Like you said, the sand would be packed so 2.0-2.3 is a much better number. Also, the sinkhole is much deeper than 30m. National geographic has it at 30m x 100m.
I've got 148.5 million kg of sand to fill this. Damn.
They need to fill it, too. Otherwise the ground will begin to subside and engulf nearby buildings . Wonder what kind of madman would build something over top of a 100m column of manmade packed sand though.