“Lateral factor of safety” means Horizontal Safety Factor at Theta.
The vertical factor of safety is computed but ignored. It is not significant if the pipe moves up and down slightly so long as it does not move horizontally as determined by the horizontal safety factor calculation.
Pipeline stability for Level 2 analysis requires that at the end of 4 hr storm buildup the horizontal safety factor at theta for U(1/100) must be 1 or greater and that at the end of the additional 3 hr storm the horizontal safety factor at theta for U(1/1000) must be 1 or greater.