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
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