As it becomes increasingly likely that the world will miss the goals set forth by the Paris Climate Agreement, civil engineers across North America are turning their attention to building road infrastructure projects that can withstand a more challenging and hostile world. Warming of 2.7 degrees Fahrenheit (1.5 degrees Celsius) means more stress on roads—whether that’s from increased and extended duration rainfall, drought, flooding, or changing soil conditions—and engineers and local governments alike are taking proactive measures to adapt

So far, engineers and contractors have made strides in both boosting resilience and reducing waste by using recycled plastic and recycled road materials to build roads. This is certainly good progress, but to further reduce infrastructure’s environmental impact and improve its hardiness, engineers will need to do more. One cost-effective and efficient solution lies deeper—all the way down to the subgrade.

Why subgrade?

When it comes to weathering an array of current climate conditions, stabilizing and stiffening the unbound aggregate layers above the subgrade with geogrids protects the subgrade and proves to be an effective strategy. Climate and soil conditions vary greatly across North America, but in the contiguous US and Canada, expansive clay soils and silty soils pose unique challenges for contractors. A closer look at how geogrids already help in these conditions can lend some insight into how climate-resilient roads might be built.

Expansive clay soil

Expansive clay soil, as its name suggests, expands and contracts according to moisture levels. This poses an especially big construction issue in southern and western US states where expansive clay accounts for a large percentage of soil composition. Winters in these regions typically see lots of rain, causing the clay to expand, while dry conditions in the summer cause the clay to shrink up, forming longitudinal cracks in roads that must be repaired.

According to a 2019 study from the University of Texas at Austin, roads stabilized using geogrids showed a significantly lower percentage of longitudinal cracks than roads stabilized through chemical methods. Spanning three years, the study monitored crack formation on a stretch of road southeast of San Antonio, Texas that used different subgrade stabilization techniques for three sections of the road.

Longitudinal cracks grew the most between September and October 2017, with the chemically stabilized section seeing a 28 percent increase in cracks. However, the two geogrid stabilized sections used in combination with a thinner section of cement stabilized subgrade saw a seven percent decrease and a two percent increase, respectively.

The period between September and October also saw one of the largest increases in rainfall following a long stretch of hot and dry conditions. This unusual weather pattern will soon become normal as the world warms up, suggesting geogrids can be used in combination with chemical stabilizers where required for meeting climate change-imposed demands on infrastructure.

Freeze-thaw in silty soil

In colder climates, especially in the northeast and around the Great Lakes, the regular cycle of freezing and thawing that occurs every winter and spring can be especially damaging to roads. Subgrade conditions in these regions are already less than ideal—glacial till comprises large amounts of the soil, making for loose subgrades that settle unevenly. What might seem like sturdy ground in the frigid winter months can turn to soup when melting ice rehydrates the soil in spring, causing ruts to form in roads.

In these unstable conditions, geogrids act like giant snowshoes for the pavement above, spreading out load pressure to preserve road longevity. This is why the Nova Scotia Transportation & Infrastructure Renewal Department (NSTIR) chose multi-axial geogrids for the construction of Highway 104 in Antigonish, Nova Scotia. NSTIR needed to ensure certain levels of foundation stiffness for the highway, and international roughness index (IRI) measurements taken after construction showed that, in the face of difficult weather conditions, the sections of the highway stabilized with geogrids were consistently smoother than the sections stabilized through traditional means.

As the regions closest to the Arctic continue to warm faster than anywhere else, proven subgrade stabilization methods like synthetic geogrids will be crucial for preventing road failures. This is especially true for northern communities impacted by glacial deposition—the transportation of rock chunks and sediment from melting glaciers.

Standing up to extreme weather events

But geogrids don’t only prepare roads for regular wear and tear. Subgrades protected by aggregates that have been stabilized and stiffened with geogrids also perform well in extreme environmental events, like earthquakes and hurricanes.

Building in the Pacific Rim

Earthquakes in New Zealand are a daily occurrence. And while not all earthquakes are strong enough to be felt, this frequency of seismic activity takes a toll on infrastructure, making it especially difficult to stabilize the foundations of roads and buildings. In these conditions, geogrids incorporated into foundation layers actually create ductile behavior in the foundation layers, preventing them from becoming brittle and fracturing. This makes infrastructure more pliable, extending the ability of foundation layers to bend before they break. Just like supple, green trees handle strong winds better than rigid, dead ones, foundation layers stabilized with geogrids have a much better chance of surviving earthquakes.

A strong earthquake can still destroy or cause serious damage to even the best-engineered road foundation layers, but these roads are still much more likely to safely bear emergency response vehicles once the dust settles. Climate change may or may not increase the frequency of earthquakes, but it will cause more flooding, which inflicts distress on infrastructure similar to what earthquakes are capable of. Now and in years to come, geogrids are a safe bet against seismic activity and overloading.

Maintaining work schedules through a hurricane

In August 2017, Hurricane Harvey battered the US Gulf Coast, dumping heavy rain on coastal communities in Texas, Louisiana, and Mississippi. As towns and cities along the Gulf of Mexico grappled with flood damage, the construction site of one petrochemical plant managed to get back to work the next day. A massive ringer crane—in fact, the third-largest in the world at the time—was installed onsite with a geogrid-stabilized platform. Despite already poor soil conditions and a high-water table, the crane’s platform withstood Hurricane Harvey without sustaining any damage.

The crane resumed operations the day after the storm, making for a cost savings of $3.1 million and seeing the construction project completed 32 days ahead of schedule. And because the crane’s platform used geogrid stabilization, construction crews were able to reuse the crushed rock backfill, something that wouldn’t have been possible with a typical concrete pad. As hurricanes intensify in a hotter world, contractors would be wise to consider geogrids for subgrade stabilization and stiffening.

Stabilization and stiffening is just the beginning

Climate change holds massive implications for the future, not least of which is how we design and build our infrastructure for a multitude of known and unknown potential stressors. In February 2020, Civil Engineering magazine released a special issue on the need for civil engineers to step up as leaders on climate resilience. One of the articles from this issue, “Leadership Required: Civil Engineers and Resilient Infrastructure,” features a roundtable discussion from the American Society of Civil Engineers’ (ASCE) International Conference on Sustainable Infrastructure (ICSI) in which panelists discussed the need for resilient design in infrastructure projects.

Near the end of the discussion, Carol Haddock, a panelist and the Director of Houston Public Works, said, “We have to be leaders in changing our approach so that any outcome that is not resilient is not acceptable as an engineering outcome.”

In this quickly changing environment, contractors must prepare to meet civil engineers’ resiliency expectations. Geogrids have already proven their value in many different conditions around the world, and now they’re ready to prove their value as crucial components of resilient infrastructure. This will be especially important for voters in the years to come as taxes increase and the quality of life on the road decreases due to aging infrastructure.

It’s going to take a lot to improve our infrastructure for the challenges presented by climate change, but by focusing first on protecting subgrades by stabilization and stiffening the aggregates they are built with, roads stand a much better chance of allowing local government officials to claim they have addressed constituents’ concerns about infrastructure.