The upgrade to a 700-meter-long section of the existing apron at Terminal 46 (T-46) for the Port of Seattle posed numerous geotechnical challenges. Since its construction in 1977, the apron, supported by prestressed concrete piles bearing in loose to dense alluvial soils, has experienced settlement and lateral movement on the order of 0.3 to 0.6 meter. The adjacent sheetpile bulkhead has lost as much as 3 meters of lateral support because of slumping of the submarine slope. The geotechnical challenges included the assessment and mitigation of the potential additional movement of the marginally stable submarine slope and damage to existing facilities caused by pile driving, and the evaluation of pile types that would achieve the required axial pile capacity within the alluvial soils. An indicator pile program, consisting of driving single and grouped, 0.6-meter, octagonal, prestressed concrete piles and single, 0.6-meter-diameter, open-ended steel pipe piles, was implemented prior to awarding the construction contract. Instrumentation was installed at several locations to measure the generation and dissipation of porewater pressures and submarine slope movement during and after indicator pile driving. The results of the indicator pile and instrumentation program allowed the designers to select appropriate pile types and to develop an installation specification that would minimize adverse impacts of pile installation to the stability of the submarine slope and apron. The piles were successfully installed within budget and without damage to the existing facilities.