Karpaf part 2 – Eruvin 24a
(excerpted from The Contemporary Eruv)
The Dvar Shmuel (cited in the Biur Halacha 358:9, d.h. Aval Im Nizra) allows a karpaf consisting of planted fields within an eruv when the significance of the inhabited sector of the enclosure outweighs the significance of the uninhabited, planted sector. Most Poskim conclude, however, that the Dvar Shmuel’s leniency only applies to enclosures consisting of real walls. This ruling is, therefore, generally not relevant in modern urban settings. The Chacham Tzvi (siman 59, also cited there by the Biur Halacha) extended the Dvar Shmuel's leniency even to cases where a tzuras ha'pesach preceded the development of a karpaf (such as a planted field) within its perimeter. In the specific case discussed by the Chacham Tzvi, however, other mitigating factors were involved. See also Nesivos Shabbos and note 50.
The Divrei Malkiel (Vol. 4 siman 3) and others rule that if an eruv was built around an area that contained a pre-existing karpaf, then the “hekef l'dira” (the act of enclosure for the purpose of enhancing human habitation) of the eruv enclosure counteracts the eino mukaf l'dira of the karpaf. Such an eruv is therefore valid. The case in question in that teshuva, however, concerned an area that was technically suitable for walking, however the gentile owner would not allow anyone to actually do so.
Furthermore, the Divrei Malkiel employs several additional reasons in validating the eruv that included this karpaf. It is therefore difficult to isolate one of his reasons and extrapolate a universal leniency based solely on that one reason.[ 3In Hilchos Eruvin , note 168, Rabbi Lange notes that the Biur Halacha, ibid., d.h. HaZera'im Mevatlim HaDira would apparently disagree with the Divrei Malkiel. Rabbi Lange therefore says that one may rely on the Divrei Malkiel's heter only “b'dochak gadol.”] The best solution is to exclude a questionable area from the eruv. Constructing a tzuras ha'pesach around the karpaf itself may accomplish this.