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In Situ Shear Wave Velocity Measurements in Rocks - Case Studies and Conclusions

 

5.0 Case Studies

Three case studies providing recent examples of applications for in situ measurement of Vs in rocks are summarized in this section. The case studies include identifying underground cavities and characterizing geologic materials (Bianchi Fasani et al., 2013), monitoring of rock mass properties and conditions ahead of tunnel excavation (Godio and Dall’Ara, 2012), and identification of fracture zones in sedimentary bedrock at a contaminated site (Ellefson et al., 2012).

5.1 Underground Cavities in Rome, Italy – Bianchi Fasani et al. (2013)

Throughout parts of Rome, Italy, Pleistocene pyroclastic deposits (ignimbrite) were mined at undocumented locations. This resulted in unmarked subsurface cavities and, as a result, made the ground surface susceptible to sinkholes and collapse, similar to sinkholes developed in areas with bedrock dissolution. The cavities are generally at depths equal to 1.5 to 3 times their diameter and were mined using the “room and pillar” technique. The figure below identifies the test sites where the Bianchi Fasani et al. (2013) study was performed. The goal of their testing was to develop a testing program that could be implemented throughout Rome to generate subsurface cavity hazard maps.

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Figure 7: Rock Outcrops and Test Sites (Bianchi Fasani et al., 2013)

 

 

In order to do this, the authors planned to integrate geophysical measurements with previous borehole logs, verify the homogeneity of the pyroclastic deposits, assess the ability of electrical resistivity tomography to identify cavities, and determine the ranges of some mechanical properties including the Vs. Using previously collected geologic information of the area, a synthetic geologic model was developed and used to determine the necessary depth of investigation and resolution for electrical resistivity tomography, which was the focus of the field testing performed. Additionally, the Vs of different geologic units at the test sites were investigated using the seismic crosshole method. These measurements were coupled with the surface electrical resistivity survey to pursue the goals of the study. The Vs measurements provided in situ data on materials that had previously only been extensively studied in the laboratory. The Vs measurement results from this study are shown in Figure 8.

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Figure 8: Resulting Vs Measurements

 

5.2 Rock Mass Evaluation Ahead of Tunnel Face – Godio and Dall’Ara (2012)

            Sonic logging allows for the measurement of seismic wave velocities in many situations. In the case study reported by Godio and Dall’Ara (2012), sonic logging was used ahead of an advancing tunnel face in order to monitor and update rock mass mechanical properties and detect zones of weakness that may intersect the tunnel. A schematic of the logging system is shown below. The data was collected during the excavation of a 52.7 km tunnel. The authors discuss the first portion of the tunnel which was primarily advanced through quartzite with some tectonized zones and intersections with a gypsum formation.

 

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Figure 9: Sonic Logging System (Godio and Dall'Ara, 2012)

 

 

Quality measurements were conducted despite significant water flow from the horizontal boreholes. The collected data was used to accurately predict the changes from hard to soft rock and identify zones weakened by faulting and joints which could inhibit tunnel excavation. This can be observed in Figure 10. In cases where the S-wave was difficult to resolve, a frequency-wavenumber analysis was used on Stoneley waves, which are vertically-polarized waves traveling along the interface (with particle motion perpendicular to the interface) between two materials, to resolve Vs from recorded data.

 

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Figure 10: P- and S-wave Velocities ahead of Tunnel Face during Excavation

 

 

5.3 Characterization of Geologic Framework at a Contaminated Site – Ellefsen et al. (2012)

            Groundwater and sedimentary bedrock at a site in West Trenton, NJ, USA, was contaminated with chlorinated solvents. The authors of the study aimed to further characterize the geologic conditions in order to more accurately predict contaminant transport through the bedrock. The primary goal was to identify fault zones that would significantly control the flow of the contaminated groundwater. Seismic refraction surveys and MASW tests to determine Vs in the bedrock were the primary geophysical methods used. An example refraction survey is shown in the figure below.

 

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Figure 11: Seismic Refraction Survey Record

 

 

 Borehole logs revealed fractures present in the bedrock but did not accurately describe their persistence in the deposit. Two dimensional Vs profiles were developed to map the Vs of the bedrock containing the contaminated groundwater. The interpretation of the depth to bedrock from the Vs data is shown below. The low velocity anomalies show the extent of fractured zones which are likely to be critical for estimating and monitoring contaminant transport.

 

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 scale

Figure 12: 2D Vs Profile with Bedrock Surface Interpretation and Identification of Fracture Zones

 

6.0 Conclusions

            There are many geophysical methods that allow for the measurement of shear wave velocity in rock materials. The importance of Vs to soil and rock characterization makes simultaneous measurement in both materials of interest. Several in situ test methods for Vs in rock, or soil, have been highlighted. These tests have been implemented with measurements for Vp and other geophysical methods for thorough site investigations and assessing the condition of a rock mass. A few examples of applications for Vs measurement in rocks have been explored in this report. Additionally, a table of Vs values for some rock types in the discussed references and others from the literature has been included. 

 

 

 

 

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