Settlement of the USS Arizona, Pearl Harbor, Hawaii

Abstract

The U.S. Geological Survey, in collaboration with the National Park Service Submerged Resources Center, undertook investigations at the USS Arizona Memorial at Pearl Harbor, Hawaii, in 2002, 2003, and 2005 to characterize geological factors affecting the deterioration and movement of the hull of the USS Arizona. Since sinking on the morning of December 7, 1941, the hull of the USS Arizona has been slowly but steadily disappearing below the surface of Pearl Harbor. Continuous sediment coring at three of four locations around the hull of the Arizona was only partially successful, but it was sufficient to identify a varied sedimentary substrate beneath the hull. A boring near the stern reveals a thick, continuous sequence of soft, gray clay to the bottom of the boring. In contrast, borings near the bow and starboard side, below about 5 meters subbottom depth, indicate the presence of very stiff, brown clay and coral debris and an absence of soft clay. Multisensor core logger scanning of the recovered cores distinguishes the lower density of the soft, gray clay at the stern from the higher density of the stiff, brown clays and coral debris at the bow and starboard side. Uniaxial consolidation testing of the soft gray clay indicates a normally consolidated sequence, whereas the stiff, brown clay and coral debris are overconsolidated. Profiles of shear wave velocity vs. depth obtained through spectral analysis of interface wave testing around the perimeter of the hull in 2005 identified areas of higher velocity, stiffer sediment at the bow and starboard side, which correspond to the dense, stiff clay recovered near the bow and starboard borings. Low shear-wave velocities at the port midship and quarter of the hull correlate with the lower density, softer sediment recovered from the boring at the stern. Cross sections of the subbottom of the Memorial combine results from the sediment borings and geophysical surveys and depict a wedge of soft clay unconformably overlying the stiff clays and coral debris beneath the aft half of the USS Arizona and thickening toward the stern. The 2008 position of the hull has been documented using both tide-based and differential Global Positioning System (GPS) measuring systems. Analysis of historical and recent photographs was done to create a record of settlement from the time of sinking in 1941 to the present. By examining shadows in suitable photos, the sun azimuth, local time of day, and tide levels were determined to derive tide-adjusted and sea-level-rise-corrected elevations for structures on the hull and from these elevations to obtain settlement and tilt trends. The settlement trends, most complete for barbette 3, have two components. An early, nonlinear component ends on December 9, 1941, and represents the initial penetration and displacement of the bottom sediment by the hull. A linear, long-term trend of normal consolidation continues to the present day. Long-term settlement rates are greatest at the stern and decrease linearly to the midship, showing that the aft half of the hull is moving as an intact, rigid body. The recent rate of settlement at the stern is about 3.5 mm/year; rates at the starboard midship and forward part of the hull are less than one-third of the stern rate. The aft half of the USS Arizona hull presently tilts about 2 degrees to port, an increase of at least 1.5 degrees since the initial sinking of the ship. The results of this study identify differential settlement of the Arizona hull, due to the wedge of soft clay underlying the aft half of the hull, as the cause of the movement of the hull beneath the surface of Pearl Harbor. Calculation of sediment consolidation using lab-determined properties of the soft clay demonstrates that the observed settlements can be reproduced by projecting appropriate clay thicknesses beneath the hull. Several of the high-quality photographs analyzed for the historical settlement analysis highlight some of the limitations of this retrospective technique for determining tide-based elevations. In these cases, calculated structure elevations do not conform to the settlement trend, indicating that there can be complicating factors affecting the interpretation of the photos. Conflicting dates for events during the salvage operations were also encountered.