You don’t lower the cross-beam – you raise the ground!
This page discusses how this happens. Specifically, the minimum width of the new ramp underneath the beam. Rabbi Joseph suggests on one handbreadth wide. Abaye suggests four. They differ on the principle. Is it wide enough to “make use” of it, or is it just wide enough to be a “distinguishing mark” ?
And the same question arises for the opposite problem: what if the cross-beam is too low (less than 10 handbreadths high)? Then you have to excavate a ditch under the beam! How wide? Rabbi Joseph says 4 handbreadths, Abaye says four cubits! (of 5 or 6 handbreadths – see previous page).
This and other issues – such as breaches in the side walls – are discussed at length. But how is a decision arrived at? They create uncertainty
in respect of a Rabbinical enactment, (as opposed to a Torah law) and in any uncertainty appertaining to a Rabbinical enactment the more lenient course is followed.