Seismic vulnerability assessments identify potential impacts to infrastructure in earthquake prone areas. These evaluations can help forecast damage as well as estimate costs for pre-emptive retrofits to upgrade critical systems to current design standards. KCI Practice Leader Kwabena Ofori-Awuah, PE, D.GE, ENV SP, worked with Walter Altman, PhD, PE and then Virginia Polytechnic Institute and State University professor J. R. Martin, PhD, to conduct an analysis of the Hanahan Water Treatment Plant in Charleston, South Carolina, using geographic information system (GIS) technology.
Although Charleston may not be the first location to come to mind when thinking about earthquakes in North America, a tremor that shook the city in 1886 is actually among the largest to strike within the contiguous United States. The results of numerous geologic studies have indicated that the area is seismically active and some sites are founded on liquefiable soils, and there are widespread anecdotal reports of liquefaction during the 1886 event. Field investigations (paleoseismic assessment) have documented at least five other strong earthquakes prior to the 19th century.
The Hanahan Water Treatment Plant is among the city’s most critical installations as it services a population of approximately 500,000. The facility dates back to the original pump station built in 1903, long before seismic design guidance was published. During or in the aftermath of a large-scale natural disaster, the water supplied by the treatment plant will be critical to residents, businesses, and especially for firefighting.
The GIS created for the seismic vulnerability assessment includes significant geologic and geophysical information, including:
- Geotechnical data for subsurface information
- Facility and utility line footprints
- Seismicity and seismic ground motion data
- Ground deformation
- Relative susceptibility to liquefaction
- Probability models predicting damages to pipes and structures
The team evaluated two scenarios: a moderate earthquake of magnitude 6.0 and a larger temblor at 7.0. Soil boring logs of the site, which had been collected from previous studies and rehabilitation projects, were then used to determine the soil composition of the area. For each scenario, the site was then modeled to determine key seismic values: peak ground acceleration, peak ground velocity, liquefaction potential index, liquefaction induced settlement, and lateral spreading. The geographic dispersion of these impacts was mapped and then overlaid with the inventory of pipes and structures, predictive damage models and ground deformation.
The results for the Hanahan Water Treatment Plant indicated damage to various infrastructure components that ranged from minor in nature to a complete loss of function. For example, the model forecasted nearly 50 pipes would leak and another 70 would break during a magnitude 7.0 earthquake.
By using a GIS, the team was able to interpret and graphically represent large amounts of data. The output was easier to understand, allowing critical areas to be easily identified and remediation efforts prioritized. This information could significantly affect decision-making that supports the facility’s long-term survivability.