Love this explanation! The forerunner builder rate had their own architecture seen in the bungie games, the 343 games architecture is seen as the warrior servant's architecture. This ring being much older could have been just much older builder style, or even maybe lifeworker or miner focused?
We know that the Ark mines other planets for minerals to create Halos. If these hexagonal pillars elevate the terrain under a mountain, they'd need a lot fewer raw minerals from other planets. Or they could just be placeholders keeping things connected or from floating off until they mine enough materials, with the hexes eventually being replaced.This Halo wasn't made by the Ark, at least not the Ark we know. It was originally created as part of an older array of 12 Halo rings, which were larger but less powerful than the current array. These rings were built at the "Greater" Ark, a structure created by the Builder caste of Forerunners to use as a way to enforce peace. When the AI Mendicant Bias went rampant and defected to the Flood, it hijacked these rings and held the galaxy at gunpoint while the Flood built up strength, using Installation 07 (the ring Infinite takes place on) as a base of operations. The Flood then destroyed the Greater Ark.
With this Halo being nowhere near the Ark's forge, they might not have the materials required to properly fix any damage. The sentinels could be using materials from another part of the ring (a large mountain) to remake the broken areas. Hex walls being boundaries of broken areas until new materials arrive.
When Mendicant Bias was eventually defeated and imprisoned, all but 1 of the 12 rings were destroyed. A new "Lesser" Ark, the one we know so well, was constructed to both house various species for re-seeding the galaxy and to create 6 new, updated Halo rings that were smaller, but more powerful. Installation 07 then underwent a refit to bring its power and size more in line with the other Halos, though retaining its old superstructure.
I don't think they have been explained yet. I'm assuming it's the foundation of the ring and the reason we're seeing them exposed is because the ring was blown up
These ideas seem on the right track, but here's three thoughts on the hexagons.
(1) Foundations for buildings can be made using friction and/or bearing piles: (a) friction piles disperse the weight and pressure of the building above sideways into the soil; (b) bearing piles conduct all the weight into the bedrock below, but combination "friction and bearing piles" carry some weight sideways into the soil and the rest straight down into the bedrock below. On a circular ring like Halo orbiting a planet, hexagonal friction and bearing piles are probably the best way to hold the foundations of the enviroment together and keep them roughly circular and bound to the Ring.
See the friction and bearing piles for the foundation of the Lakhta Tower:
Meanwhile, Cofferdams are constructed using interlocking hexagonal sheets to hold back soil or water when building, as seen around many a construction site - so seeing hexagons often indicate "under construction or repair":
(2) Or like Cortana said in Halo: CE "[These hexagons] are not a natural formation, someone built it, so it must lead somewhere...".
(3) Or since Halo Infinite is the "spiritual reboot" of a "classic game", maybe 343's hexagons in Halo Infinite were inspired by that most hexagonal of all reboots - creating "Chief-bert"?
Take your pick, but one of those three is surely the right answer. ;-)
Personally I'm betting on the boring one about gravitational astrophysics dictating hexagons as the optimal Friction And Bearing Piles for the surface of a ring structure that will orbit a planet. The ring was then hit by an explosion which shook the loose ends of the ring up and down, overcoming the friction of the sides of the hexagons rubbing against each other to hold them in place, so causing them to ride up out of the ground. If so, it's unlikely these hexagons will be visible away from the damaged part of the ring.
Meanwhile, on a Narrative Design note about 343i's use of artistic symbolism to parallel their story, then see this clue in Halo 5: the ice and snow cave in to hint that "There's a much deeper Forerunner story hidden here", because as she should, Exuberant Witness serves her installation before the Holder of the Mantle and "Cortana" is suddenly powerless before her - as to a fanfare we see the scene I call "Behold! An Archeon-class Ancilla is revealed!"
So too in Halo Infinite, the hexagons might hint that "the hidden underlying structure" of the Halo 4 & 5 story arc will finally be laid bare in Halo Infinite...
Hope this helps, and/or amuses... :-D