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a working platform for the cranes to install the piers and erect the bridge. The Bridge. # I Barge Alternative is the Preferred Alternative since it would have fewer potential impacts to the aquatic environment than the Bridge #2 Trestle Alternative. Under the Bridge #1 Barge Alternative, shallow construction barges would be used to float in the piers, which would be driven by a barge crane with a pile driving rig. The concrete piers and four bridge pieces would be transported to the site via "flexi-float" construction barges, which are capable of navigating very shallow depths. The flexi-float barges would bring the piers and bridge pieces into the channel during high tide and at low tide the barges would sit on the mudflat bottom of the channel. The bridge piers would be driven into place with a barge crane and the four bridge truss segments would be lifted into place from the water using the same crane. According to the engineering report for the bridge (Creegan + D'Angelo, 2004), the following equipment and construction methods would be used for the Bridge #1 Barge Alternative. For the piers, flexible floats would be floated and towed to the north pier site and ballasted to temporarily rest on the Bay floor. A pile driver and hammer would be transported by truck to the construction site and would be driven onto the flexible float platform to access the pier location. The large diameter steel pipe for the piers would be delivered on a barge. The north pier pile would be installed with a large pile driving hammer. The floats and pile rig would then be moved to the south pier location and the operation repeated to install the south pile. For the abutments on either end of the bridge, it is assumed that the same crane would be used to install the abutment piles. These piles are located out of the water and would require a smaller hammer. After the piles are installed, a reinforced concrete pile cap and bridge seat would be constructed. For the bridge installation, the prefabricated pieces of the bridge would be delivered to the site on the water and lifted into place with the crane. The prefabricated bridge truss would come with steel decking that provides the formwork for the concrete deck. After the trusses are in place, lightweight concrete would be poured in the steel decking and fInished in place. Under the Bridge #2 Trestle Alternative, a temporary 20-foot (6-meter) wide and 250-foot (76.2- meter) long trestle would be constructed on the east side of the bridge alignment. The temporary trestle would be constructed by driving steel piles into the slough, and steel beams would be placed and welded on top of the steel piles. Timber blocks would then beplaced on top of the steel beam to provide support for the construction equipment. A preliminary engineering report (AGSt 2002a indicates that the trestle could be constructed with 16- inch (OA-meter) open ended steel pipe piles spaced approximately 15 to 20 feet (4.6 to 6.1 meters) apart. The piles would support a l6-inch (OA-meter) wide flange beam framing to support 12-inch by l2-inch (OA-meter by OA-meter) timber lagging. Open ended piles are recommended as they cause the least amount of disturbance to the soil and can be easily vibrated out while dismantling the trestle (AGS, 2002a). It is estimated that six steel piles would be required at each support of the trestle and the support could span 20 feet (6 meters). A total of approximately 13 supports would be required, which means Y4204IS.00693.doc - 4/5/07 -4-