Construction crews have poured the first of 12 concrete caissons that will support the new Clarks Bridge on Ga. 284/Clarks Bridge Road at Lake Lanier. The first caisson was drilled Monday to a depth of 103.88 feet when it reached bedrock, and 108 cubic yards of concrete were poured into the 5«-foot diameter steel liner for vertical bridge support, said Harold Mull, Georgia Department of Transportation district construction engineer. Other work this week includes moving a barge into position so workers can begin drilling for the parallel caisson, Mull said.