1: in the gemorah 14: higbiah lechi mekarca 3 since giduim bokim it`s not a mechitze the question is how can we hold koireh meshum mechitzeh if everybody walks undernith it (including giduiem)?
2: that roveh who says geduim bockim says himself 15. lechi meshum hekeireh?
Excellent questions! Let us try to address them:
(a) Korah, if it works because of Mechitzah, works because of the principle of "Pi Tikrah Yored v'Sosem" (see Rabeinu Yehonasan, 2a). When the Mechitzah is being made through "Pi Tikrah Yored v'Sosem," there is no problem of Beki'as Gedi'im (as far as why there is no problem of Beki'as Gedi'im, see Insights to 25a).
(b) Rava invalidates a Lechi that was placed more than three Tefachim away from the wall of the Mavoy, or is raised three Tefachim above the ground, because of Beki'as Gedi'im. You are asking that we find that Rava himself maintains that Lechi works because of Heker, and if so, why should Beki'as Gedi'im be a problem?
The TORAS CHAIM asks this question. He answers that when the Gemara says that Rava invalidates a Lechi when there is a problem of Beki'as Gedi'im, it should actually say " Rabah ," because it is Rabah who maintains that Lechi works because of Mechitzah (12b). This is indeed the Girsa of the Magid Mishnah (Hilchos Shabbos 17:25). (The Dikdukei Soferim has the Girsa "Rav" instead of Rava; Rav is also of the opinion that Lechi works because of Mechitzah, Daf 15a.)
(According to the opinion cited in the ROSH 1:10 that it is Rava on 12b who maintans that Lechi works because of Mechitzah, the question does not begin.)
I hope this was helpful. (By the way, what is your full name and where are you writing from?)