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<br />p <br /> <br />SUMMING THE VERTICAL FORCES <br />ON HALF OF THE PIPE AT A TIME <br />SHOWS THAT <br /> <br />C" P. ~ <br />2 <br /> <br /> <br />WHERE <br /> <br />c <br />c <br /> <br />c <br />c <br /> <br />C= COMPRESSIVE THRUST IN <br />THE CULVERT WALL <br /> <br />P= SUM OF SOIL PRESSURE <br />ACTING ON THE CULVERT <br /> <br />S" THE SPAN OR DIAMETER <br /> <br />~= THE RADIUS (R) <br /> <br /> <br />s~ <br /> <br />Exhibit 22. <br /> <br />Formula for ring compression. <br /> <br />An arc of a flexible round pipe, or other shape will be stable as long as <br />adequate soil pressures are achieved, and as long as the soil pressure is <br />resisted by the compressive force C on each end of the arc. Good quality <br />backfill material and proper installation are critical in obtaining a stable <br />soil envelope around a flexible culvert. <br /> <br />In long span culverts the radius (R) is usually large. To prevent excessive <br />deflection due to dead and/or live loads, longitudinal or circumferential <br />stiffeners are sometimes added. The circumferential stiffeners are usually <br />metal ribs bolted to the outside of the culvert. Longitudinal stiffeners may <br />be metal 0,' reinforced concrete, as shown in exhibit 23. The thrust beams are <br />added to the structure prior to backfill. Concrete thrust beams provide some <br />circumferential stiffening as well as longitudinal stiffening. They also <br />provide a solid vertical surface for soil pressures to act on and a surface <br />which is easier to backfill against. The use of concrete stress relieving <br />slabs is another method used to achieve longer spans or reduce minimum cover. <br />A stress relieving slab is cast over the top of the backfill above the <br />structure to distribute live loads to the adjacent soil. <br /> <br />33 <br />