Hydraulic Design of Highway Culverts - DOT On-Line Publications

Tables, charts, and formulas are available from manufacturers to streamline the process of
structural design. The "Structural Design Manual for Improved Inlets and Culverts," published by
FHWA, is an excellent reference (26). Methods for hand calculation and computer solution are
presented for reinforced concrete box culverts, and for circular and elliptical pipe culverts. The
structural design of improved inlets is included, along with standard plans for headwalls,
wingwalls, side-tapered and slope-tapered culverts.
2. Floatation and Anchorage. Flotation is the term used to describe the failure of a culvert
due to the tremendous uplift forces caused by buoyancy. The buoyant force is produced when
the pressure outside the culvert is greater than the pressure in the barrel. This occurs in a
culvert in inlet control with a submerged upstream end. The phenomenon can also be caused
by debris blocking the culvert end or by damage to the inlet. The resulting uplift may cause the
outlet or inlet ends of the barrel to rise and bend. Occasionally, the uplift force is great enough
to dislodge the embankment. Generally, only flexible barrel materials are vulnerable to failure of
this type because of their light weight and lack of resistance to longitudinal bending (38). Large,
projecting or mitered corrugated metal culverts are the most susceptible (Figure VI-33). In
some instances, high entrance velocities will pull the unanchored inlet edges into the culvert
barrel, causing blockage and additional damage. Events have been recorded in which the
culvert barrel has been turned inside out by the forces of the flow.
Figure VI-33--Unanchored Mitered End
A number of precautions can be taken by the designer to guard against flotation and damages
due to high inlet velocities. Steep fill slopes which are protected against erosion by slope paving
help inlet and outlet stability (Figure VI-34). Large skews under shallow fills should be
avoided. Rigid pipe susceptible to separation at the joints can be protected with commercially
available tie bars.
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