Iowa Department of Transportation
Iowa Highway Research Board
Zhengyu Liuzhengyu@iastate.edu email >
Research Engineer, BEC
Brent Pharesbphares@iastate.edu email >
Bridge Research Engineer, BEC
About the research
Earth pressure on concrete box culverts is a key component of design and load rating decisions. Currently, the Iowa Department of Transportation (DOT) Office of Bridges and Structures uses the maximum and minimum lateral earth pressure of 36/18 lb/ft3 specified for load factor design (LFD) and allowable stress design (ASD) and 60/30 lb/ft3 for load and resistance factor design (LRFD).
The objective of this project was to analyze collected data to determine the actual earth pressures associated with Iowa soil conditions to determine which load pressures are more realistic for typical Iowa soil conditions and construction methods for concrete box culverts.
A brief literature review was conducted to identify potential factors related to the vertical and lateral earth pressures. To gain a better understanding of actual pressures on concrete box culverts in Iowa, a culvert in Ida County was instrumented in 2016. Two concrete box culverts—the one in Ida County and the other in Crawford County—were monitored for more than 2.5 years and 1 year, respectively, to identify the realistic design soil pressure for Iowa soil conditions.
The captured pressure, strain, and temperature data were analyzed to find the relation between the temperature and earth pressure experienced by the culverts. The measured vertical and lateral pressures were compared with specified design pressure loading.
The monitoring results from both culverts led to the consistent conclusion that the earth pressure experienced by the culverts were 2 to 6 times that of the design values with the LRFD and LFD/ASD methods. Extensive longitudinal cracks (parallel to the flow) were observed at the bottom surface of the top slab on both culverts.
Given the findings from this research, further research is recommended, as outlined at the end of this report.