Seismic Retrofit and Improvement of Alpha-Bravo Wharves in Guam

Guam has experienced several large earthquakes in the past several decades. The 1993 earthquake with a moment magnitude of 7.7 caused significant damage throughout the island. Liquefaction and lateral spreading caused damages to commercial and naval port facilities at Apra Harbor. The Alpha-Bravo wharf, built in 1947 without proper seismic considerations, suffered some damage during the 1993 earthquake, including backland liquefaction. The U.S Navy commissioned a seismic improvement program that included upgrade of the Alpha-bravo wharf. The improvements to the wharf included increasing the water depth from approximately 10.7 m to 12.8 m, and retrofitting the existing wharf to meet current seismic and future operational requirements. A two-level seismic performance criterion was used: under the Level 1 earthquake associated with a 72-year return period, wharf elements sustain only minor or no structural damages with no interruption to operations; under the Lever 2 earthquake events with 475-year return period, the wharf elements undergo controlled inelastic structural behavior with repairable damage. Sheet piles and tie-back system were designed to accomplish the seismic retrofit and improvement of the wharf. In order to withstand high tie-back forces, the anchor blocks in the backlands were supported on battered, steel H piles. During construction, the test piles driven at anchor blocks showed softer driving, and dynamic pile analyses results indicated lower axial capacities than the design capacities. Adjustments were made by adding lugs to piles and increasing pile lengths to incorporate the findings from test piles. Partnering meetings and proactive participation of all interested parties including the owner, designers, and contractor throughout the construction of the project helped minimize project delays and cost overruns.