Any barrier, even underground, could be circumvented if given enough time and resources. The real protection is coupling a barrier with hyper-sensitive monitoring equipment that can detect tunnel construction. I suspect that the IDF has sensors that can detect an ant burrowing under the surface.
Gaza is dangerous. I was there in 2006 to do some work. While we were outside on the road the runs adjacent there the IDF had rolled up across our position inbetween us and the built up area. They did this due to military movement in Gaza directed towards us to head off any incident.
On the 23d of December in 2005, I got to go to Bethlehem. There were six of us escorted by 1.5 dozen rental-infantrymen... Between the Church of the Holy Serpecher (?) and the manger/Milk Grotto the Pali’s armed on rooftops shadowed our movements. There were anti-jew, anti-Israeli posters everywhere. Our team of security literally went into guns up and cover.
Personally I have no use for the militants running the West Bank and Gaza.
Maybe a 300 foot deep by half mile wide moat around all of the Gaza Strip on the Israeli side.