The situation for Case-I is depicted in Figure 1.2. The friction force per unit length of pipe in
the entire length of PGD zone (L) from point B to point D, acts to the right due to ground
displacement δl. By symmetry and equilibrium, the friction force per unit length acts to the left, over
a distance of L/2 before the head of the PGD zone (from point A to point B) and over a distance of
L/2 beyond the toe of the PGD zone (from point D to point E). In the pipe, the maximum tensile
strain occurs at point B and maximum compressive strain occurs at point D. As a result, the
maximum stress (tensile or compressive) in the pipe is the stress induced due to friction force over a
length of L/2. Hence, the maximum tensile/compressive stress in the pipe can be calculated as: