Experimental investigation of the Horizontal-to-Vertical Spectral Ratio (HVSR) method for estimating depth of bedrock in Central Missouri
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Determining the depth to bedrock is important in geotechnical site investigations, especially for foundation, slope stability, and settlement problems. Although methods such as drilling can be used to determine the depth to bedrock, geophysical methods are excellent supplemental tools to fill in the space between borings. However, many geophysical methods require extensive equipment to deploy and expertise to interpret the data. A recent, simple method to estimate the depth to bedrock is the Horizontal-to-Vertical Spectral Ratio (HVSR) method, which is a single-station measurement that only requires ambient noise and can be easily deployed and completed by a single person within 15 minutes. The objective of this study is to determine if a reliable relationship of depth to bedrock could be developed for the University of Missouri (MU) campus using the HVSR method alone without shear wave velocity measurements. In total, 65 HVSR measurements were performed around the MU campus over an approximate area of 1,200,000 m2. Measurements were performed with a three-component geophone, hand-held data acquisition system, and laptop computer. Relationships were developed between HVSR frequency versus depth to bedrock for all data and subsets of data based on the bedrock conditions. The results showed a reliable relationship can be developed for soil over limestone profiles, where the average errors were within 12 percent, which is consistent with similar past studies. However, when shale was present, the bedrock depth predictions were unreliable with errors as large 58 percent. Therefore, in practice, the HVSR relationship developed in this study should only be used at sites where shale is known to be absent, based on supplemental information. The HVSR relationship developed in this study was applied at a major construction project near the University of Missouri campus and showed good agreement between the prediction from HVSR and refraction results performed by another contractor. The errors in the depth predictions were within the range of 0.5 to 3.2 ft.
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M.S.