Sukkah 7B

סיכך על גבי מבוי שיש לו לחי ⁠— כשרה.

If one placed roofing over an alleyway that has a side post, it is fit for use as a sukka.

ואמר רבא: סיכך על גבי פסי ביראות ⁠— כשרה.

And similarly, Rava said: If one placed roofing over upright boards surrounding wells, it is fit for use as a sukka. A well is usually at least 4 handbreadths wide and 10 handbreadths deep. Therefore, it is considered a private domain, and it is prohibited to draw water from it on Shabbat, as that would constitute a violation of the prohibition to carry from a private domain into a public one. In order to permit drawing water from the well, the surrounding area must be partitioned off and rendered a private domain. For the benefit of Festival pilgrims, the rabbis instituted a special leniency that full-fledged partitions need not be constructed around the well for this purpose. Rather, it is sufficient if there are 4 double posts at the 4 corners of the area surrounding the well. Since these symbolic barriers are considered partitions for the halakhot of Shabbat, they are considered partitions for the halakhot of sukka on Shabbat as well.

וצריכא. דאי אשמעינן מבוי, משום דאיכא שתי דפנות מעלייתא. אבל גבי פסי ביראות, דליכא שתי דפנות מעלייתא ⁠— אימא לא.

The Talmud notes: And it is necessary for Rava to state the halakha in each of the two similar cases, as if he had taught us only that the sukka is fit in the case of the alleyway, one could say that it is due to the fact that there are two full-fledged walls; however, in the case of upright boards surrounding wells, where there are not two full-fledged walls and most of the area is breached, say no, it is not considered a fit sukka.