Rif Eruvin 8b {Eruvin 34b continues; 32b; 35a}

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8b

{Eruvin 34b continues - Mishna}
it is a valid eruv.
If he placed it in a turret {=tower shaped closet}, closed it, and lost the key, it is an eruv.
Rabbi Eliezer says: If he does not know that the key is in its place, it is not an eruv.

{Eruvin 32b}
Gemara:
This tree, where does it stand?
If you say that it stands in the private domain, what does it matter higher or lower {than ten handbreadths}? A private domain extends until the heavens!
But rather, that it stands in the public domain, and he intends to establish Shabbat rest {shevut} there? How so? If he intends to rest {=shevut} above, only then are he and his eruv in a single place. But if he intends to rest below, then he will end up having to make use of the tree {which is impermissible Rabbinically}!
In truth, the case is one in the public domain, and he intends to rest below, and who is this? Rabbi, who states that any Rabbinic matter they did not decree at twilight {when he must establish his place of Shabbat rest}.

"Ih he placed it in a pit":
This pit, howso?
If you say that it is in the private domain, this is obvious {and the depth would not matter}, for just as a private domain extends and reaches up to the heavens, so too it reaches down below.
Rather, it is in the public domain, where does he intend to establish his Shabbat rest? If above {and not in the depths of the pit}, he is in one location and his eruv is in another location. If below, it is obvious, for he and his eruv are in one location! Rather, here we are dealing with a pit situated in a karmelit {which is Rabbinic}, and he intends to rest above, and it is Rabbi, who states that any Rabbinic matter they did not decree at twilight.

"If he placed it on the head of a reed or on the head of a pole":
If it is detached and stuck in {the ground}, yes, but that which is not stuck in and detached {but rather grew as such}, no, as a decree lest he break off the top.

{Eruvin 35a}
"If he placed it in a turret {=tower shaped closet}, closed it...":
Why {should it be a valid}? He is in one location and his eruv is in another location!
Abaye and Rava both say that we are dealing here with a key hanging from the lock by a cord and we need a knife to cut it. And Rabbi Eliezer holds like Rabbi Nechemia that even a spoon and even a talit may only be handled for their intended purpose - therefore it is forbidden; and the Sages maintain that all vessels may be handled, whether for their intended purpose or not for their intended purpose.