[blue goats]

Yes I am familiar with placing a 6-8" drainage layer of sand. It is placed when clay ,springs, or bedrock is encountered during road excavation . In my line of work we do this under new roadboxes below the GABC (crush & run) and extend it to the new ditch invert(below topsoil), or make sure it is above the new u-drain if it's a curbed section.You can do this in a yard to help drain it but it won't lower the water table unless he runs the sand bed somewhere.Regarding the u-pipe, it will never clog if the stone is wrapped with filter fabric not the pipe. Now about the yard basin deal,If the storm sewer is 5' below grade and you have a drainage inlet (catch basin) within close proximatey I would recomend placing your yard box pipe to the road basin high enough that most storm events would not affect your drainage. I'm thinking that even if this is a 42" pipe in the road that there still is almost 2' of drop into that catch basin. Tying your yard pipe into another pipe is not the preferred way to do it . That existing drainage system may have been overdesigned when it was placed originally but probably not now . How did you fare with all the rain we got yesterday?
Hope this helps and doesn't confuse you,
George