Buried Pipe Design
Specifying Pipe Stiffness
Fibreglass pipes are considered to be 'flexible' pipes. They are designed to deform or deflect
diametrically within specified limits without structural damage. The degree of side support provided by the backfill used is a major determinant of the pipe stiffness required.
Parameters for Pipe Stiffness Specification:
- Type of backfill and compaction control used
- Native soil type
- Trench depth, width and wall angle
- Presence and height of ground water
- Pipeline operating pressures (+ve or -ve)
- Applied loads, e.g. traffic.
The site geotechnical engineer and consultant will generally be responsible for assessing these parameters, and will specify the pipe stiffness required. Note: the 'stiffness' of a pipe wall is proportional to the cube of the wall thickness, hence the wall thickness is the major parameter in achieving a high stiffness rating.
Low Stiffness Pipe Options
Installation must be carried out with great care when using low stiffness pipe such as SN 1250 or SN 2500. It is critical to carefully select backfill material, and ensure compaction is done to strict standards. The site geotechnical engineer and consultant must specify and approve pipe stiffness, excavations, backfill material, and supervise backfill installation. Even then, with low stiffness pipes, long term problems with pipe deflection and eventual cracking can occur. Generally, backfill requirements lessen as the pipe stiffness increases.
A Typical Project
In many drainage projects, there is ground water at ground level, traffic loads, buried depths of 1 to 3 metres, normal trench widths, and poor native soils. Projects are often done in difficult weather and to strict timetables. Compromises occur, so Armatec usually recommends:
- Pipe to be a minimum stiffness of SN 5000
- Normal trench widths to enable compaction along the sides of the pipe - wider at butt and strap joint locations
- Pipe bedding with 100mm minimum of compacted gravel/scoria AP7/quarry fines
- Backfill with compacted gravel/scoria AP7/quarry fines, compacted in 300mm layers
- Ensure large or sharp objects are not in contact with FRP
- Backfill as above to at least 300mm above top of pipe
- Use native soil to backfill to ground level. Compact in 300mm layers
For deeper buried depths, or where the pipe has large applied loads, then custom design will be required.