In Part 1 of this blog article series, we touched on the basic fundamentals related to selecting material type, structure type, structure shape and size selection for culverts and buried bridge structures. Part 1 of this series was intended to be a brief overview of the various key factors related to the process of logical structure selection.
In the second part of this series, we will delve deeper into several areas – specifically the practical differences of shape type for a particular site location. The primary focus here will be on corrugated metal and structural plate offerings.
Buried bridge and culvert structures come in a variety of shapes – including round, arch, elliptical, pipe arch, underpass and box culvert shapes. While this variety does provide developers and specifying engineers a number of choices and options for their given project, consideration needs to be given to the practical differences between shape type and how those differences might affect shape selection for a project.
One of the key governing factors will be the headroom – the distance from the invert of the buried bridge or culvert to finished grade - available at the site in question.
- Will a round pipe fit within this headroom and still have sufficient cover height over the pipe to carry anticipated design loads?
- Can a round pipe be buried, thus protecting the invert from daily flow forces and abrasive / corrosive forces?
- If not, would a horizontal ellipse or a pipe arch structure be a better choice?
- Conversely, if a more extreme geometry such as a pipe arch or horizontal ellipse or underpass structure is selected, will there be an inordinately high amount of overburden above the structure that would produce significantly greater pressures on the surrounding soil envelope (specifically in the corner or haunch areas of the structure), pressures that could exceed the allowable bearing capacity of the soil and the underlying foundation and lead to shape distortion or settlement?
Pipe arch and underpass shapes were developed to address several practical needs, primarily the need to maximize the flow area or interior shape clearance dimensions within a limited amount of vertical height or rise of the structures. These were necessary shape modifications designed to fit within tight headroom constraints. Unfortunately, project engineers and developers would choose pipe arch and underpass shapes for situations where a round pipe would be a better selection, leading to significantly higher shape factors (the ratio of the crown radius to the corner or haunch radius of the structure) and consequently, significantly higher pressures on the soil surrounding the structure. These high soil pressures in the haunch areas of the structure contributed to foundation issues and led to shape distortion. Also, proper placement and compaction of the backfill in the critical haunch support areas is more difficult with these more extreme shapes when compared to round pipes or horizontal ellipses. While it certainly is possible and feasible in many cases to use higher quality backfill material, i.e., material with greater bearing strength and that is easier to place and compact (or that may not need compaction – such as soil-cement, CLSM, slurry cement, etc.), such precautions are sometimes overlooked, leading to structure support and stability issues.
Selection of a slightly oversized round pipe as opposed to a pipe arch or an underpass shape might be a more logical choice for a given site. This might mean the invert will be buried when compared to the pipe arch or underpass structure, but this burial depth has the added benefit of protecting the invert of the structure from environmental factors and abrasive flow forces, thus prolonging the service life of the structure. Also, site restrictions may require a natural stream bottom as opposed to the invert of a pipe, and burying the invert and allowing it to silt in gradually creates that natural stream bottom.
An additional potential problem related to pipe arch and underpass structures – and one that can be an issue with other shape geometries as well - is attention to end treatment and invert protection. Consideration of adequate cut-off walls, slope collars or headwall treatment to guard against erosion and flow forces is part of the site design and structure selection process and should not be overlooked. End treatment is a topic within itself and has been covered in a previous blog article.
Arch structures are another shape choice available. These are typically installed on footings. There are a number of practical factors related to the selection of a logical arch shape and footing layout to fit a particular site, factors that will provide benefits – or that can potentially contribute to problems - that are not always obvious. We will leave the discussion of that aspect of shape selection to a subsequent blog article however.