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<br />As the trench width increases, the effect of the friction at the sides of the <br />trench is reduced and dead load on the pipe is increased. The amount that the <br />loading is increased depends on trench width and the amount of backfill <br />settlement, which is related to compaction. Poorly compacted soil will settle <br />more than well compacted soil. In a trench that is too wide, poor compaction <br />can result in an increase in the dead load on the pipe. Pipes placed in a <br />shallow bedding on top of the original ground surface and then covered by the <br />embankment material will have loads similar to the very wide trench. Pipes <br />placed in trenches in the original ground prior to being covered by embankment <br />have reduced earth loads similar to those described for the narrow trench <br />installations. <br /> <br />3-3.5 Construction and Installation Requirements. <br /> <br />The structural behavior of flexible and rigid culverts is often dependent on <br />construction practices during installation. Items which require particular <br />attention during construction are discussed briefly in the following text. <br /> <br />a. Compaction and Side Support--Good backfill material and adequate <br />compaction are of critical importance to flexible culverts. A well-compacted <br />soil envelope is needed to develop the lateral pressures required to maintain <br />the shape of flexible culverts. Well-compacted backfill is also important to <br />the performance of rigid culverts. Poorly compacted soils do not provide the <br />intended lateral support. <br /> <br />b. Trench Width--Trench width can significantly affect the earth loads on <br />rigid culverts. It is therefore important that trench widths be specified on <br />the plans and that the specified width not be exceeded without authorization <br />from the design engineer. <br /> <br />c. Foundations and Bedding--A foundation capable of providing uniform and <br />stable support is important for both flexible and rigid culverts. The <br />foundation must be able to support the structure at the proposed grade and <br />elevation without concentration of foundation pressures. Foundations should <br />be relatively yielding when compared to side fill. Establishing a suitable <br />foundation requires removal and replacement of any hard spots or soft spots. <br />Bedding is needed to level out any irregularities in the foundation and to <br />insure uniform support. When using flexible culverts, bedding should be <br />shaped to a sufficient width to permit compaction of the remainder of the <br />backfill, and enough loose material should be placed on top of the bedding to <br />fill the corrugations. When using rigid culverts, the bedding should conform <br />to the bedding conditions specified in the plans and should be shaped to allow <br />compaction and to provide clearance for the bell ends on bell and spigot type <br />rigid pipes. Adequate support is critical in rigid pipe installations, or <br />shear stress may become a problem. <br /> <br />d. Construction Loads--Cu1verts are generally designed for the loads they <br />must carry after construction is completed. Construction loads may exceed <br />design loads. These heavy loads can cause damage if construction equipment <br />crosses over the culvert installation before adequate fill has been <br /> <br />36 <br />