General Buried Design
For the purpose of calculation, buried installations of PE pipe can be separated into four categories depending on the depth of cover, surface loading, ground water level, and pipe diameter, as follows:
- Standard Installation-Trench or Embankment. Maximum cover of 50 ft, with or without traffic, rail, or surcharge loading; minimum cover of at least one diameter or 18-in., whichever is greater. Earth pressure applied to the pipe is determined using the prism load (geo-static soil stress). This design category includes the AWWA Design Window conditions.
- Shallow Cover Vehicular Loading Installation. Minimum cover of at least 18-in., but less than one pipe diameter. Similar design procedures as Standard Installation, but also considers effect of wheel load.
- Deep Fill Installation. Applicable to depths exceeding 50 ft. Differs from Standard Installation design procedure equations by considering reduced soil pressure due to soil arching, as well as less conservative calculation of pipe deflection and buckling. (The soil pressure calculation may also be used for profile pipe in trenches less than 50 ft, due to arching effects activated at lower depths for the relatively flexible profile pipe.)
- Shallow Cover Flotation Effects. Applicable where insufficient cover to prevent flotation or avoid hydrostatic collapse due to unconstrained buckling.
Detailed design calculations and examples of these four categories are provided in Chapter 6 (Section 3) of the Plastic Pipe Institute Handbook of Polyethylene Pipe.