Yes to be truthful I don't know how all this phase stuff works or how to interpret it. I know because of my background in the building trade and groundwork I have done, that you can only go so far before you have to compact everything so that what you have so far is stable. Once done off you go again. Now whether that is what is regarded as a 'phase' I don't know but maybe Fachtna could put us straight here!
When you compact a base for a building you 'blind' it with sand or grit or at least something that fills the gaps left by larger materials and it forms a layer. Anything over that is considered another layer but still part of the same structure so not a separate build but yes another phase.

Something else we don't seem to hear about anymore is the 'wedding cake' or drum formation that Prof Atkinson mentioned and the series of radial walls which divided each drum into compartments that were then filled with compacted chalk. Those walls were supposed to have been built with chalk blocks weren't they! Maybe JL mentions this in his book.