Soil pressure is one of the external loads on buried pipes. The load from soil pressure depends on both the stiffness of the pipe and the surrounding soil. The calculation of soil pressure is statically indeterminate since the pressure of soil produces deflection which again determine the soil pressure. The calculation of soil pressure depends on whether one have a rigid pipe or a flexible pipe;

Rigid pipe: Assume the pipe is only affected by the vertical pressure (soil, traffic etc.)

Flexible pipe: The vertical load causes deflection which results in horizontal supporting soil pressure. If the horizontal and vertical pressure are close to equal, one can approximate the hydrostatic load, which leads to mainly circumferential (hoop) compressive stresses.