Combining Different Types of Structure — Eruvin 93a-b
תלמוד בבלי מסכת עירובין דף צג/א-ב
אמר רב חסדא גידוד חמשה ומחיצה חמשה אין מצטרפין עד שיהא או כולו בגידוד או כולו במחיצה... דרש מרימר גידוד חמשה ומחיצה חמשה מצטרפין... והלכתא גידוד חמשה ומחיצה חמשה מצטרפין.
Why does R' Chisda hold that a mound that is five tefachim high cannot be combined with a wall (atop it) that is five tefachim high to comprise a ten-tefach mechitzah?
Teshuvos Chacham Tzvi (§5) explains that it is because the fundamental halachah that underlies the creation of a reshus ha'yachid on the top of a mound is gud aseik mechitztah — viz., the sides of the mound are seen as extending upwards to create the legal fiction of a walled area; while the fundamental halachah that underlies the creation of a reshus ha'yachid within walls is the enclosure created by the walls themselves
R' Yosef Engel (Gilyonei HaShas here) adds that it may be that R' Chisda is of the same opinion as R' Meir (above, 4b) that the halachah of true walls is derived from the text of the Torah (from the dimensions of the Aron); while the halachah of gud aseik originates in an Halachah l'Moshe me'Sinai. Accordingly, he holds that a halachah that is [Torah-]text based cannot be combined with a halachah that is Halachah l'Moshe me'Sinai based.
On the other hand, suggests R' Yosef Engel, the reason why the halachah is that the five tefachim of the mound may be combined with the five tefachim of the wall, is that the halachah is in accordance with R' Yehudah (also above 4b) — viz. that the halachah of true walls is also an Halachah l'Moshe me'Sinai. Hence, originating in the same fundamental halachah, the two structures may be combined to comprise a ten-tefach mechitzah (see Gilyonei HaShas for a consideration of the application of this principle to other areas).