Red - as myself and others have pointed out there are many unknowns you haven't specified. If your question is how to analyse for a horizontal load being applied, which is irrelevent what causes it, then this is basic structural anaysis. and you can get this from a suitable text book. Same if you are looking at retaining wall behaviour. its just analysis of equilibrium and stresses etc like any other problem.
Without more context its impossible to give you any more info. but a line load will have the same effect any any othe, but its point of application is critical and therefore resulting forces depend on the position. Like you assume a triangle pressure diagram for earth and the force is taken as 1/3 up.
of course you cannot guarentee where the force will strike maybe in vehicle impact. which is why relevent standards related to highway structures state forces and point of application to provide a effective 'model' of behaviour. Again as for my first reply. this may or may not be relevent in your case. but i doubt a 'worked example' outside of this that prefectly reflects your condtion. but we dont know because you havent given enough info.