Klohn Crippen Berger (KCB) was retained by the US Navy as part of the Moffatt & Nichol team in early 2006 for detailed engineering for the Kilo Ammunition Wharf Extension project on Guam, Marianas Islands. Kilo Wharf, located in the outer Apra Harbour, is the primary and most strategically important ordnance facility within the Pacific fleet area of operations. The receipt and storage of containerized ordnance at Guam is essential to the Navy’s mission in the Western Pacific Region. The Kilo Wharf extension will support the new T-Class combat logistics force ships (T-AKE), currently under construction.
The original Kilo Wharf, 122 m long and built in 1987, comprises five large concrete box caissons with a mooring dolphin at either end. An additional caisson was included to combat the severe wave climate from typhoons, but it was never decked over. The extension will also utilize caisson construction to satisfy a variety of difficult site conditions, bringing the overall berth length to 244 m. Four new caissons will be built to form the wharf face, plus another smaller caisson to provide end fill retention, and the original sixth caisson will be decked over to complete the upgraded wharf. One of the existing mooring dolphins will be re-floated, repaired in dry dock and then reinserted into the wharf extension.
Guam is subject to frequent major earthquakes, being fairly close to the Marianas Trench. The existing caisson wharf was hit by a magnitude 8.1 earthquake in 1993, with minimal damage being incurred and the wharf remaining operational. The client’s brief for the berth extension project included seismic upgrade of the existing wharf to A475 design level, using the US Navy’s displacement-based seismic design criteria.
Displacement-based seismic design allows lateral sliding of the caisson wharf structure within prescribed limits, which results in a more economic design. This evolved into a unique seismic upgrade: the existing caissons will be partially excavated and some ballast rock will be removed on a permanent basis to reduce the inertial mass of the system. This “de-ballasting” operation also reduces foundation reactions as a precaution in case of partial liquefaction at subgrade level.
Apra Harbour is also subject to frequent tropical storms and typhoons. The 20 ft design wave height against the berth face was a major factor in the selection of a caisson design, to avoid the uplift forces associated with a piled deck. Design against typhoon winds necessitated the provision of crane tie-down points within the deck structure. Coral reefs are located immediately adjacent to the planned extension, imposing severe environmental restrictions on construction means and methods.