Packers are normally introduced in pipe joints in pipe jacking. Why should packers be kept 20mm back from the edge of concrete?

Joint stress is induced in pipe joints during pipe jacking. Packers are normally installed in pipe joints to avoid localized stressing of joints leading to concrete crushing. In essence, packers should be elastic enough to take the reloading jacking force. Moreover, it should be thick enough to take the compression of maximum joint stress. Theoretically speaking, packers should be provided in all of the joint area except 20 mm back from the edge of concrete. The reason of such provision is to reduce the risk of local spalling of side edges.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.