PE piping is considered "flexible", indicating such products can withstand large amounts of deflection without damage. However, since their flexibility is directly proportional to the dimension ratio (outer diameter divided by minimum wall thickness), different installation requirements may be necessary for different DR values to achieve successful and economical installations. The depth of cover and anticipated surface loads affect the particular installation requirements. Therefore, the engineer has to make an assessment of the application and site conditions to determine the best and most economical installation design.
There are two objectives to achieve in an installation. The first is to provide an envelope of embedment to protect the pipe from mechanical damage from impact or hard objects (cobbles, boulders) in the soil. The second is to provide support against earth and live load pressures, where required. The earth and live loads are supported by the combination of the stiffness of the pipe and the stiffness of the surrounding embedment material. Lower DR pipes will carry more of the load and require less support from the soil in haunch areas. When support from the embedment is needed by the pipe to resist earth and live loads, the embedment material is often compacted. The trench backfill placed on top of the embedment material may also be compacted. Compaction of trench backfill immediately above the pipe facilitates the redistribution of some of the load away from the pipe and into the side-fill soil.
Generic Pipe Trench
The engineer must evaluate the site conditions, the subsurface conditions, and the application objectives to determine the extent of support the pipe may need from the surrounding soil. Where the pipe burial depth is relatively deep, where subsurface soil conditions are not supportive of pipe, where surface loads or live loads are present, or where the pipe DR is high, the engineer will generally want to prepare an engineered trench specification. On the other hand there are many applications that meet the criterion for using the AWWA Design Window.