GRS bridge abutments show potential for earthquake-prone regions


UNIVERSITY PARK, Pa. – Geosynthetic reinforced soil (GRS) bridge abutments may provide an effective alternative to conventional designs, even in regions with high seismic activity, according to researchers from Penn State and the University of California, San Diego.

According to the U.S. Federal Highway Administration, GRS bridge abutments can help states and local public agencies meet the country’s demand for small- to medium-span bridges by delivering low-cost, strong and durable structures in less time; however, little was known about their performance in regions where earthquakes threaten the safety and durability of even the sturdiest bridges.

That’s what the team of researchers wanted to find out.

GRS bridge abutments consist of three main components: a GRS foundation; a retaining wall of concrete facing blocks and GRS backfill; and a concrete bridge seat, which transmits the weight of the bridge to the underlying GRS backfill.

 “To create geosynthetic reinforced soil, we basically alternate layers of sand and high-strength polymer products called geosynthetics,” said Patrick Fox, the John A. and Harriette K. Shaw Professor and Head of the Department of Civil and Environmental Engineering at Penn State and co-author on the study. “The geosynthetic layers provide much higher strength and stiffness to the soil mass, which allows it to carry the heavy bridge load with minimal deformation.”

In their experiments, recently published in several leading journals, the researchers used a polyethylene geogrid as their geosynthetic material. Geogrids are made of polypropylene, polyethylene or polyester and are extruded in the factory as a thin sheet with a pattern of holes. These holes allow the soil above and below the sheet to interlock together, creating a stable, bonded material that is capable of carrying heavy bridge loads with predictable and reliable performance.

“Soil has no tensile strength of its own,” said Fox. “So if you try to pull apart a soil mass, like a sand castle, it will rupture and separate right away. The geosynthetic is added to give the soil tensile strength, which changes the properties completely and makes it much more useful as a construction material.”

In conventional pile-supported bridge abutment designs, the bridge sits directly on steel or concrete piles, with footings buried roughly 50 feet below ground, which are independent of the soil holding up the roadway.

“In some ways that's not a good idea,” said Fox. “It’s been a problem for decades. If you set the bridge on piles, and the abutment soil settles for whatever reason, you get a bump in the roadway at both ends of the bridge. This affects travel comfort, can cause safety issues and definitely increases long-term maintenance costs for the bridge.”

In a GRS bridge abutment design, the bridge load sits directly on the GRS backfill, solving the settlement problem because everything moves together. This design concept is commonplace in low seismicity areas of the country, like Pennsylvania, but the California Department of Transportation has been hesitant to adopt it due to concerns about bridge survivability in a major earthquake.

To better evaluate GRS bridge abutment performance under seismic conditions, the research team conducted a series of large-scale laboratory tests using the indoor shaking table at the University of California, San Diego, Powell Structural Laboratory. During testing, six half-scale GRS abutment specimens were subjected to earthquake shaking motions.

According to Fox, the systems performed very well during the physical model testing. In fact, the experiments showed little bridge seat settlement and deformations of the retaining wall – a few millimeters of movement and well within acceptable limits.

In addition to their structural reliability, GRS bridge abutments are much more sustainable than traditional abutments. No steel or heavy equipment is used in their construction, meaning they have a significantly smaller carbon footprint than pile-supported designs.

Based on the research findings, this technology may also provide a more viable, cost-effective design alternative for transportation infrastructure applications, even in areas of high seismicity.

To provide more insight on the effects of other design parameters such as geometry, reinforcement stiffness, reinforcement vertical spacing and bridge load, the research team plans to complete additional experiments and numerical modeling studies.

“It’s exciting to be able to expand the application space for GRS abutment technology to seismic regions,” Fox said. “This can provide a great savings to the state of California and similar high seismicity regions of the world.”

Financial support for this study was provided by the California Department of Transportation (Caltrans) and an FHWA Pooled Fund Project.


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Jennifer Matthews 



The Penn State Civil and Environmental Engineering Department, established in 1881, is internationally recognized for excellence in the preparation of undergraduate and graduate engineers through the integration of education, research, and leadership.

Department of Civil & Environmental Engineering

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The Pennsylvania State University

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Phone: 814-863-3084