In this paper a unique solution is presented for anchoring large Naval vessels in a difficult soil condition. Deep embedment steel plate anchors, originally designed by the Navy Civil Engineering Laboratory for explosive embedment, were installed using a long pipe pile follower. Anchor holding capacities in excess of 200 kips were obtained in 25 feet of water where 20 to 90 feet of very soft Iagoonal sediments overlie a stiff clay. Design, installation, and field testing are described. Experience and retrospective analysis are combined to offer guidelines for future deeply embedded anchor designs.
The Naval Inactive Ship Maintenance Facility exists in Middle Loch, Pearl Harbor, Hawaii, to preserve decommissioned naval vessels until such time as they are called up for service or scrapped. Since the vessels are unmanned and unpowered, they must remain moored in the hurricane force winds common to the site. A location map is shown in Figure 1.
The Inactive Ship Facility has long relied on large concrete deadweight anchors. A typically spread mooring system uses four or more mooring buoys around the moored vessel(s). The buoys are connected via riser chains to 30 ton octagonal central clumps weights. Ground leg chains connect each clump to between three and five satellite anchors. The satellite anchors, commonly known as Pearl Harbor Anchors, are generally rectangular, with a wedge-shaped leading edge to help them dig into the mud as they are dragged along the bottom. They weigh either 30 or 60 tons and have been used in Middle Loch for over 40 years, A Pearl Harbor Anchor is shown in Figure 2.
In 1982 Hurricane Iwa caused many of the moorings in Middle Loch to be displaced. An anchor test program was conducted jointly by the Naval Civil Engineering Laboratory (NCEL), Port Hueneme, California, and the Ocean Engineering and Construction Project Office, Chesapeake Division, Naval Facilities Engineering Command, to determine the red capacity of the concrete anchors. By test pulling one mooring against another, the concrete anchor systems in use were demonstrated to have only 20 to 30 percent of the previously predicted holding capacity. Chesapeake Division was then asked to design 63 safe and cost effective buoy mooring systems for ships up to 25,000 long tons displacement. These would be installed over a five-year cycle starting in 1988. The first two mooring spreads were planned to be for two Material Support (AR's) vessels and four Destroyers (DD's). These moorings are the subject of this paper.
In the northern end of Middle Loch, where these moorings are located, the water depth is consistently around 25 feet. The visible bottom is featureless and flat. The uppermost sediments are recent harbor deposits with undrained shear strength increasing uniformly with depth from around 5 pounds per square foot (ps9 at the mud line, at a rate of about 6 psf per foot of depth.
Beneath the silt is a firm sandy clay. The anchor design has to accommodate silt depths in the range 10 to 90 feet,