Reshus HaYachid Mounds — Eruvin 78a
תוספות עירובין דף עח/א
ד"ה אלא מאי אית לך למימר דמנח עליה מידי ומשתמש:
ולא קאי האי טעמא אלא היכא דאין הבור רחב ד' מבפנים אלא ע"י צירוף חוליא והחוליא גבוה עשרה מבחוץ לרה"ר דאע"ג דתוכו לאו רה"י הוא על גביו רה"י משום דאי בעי מנח עליה מידי ומשתמש
The Tosafos state that the Gemara only needs to resort to the possibility of placing something across the mouth of a hollow mound in a case in which the inner dimensions of the mound do not suffice to constitute a reshus ha'yachid (viz. the areas if the mound is less than 4x4 tefachim) — while the outside dimensions are sufficient. Since there exists the possibility of placing something, such as a plank, across the mouth of a mound, the top of the mound — which is surrounded by ten-tefach walls and is itself 4x4 — is a reshus ha'yachid.
R' Chaim Soloveitchik of Brisk (Hil. Shabbos 14:1) finds this approach difficult: If the top of this mound is a reshus ha'yachid, why is the inside of the mound not also considered a reshus ha'yachid on the basis of the principle of חורי רה"י כרה"י — the holes [extending] from a private domain are like a private domain (see Shabbos 7b)?
In order to answer this question, he distinguishes between two types of walls: Walls that create a reshus ha'yachid within their confines, vs. walls that create a reshus ha'yachid above their confines (through the principle of gud asek mechitzta [literally: extend the walls up]). When walls encompass an area that is itself of sufficient dimensions to be a reshus ha'yachid, then the holes coming off that area s are also considered part of that reshus ha'yachid (even though in themselves the holes do not posses those dimensions). However, where the walls only create a reshus ha'yachid when their thickness is included — and therefore only create that reshus across their top (by means of the virtual plank) — the holes in the walls are separate from the area in which not a part of a reshus hayachid and therefore are not subject to the principle that the holes [extending] from a private domain are like a private domain.