Open Cell™ Bulkhead: An economical port expansion solution

Authorship

Todd Nottingham & David Pierce, PND Engineers, Inc., Seattle, WA, USA

Publication

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Open Cell introduction

Port expansion necessitates ever-deeper water to meet the increasing vessel sizes dictated by economics. The industry has met this challenge by introducing new and better materials along with improved construction and fabrication methods.

Typically filled bulkheads have been desired over pile supported platform docks because they provide a lower initial construction cost and lower maintenance costs. These bulkheads have generally relied upon tied back z-sheets or combi-wall systems. Alternatively, in more expensive applications, the use of closed cell or diaphragm systems, with flat sheet piles have been used. However, the increase in water depths is approaching or surpassing the practical limit of these wall types. The Open Cell structure provides an economical alternative to other dock types that can meet these challenges.

Port of Anchorage

The Port of Anchorage, Alaska is the state’s largest port and serves its most populated region. All types of commercial cargo and petroleum products pass through this port and the military is an important user as well.

Space limitations on the 105-acre port site prompted investigation into ways to expand the acreage an additional 140 acres. By locating a new dock face seaward about 400 feet and expanding the deteriorating existing 2,000-foot long dock face to a new 10,000-foot dock face, a number of benefits can be realised (See Figures 1 and 2). Annual dredging would be reduced and the port could be deepened from the current El-35 to El -45-feet Mean Lower Low Water (MLLW). The longer dock face would be able to accommodate tour ships and barges as well as RO-RO and container ships of increasing size.

With geometric and use criteria established, this project has some of the world’s toughest technical issues:

Tides: El+35-feet to El-5-feet MLLW

Ice: 3-foot thickness

Seismic: 0.46g peak ground acceleration, 5 minutes duration

Phreatic Water: El 19-feet

Dock Face Height: 80+ feet

Economics ruled out conventional pile-supported platform
docks in favour of a new Open Cell Bulkhead. These structures
can have configurations with narrow open cells and long tail walls,
which will address even the most severe seismic, height and soft
soil conditions. This Open Cell system will utilise about 30,000
tonnes of flat sheet piles along the 10,000-foot dock face.
In support of Open Cell technology there are about ten such
structures totalling over one mile in length bordering Cook Inlet,
and nearly 140 structures world wide. Port MacKenzie, with a face
height of 60 feet and length of 500 feet, was built directly across
Knik Arm from the Port of Anchorage in 1998. The bulkhead has
weathered several earthquakes with no adverse affects. In addition,
it is supporting increased use primarily for bulk cargos such as
wood chips and gravel. This important structure is a prototype for
the new Port of Anchorage.

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