Roads and highways are the lifeline of a country. The United States has over 4 million miles of public roads, including the Interstate Highway System. It's critical to prevent damage and maintain roads and highways to support the longevity of the sprawling transportation infrastructure within the United States.
Toward that effort, Texas A&M researchers recently published a report in the journal Transportation Geotechnics on a forensic investigation of a collapsed slope of a Texas highway embankment using a variety of methods, including numeric modeling and laboratory tests.
"Failures of embankment slopes are quite common every year," said Dr. Anand Puppala, professor in the Zachry Department of Civil and Environmental Engineering . "Repairing these failures involves a tremendous amount of work, especially if the highways are in the middle of a city, for example. In our study, we investigated the reasons for failure of embankment slopes and the best way to repair such damages."
According to the Federal Highway Administration, embankments are structures that are made of compacted earthen materials, like soil, rock or aggregate. They raise roadways above the level of the existing surrounding ground surface to provide structural support, prevent flooding and improve drainage from roads by directing water through defined paths.
But even embankments can fail over time due to heavy rainfall, snowmelt, seismic activity and construction defects. These failures can lead to loss of life and property damage. Highway embankments made of heavy materials are often built over soft ground and this can sometimes lead to settlement and instability of the embankment.
"Most of the embankments in Texas, for example, are built with the local clayey soils," said Puppala. "Embankment slopes made with compacted clayey soils are highly problematic. After 10 to 15 years of continuous use along with the natural impact of weather, embankments tend to fail."
For their study, the researchers selected a collapsed highway embankment slope in Houston. They performed in-situ site investigation, collected the soil specimens using a coring barrel and transported the specimens to the laboratory for testing and analysis. Here, the team performed a variety of tests, including basic soil characterization and laboratory experiments, to determine the shear strength of the soil.
Next, they used the results from these tests, such as the soil strength, as parameters for a numerical model that can analyze the stability of the embankment slope. In addition, they also used the model to explore scenarios of extreme rainfall on the embankment slope.
"The model was telling us that a 24-hour rainfall or an extreme flooding event, for example, can lead to the development of a water table on the top surface, which generally results in the failure of the slope," said Ayush Kumar, a graduate student in Puppala's laboratory and lead author on the study. "Texas soils tend to be quite plastic that increase and decrease in volume based on wetting and drying cycles, making them more susceptible to moisture change."
Based on their findings, the researchers noted there are practical solutions that could have prevented collapse of the embankment slope investigated in this study. Their recommendations include using stabilizing agents, like cement, to reduce the impact of moisture and the placement of perforated pipes to drain the water quickly.
Although the study is a forensic analysis on the reasons underlying the failure of a specific embankment slope, the researchers noted that a similar comprehensive analysis can be carried out for different slopes and weather scenarios.
Thus, this research has the potential to be useful for transportation workers tasked with maintaining roads and highways across the country.
"This study is a result of good teamwork," said Puppala. "We've been fortunate to get a lot of help from TxDOT Houston district personnel to get the right samples and then point us to places where the embankment failures have happened. We hope that our study in turn provides a rehabilitation strategy that can be used by different transportation agencies to prevent failure in the future."
Contributors to the research include Nripojyoti Biswas from the University of Massachusetts and Benamar Mebarkia from the Texas Materials and Tests Division at the Texas Department of Transportation.
This research was supported by the Houston District of the Texas Department of Transportation.
Texas A&M University College of Engineering
