If you can schedule your trip to be after the 15th, that might be better for you. Visiting is a very important part of the grad school decision process, in my opinion. However, I think you said before that you are from France, and usually schools do not fly international students out for visits (unless you can combine many schools in one trip so that each school's share is about the same as they would have paid for a domestic student), so you might not have to worry about that. In addition, some schools might want to do skype interviews with you.
If it's really important that the trip is in March, then if you note this in your application, they should take it into consideration. Depending on the school (and how many applicants they get), they might be able to make special arrangements for you -- for example, interview you early anyways whether or not you would have been offered one. You should probably arrange for someone else to check your email and let schools know that you are away from the internet. If you could call someone at home every few days during your hike and check in and relay instructions, that could work too.
Although it's unusual, I don't think this is an impossibility. The school should not treat people without access to Internet (even though it's by choice) differently and if other disciplines where students are away for extended periods of times on field work can figure it out, then this should be doable. However, what "should" be the case probably won't actually be so -- leaving during the busy Mar-Apr season may get your application "lost between the cracks" (either accidentally or on purpose) at big schools.