Don't think they'll expand any much larger than the current extent at this stage. Probably just waiting for the sediment to sink and stabilize before building heavy structure on it. The dark blue waters around the reef is actually quite deep from a map I saw - to the orders of hundreds of meters and at some locations thousands of meters deep, not even that far from the reef itself.
No need for any specific military installations!
Think diplomacy!
Now it's just a civil installation; any hostile military action would stir (justified) international outrage!
On the other hand the size of the runway and the corresponding installations means that in the event of any threath, China is able to deploy troops and material in sizeable quantities within a really VERY short notice!
A steel hand within a silk glove.