In the design analysis, the soil engineer usually assumes a total
unit weight gt of 120 pcf (18.9 kN/m3) and a friction angle f of 30° for the granular backfill. Using
Eq. 11.2 and f = 30°, the active earth pressure coefficient kA is 0.333. Multiplying 0.333 times the
total unit weight gt of backfill results in an equivalent fluid pressure of 40 pcf (6.3 kN/m3). This is a
common recommendation for equivalent fluid pressure from soil engineers. It is valid for the conditions
of clean granular backfill, a level ground surface behind the wall, a backdrain system, and no
surcharge loads. Note that this recommended value of equivalent fluid pressure of 40 pcf (6.3 kN/m3)
does not include a factor of safety and is the actual pressure that would be exerted on a smooth wall
when the friction angle f of the granular backfill equals 30°. backfill