Date of Award

5-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Andrus, Ronald C

Committee Member

Juang , C. Hsein

Committee Member

Pang , Wei C

Abstract

Two geology-based probabilistic liquefaction potential maps are developed for the 7.5-minute Charleston, South Carolina quadrangle in this thesis. Creation of the maps extends the previous liquefaction potential mapping work of the Charleston peninsula by Hayati and Andrus (2008) and Mount Pleasant by Heidari and Andrus (2010), and improves upon the previous maps by using peak ground accelerations that vary with local site conditions. The GIS software package ArcGIS 10 is used to develop the maps.
Development of the liquefaction potential maps involves the creation of four additional maps needed as inputs. The four additional maps are (1) depth to the top of the Tertiary-age Cooper Marl (dMarl); (2) average shear wave velocity in the top 30 m (VS30); soft-rock peak ground surface acceleration (PGAB-C)for about a 500-year return period; and site-adjusted peak ground surface acceleration (PGASite). The dMarl map is created using the elevation contour maps by Weems and Lemon (1993) and Fairbanks et al. (2008), and topographic information from the South Carolina Department of Natural Resources GIS Data Clearinghouse. Values of dMarl range from 2 m to 24 m.
The VS30 map is created by combining the dMarl map, the surficial geology map by Weems et al. (2011) and the average shear wave velocity values reported in Andrus et al. (2006), and assuming a simple two layer model. The initial VS30 map is refined locally with calculated VS30 values from specific test sites. Mapped values of VS30 range from < 140 m/s to 350 m/s. These VS30 values correpsond to seismic Site Classes D and E, assuming no special Site Class F conditions.
The VS30 map is used with site factors derived by Aboye et al. (2011, 2012) to adjust the map of PGAB-C created from the 2008 USGS National Seismic Hazard Map. The resulting map of peak ground surface acceleration adjusted for site conditions (PGASite) consists of values 15 to 50% higher than values of PGAB-C.
Liquefaction potential for the area is expressed in terms of the liquefaction potential index (LPI) and calculated using relationships by Heidari (2011). These relationships correlate the probability of LPI > 5 (PLPI>5) with the ratio PGASite/MSF (where MSF is the magnitude scaling factor), depth to groundwater table (GWT), and dMarl. To match relationships by Heidari (2011), the geology of the area is grouped into the categories of artificial fill, younger natural sediments, and the Wando Formation.
An analysis of GWT for the quadrangle is conducted using data from Fairbanks et al. (2004) and Mohanan et al. (2006). From this data, a conservative estimate of 1.0 m is initially used for the GWT depth for all areas.
The first liquefaction potential map is based on a moment magnitude (MW) = 7.3 and GWT = 1.0 m for all areas. The second liquefaction potential map is based on MW = 6.9, GWT = 2.0 m for the Wando Formation, and GWT = 1.0 m for areas covered by younger materials. Liquefaction potential values for the MW = 7.3 map are too high when compared to field performance during the 1886 earthquake. Values of liquefaction potential for the MW = 6.9 map coincide more closely with observed field behavior and previous maps for the Charleston peninsula and Mount Pleasant. The highest risk of liquefaction on both maps is found to be in areas with the largest dMarl depths and covered with artificial fill and the younger natural sediments.
A potential use for the liquefaction potential maps is discussed with respect to the resiliency of the roadway and bridge infrastructure of Charleston. All bridges in the quadrangle have abutments located on areas with PLPI>5 = 60 - 100%. The roads with highest risk of liquefaction-induced (i.e., areas of PLPI>5 = 80 - 100%) are located on the Southern end of the peninsula, James Island, and western Mount Pleasant. General recommendations are given for improving the resiliency of bridge infrastructure by taking preventative measures with existing and future structures, and by insuring that inspection and repair of damaged bridge structures take place in a timely manner after an earthquake. The liquefaction potential maps can be used to prioritize areas to be inspected following the next strong earthquake event.

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