There is actually a technical term for the caking of DE that your pictures show. It is called 'bridging".

I have said many times before in the forum that backwashing a DE filter is NOT a good idea and that they really should be torn down when they needs to be cleaned. Not a popular opinion in the forum, btw! Your pictures illustrate my point perfectly! The problem with backwashing is you never really know how much DE is washed out and so you never really know how much to put back in. If you put in too little you run the risk of clogging the grids with dirt and organics. If you put in too much you can cause bridging between the grids and the DE can cake into a cement like mess....sort of like in your pictures! The errors can compound with each backwash.

It really does't take THAT much time to break it down and clean it properly if it is done on a regular basis! Save backwashing for time that you are killing an algae bloom and are trying to get suff out of the pool and when you are done break it down, clean it, and recharge it with the proper amount of DE!

Just my 2 cents (for the umteenth time it seems! Maybe someone will take note of what I have been trying to say!)