Bava Batra 18 - Preventing Future Damage

When one observes certain distance between his potentially damaging objects and his neighbor's wall or pit, does he have to do it only when the wall or pit are already there, or is it true even before the wall is built or the pit is dug out? Abaye says, "Only when it's there." Rava says "Even before it's there."

Question against Rava from a previous ruling: Olive press refuse... must be distanced three hand-breadths from a neighbor's wall , which  implies that the wall must be there! Rava answers: "No, this is true even before the wall is there, and this ruling is still needed to teach us that these materials are harmful to a wall."

Art: Eastman Johnson - Five Boys on a Wall