Design, Construction, and Installation of a Floating Caisson Used as a Bridge Pier

A suspension bridge over a river is generally supported on a structure mounted on large concrete piers placed on the river bottom near the two banks. Because of its massive size the piers are built by pouring concrete at site in a floating position. During this construction period, the floating caissons are moored in place to withstand any environmental currents and waves, which may be prevalent at the site. Since the draft of the caisson changes during construction, the mooring system must be capable of holding the caisson at all these drafts. Once the construction sequence reaches the required height, the caisson is ballasted down to rest on the river bottom and subsequently during further ballasting penetrates the soil to its desired depth. The design and construction of the piers in place and their mooring system provides several technical challenges. This paper describes these challenges experienced in the design of a caisson system for a new bridge at Tacoma Narrows, close to Seattle. This design process included towing the caissons at a shallow draft from the dock to the site, design and installation of the mooring system, construction of the caissons in place, and final touchdown and penetration to soil. The current at the Narrows was extremely high, which gave particular challenges in the construction sequence. Some of the critical areas of the associated design and their solution techniques are highlighted. It should be noted that the dimensions of the bridge caissons and their mooring system are of similar order of magnitude as typical offshore structures exposed to severe environment.