Gemara

(Mishnah): One may not dig a pit close to his neighbor's pit, or an irrigation channel, or a laundry ditch, unless he distances three Tefachim (handbreadths) from his neighbor's pit, and plasters the wall of the pit;

One must distance Gefes (the refuse of olives), manure, salt, lime, and rocks three Tefachim from his neighbor's wall, and/or apply plaster;

One may not plant seeds, plow, or put urine within three Tefachim of his neighbor's wall.

19a (Mishnah): V'Sad (and he plasters the wall of the pit).

Question: Does this mean 'and he plasters', or 'or he plasters'?

Answer #1: Clearly, it must be 'and'. If not, the Reisha (one who digs a pit, irrigation channel... must distance v'Sad) would have been taught together with the Seifa (the next clause, one must distance Gefes, manure...)!

Rejection: Even though also in the Reisha, either suffices, the Tana teaches separately damage from water, and damage from heat.

Answer #2 (Beraisa - R. Yehudah): In land with brittle dirt, one may dig (a pit) in his own property, but must distance three Tefachim and plaster it.

Inference: He must distance and plaster only because the dirt is brittle!

Rejection: No. Even if the dirt is not brittle, he must distance and plaster. One might have thought that brittle dirt requires more than three Tefachim are required. The Beraisa teaches that this is not so.

Rishonim

The Rif brings our text of the Mishnah (the Seifa says or plasters).

Rambam (Hilchos Shechenim 9:1): One may not dig a pit, irrigation channel, or laundry ditch near his neighbor's wall, unless he distances three Tefachim and plasters the wall of his own pit or ditch or channel on the side near his neighbor, lest water be absorbed and damage his neighbor's wall.

Magid Mishneh: The Rambam requires both distancing and plastering. Meforshim say that even though the proof was rejected, since we (initially) said 'clearly, it must be 'and'', this is the Halachah.

Lechem Mishneh: According to Rashi and Tosfos, even though the Seifa does not explicitly say 'or', the Gemara knew that the damagers in the Seifa are not so potent to require both. The Rambam requires both for a pit, like the Magid Mishneh explained. There was never a question about Gefes. If there was, what was the attempted proof 'if not, the Reisha would have been taught together with the Seifa'? Perhaps for both of them, one must distance or plaster! We cannot say that the Gemara meant that since they were taught separately, surely just one requires both, but we are unsure which. Surely, we need more to stop the damage of a pit than of Gefes! The Beis Yosef says that the Rambam rules like the Rosh, that the Gemara asked about Gefes, and since it did not resolve the question, it suffices to do one of them. This is astounding. The question was only about a pit! The Gemara suggested that if the Mishnah means one of them, both clauses should have been taught together. If we ask about Gefes, this means that surely a pit needs only one of them! Also, the Beis Yosef says that Rashi holds that the question was about a pit, and later he says that it was about Gefes! Rather, all agree that Gefes needs only one, and the question was about a pit. The Rosh holds that the question was not resolved, and the Rambam holds that since the Gemara said 'clearly', we require both.

Rambam (2): One must distance the refuse of olives, manure, salt, lime, and rocks three Tefachim from his neighbor's wall, or apply plaster. One must distance a cavity in which urine gathers three Tefachim from a wall.

Rambam (10): If Ploni wants to dig a pit at the edge of his property near his neighbor's (Levi's) field, if it is normal to have pits in a field like Levi's, Ploni must distance his pit three Tefachim from the border.

Rosh (2:4): The Gemara did not resolve whether he must distance and plaster, or if one suffices. Therefore, if he did either, we cannot force him to do more.

Tosfos 17a (DH v'Sad): The text in both clauses must be v'Sad. If not, surely the Tana said v'Sad only in the Reisha, for only in the Reisha both distance and plaster are required! The Gemara knew that the damagers in the Seifa are not so potent to require both.

Poskim

Shulchan Aruch (CM 155:4): One must distance Gefes, manure, salt, lime, rocks or wet sand three Tefachim from his neighbor's wall, or apply plaster.

Beis Yosef (DH Kosav Rabeinu): The Gemara did not resolve the question. The Rosh says that therefore, if he did either, we cannot force him to do more. Also the Rambam says so, for in every monetary Safek we are lenient for the defendant.

Shach (4): The Mechaber requires also plaster for wet things, like the Rambam. If so, wet sand should require both! The Rambam and our Mishnah require both for an irrigation ditch (and surely, also for wet sand).

Shulchan Aruch (5): A cavity in which urine gathers must be distanced three Tefachim from another's wall.

Rema: One must also plaster it; see Sa'if 10.

SMA (21): If there is no cavity, and one merely pours urine on the ground, it suffices to distance three Tefachim.

Taz: The Rambam holds that when the Mishnah discusses urine, it refers to a cavity for urine. If so, it should have taught urine together with things for which it is feasible to plaster, but not with seeds and plowing!

Shulchan Aruch (10): One may not dig a pit or irrigation or laundry ditch near his neighbor's wall, unless he distances three Tefachim and plasters the wall of his own pit or ditch on the side near is neighbor, lest water be absorbed and damage his neighbor's wall.

SMA (28): The Rambam requires distancing and plastering even near the wall of another's house. The Rosh and Tur hold that it suffices to distance from a house wall.

Question (Shach 7): The Rosh and Tur never require both, even near the wall of another's pit!

Answer (Ketzos ha'Choshen 5): The Rosh and Tur hold that the question was resolved. If so, l'Chatchilah one must do both. B'Di'eved, if one did only one, Beis Din cannot force him to do more. Regarding a house wall, even l'Chatchilah one must do only one.

Beis Yosef (DH umi'Kol): Why did the Tur bring the Rambam's opinion, who requires distancing and plastering, but not the Rosh, who requires only one? Perhaps he holds that the Rosh relied on Rashi, that the question was only about Gefes, but surely both are required for a pit. However, the words of the Rosh connote that he discusses also a pit. This requires investigation.

Taz (Sa'if 18): The Tur did not say so here, for he relies on what he says later, which is the primary place for this law. However, the Rema should have said here that some disagree!

Shulchan Aruch (18): If Ploni wants to dig a pit at the edge of his property near his neighbor's (Levi's) field, if it is normal to have pits in a field like Levi's, Ploni must distance his pit three Tefachim from the border.

Bedek ha'Bayis: Why did the Rambam not mention plastering regarding one who digs a pit near the border? It seems that the need for both is only near a wall, lest it be ruined, but when there is no wall, plastering does not help. In any case, one must distance three Tefachim.

Rema: Some say that he must also plaster the wall of his pit, like we said in Sa'if 10. Some disagree.

Gra (26,51): The Rambam holds that it was a mere Dichuy to say that perhaps only one is needed for a pit. The Rosh holds that the question was not resolved, so l'Chatchilah it suffices to do one. Above, the Mechaber (Sa'if 10) and Rema (Sa'if 5) rule like the Rambam.