and this causes a frictional force of
, acting upward. The cohesive or shearing resistance on the area,
, acts upward, and its amount is
. Placing the sum of the vertical forces acting on the slice equal to zero.
In reality, an arch or dome of the earth should be considered in place of the horizontal stratum, but the result is the same, because the same vertical forces act in either case. Simplifying the above equation, and dividing by
