Building Resilient Infrastructure
By Eric Ferrebee, Contributing Author
With flooding and other natural disasters on the rise, building resilience and durability into infrastructure is a priority. Rigid pavements are part of the solution.
Data from the Environmental Protection Agency (EPA) shows that the total annual precipitation has increased over land areas in the United States and worldwide since 1901, and in recent years, a higher percentage of precipitation in America has come in the form of intense single-day events.
Incidence of river flooding has varied by geographic region, with "floods generally having become larger across parts of the Northeast and Midwest and smaller in the West, southern Appalachia, and northern Michigan. Large floods have become more frequent across the Northeast, Pacific Northwest, and parts of the northern Great Plains, and less frequent in the Southwest and the Rockies,” according to the EPA.
With high-precipitation storm events and more flooding come closed roads. In 2018, Hurricane Florence resulted in approximately 2,500 road closure sites in North Carolina. The state's Transportation Secretary, Jim Trogdon, was quoted in a North Carolina Department of Transportation press release saying, "I have been in state transportation 30 years – through hurricanes Fran and Floyd and others -- and I’d never seen flash flooding like we experienced with Florence or the widespread impacts across the state.”
Given the safety risks and the costs associated with disaster recovery, it is becoming imperative to design infrastructure with resilience in mind.
Resilience, as defined by the U.S. Army Corps of Engineers, is “the ability to anticipate, prepare for and adapt to changing conditions and withstand and recover from disruptions."
Roadways and airfields that maintain their structural integrity during a flooding event will help preserve evacuation routes and provide ground access for rescue crews—fundamental aspects of resilient infrastructure. The ability to recover quickly after a flooding event and not degrade through recovery efforts is also critical to resilience.
Designing Resilience into Road Networks
The first step in addressing resilience for new and existing infrastructure is to perform a risk assessment. Many flood maps are outdated or inaccurate, so it is important to review multiple data sources to determine the true risk of flooding. Armed with accurate information, planners can identify where design efforts should be focused. Identifying which pavements are most critical for life safety needs can also help focus efforts.
It is important to determine the type of flooding that is likely to occur in a given location. If road damage occurs as a result of rapid flowing currents that erode the soil and roadbed—a circumstance known as a “washout”—the solution or preventative action will extend to earthworks, such as designing culvert systems to accommodate high water flows.
Flooding attributable to rising water levels that submerge roads, on the other hand, can be proactively protected against with proper pavement design and selection. This is because road submersion causes the subgrade to become supersaturated, with water pushing the subgrade particles apart and reducing subgrade strength.
Soaked soils typically represent a loss of bearing capacity between 20% and 50% (depending on clay and silt content), according to research presented in “Comparison Between Soaked and Unsoaked CBR” by Sathawara et al.
A pavement system that is rigid, as opposed to flexible, will help mitigate this type of subgrade damage, since rigid pavements rely minimally on the strength of foundation layers. Concrete, a rigid pavement system, maintains its structural integrity even when submerged. It will deflect less than flexible pavements and distribute loads over a larger area.
Under a 7,000-pound load from a tire, flexible asphalt pavements concentrate the load, transmitting it through the base and subbase layers into the subgrade, at a pressure of approximately 15-to-20 psi.
The same 7,000-pound load on a rigid concrete pavement will significantly reduce the pressures on the subgrade, and the distribution of the load will restrict pressure to 3-to-7 psi. An asphalt pavement system relies on the underlying layers to help carry the load, whereas a rigid system naturally spreads the load across a wider area.
Even after flood waters recede, the threat to pavement integrity is not over. Research has shown that it can take up to a year for the subgrade under a pavement structure to completely dry out and the strength to recover.
Loading that happens during the one-year recovery period further accelerates damage to the pavement. This is especially concerning considering the fact that large numbers of extremely heavy vehicles, such as articulated trucks, can be deployed as part of rescue and recovery efforts. The result is reduced lifespan for flexible pavements.
A study titled “Impact of Hurricane Katrina on Roadways in the New Orleans Area” by Gaspard et al., showed an overall strength loss in asphalt, with damage occurring regardless of the length of time the pavement was submerged, whereas concrete pavements suffered little relative loss of strength due to flooded conditions. Concrete also maintained a resilient modulus that was similar for submerged and non-submerged pavements.
With data showing pavement deterioration is lessened—and pavement life expectancy extended—by engineering stiffer pavement systems, agencies would be wise to consider modifying design standards to use soaked subgrade strength values during pavement design. Doing so could help ensure road networks that are built to withstand flooding scenarios—scenarios that are increasingly business-as-usual, instead of merely worst-case.
Constructing Rigid Pavements
When constructing new pavements, stiffness can be built in from the ground up using concrete pavements and cement-stabilized bases. In addition to conventional concrete pavement placement, roller-compacted concrete (RCC) may be a good option in some locations.
RCC, which is compacted using high-density pavers and vibratory rollers, is dense and strong while offering ease and expediency of placement, cost savings and low maintenance.
Existing pavements can be stiffened in several ways. Placing concrete overlays on existing asphalt pavements can reduce loading pressures significantly at the top of the asphalt layer, and the additional height contributes to elevating the pavement surface above flooding conditions. Overlays can provide a cost-effective retrofitting solution that requires no demolition and little use of new raw material.
Performing full-depth reclamation (FDR) with a cement-stabilized base (CSB) is another option. This method creates a deep recycled layer and provides a new stabilized base.
The base offers strengths associated with a rigid road cross section, spreading loads over a larger area and maintaining a high level of pavement strength and stiffness even in saturated conditions.
A 100 psi load on an unstabilized granular base will transmit to the subgrade at 15 psi, whereas the same 100 psi load on FDR with CSB will reduce that transmission to 4 psi.
CSB also reduces permeability to moisture coming from high water tables underneath the road’s structure. By utilizing in-place materials and reducing trucking requirements, FDR with CSB can be a cost-effective solution.
Recent climate trends suggest that future conditions will be more extreme than in the past. In addition to increased precipitation intensity and frequency, global mean sea levels are rising, a situation that poses particular hazards to shoreline areas, which are often heavily populated. Extreme heat and drought conditions are making some regions more prone to forest fires.
Resilience is about good engineering. Pavements designed with long-term resilience in mind offer economic and social benefits. Their durability reduces life-cycle costs, while their reliability supports public safety. RB
Eric Ferrebee is the senior director of Technical Services at the American Concrete Pavement Association.