the displacement is
maximized when the horizontal and vertical components are 180°
out of phase, i.e., when the vertical inertial force acts downward
~hence, with a downslope tangential component! while the horizontal
inertial force acts in the downslope direction ~also with a
downslope tangential component!. Because the true tangential
procedure does not account for the increased frictional resistance
associated with the downward acting vertical force, it produces no
‘‘penalty’’ on displacement. In the true two-dimensional approach,
the downward acting vertical force will produce a signifi-
cantly increased resistance that will greatly reduce the computed
displacement. For the true two-dimensional case, displacements
are maximized when the vertical inertial force acts upward at the
same time the horizontal inertial force acts in the downslope direction.
In that case, the vertical component will reduce the driving
and resisting forces, but displacements will be maximized
because it reduces the resisting force much more than it reduces
the driving force. This behavior is not observed when the interface
has purely cohesive resistance because changes in normal
force do not influence the resisting force.