[buffaloguy]

29 million to fill tunnels would most likely involve pumped concrete. Possibly from the mainland.

[EMI_Guy]

I was thinking a suction dredge just offshore. Pump them full of sand and whatever size rocks will pass through the impellers.

[Axenolith]

With those kinds of volume and a need to merely fill ( as opposed to construct) tunnels you’d just put a portable batch plant there and mix native soil/sand into a .5-1 sack slurry and pump them full...