United States Department of Transportation
i
Continuation of down-hole geophysical testing for rock sockets.
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2013-11-01
[PDF-1.57 MB]
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Abstract:Site characterization for the design of deep foundations is crucial for ensuring a reliable and economic substructure design, as unanticipated site conditions can cause significant problems and disputes during construction. Traditional invasive exploration methods sample a small volume of material and insufficiently assess spatial variation in subsurface conditions. Established and emerging surface-based geophysical exploration methods may identify large-scale spatial variability, but fail to provide a detailed picture of the rock quality at depths where a socket is required for the design of a drilled shaft foundation. In order to compensate for the shortcomings of these methods, a new borehole-based characterization method has been developed, which creates images of the shear wave velocity profile along and around the borehole to provide credible socket material analyses and detect nearby anomalies. The proposed imaging technique is based on the time-domain full waveform inversion of elastic waves generated inside a borehole, which are captured by a string of sensors placed vertically along the borehole wall. This approach has the ability to simulate all possible wave types of seismic wavefields, and then compare these simulations with observed data to infer complex subsurface properties. This method formulates and solves the forward model of elastic wave propagation within a borehole using ABAQUS, a commercially available finite element package. The inversion is cast as a Least Square optimization problem solved using the regularized Gauss-Newton method. To test the proposed imaging technique, the present study performed comprehensive numerical studies. First, the accuracy of the forward model was validated. Then, the capability of the proposed imaging technique was evaluated by inverting a series of three-dimensional (3-D) synthetic data sets, including a homogeneous model, a horizontally layered model with high impedance contrast, a vertically layered model that mimicked borehole preparation, and simplified earth models containing ring-type anomalies and isolated anomalies. Good models were recovered for each case presented herein.
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