methods are used to evaluate the in-situ small-strain stiffness of soil and rock for geotechnical earthquake engineering analysis and design. These methods provide localized measurements of the constrained compression and shear wave velocities, V [subscript P] and V [subscript S], respectively. Early invasive testing methods, including crosshole (CH) and downhole (DH), were exclusively borehole-based (i.e., the test was performed in a pre-drilled borehole). The time and costs associated with the preparation of one or more boreholes are notable disadvantages of conventional CH and DH testing, significantly limiting their use. Direct-push variants of these testing methods have been developed, where the instrumentation can be installed in conical probes and a directly into the ground. This dissertation documents recent advancements in direct-push invasive seismic testing. One such advancement is the development of the direct-push crosshole testing (DPCH) method where P- and S-waves are propagated between instrumented cones, pushed directly into the ground. Each instrumented cone contains a sensor package including geophones to measure the seismic waveforms and a three-component MEMS accelerometer to track the cone deviation/position during the test. DPCH testing enables high-resolution profiles of V [subscript P] and V [subscript S] to be measured over the top 20 – 30 m of the subsurface. In addition to developing a high-resolution velocity profile, it is important to quantify the epistemic uncertainty associated with these measurements. Often, a single velocity profile is provided to the engineer with no indication of uncertainty leaving the engineer to potentially over- or under-predict this uncertainty. Yet, through the consideration of multiple data reduction techniques and analysis methods, a robust and meaningful quantification of the epistemic uncertainty may be developed. A direct application of DPCH high-resolution V [subscript P] and V [subscript S] measurements is the estimation of in-situ void ratio in granular soils using a relationship developed by Foti et al. (2002). The effectiveness of using DPCH measurements to estimate in-situ void ratio in granular soils is critically examined through comparisons with current in-situ penetration based estimates and laboratory measurements developed at several case history sites. The limitations associated with these estimates are explored in-depth.One such advancement is the development of the direct-push crosshole testing (DPCH) method where P- and S-waves are propagated between instrumented cones, pushed directly into the ground. Each instrumented cone contains a sensor package including geophones to measure the seismic waveforms and a three-component MEMS accelerometer to track the cone deviation/position during the test. DPCH testing enables high-resolution profiles of Vᴘ and VS to be measured over the top 20 – 30 m of the subsurface. In addition to developing a high-resolution velocity profile, it is important to quantify the epistemic uncertainty associated with these measurements. Often, a single velocity profile is provided to the engineer with no indication of uncertainty leaving the engineer to potentially over- or under-predict this uncertainty. Yet, through the consideration of multiple data reduction techniques and analysis methods, a robust and meaningful quantification of the epistemic uncertainty may be developed. Invasive seismic testing methods are used to evaluate the in-situ small-strain stiffness of soil and rock for geotechnical earthquake engineering analysis and design. These methods provide localized measurements of the constrained compression and shear wave velocities, V [subscript P] and V [subscript S], respectively. Early invasive testing methods, including crosshole (CH) and downhole (DH), were exclusively borehole-based (i.e., the test was performed in a pre-drilled borehole). The time and costs associated with the preparation of one or more boreholes are notable disadvantages of conventional CH and DH testing, significantly limiting their use. Direct-push variants of these testing methods have been developed, where the instrumentation can be installed in conical probes and a directly into the ground. This dissertation documents recent advancements in direct-push invasive seismic testing. One such advancement is the development of the direct-push crosshole testing (DPCH) method where P- and S-waves are propagated between instrumented cones, pushed directly into the ground. Each instrumented cone contains a sensor package including geophones to measure the seismic waveforms and a three-component MEMS accelerometer to track the cone deviation/position during the test. DPCH testing enables high-resolution profiles of V [subscript P] and V [subscript S] to be measured over the top 20 – 30 m of the subsurface. In addition to developing a high-resolution velocity profile, it is important to quantify the epistemic uncertainty associated with these measurements. Often, a single velocity profile is provided to the engineer with no indication of uncertainty leaving the engineer to potentially over- or under-predict this uncertainty. Yet, through the consideration of multiple data reduction techniques and analysis methods, a robust and meaningful quantification of the epistemic uncertainty may be developed. A direct application of DPCH high-resolution V [subscript P] and V [subscript S] measurements is the estimation of in-situ void ratio in granular soils using a relationship developed by Foti et al. (2002). The effectiveness of using DPCH measurements to estimate in-situ void ratio in granular soils is critically examined through comparisons with current in-situ penetration based estimates and laboratory measurements developed at several case history sites. The limitations associated with these estimates are explored in-depth.

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