The last time I faced this issue I bonded to the big steel above ground.
It was no hassle, and a cheaper procedure.
Because of water bonding and gas bonding, above ground bonding was in the plans.
Jumping over a couple of feet to clamp the big steel was no effort.
I'm scratching my head trying to figure out why you'd be so interested in having this bond done underground. From my experience, it would almost always be more expensive. Only exothermic or listed trick connectors would meet Code. By comparison it's quick and cheap to 'tag' the big iron above ground - usually in the electrical room -- or right next door.
Explain it to me.
