What Lies Beneath:
Designing Earthquake Resistant Bridges From the Ground Up.
Clint Wood has traveled all over the world studying the effects of earthquakes on buildings and infrastructure. Whether in Haiti or New Zealand, Japan or Ecuador, architects and engineers are using data from his research to design safer buildings, bridges and roads.
It’s here in Arkansas, however, where his work to help build more resilient and cost effective structures is making the biggest difference.
In 2016, officials with the Arkansas Department of Transportation commissioned Wood to conduct soil tests to estimate the seismic demand placed on bridges in the event of a large-magnitude New Madrid Seismic Zone earthquake in eastern Arkansas. The goal of the project was to determine the optimal seismic design specifications for new and replacement bridges in that part of the state.
Wood, an assistant professor of civil engineering, grew up in Elkins, but his research has focused on the New Madrid Seismic zone in and around the mostly small communities within an area geologists call the Mississippi Embayment, stretching from Corning near the Missouri border, down to Marianna and east to the Mississippi River.
The Mississippi Embayment is the wide and ancient riparian zone of the Mississippi River. Soil there is extremely deep, deposited over millennia by floods and channel shifts of the river. In some areas of this zone, bedrock lays beneath more than 3,500 feet of soil. In northeast Arkansas, the embayment lays within the New Madrid Seismic Zone, a large area that has some of the highest expected ground motions in the United States. In 1811 and 1812, a series of earthquakes with estimated magnitudes between 7.5 and 7.9, destroyed homes and infrastructure in this area and even altered the flow of the Mississippi River.
Wood is a geotechnical engineering specialist. His research focuses on characterizing the depth, composition and layering of soil and rock formations on which buildings and infrastructure are built. He does this by using seismometers to measure low-intensity stress waves as they move through the soil and rock formations.
Measuring Seismic Risk
By measuring the frequency and wavelength of these stress waves, Wood estimates the shear-wave velocity and layering of the soil underneath the seismometers. With these measurements, he creates what are called shear wave velocity profiles, which are simply site-specific data about soil stiffness and depth. These profiles help engineers understand ground motions recorded during earthquakes and are used to better design buildings, bridges and roads that can withstand the powerful forces generated by earthquakes.
“The implementation of this research could lead to millions of dollars in savings for Arkansas Department of Transportation and Arkansas taxpayers.”
Through his evaluation of the soil structure and current structural design of bridges in the area, Wood has identified potential areas of cost savings for the state. By better characterizing the hazard from a future seismic event, the overall structural requirements of bridges can be reduced ultimately reducing the cost of constructing bridges without sacrificing safety.
Safer Bridges for Less Money
For example, Wood recently analyzed a newly constructed, four-lane, bridge in Monette, 25 miles east of Jonesboro. This seven-span bridge stretches 330 feet across Cockle Burr Slough along Arkansas Highway 18. Using the shear wave velocity profile Wood measured at the site, his team conducted a site response analysis, which models how seismic waves move through the rock and soil layers. This allowed his team to better understand the seismic demand placed on the bridge during a real earthquake. The results indicated that the 2,200 feet of soil below the bridge would dampen the short period seismic waves’ effect on the bridge and this in turn would reduce the seismic demand for the bridge’s structural columns, restrainer blocks and approach embankments.
Wood then re-designed these bridge components based on the decrease in seismic demand determined from the site response analysis and compared the construction cost of the two bridges. He estimated a potential cost-savings of $205,000, or a 7 percent reduction of the total bridge project cost was possible.
“Overall, this collaborative project assembled the tools and information necessary for conducting site response analyses at bridge sites and demonstrated the potentially dramatic cost savings possible for future projects,” Wood said. “The implementation of this research could lead to millions of dollars in savings for Arkansas Department of Transportation and Arkansas taxpayers.”