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
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.