In typical bridge construction, the roadway must transition from an area supported by compacted but flexible soil embankment layers (backfill) to a more rigid bridge abutment support. Due to several factors, including seasonal temperature changes, loss of backfill by erosion, poor drainage, and high traffic loads, the portion of the backfilled roadway can settle over time while the abutment resists settlement, creating an abrupt elevation change, or "bridge bump," at the transition point. Of the United States' approximately 600,000 bridges, about 25 percent suffer from bumps at their ends, and state transportation agencies spend an estimated $100 million per year fixing them.
How Does It Work?
Typically, roadway backfill is placed in layers, with compaction occurring after each layer. Additional minor settlement of this backfill over time is normal, but it becomes noticeable to drivers as a bump at the transition point onto a bridge abutment. A 2005 Iowa DOT research study demonstrated the effectiveness of enhancing backfill with layers of porous backfill (open-graded aggregate) bound by geosynthetic fabric to minimize "bridge bump." The fabric and backfill work together to resist the settlement problems encountered with typical backfill, keeping the backfill-supported roadway at the same elevation as the adjacent abutment-supported roadway.
What Are The Benefits?
The primary benefits of GSBA include reduced future maintenance costs and improved roadway rideability at the transition to the bridge. PennDOT maintenance forces utilize asphalt patching or minor grinding to address "bridge bump" — neither of which addresses the underlying cause. The more robust maintenance solution requires excavation and replacement of the subgrade and pavement, which is costly and requires long-term traffic control setups that inconvenience motorists.
If bridges are already targeted for replacement or significant rehabilitation that will require substantial traffic control, GSBA presents a low-cost, low-risk enhancement that will pay for itself. For example, the construction cost to repair a settled approach slab can cost up to $60,000. Compare that to the average price of $4,000 to upgrade the standard bridge approach backfill with the new geosynthetic-reinforced backfill option.
It also helps to ensure motorists get the ride quality they expect while enhancing safety and minimizing damage to their vehicles.
Innovation in Motion
PennDOT District 2 began utilizing the technique on bridge approaches in 2015. Since then, the construction of over 50 GSBA has provided a large-scale field demonstration that has proven the concept effective. The enhanced backfill construction only adds about $5,000 in cost per approach.
Based on the success of District 2's efforts to date, as well as additional research around the country, PennDOT is developing a new roadway construction standard that will facilitate statewide use of GSBA technology; the new standard should be approved for use by the end of 2021. Future efforts could include developing additional standards, including reinforcing the original ground with geosynthetic material to the streambed elevation.