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<br />INLET FAILURES <br /> <br />Most of the inlet failures reported have occurred on large <br />flexible-type pipe culverts with projected or mitered <br />entrances without headwalls or other entrance protection. <br />The mitered or skewed ends of corrugated metal pipes, cut to <br />conform with the embankment slopes, offer little resistance <br />to bending or buckling. When soils adjacent to the inlet <br />are eroded or become saturated, pipe inlets can be subjected <br />to buoyant forces. Lodged drift and constricted flow <br />conditions at culvert entrances cause pressures which, while <br />difficult to predict, have significant effect on the stability <br />of culvert entrances. <br /> <br />To aid in preventing inlet failures of this type, protective <br />features generally should include full or partial concrete <br />headwalls and/or slope paving (Figures II-A-I and II-A-2). <br />Riprap can serve as protection in some instances, but con- <br />crete inlet structures anchored to the pipe are safer. <br />Preformed concrete or metal end sections may be used in <br />lieu of the inlet structures shown. Metal end sections for <br />culvert pipes larger than 54 inches in height must be anchored <br />to increase their resistance to failure. The figures also <br />show inlet designs which should be used if such protection <br />is considered necessary for pipes smaller than 4 feet in <br />height. <br /> <br />Failures of inlets are of primary concern, but other types <br />of failures have occurred. Seepage of water along the <br />culvert barrel has caused piping or the washing out of <br />supporting material. Hydrostatic pressure from seepage <br />water or from flow under the culvert barrel has buckled <br />the bottoms of large corrugated metal pipe arches. Good <br />compaction of backfill material is essential to reduce the <br />possibility of these types of failures. Also, where soils <br />are quite erosive, special impervious bedding and backfill <br />material should be placed for a short distance at the <br />entrance, and further protection may be provided by cutoff <br />collars placed at intervals along the culvert barrel or <br />by a special subdrainage system. <br /> <br />II-B - EROSION HAZARDS AT CULVERT OUTLETS <br /> <br />Erosion at culvert outlets is a common condition. Determination <br />. of the flow condition, scour potential, and channel erOdibility, <br />should be standard procedure in the design of all highway <br />culverts. <br /> <br />11-3 <br />