Sewer design requires prior knowledge of soil and site conditions to determine overburden loads that will be placed on buried pipes.

It depends upon following factors:

• Trench Width (B)
• Depth of Burial i.e, depth of fill above pipe (H)
• Unit Weight of Fill Material  (w)
• Frictional Characteristics of Backfill

Live Loads on the Surface rarely influence design of sanitary sewer because of their Great Depth and Small Size.

Backfill load on buried pipes can be calculated using Marston’s equation.

W = CwB2

• W = Load on the pipe per unit length, Kg/m
• w = Weight of the backfill material per unit volume, Kg/m3
• B = Width of trench, m

B = 1.5 D + 0.3 m (as minimum)

• D= Diameter of the sewer in m
• C = a coefficient depending upon depth of fill on top of the pipe and character of construction fill materials.
• μ = Coefficient of sliding friction
• k = Ratio of active lateral pressure to vertical pressure
• kμ = 0.1 to 0.16 for most soils (0.11 for saturated clay)
• H = Depth of fill above the top of the pipe, m
• B = Width of the trench, m

For structural stability, strength of sewer as determined by 3-Edge Bearing test should be greater than backfill load on the Sewer.