The basic analysis developed by M.G. Spangler includes frictional forces between the trench wall and the backfill. This permits the weight of the overburden to be partially carried by the surrounding soil and reduces the total soil load on the pipe. The equations require the following information to determine internal friction.
- Compaction/Settlement of the soil
- Type of Soil and density
- Years pipeline has been in service
- Depth of Cover (DOC)
- Moisture Content
- Uniform Support Underneath Pipeline i.e., Bedding
- Overbends and Sags
- Horizontal Bends – Point of Intersections (PIs)
Note 1: There are rules of thumb that most US pipeline operators use for determining trench width with no saturated soil:
- Newly installed pipe less than 5 years – original trench width at bottom
- Compacted backfill after 5 years – diameter plus 100 mm (4 inches)
- Bored pipelines – diameter plus 50 mm (2 inches)
Note 2: Technical Toolboxes is not responsible for Note 1 and is only presenting this information of what many pipeline operators do in the US.