Assessing the impact of fill material in box culvert design. A case study: single cell box
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Culverts are small structures under cross draining structures that are an important part of infrastructure. Culverts are generally constructed as crossing channels across streams, below highways, railways, roads and flyovers basically to provide natural drainage across these structures. However, they serve a bigger purpose of balancing the flood water on both sides of the roads there by reducing the flood head and in turn decreasing the flood menace. Since culverts pass majorly through earthen embankments, many times they are subjected to traffic loads just as the one’s roads carry and therefor are required to be designed for loads in practice. Culverts have often been preferred to bridges because they can be used in soils with low bearing capacities, and are more economical than bridges where the discharges are in the openings of 18 sq. meters; however, this fact depends on the number of cells which are generally to be used where the road way crosses a high embankment. In this project, we focused on studying the design of a case study, single box culvert of span 3m and height 2m. We modelled this case study and analyzed its 3D model in Midas Civil software for different load combinations both with and without cushion/ fill and obtained the shear and axial forces, and the bending moments induced within the structure by the loads. Our load considerations included; Dead loads (Self weight, Earth pressure, Surcharge) and variable loads (Vehicle loads and cushion loads of varying depths starting at 1m to 10m). By varying the cushion depths, we ascertained the point at which the effects of the vehicular loads become insignificant to the structure. From the observations made from the analysis of results, we concluded that at fill heights in the range of 2.2m, the cushion weight starts to become more significant as compared to the vehicular loads and that the vehicle loads can be neglected when they contribute about 20% of the entire loads on the box. Therefore, we belief that for locations where the box is covered in cushion of 6m and above, the design can be done based on the cushion load and vehicular loads ignored.