Why Does The Gasket Move?
There are a variety of reasons the rubber gasket relocates to the AS1. The most common reason is improper lubing of the joint. When the lubricant is not applied properly to the joint and/or gasket, when the joint is assembled it pulls the gasket into the clearance annular space, AS1, and bad things can happen. So, you ask, how do you correctly apply lubricant to the joint. I am glad you asked. See the below images to see how to do it correctly. Without going through all the procedures, there is one area that needs special care and that is the entrance slope of the bell.
The entrance slope is where the gasket hits the bell first. If this is not lubed well, and by well, I mean the entire circumference of the entrance slope. If this is not done properly, the gasket is going to hit dry concrete and immediately be pulled into the clearance annular space, AS1. If it is a manhole riser section, you are probably going to hear that awful cracking noise which means the bell has broken. If it is a pipe you may hear that cracking noise or you will see the joint push back (separate) after the equipment holding the joint together is removed.
Another prime reason joints do not assemble properly and can lead to bell breakage is not "equalizing" the gasket before assembling the joint.
What is equalizing? Imagine taking a rubber band and pinching one side and pulling it. Is the rubber band the same thickness on the opposite you pinched? No, it isn't, it is thinner. The same thing happens when you stretch a rubber gasket over a joint, the rubber becomes thinner on the side that is attached to the joint and the other side is being pulled away from it.
The rubber gasket is thinner on one side of the joint than the other. The rubber gasket volume must be "equalized" around the joint before joint assembly or you risk pulling the thicker part of the unequalized gasket into clearance annular space, AS1. Even if you are able to assemble the gasket you are also risking that the thinner part of the gasket may leak.