Engineered Trench Specification
For applications where the simplified installation guidelines do not apply -- such as gravity flow pipes that are relatively deep, shallow cover applications where the pipe is subject to vehicular or train loading, pipes placed in unstable, soft, or wet soils, high DR pipes, or pipes in deep applications such as landfills and embankments -- the engineer must prepare a specific embedment specification. In these cases, detailed attention must be paid to the native (insitu) soil, the embedment soil, and the placement of the embedment soil. The key objective of installation is to minimize or control subsequent pipe deflection. A limit of 5% is commonly used, which provides an additional safety factor, as most gravity flow PE pipe can withstand higher deflection without damage.
The load carrying capability of a PE pipe, particularly a pipe with a high DR, can be greatly increased by the soil in which it is embedded, including the installation procedure. When the pipe is loaded, load is transferred from the pipe to the soil by a horizontal outward movement of the pipe wall. This enhances contact between pipe and soil and mobilizes the passive resistance of the soil. This resistance aids in preventing further pipe deformation and contributes to the support for the vertical loads.
The factors to be considered in a properly engineered installation specification include pipe embedment materials, compaction of embedment materials, trench dimensions, and trench design and construction. These four items are discussed separately, with greater detail provided in Chapter 7 of the Plastic Pipe Institute (PPI) Handbook of Polyethylene Pipe, which is available at www.plasticpipe.org/pdf/chapter07.pdf. In addition, the burial of PE pipe for gravity flow applications is covered by ASTM International D 2321, Standard Practice for Underground Installation of Thermoplastic Pipe for Sewer and Other Gravity-Flow Applications. ASTM International D 2774, Standard Practice for Underground Installation of Thermoplastic Pressure Piping, covers water pipe and force mains.
Mobilization of Enveloping Soil through Pipe Deformation