Date of Award
Doctor of Philosophy (PhD)
Ronald D. Andrus
Assessing earthquake hazard in the State of South Carolina is important because it is one of the most seismically active regions of the eastern United States and has experienced earthquakes of damaging levels in the historical past. Examples of these damaging seismic events are the 1886 Charleston earthquake (M 6.7 to 7.5) and the 1913 Union County earthquake (M 4.5 to 5.5).
Small-strain shear-wave velocity (VS) is an important parameter in performing site response analysis. The deep nature of the top of reference firm rock (i.e., VS ≥ 760 m/s or B-C boundary) due to the thick sediment cover (> 150 m) in many locations in the Coastal Plain region makes it difficult and expensive to make all the needed VS measurements. Similar difficulties are encountered in the Blue Ridge and Piedmont region, where it can be expensive to make VS measurements from the ground surface to the top of hard rock (VS = 3,000 m/s) or weathered hard rock (VS = 2,500 m/s). Due to this, a geology-based VS model of South Carolina is needed for performing more accurate and economical site response analyses.
This dissertation presents work on a geology-based, base case VS model of reference conditions in South Carolina. The model is derived from 55 VS profiles that extend into basement rock and 113 VS profiles that extend into Coastal Plain Cretaceous and/or Tertiary sediments. Of the 55 VS profiles that extend into rock, 18 are from predominantly non-foliated crystalline rock sites, 26 from predominately foliated crystalline rock sites, and 11 from sedimentary basin rock sites. Of the 113 profiles from the Coastal Plain, 55 extend into Tertiary sediments above the base of the Santee Limestone Formation, and 70 extend into Cretaceous and/or Tertiary sediments older than the Santee Limestone Formation.
Also presented in this dissertation is a three-dimensional geological model of reference site conditions in the South Carolina Coastal Plain (SCCP). Kriging is used as the geostatistical interpolation method in ArcGIS Pro Software to generate the depth to the top of two geological layers with high VS constrast. The State is divided into 14 zones microzones based on geology and depth to the top of the basement rock. Representative shear-wave velocity and geologic profiles are created for each microzones.
Selected geotechnical properties are characterized in the form of total unit weight (γT), plasticity index (PI), and unconfined compressive strength (UCS) profiles. Median γT values for the non-foliated crystalline, foliated crystalline and sedimentary basin basement rock range from 25 to 28 kN/m3, and for Cretaceous and Tertiary older than the Santee Limestone, and Tertiary above the base of the Santee Limestone in the SCCP range from 16 to 25 kN/m3. Using six boreholes in the SCCP that range in depths from 136 to 158 m, the PI of SCCP sediments is characterized by the 10th percentile = 0 %, 50th percentile = 15 % and 90th percentile = 45 %. Median UCS values for three rock groupings are in the range of 13 to 168 MN/m2.
Site response analyses are conducted on three VS profiles that are characteristic of the reference site conditions in the Blue Ridge and Piedmont regions of South Carolina using linear and equivalent linear methods. The VS profiles are the non-foliated crystalline rock and foliated crystalline rock profiles developed in this study and the portion of the Darragh et al (2015) “Piedmont Saprolite.” The response spectra and amplification factor plots showed that VS has significant influence on the results of site response analyses, whereas shear modulus and material damping has less effect on the results.
The results of this study have been used to develop probabilistic ground motion maps for the reference outcrop conditions for use by the South Carolina Department of Transportation in the evaluation and design of their facilities.
Amevorku, Camilius, "Geology-Based Shear-Wave Velocity Model of Reference Site Conditions in South Carolina for Seismic Site Response Analysis" (2022). All Dissertations. 3208.