A flexible structure should be checked to ensure that the cross-sectional shape it was designed for is intact. If the flexible culvert, whether it is a round pipe, a pipe arch, an arch, a horizontal ellipse, or any other structural shape, deflects from its design shape, it is not receiving the required support from the backfill. It is assumed in the design of flexible structures that moment in the structure is negligible and that due to the thrust forces, the structure is in compression throughout. If the deflection is large enough to cause a flattening of the structure, these assumptions will not hold true and the structure may collapse. Larger structures with large top radii, such as long-span structures, can withstand a smaller percentage of deflection before reverse curvature occurs than can round structures.
Visual observations of the culvert shape may reveal only large distortions and deflections; deformations may not be readily apparent until they reach approximately 10 percent. For this reason, if excessive deflections in the cross-sectional shape are suspected, physical measurements should be taken and documented with changes over time. Reference points should be permanently marked, and for a corrugated structure, measurements should be taken to inside corrugations for consistency. General deflections of round pipe greater than 5 percent should be investigated and monitored; reversal of curvature is expected at 20 percent for a metal culvert, but it may occur at a lesser value for a large structure. Localized flat spots or reversals of curvature are matters of special concern. It is necessary to determine, over time, if the structure deflection has stabilized. This information is critical in determining how serious the deflection is, if rehabilitation is necessary, and in determining if the rehabilitiation method needs to offer additional structural support to the culvert. A computer program is available to aid in the investigation and evaluation of multiple-radius metal structures. (See D. C. Cowherd et al., “Application of the Program MULTSPAN/SOILEVAL to Analyze Problem Structures,” Proceedings of the Second Conference on Structural Performance of Pipes, Ohio University, Athens, Ohio, 1993, A. A. Balkema, Rotterdam, 1993.)
All metal culverts should be investigated for evidence of corrosion and erosion. With a general loss of section there will be an accompanying loss of structural capacity. Wear will first be noted by a loss of the galvanized or other coating. If this occurs, then the unprotected metal may be expected to deteriorate more rapidly because of the erosive effects of the bedload. Corrugated metal pipe should be checked to ensure joint integrity (see Art. 5.11.4).
Bolted longitudinal seams of structural-plate culverts should be inspected for cocking, cracking, and bolt tipping. Cocking occurs where the structure deflects inward at the seam, causing a significant change in the structure’s shape or appearance. This may be caused by improper erection or fabrication of the plates and can result in loss of backfill due to piping and a reduced allowable compression strength of the structure due to the distortion. Cracking may occur where there is excessive deflection at the seam. This could ultimately lead to a disjointing, which would result in loss of ring thrust.
Bolt tipping is rare; it occurs where the plates slip because of high compressive forces. However, if the structure is under high fill and the plates slip, the bolt holes could become elongated, with the result that the bolt is eventually pulled through the plate.
Plastic pipe should be inspected for excessive deflection, joint integrity (see Art. 5.11.4), and cracking.
