Research update: How can we prevent erosion of coastal roads after storms?

Montoya and her research group

Roadbeds supporting coastal highways in North Carolina are susceptible to erosion during large storms, when floods and waves can erode the soil and undermine the highway. Erosion at the interface of pavement and the subgrade can lead to the collapse of pavement and development of channels for flow across or along a barrier island. Preventing erosion of the subgrade soils and adjacent slopes can reduce damage to coastal highways and help maintain open and connected routes during the recovery period after major storms.

Associate Professor Dr. Brina Montoya, alongside graduate student Pegah Ghasemi (with fieldwork assistance from the Geo-Institute Graduate Student Organization), led a study funded by the North Carolina Department of Transportation (NCDOT) to assess the potential for biologically based methods to reduce erosion susceptibility of coastal subgrades. Her research group used natural soil bacteria to cement the soil together, a process called microbial-induced carbonate precipitation (MICP). The MICP process was applied to a sandy slope in Hertford County, North Carolina, at a NCDOT facility to study its ability to prevent erosion when subjected to the elements, including rain, wind and snow. Several methods to apply the MICP treatment were assessed for their ability to treat the surface of the slope over the design area, ranging from simple surface spraying to more complex shallow injections through geotextile drains. After the sandy slope was treated with MICP, the researchers monitored the slope over the course of a year to assess its performance when exposed to the elements.

The researchers found all application methods provided sufficient levels of cementation to the soil, and the simplest method, spraying the surface of the slope, resulted in the most uniform distribution of cementation. The sandy slope demonstrated a marked increase in erosion resistance, which was assessed immediately after the treatment and throughout the year of monitoring. The compressive strength of the cemented sandy slope also increased, which has implications for improved bearing capacity of the coastal rights-of-way.

Ph.D. students in Montoya’s research group working at NCDOT field site. Foreground: Pegah Ghasemi, Qianwen Liu; Background: Ashkan Nafisi, Rowshon Jadid, Thomas Na and Jinung Do.

Perhaps the most exciting result from this study was the performance of the MICP-treated slope. During the one- year monitoring period, the study area received cumulative precipitation of 738 mm (29”) and experienced maximum and minimum temperatures of approximately 35°C and -8°C (95°F and 18°F), respectively. The field site was also subjected to a maximum average wind velocity that was higher than that needed to initiate wind-induced erosion. Researchers monitored soil performance periodically and after major events such as Hurricane Dorian. The cemented sandy slope behaved well with no apparent erosion. This study is the first to monitor a MICP-treated site for an entire year. The fact that no degradation of cementation took place when exposed to the natural elements helps advance the MICP technology toward mainstream implementation for soil improvement projects.

The work was recently published in the Journal of Geotechnical and Geoenvironmental Engineering.