The holes were dug into the street for the foundations, and concrete poured in a large block. On top of this, a steel form has been put in place to hold the rebars and the poured cement that will form the column holding up the arch.
Looking into the hole, the size of the concrete block is apparent. It sits on bedrock. Notice the four holes in the block.
A boring machine drills through the four holes in the cement block, into the bedrock. The new holes go 28' down through the bedrock. Anchors will be installed to hold the block to the bedrock.
The anchors through the foundation into the bedrock will make the foundation earthquake (tremblor) proof, so the arch should really be eternal.