Mooring Bridge Caissons During Construction Using Jetted-In Driven Plate Anchors-PDF

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The paper describes the design, installation, and testing of jetted-in driven plate anchors used to moor floating foundation caissons during construction of the new Tacoma Narrows Bridge. The caissons were exposed to deep water and tidal currents reaching 7–9 knots, making reliable temporary mooring essential.<br /><br />Each anchor comprised a steel plate attached to a structural beam. A vibratory hammer drove it into the seabed, with water jets loosening the dense sand. After the follower was withdrawn, proof loading rotated the anchor into a horizontal position (“keying in”), maximizing resistance to pullout. Anchor design considered soil conditions, embedment, drivability, and installation effects. The team used established design charts and analytical methods to estimate capacity, then validated the design with full-scale tests.<br /><br />The anchors were designed for a total holding capacity of 4,450 kN (1,000 kips), including the contribution of the embedded chain. Required embedment was approximately 10.5 m after keying, with a minimum driving depth of 14.3 m. Installation typically took 20–40 minutes. Horizontal and vertical proof tests were used; vertical testing proved faster and required less force. All anchors passed their required proof loads, with negligible movement at the final test load. Most keyed in after 3.0–3.7 m of chain movement, consistent with predictions.<br /><br />The project demonstrated that driven plate anchors can be installed in very dense sands and provide high holding capacity at substantially lower cost than alternative anchor systems. The authors emphasize that successful use depends on sound site characterization and careful evaluation of installation effects, drivability, keying behavior, and capacity.

Keywords

driven plate anchors

Tacoma Narrows Bridge

caisson mooring

vibratory hammer

jetted installation

deep water

tidal currents

proof loading

dense sand

anchor capacity