Wingwall Design of Skewed-Crossing Concrete Box Culverts

Background

The Society of Afghan Engineers (SAE)—specifically through the expertise of our former Vice President, Eng. Amanullah Mommandi—authored and published technical guidance in our April 2021 eNewsletter on the proper design of wingwalls for skewed concrete box culverts. Recognizing that many existing culverts on Afghan roadways lack hydraulically efficient configurations, we utilized established design standards from the Colorado Department of Transportation to provide practical, step-by-step design criteria. This guidance was directly targeted toward hydraulic engineers at the Afghan Ministry of Public Works (MPW), the Ministry of Rural Rehabilitation and Development (MRRD), and local engineering consultants to help prevent future infrastructure failures in Afghanistan. In addition to publishing this research, we also delivered targeted online technical seminars on wingwall design to Afghan engineering students.

The Challenge of Skewed Culverts in Afghanistan

Ideally, a stream crosses a roadway at a perpendicular, 90-degree angle; however, various legal, environmental, and right-of-way restrictions frequently prevent this alignment, forcing engineers to design culverts that cross roadways at skewed angles. Based on our inspections of constructed culverts across Afghan roadways, we identified that many of these skewed culverts carrying water under the roads were built without hydraulically efficient wingwalls at their inlets or outlets.

To address this critical deficiency, we developed practical design examples based on Chapter 9 of the 2004 Colorado Department of Transportation Drainage Design Manual (CDOT DDM). Our goal was to provide simple, easy-to-understand methodologies that Afghan hydraulic engineers and consultants could easily adopt for roadway, railroad, and other infrastructure crossings.

Technical Guidance and Best Practices

In our technical paper, we provided specific engineering parameters to ensure the stability and efficiency of these structures:

• Addressing Culvert Skew: We defined culvert skew as the acute horizontal angle to the left between the roadway centerline and the culvert centerline. We advised that culvert skew should not be less than 45 degrees without explicit approval from a hydraulic engineer, as angles below this threshold significantly decrease the hydraulic efficiency of the structure. Furthermore, because the wingwalls for a skewed culvert are not equal in size, the wingwall located at the attack angle of the stream must be intentionally designed to be longer to properly protect the roadway embankment from failure.
• Wingwall Flare Angles: To smoothly direct flow into the culvert entrance and retain the roadway embankment, we established that wingwalls provide the optimal hydraulic efficiency when their flare angle is designed between 30 and 60 degrees.
• Aprons and Cut-off Walls: To reduce scour caused by high flow velocities, we directed that concrete aprons must be used at both the inlet and outlet of culverts with wingwalls, and that these aprons must terminate with cut-off walls.

Preventing Wingwall Failure

A major focus of our guidance was analyzing the root causes of culvert failures. We determined that the primary cause of failure for many wingwalls in the region is the absence of a concrete apron and cutoff wall between the two wingwalls.

To protect against structural undermining, we provided detailed specifications for placing concrete aprons or heavy rocks (riprap) between the wingwalls. We also established the structural requirement to connect the wingwalls to the main culvert walls using number 4 rebars that are at least two feet in length.

Strategic Recommendations for Afghan Ministries

Our technical paper concluded with actionable policy recommendations for Afghanistan’s governing infrastructure bodies. We formally recommended that the Ministry of Public Works (MPW) and the Ministry of Rural Rehabilitation and Development (MRRD) adopt these standards and make it a mandatory requirement within their design manuals to place wingwalls, headwalls, aprons, and cutoff walls at the outlet of all culverts under roadways. We also recommended that the depth of the culvert cut-off walls at the outlet be designed to reach at least two feet below the calculated scour depth to prevent the undermining of the apron.