Manholes may be acted upon by a variety of forces which may change with time due to the surrounding environment and any physical changes which may be acted upon it. Due to being installed below ground level, two of the primary forces acting on manholes are uplift forces due to ground water and downward forces due to the weight of the manhole inclusive of upper and lower plates and the frictional forces from both backfill and surrounding soil conditions.
Safe installation of the manhole requires a resultant downward force to balance and retain the manhole at equilibrium. This is represented by a safety factor, which is derived from the ratio of the total force acting downwards when compared to the uplift forces.
The table illustrates the safety factor for manholes of different heights and depth of immersion in ground water.
